Multiple bottlenecks, allopatric lineages and badlands bison bos bison by joelbergerconservation

0006-3207(94)00006-9

MULTIPLE BOTTLENECKS, ALLOPATRIC LINEAGES AND BADLANDS BISON Bos bison: CONSEQUENCES OF LINEAGE MIXING Joel Berger & Carol Cunningham Program in Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, Nevada 89512, USA

(Received 9 April 1992; revised received 7 January

1994; accepted 2 February

1994)

populations to be selected for re-introduction. This applies to both wild and captive populations. The prevalence of outbreeding and inbreeding tolerances within populations of managed and protected species needs verifiable documentation.

Abstract While ecological and conservation consequences of combining animals of varied genetic backgrounds have been widely discussed, the demonstration of eflects that stem from lineage mixing remains elusive. Since management agencies relocate populations or supplement them with individuals regularly, the opportunity for either inbreeding or outbreeding depression may be high: still, any putative efl&ts will go unnoticed without detailed knowledge of life-history and behaviour. Here, we report potential consequences of lineage mixing in a restored population of North American Bos bison studied for jive years. In 1984 two allopatric lineages became sympatric in Badlands National Park, South Dakota; they difSered in both founding population size and the number of demographic bottlenecks experienced since 1907. Measures of reproductive variance in both sexes were employed to estimate eflective population size based on 261 copulations and the survivorship of calves between 1985 and 1989. We assumed that the reproductive variance and mortality documented in this study are representative of the bison’s recent past and based on this assumption we calculated N, separately for each generation for which the lineages were allopatric. Four potential correlates of fitness were studied in the new sympatric population: (1) $emale fecundity; (2) juvenile survival; (3) growth rates; and (4) female age at puberty. Of these, neither female fecundity nor juvenile survival was associated with lineage but growth rates were more rapid and ages at puberty were lower for F, purebred (inbred) juveniles than for F, hybrid (outbred) juveniles. Possible consequences of this variation in the F, generation include (I) higher winter mortality in the slower growing line as well as (2) decreased lifetime production of young; both are life-history parameters that could be interpreted as long-term selection against outbreeding. However, these data by themselves do not constitute support for an outbreeding depression hypothesis. The failure of males from one lineage to mate at all prevented the possible combinations of the F, generation needed for the appropriate statistical contrasts. Nevertheless, these interpretations (I) substantiate a level of variation in life-history parameters stemming from lineage mixing: and (2) suggest that advice regarding prudent conservation strategies must be sought concerning the genetic histories of individuals and

Keywords: population outbreeding, fitness, ogy, bison.

bottlenecks, genetics, inbreeding, re-introduction, behavioural ecol-

INTRODUCTION The mixing of discrete lineages to enhance genetic diversity in mammalian populations has been controversial for at least 125 years (Shirley, 1867; Smith, 1979). Even today, effects of inbreeding and outbreeding in wild mammals are still uncertain (Shields, 1982, 1993; Ralls et al., 1986). In species whose extant populations were derived entirely from single founding events, such as wisent Bos bonasus or black-footed ferrets Mustela nigripes (Olech, 1987; Ballou, 1989), the potential for deleterious effects exists if new populations are discovered and/or distantly related animals are bred with descendants of the founders. In fact, the same result can easily be envisioned for populations of more common species given the rapidity with which management agencies practise translocation. For example, lions Panthera leo in Kenya have been transplanted to existing populations in Botswana; more striking perhaps is that the endangered Sonoran pronghorn Antilocapra americana sonoriensis has been maintained with pronghorn from northeastern California in at least one Mexican reserve (Berger, unpublished data). One way to gauge strategies of demographic recovery (i.e. translocation, re-introduction, or supplementation) is to assess the structure of already established populations after new individuals have been introduced. Nevertheless, to develop realistic criteria for judging the success of such conservation actions will be complex - it will require knowledge about the extent to which populations have been fragmented and for how long (generations of isolation). In North America, few mammals exist where data on population sizes have been known with precision over long periods. One rare exception is North American bison Bos bison, a species estimated to number between 13

J. Berger, C. Cunningham

30 and 60 million individuals last century but reduced to less than 1000 animals by the 1880s (Roe, 1970). Populations today are descended from far fewer individuals. Even so, reliability and consistency in data on reproduction and population translocations are often poor. Here, we report on the fates of discrete lineages separated for a minimum of 77 years and later restored to Badlands National Park, South Dakota. Specifically, we use field data gathered over 5 years (1) to describe the mixing of two lineages; (2) to offer retrospective estimates of N, for past allopatric generations; and (3) to examine four fitness correlates in the F, descendants of the new population. BACKGROUND

