68 Reproductive Strategies

Key Idea: The way in which an animal allocates its reproductive effort is part of its reproductive strategy