AND METHODS

Study area, population history, and differentiation of lineages The current population of plain’s bison in Badlands National Park, South Dakota, USA (40”50’ N, 102”20’ W) was founded in 1964 by 28 animals whose origins were traced to six Nebraska animals (two males and four females in 1907) in what is now the Fort Niobrara National Wildlife Refuge. Between 1913 and 1964 at least 11 males from other states (Montana, South Dakota, and Wyoming) were added to this lineage. Subsequently, the population experienced removals resulting in multiple bottlenecks (see below). In 1984 20 Colorado animals supplemented the Badlands population; all were descended from three animals (one male and two females) traced to Colorado National Monument around 1925 and originally from Denver City Park, in at least 1907 (Berger & Cunningham, 1994). For simplicity animals of the two lines are designated NL (Nebraska lineage) and CL (Colorado lineage) respectively. Data on changes in population size for both allopatric lineages and for the sympatric population (after re-introduction into Badlands National Park) stem from four sources: (1) reports of the American Buffalo Society housed in the Archives of New York Zoological Society (Bronx, New York); (2) unpublished data in the US Fish and Wildlife Service files (Valentine, Nebraska); and unpublished reports or files of the National Park Service at (3) Colorado National Monument (Grand Junction, Colorado) and (4) Badlands National Park (Interior, South Dakota). Badlands National Park (approximately 1000 km2) is the largest short-grass prairie national park with bison in the United States, although it covers a mere fraction of the area used historically by bison. The study region within the park was the 250 km2 Sage Creek Wilderness Area. During the first 4 years of the j-year (1985-1989) field project the bison were unmanaged. During the final year, a portion of the growing population was translocated elsewhere. During the study, the population increased from about 300 to 775. We and our 16 co-workers observed bison for more than 8750 h which resulted in information on 261 copulations (Berger & Cunningham, 1991). More than 200 animals were known individually

based on ear-tags, brands, and combinations of distinguishable head hair and horn patterns. Animals were handled more than 1400 times during capture operations involving immobilization or driving animals into a fenced area. Bison were then weighed (on cattle scales), aged, and measured morphometrically (Berger & Peacock, 1988; Berger, 1989). Individuals of the two lineages were identified as follows. All of the CL bison received conspicuous red, numbered cattle ear-tags prior to their release at Badlands and males were freeze-branded with numbers on their hips. None of the resident Badlands bison (all of NL descent) had brands; some NL animals initially received cattle ear-tags or numbered silver clips in their ears following brucellosis vaccinations several years earlier. Additionally, the animals of CL ancestry had darker pelage. Therefore, an animal’s ancestry was known by noting fur color, the presence or absence of tags (128 were attached during this study to facilitate identification of NL individuals), and brands (for males only). Assessment of growth, reproduction, and effective population size The birthdates of neonates were known precisely or estimated to within 3 days (Berger, 1992). Juvenile growth rates were assessed by using a digital caliper (Mitutoyo 500 Series, Arcata, CA) attached to a Nikon 300-mm telephoto lens. This technique measures with up to 99% accuracy the distance between an object and the photographer (Jacobsen, 1986). At every opportunity we photographed known-aged calves, and scaled the prints on a digitizer to predict the relationship of body mass to head length and width. For 29 captured juveniles, head measures accounted for 84 and 87% of the variance in weight, Y,,,,,, = 4,03X, + 6,86X, 171.95; Y,,,,, = 4,19X, + 5,84X, - 105.24, respectively, where X, is head width and X2 is head length in cm. To determine body condition in adult females, we fit data to a multiple regression using body mass (dependent variable) and head width, date of weighing, and parity as independent variables. A single morphological variable, head width, was then used as an indicator of body mass since the exponential equation (Y = ABX) explained the greatest amount of variation in mass (r2 = 0.76; Berger & Peacock, 1988). It was necessary to rely on multiple regression to control for effects of mass change at different times of the year. For instance, on average, the same-sized females at winter’s end weigh only about 80% of what they will in summer (July). Because head size and mass were known, points falling above or below the exponential regression were assigned to an ‘above’ or ‘below’ average body condition category with date controlled. While this method is admittedly crude, it offers an alternative to evaluating breeding performances in relation to body mass per se. Just as large animals may be in poor condition, small animals may be in good condition. The potential effect of maternal dominance on offspring growth was examined by construction of matrices of social interactions. The effect of age was

Bottlenecks, lineages, and bison discounted by using only females aged 5-13 years, the cohort in which reproductive variation was undetectable (see below). Dominance and subordinate relationships were determined by noting whether each participant in dyadic encounters was either a winner (e.g. supplanting) or a loser (e.g. moving from the other individual). Dominance was ranked ordinally by the outcomes of these encounters annually; the highest ranking individuals had the greatest disparity in ‘wins’ versus ‘losses’ per year. When ranks tied, individuals were given a higher status if an additional win gave them a higher percentage. Lineage dominance for mothers was gauged in two ways. First, using the total number of interactions, irrespective of the lineage of the dyadic partner, we asked whether an association between rank and lineage existed. Second, when encounters were with dyad members of the opposite lineage we checked for any relationship between lineages using only data when interactions were with dyad members of the opposite lineage; that is, between-lineage dominance was determined by excluding within-lineage interactions. For instance, cows interact with conspecifics irrespective of lineage. We simply evaluated whether one lineage was consistently dominant by focusing on interactions between mothers of the two lines. Reproductive behaviour was evaluated using both focal animal (continuous) and instantaneous (point) techniques. During the rut (July-August) we and our co-workers gave the highest priority to focal consorts in which members of both sexes were identified. Second and third priorities, respectively, were given to consortships in which males were known, and in which only females were known. Our sampling scheme could not be ideal under the circumstances (not every individual in the population was known, thus we relied on a subsample) and undetected biases may have been introduced. For unknown males we had no way of knowing whether the same individuals were repeatedly observed or for what periods of time, irrespective of whether or not they had copulated. Nevertheless, we attempted to minimize these problems by focusing on known animals as much as possible and using rates of behavior (number of events or observations of known individuals/time). We estimated reproductive success (RS) directly in females by counting the number of offspring produced over the study duration. Calf survivorship at Badlands exceeds 98% (Berger & Cunningham, 1994). For males we counted the number of copulations each had with different females. This observational method suffers from several potential biases which we attempted to control for. (1) Copulations might occur at night. If this were primarily the case, we might miss a high proportion of matings. To check this possibility, we compared over IO-day periods in 1986, 1987, and 1988 the change in proportion of cows with fully erect ‘tail up’ postures (a sign of being recently copulated with, lasting up to 6 h;

Berger, 1989) at two periods, 0800-2000 h and 2000-0800 h. If more copulations occurred during the night than the day, then the relative number of cows with new tail ups should be higher early in the morning. For these analyses, we used only data gathered on groups with more than 200 animals that did not move more than 1 km during the night or day, thus assuring us of approximately the same group composition. Differences between night and day were not detectable [Mann-Whitney U Test: 1986 (U = 38, NS), 1987 (U = 36, NS), 1988 (U = 41, NS)]. (2) Copulations might not equate to RS per se since males may mate with females more than once and more than one male may copulate with the same female. The proportion of females mating with different males has generally been assumed to be less than 5% (Lott, 1979). However, our data suggest a value closer to 10%. We could not be confident of which male actually fertilized a female, so we assigned each copulating male a proportion of the putative offspring fathered. Thus, if two males copulated with the same female each was assigned 0.50 offspring. In only two of 261 (0.8%) cases were more than two males observed to copulate with the same female. Finally, we assumed that the males who copulated the most had relatively higher reproductive success. (3) Because individuals were observed for different periods, it was possible that whichever was watched the most could have the highest RS. To standardize comparisons across males, we restricted analyses to focal animals and recorded the number of copulations observed hourly. Otherwise, without sampling animals for every active hour throughout the breeding season it would be impossible to derive a reasonable expectation for the total frequency of copulations achieved by individuals over the entire rut. In attempting to circumvent this problem, we relied on data gathered in two ways. First, we evaluated on a yearly basis the relationship between copulations and observation time, using males that had copulated at least once. The asymptotic point, at which copulations no longer increased with observation time, was used to mark when copulations per hour would no longer scale to total hours observed. Second, because we knew how long (in days) males associated with females, we considered the total time that the male participated in the rut. By taking into account participation time and knowing the asymptote, we could derive an estimate of total copulations per male per season. For instance, in 1986 there was no relation between copulation frequency and observation time beyond 27 h. Therefore, a bull observed for 30 h and which mated six times during the 1986 season would be credited with a total of six copulations. In contrast, an individual observed for 20 h and which copulated three times (0.15/h) would receive a seasonal total of 3 (actual copulations) + 7(0.1 S)(prorated copulations) or 4.05 copulations. Actual copulation frequency was counted in cases where individuals exceeded the asymptote. In cases where individuals fell below the asymptote, we estimated the number of days at the rut and then multi-

J. Berger, C. Cunningham

plied this value by their copulation rate based on 15 h/day (the amount of time data were gathered daily). Two additional points are worthy of mention. (1) Although we initially tried to sample all known individuals for equal periods, it soon became obvious that some had a much higher probability of mating than others. Therefore, we deliberately oversampled males from both extremes (e.g. high and low probability of mating) at about the same rate, a procedure important not only in assessing copulatory rates of individual males but in contrasting mating performances between lineages. (2) Also, a question arose as to whether males that were sampled for only a few hours and were not observed to breed would have eventually done so had our sampling of those individuals been longer. Following PruettJones and Pruett-Jones (1990) we retrospectively compared sampling efforts for non-mated males with the number of elapsed hours until the first mating of breeding males (matched by age) was detected. The latter was indicative of the necessary sampling effort to observe at least one mating. In 3 of 4 years, no differences existed while, in the fourth year the time needed to observe the first copulation was less than the number of observation hours (sampling effort) for non-mated males (1986: t = 1.46; d.f. = 16; NS; 1987: t = 2.02; d.f. = 26; p c 0.05; 1988: t = 0.97; d.f. = 36; NS; 1988: t = 1.15; d.f. = 40; NS). These comparisons suggest that our sampling efforts were sufficient to detect any breeding in the unmated males, had they competed successfully for copulations. In estimating N, in the extant population we contrasted values derived by two commonly employed methods. The first is based on breeding sex ratios, rather than simply using the number of adults in the population, in which N, = 4N,NF\I, + N, when N is the number of breeding males (m) and females (f), respectively (Wright, 1931). The second, N, = 4N-2/V, + 2, accounts for variance in progeny production (V,) in each sex (Crow & Kimura, 1970). We then combined the separate measures for males and females to achieve an ‘ideal’ population (e.g. assuming a binomial distribution) by using the first equation (after Koenig & Mumme, 1987). To consider potential losses in heterozygosity in the two lineages prior to their sympatry at Badlands, it was necessary to estimate N, over multiple, unstudied generations. Because all other demographic data, except population sizes, were lacking over most of these periods, several simplifying assumptions were necessarily employed. This is a common practice for large mammals (Harris & Allendorf, 1989). (1) The generation interval is 6.75 years (the mean of the eight populations simulated by Shull & Tipton, 1987). (2) The birth sex ratio is 50: 50 (Green & Rothstein 1991). (3) Neonate survivorship approaches 100% (Green & Rothstein, 1991). (4) More than twice as many adult males were removed than females (given that the sex ratio of reintroduced CL adults consisted of twice as many females). (5) Breeding ages for females and males begin at three and four years respectively (Berger & Cunning-

ham, 1994). Additionally, since information on population size was not available for every past generation, we used data from the year determined to be the ‘next generation’. This was done by devaluing a lineage’s population size by the exponential rate of increase for years when population counts were unavailable, which we determined to be 16.7% for the Badlands population during our field study. Following Lande and Barrowclough (1987) we estimated N, as l/2 [1-{i,[l-l/ZN,(i)]}“], where t is the number of generations; use of the geometric mean accounts for fluctuating population size.

RESULTS

AND DISCUSSION

Reproductive variation The maximum number of calves expected by females over the 5-year study was five since bison are capable of producing one calf per year and twinning is rare (Green & Rothstein, 1991). From 1985 to 1989, 17% of the females 3 years and older in 1985 produced no progeny; 30% had four calves and 26% had five. When only the 4-year period, 19861989, is considered 7% never had calves while 24% produced the maximum, four calves (e.g. one per year). Age influenced reproduction (Fig. 1). Analyses of variance on data normalized by square root transformation revealed that after the age of 14 years, female reproduction declined (F,,,,, = 198.13; p < 0.0001) but between the ages of five and 13 years, cows did not vary (Student-Newman-Keuls (SNK) test between cohort contrasts; NS). For males 4 years or older when the study began, the number of putative offspring sired varied considerably. Six of 54 bulls (11% of the total) accounted for 48% (92 of 192) of the progeny. Considerable age-graded variation existed among males (F,,,,, = 5.13; p < 0.001). Those in the 7-12-year-old cohort were the most fecund followed by 13-14, 15+, and 6-year-olds (Fig. l), although these last three cohorts did not differ from one another (SNK tests). The most successful male in a given year copulated with a minimum of nine different females, four in 24 h, and was estimated to have mated with 12 females. However, large inter-age differences existed and the median number of calves sired was zero in six of the seven age cohorts; it was only in the 11-12-year-old cohort that males achieved more than 50% of the copulations (Fig. 1). To control for age-related differences, we contrasted the breeding performances of seven males (between the ages of 7 and 9 in 1985) over the study’s duration. The most successful male mated with 28 females and the least successful with none. The variation among the seven animals was substantial (Kruskal-Wallis Analysis of Variance: H = 19.92; p < 0.001) and likely to result in lifelong reproductive differences, since one unmated male died and another suffered a fractured pelvis. The evidence also indicates that the greatest differences in

Bottlenecks, lineages, and bison reproduction arose once individuals reached prime age (7-12 years). The within-cohort variance in RS was highest for prime males than for males 213 years (test for Homogeneity of Variance: F,,.,, = 1.79; p < 0.02) and interindividual differences in RS were not detectable for six old males aged at 2 13 years in 1985 (H = 8.71; NS) although such variation occurred among prime males (see above). The extent to which males and females differed in their reproductive contributions to future generations was gauged as follows. We contrasted the number of offspring produced by individuals during their prime (5-13 years in females and 7-12 years in males) as this was the period of greatest reproductive variance and likely to have substantially greater effects on the next generation. This contention would be untrue if age-dependent mortality occurred but this was not the case at Badlands. Additionally, where females have relatively minor variance in RS, lifetime measures may be unnecessary (see Nishida, 1989). In any given year, RS for prime males was higher than that for females. Means (variances and sample size) for males and females, respectively, were: 1985, 0.88 (3.2, IO), 1 i 09

080.7 I 5 u

06; 05

5 5 z

04 03 02

0 I

6-7

lo-11 12-13 14-15 16+

8-9

Age (YW 2.6

Table 1. Effects of different multiple-year periods on the mean and variance in reproductive success of prime-aged male and female bison as described in the text Males

1985-1987 19861988 1985-1988 1985-1989

Females

Mean

Variance

Mean

Variance

5.7 4.0 4.9

38.2 19.5 33.5

1.9 2.1 2.5 2.9

1.1 0.9 1.9 2.5

0.72 (0.20, 28); 1986, 1.38 (6.1, 20), 0.58 (0.24, 40); 1987, 1.36 (5.2, 25) 0.70 (0.21, 51); 1988, 1.02 (4.0, 34), 0.64 (0.23, 60); 1989, (females only) 0.55 (0.25, 41). There is a discrepancy between the sexes because our sampling focused on a subset of the population (e.g. known individuals only). Not only were variances in progeny production smaller than average for females but annual variances ranged from only 0.20 to 0.25 whereas for males they varied from 3.2 to 6.1. However, measures based on only a single season inflate differences among individuals (Clutton-Brock et al., 1982; Arnold & Wade, 1984). To maximize the reproductive period spanned by individual lives during our study, we contrasted reproductive variance over differing but continuous 3- and 4-year periods. The measures are likely to be conservative since any mortality which might have occurred at young ages was omitted and the exclusion of nonbreeding animals is apt to result in heightened selection (Howard, 1988). Male values were considerably higher and more varied than those for females (Table 1). The change in reproductive variance that occurs by including multiple years is dramatic. For males the most conservative disparity (e.g. the lowest three measures of annual variance relative to the highest) was 19.516.1 or more than a magnitude of 3. However, for the 4-year period (1985-1988) the magnitude of differences was 5 l/2 times. Despite their overall lower variance, female differenees were more impressive, reaching 1.91 for the 1985-1988 period, an increase of eight times over the variance in annual RS during 1986.

22 I

2.42 I 184

116+ : ;

121 143

s =

11 084 0.4 02 06 0 I. 4-5 N

7-8

l.t

9-10

11-12

13-14

34 Age

15+ 22

(w)

Fig. 1. Mean fecundity in cows (1985-1989, top) and bulls (1985-1988, bottom) expressed as calves/year. Bars are standard errors and dots are median values (from Berger & Cunningham, 1994).

Effective population size of the lineages during allopatry and sympatry We estimated N, for each of the 4 years that male RS data were available from Badlands, first contrasting the estimates invoking only the breeding sex ratio with those based on reproductive variance (Table 2); in comparing annual variation, we adjusted for fluctuating population size as recommended by Lacy and Clark (1989) since the Badlands population was growing. However, because N, varies with choice of annual or lifetime measures and is sensitive to sample size, we also recalculated N, using data on only prime-aged animals during the 3-year period with the greatest number of animals (1986-1988). Total population size (harmonic mean) over this period was 137 which included 68 adults. The NJN

J. Berger, C. Cunningham

Table 2. Summary of annual N, and NJN estimates based on breeding adults only and variance in reproductive success as described in the text employing adjustments for fluctuating population size (from Berger & Cunningham, 1994)

Reproductive variance

Breeding adults only Year

No. known (and breeding animals) in the population

N,/N

NJN

1985 1986 1987 1988

112 (30) 139 (40) 169 (58) 181 (58)

16.00 33.71 44.37 44.51

0,286 0,449 0,431 0.420

23.91 28.06 41.42 44.67

0.427 0.374 0.402 0.42 1

based on breeding sex ratio and reproductive variance is 0.61 and 0.68, respectively. Note that the NJN (Table 2) changes substantially with measures based on multiple years irrespective of method. Given a population of 137 bison with 68 adults, the N, ranges from 21 to 46 depending upon the selected measure of N,. The V, reduction in NJN over this period was greatest, 25% over that of annual breeding seasons (a mean of 0.40 + O-32 - 1.0); when LRS was taken into account by Koenig (1988), the reduction over annual breeding in acorn woodpeckers, for example, was approximately 60%. Assuming that the mating asymmetries we report are representative of bison in general (undoubtedly this is not the case since variation amongst sites will occur for many reasons), the N, for prior generations of bison descended from the CL and NL can be estimated retrospectively employing the methods outlined earlier. During the nine generations from 1925 and 1981 the CL fluctuated very little in population size, numbering

most often between 30 and 40 animals. On a per-generation basis, N, never exceeded 3 (Fig. 2). For the NL, fluctuations in population size and N, per generation were much greater, reaching a high of 82 in 1977 (the 12th generation; Fig. 2). While at Badlands, N,s were also estimated for each lineage, separately (generations 9 for CL and 10-14 for the NL); when based solely on the sympatric population size during the 14th generation (1990), N, approached 100 (Fig. 2). Nevertheless, the cumulative number of generations must be accounted for because population sizes may increase substantially over even a few generations and the overall N, is strongly affected by the number of animals during the first bottleneck (Lande & Barrowclough, 1987). For the two allopatric lineages, allopatric overall N,s are 2.42 (CL) and 9.46 (NL). Once sympatric, the combined N, is 10.10 (the triangle in Fig. 2). Clearly, both lines have been inbred for a long time. A different way to project the expected magnitude of per generation loss in heterozygosity is by comparing actual lineage data with the theoretical number of breeding males and females needed to prevent per generation losses greater than 1% and 05%. This is typically expressed as F = 1/2N, where F is the rate of change of selectively neutral heterozygosity (Frankel & Soul& 1981). In eight of 13 generations, the NL had an insufficient number of breeding adults to minimize potential losses of greater than 0.5% (Fig. 3). More striking is the extent to which the CL animals were below the critical values necessary to minimize less than 1% loss per generation (Fig. 3). Does it automatically follow that no deleterious effects of inbreeding occur because electrophoretically detectable genetic variation is lacking in the two lineages (McClenaghan et al., 1990)? At least one argument favors the idea. Both bison and wisent passed through successive bottlenecks and, since current repro-

100 90

(NL) = 9

46 (m)

120

60 70

1977

l~cL‘1953

100 P

110

i! L

90 80

40 30 20 10 0 1

234567

Generations

OI’I Fig. 2. Changes in estimated N, or fluctuating

NL and stable CL bison populations prior to sympatry (in 1984) and after (generations 13 and 14 on the NL scale reflect combined values). Bottlenecks refer to reductions in population size when animals from the source population were transplanted to a new site prior to arriving at Badlands National Park. Mean N, for each lineage accounts for the number of generations (as described in the Methods) and the triangle at 10.10 is the combined value for both lineages when sympatric in 1990 (from Berger & Cunningham, 1994).

100

Number of breeding males

Fig. 3. Relationships

among number of breeding males and females needed to account for less than 1% (N, = 50; lower continuous line) and 0.5% (N, = 100; upper line) loss of heterozygosity per generation. Points represent each generation for NL and CL bison by year (designated for NL based on actual population estimates but omitted for CL)(from Berger & Cunningham, 1994).

Bottlenecks, lineages, and bison 190

duction is robust, it has been suspected that harmful alleles could have been purged (Slatis, 1960; Hall, 1990). A different issue, however, is whether it would be prudent to neglect the absence of variation in populations that have obviously become inbred, especially since potentially negative effects of consanguineous matings have been alluded to in species with little electrophoretically noticeable variation (Ralls et al., 1986; Ballou, 1989).

19 -

170 150 2 E .g

130

110

Lineages and potential fitness correlates To assess whether the Badlands lineages differed in traits associated with fitness, we examined four characteristics - female fecundity, juvenile survivorship, juvenile growth rates, and female age at puberty. Except for the first of these, our analyses focused on juveniles of the F, generation. It is essential to note that of 32 NL males and five CL males matched by age to correct for age-graded effects on reproduction (Clutton-Brock et al., 1982), none of the five CL males were observed to copulate over the study period, whereas 19 NL males bred. The binomial probability that the five CL males would be selected at random from the total population of non-breeding males is 0.01. This result is unlikely to have arisen from a sampling bias because CL males were observed on average for nearly 50% more hours than NL males (67.6 vs. 45.9, respectively; t = 2.15; d.f. = 17; p < 0.05). Although we had no way to evaluate whether these non-breeding males were infertile, they attempted to compete (albeit unsuccessfully) for mates, and their lack of breeding appeared to result from their low social status. Nevertheless, the more important point is that the F, juveniles were products of either NL male X NL female (purebred) or NL male X CL female (hybrid) matings, designated respectively as inbred or outbred. Neither annual juvenile mortality (x2 = 0.005; n = 236; NS) nor calf production/year for females 3 years or older (xl = 0.09; n = 161; NS) varied between lineages. However, juvenile growth differed dramatically (Fig. 4). Simple linear regressions of weight (Y) on age (X) [Y,, = 132.95 log X - 179.12 (r* = 0.66, p < 0.001) and Y,, = 176.33 log X - 225.68 (r* = 0.75, p < O.OOl)] differed between the inbred and outbred juveniles (t = 3.88; p -c 0,001). At 180 days of age, juveniles of NL descent weighed 172 kg, whereas those of CL mothers were 121 kg; in other words, outbred calves were only 70% the size of the inbred juveniles.

50 30 L 1.4

: 1 6

1.8

Log age (days)

Fig. 4. Growth rates of F, inbred and outbred calves. Regression

lines are both significant at p < 0,001 and the slopes differ from one another @ < 0.001) (from Berger & Cunningham, 1994). On average CL mothers were smaller than those from the NL. A mean maternal mass per lineage for five to 13 year olds during July was estimated at 401 (SE + 7.1; n = 29) for CL and 462 (SE + 4.4; n = 56); these differences were highly significant (t = 6.74; p < 0.001). To test for possible confounding effects of maternal mass, juvenile age, and year of birth on juvenile growth rate, we used partial correlation analysis; second-order correlation coefficients differed only for juvenile age (t = 18.01; p < 0.001). This analysis suggests that differences between lineages could not be explained by maternal mass or year of birth. Within-lineage sex differences in growth did not occur (outbred: t = 1.78, d.f. = 40, NS; inbred: t = 1.38, d.f. = 23, NS). In addition to differences in growth, the ages at which young females produced their first calves differed between outbred and inbred groups. The F, outbred females had their first young later (X = 3.9 * (0.2)) than did F, inbred cows (3.2 + 0.1 years; t = 2.10, d.f. = 28, p < 0.05), an unsurprising find given the slower growth rates of the outbred juveniles. Yet what was unanticipated was that outbred young females were more than 30 kg lighter (X = 385.1 f 9.8) than their inbred counterparts (X = 416.2 f 12,3)(t = 1.72; d.f. = 25; p < 0.10) at their first parturition despite being older than those of pure NL descent. Our results concerning direct and indirect correlates of fitness are summarized in Table 3. We detected no

Table 3. Summary of possible and actual mating relationships of male and female bison of Colorado (CL) and Nebraska (NL) lineages introduced into Badlands National Park, and effects of mating relationships on four life-history traits for the F, generation (from Berger & Cunningham, 1994)

Male (A) (B) (C) (D)

CL CL NL NL

x x x x

Female CL NL CL

F, Pure (inbred), none produced Hybrid (outbred), none produced Hybrid (outbred) Pure (inbred)

Comment or parameteP CL males did not mate males did not mate Growth slower*, puberty later*, fecundityNS, juvenile mortalityNS Growth faster*, puberty earlier*, fecundityNS, juvenile mortalityNS

“Asterisks indicate differences between F, outbred (e.g. CL X NL) and inbred (NL X NL) progeny are significant at p < 0.05. Not significant, NS.

J. Berger, C. Cunningham

differences in juvenile survivorship nor fecundity of mothers of respective lines. However, outbred F, juveniles did not grow as fast and puberty was later than it was for juveniles of the inbred line. Although we found (1) retarded growth in male and female calves, and (2) later puberty in females of the CL lineage (which had fewer founders and no immigrants), the data were restricted to the F, generation only. Why the effect arose is difficult to say. If the differences were a consequence of inbreeding, then the CL X CL and NL X NL matings would have produced purebred offspring that were slower growing than the hybrids. Alternatively, if outbreeding depression occurred, hybrid (=outbred) offspring of reciprocal NL X CL matings would be slower growing than those of purebred lines. Unfortunately, the absence of breeding by CL males (Table 3) prevents discrimination between the conflicting predictions. Alternative explanations and the possible effects of other variables As is true of virtually any field study, particularly those of large mammals, data are often unavailable to allow a clean separation of environmental from genetic effects. Of the many potential sources of variation which might account for the diminished growth we detected, four stand out. First, although maternal size had no effect on growth trajectories, offspring growth could have been compromised if CL and NL mothers differed consistently in body condition rather than mass per se. However, a lack of differences in body condition indices between parous NL and CL females (G = 0.38; n = 65; NS) suggests that this was not the case. Therefore, there is no evidence that a source of variation in juvenile growth rates stems from maternal size or condition. Second, the CL animals may have become locally adapted to desert-like conditions in western Colorado prior to their translocation to the Badlands. This too seems improbable since the CL and NL animals shared the same home ranges (Appendix 1) and predators capable of killing bison do not occur at Badlands. Additionally, unlike several other ungulates that use discrete home ranges or practice resource defense (Berger, 1986; Gosling, 1986) bison do not, making it unlikely that growth-related variation among juveniles was a consequence of differences in home range quality. However, we cannot dismiss the possibility that, because CL mothers were exposed to different foods during their ontogeny in Colorado, they may have learned to eat different grasses than did NL mothers when at Badlands; if so, neonatal growth would have been a consequence of the indirect effects of milk provisioning by mothers during their calves’ first 6 months. Nevertheless, this scenario also seems unlikely since both wild and domestic herbivores tend to select the most nutritious forage (see Hudson & White, 1985) and, in cattle, stronger inbreeding effects are usually most apparent prior to, rather than after, weaning (Dinkel et al., 1968).

Table 4. Comparison of annual differences in dominance status (percentage of contests won, assessed ordinally) as described in text between CL and NL bison mothers, 5-13 years old Number of mothers Cl

7 7 7 9

10 12 12 9

1985 1986 1987 1988

Total number of contests

610 546 731 571

Probability” (a)

(b)

0.58 0.40 0.21 0.32

0.58 0.55 0.61 0.08

“Probability (Median test) of members of one lineage being dominant based on interactions (a) irrespective of affiliation of dyad partner and (b) with members of the opposite lineage only. Third, non-linear equations are often employed in analyses of growth (Zullinger et al., 1984) but, where the aim is to assess short-term changes, linear models are appropriate (Falconer, 1984; Altmann & Alberts, 1987; Smith et al., 1987). Finally, either non-inherited or genetically based maternal effects might account for lineage differences in calf growth and correlated ages at puberty. For instance maternal nutrition affects neonate body mass (a non-genetic influence) and it is possible that individuals who display effects of a particular gene may not even possess that gene (Riska, 1991). While we examined for effects of the former (see above), it was not possible to judge the latter. Our data permitted an assessment of whether mothers of one lineage were consistently dominant to those of the other as described in the Methods. Because calves of outbred descent grew slower (Fig. 4) we expected CL mothers to be subordinate to NL cows. Based on dominance interactions no association between dominance and lineage was found during any of the years from 1985 to 1988 irrespective of lineage of dyadic members (Table 4). And, in only one year did the dominance rankings of mothers of one lineage approach statistical significance (Table 4). These were CL, not NL, mothers, the opposite of what we predicted had maternal dominance contributed to the enhanced neonatal growth of calves of inbred descent. Thus, our data do not allow rejection of the hypothesis that outbreeding may have contributed to diminished growth of Fl calves of the CL mothers even though all variables could not be controlled. Consequences

of lineage

mixing

Two not necessarily exclusive life-history consequences might result from the lineage mixing that has occurred in the Badlands population. First, because additional body mass is known to buffer against mortality when environmental conditions are extreme, it is reasonable to expect that descendants of the outbred line might experience strong negative selection during exceptionally cold winters even though none occurred during the present study. Such buffering has been documented in other mammals (Murie & Boag, 1984; Iason, 1989). Second, and more importantly, is that differences in

Bottlenecks, puberty usually result from tradeoffs between growth and reproduction during later life. If, by maturing when older, females of CL (outbred) descent were larger and produced more lifetime offspring, then costs of delayed reproduction would be minor. For instance, northern elephant seal cows that gave birth as 3-year olds experienced greater mortality and were less likely to achieve high reproductive success than those who pupped later (Reiter & Le Boeuf, 1991). However, for bison, the opposite appears true. Over a 7-9-year period, female bison that matured early had more offspring; somatic growth was sacrificed for reproduction, and age at first reproduction explained 33% of the variance in offspring production (Green & Rothstein, 1991). The differences in lifetime reproduction likely to accrue with early offspring production, at least in bison, are well in excess of the 1% advantage that Fisher (1930) maintained would be incorporated in 100 generations if the population were large. Hence, the differences in growth trajectories and correlated age at first reproduction between F, inbred and outbred juveniles would be likely to result in long-term selection against the animals of outbred descent. The conflicting results from other studies may arise due to true interspecies differences or they may stem from variation in the relative abundance of food resources as bison from the Green and Rothstein (1991) study area are maintained on a high nutritional plane by bi-annual round-ups. With the possible exception of African lions (Packer et al., 1991) and Arabian oryx (Stanley-Price, 1989) it has proved enormously challenging to detect any effects of inbreeding or outbreeding in natural, restored, or supplemented populations of mammals (Shields, !982, 1993; Rails et al., 1986; Pemberton et al., 1988). However, studies in zoos substantiate harmful effects of mating by close relatives (Ralls et al., 1979; Lacy et al., 1993). For North American bison, any putative demonstration of bottleneck effects has suffered by widespread claims that a long history of inbreeding has resulted in deleterious alleles being purged. For wisent, the evidence is mixed. Both statistically significant and insignificant inverse relationships between juvenile survival and inbreeding coefficients have been reported (Olech, 1987; Lacy, et al. 1993). For plain’s bison our data are insufficient to determine whether lineage mixing is the cause of the observed differences in juvenile growth and puberty. Although the possibility of outbreeding depression exists, it cannot be tested with the present data. An absence of comparable F, juveniles (let alone F,) in the Badlands sample was prevented by the failure of CL mates to mate.

CONCLUSIONS The results presented here are not from truly wild populations but are the product of bison lineages that experienced multiple, human-induced bottlenecks in the process of recovery. Badlands National Park has provided a rare and sufficiently large, quasi-experimental setting to accommodate the lineage mixing produced

lineages, and bison

by life histories and behaviour. The extent to which our findings apply to truly wild bison is, for all practical immaterial because free-roaming North purposes, American bison do not occur in the absence of human effects. Additional conservation measures will rely on the continued expansion of small (de facto) captive populations. Since many of the world’s restorations involve related groups of different geographic origins, it is imprudent to ignore the potential consequences of lineage mixing since the demographic outcomes are likely to be influenced by different levels of inbreeding and behavioural asymmetries. Given the frequency with which responsible agencies rely on consultation to import animals of unknown history and genetic background into resident populations, our findings have particular relevance to biological conservation in three areas; (1) viability following restoration; (2) viability as affected by gene flow; and (3) viability dependent upon life-history characteristics. Analysis of genetic change in resident populations depends on knowledge of the fates of introduced individuals and relationships among individuals, information that traditionally has not been available. Strict methodologies for such recording are prerequisite. The full bison population should be treated as one metapopulation. Finally, descriptions of inbreeding and especially outbreeding in wild (or restored) populations are likely to be less straightforward than is typically supposed. Therefore it is critical to develop sound methods for monitoring detailed life-history phenomena especially where population recovery is the goal. Conservation strategies of the future must involve not only knowing the genetic histories of individuals used in restoration, but also plans for documenting gene flow and for monitoring resident populations under all circumstances in which species survive within the context of ‘managed’ populations only.

ACKNOWLEDGEMENTS The National Park Service, National Geographic Society, Smithsonian Institution, Wildlife Preservation Trust, and University of Nevada provided generous support. Comments from or discussions with Fred Allendorf, Jon Ballou, Peter Brussard, Dave Cameron, Mary Peacock, Aron Rothstein, Andrew T. Smith, Peter Stacey, Ernie Vyse and three anonymous reviewers extended our initial thoughts by a large margin. The initial re-introduction of bison of Colorado descent would not have been possible without the guidance and cooperation of Lloyd Kortge (National Park Service), Christine Schonewald (National Biological Survey), and Robert Malcom (United State Fish and Wildlife Service). Columbia University Press allowed reproduction of figures. To all, we are grateful.

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Bottlenecks, lineages, and bison APPENDIX 1. ESTIMATION OF HOME RANGE LOCATIONS AND SPATIAL OVERLAP OF FEMALES OF TWO LINEAGES The analyses of home ranges were designed to illustrate patterns of land use at two levels: (1) home range size during the most common sampling period; and (2) degree of spatial overlap among individuals and between lineages (Fig. 5). The data set is derived from four adult females of each lineage during 97 field days from 9 May to 17 September 1988. While the NL females

CL - Females

Fig. 5. Home ranges of eight cows of two bison lineages in the 250 km’ Sage Creek Wilderness Area of Badlands National Park. Sample sizes as follows: CL A (32), B (38), C (36), D (36); NL A (65), B (65), C (37), D (37) (from Berger & Cunningham, 1994).

were selected at random, the CL cows were those who were re-sighted most often. An individual’s home range was arbitrarily designated as the area it used 90% of the time (shown below as frequency use polygons). The first sighting per day was plotted on maps gridded into 0.17 km2 quadrats. Spatial overlap by females within and between lineages was examined in two ways: (1) Percent overlap was expressed as the area in common divided by the sum of exclusively used regions plus the area in common; no initial adjustment was made for sampling intensity. Spatial overlap for the two lineages was 76%, a value derived by pooling the home ranges of cows of each lineage separately and then comparing the distributions between lineages. We also constructed matrices for each lineage so that mean spatial overlap between each pair of cows could be determined. By incorporating the pairwise contrasts, mean (within-lineage) overlap for NL and CL cows was reduced (68.5% (SE f 4.7) and 57.8% (SE * 5.5) respectively) (t = 1.19, NS) although, given the small sample, significant differences between lineages would be difficult to detect. However, without accounting for variation in sampling intensity, any firm conclusions would be hard to reach. For instance, home range size (Y) of the eight cows was positively associated with the number of re-sightings (X), a relationship best described by 55.99 log X -97.68 (r = 0.59). (2) We adjusted for the total number of sightings and then used simulation to estimate home range size and overlap. A random number generator was selected for 32 dates that each female was seen (the minimum for the female seen the least), so that home range locations could then be produced and compared for the eight cows. This procedure was repeated 50 times. While home ranges were reduced in size by 26.8% and 14.7% for NL and CL females, respectively, sampling intensities were equilibrated and mean spatial (within-lineage) overlaps were then determined. For the two lines, spatial overlap was 63%. With the simulations, mean (within-lineage) percent overlap for the NL and CL cows was 50.1 (kO.7) and 49,3(M,7)(t = 0.62; d.f. = 299; NS). In other words, based on the simulations the NL and CL cows used the same areas half the time and neither lineage was characterized by females whose ranges were more disparate than the other.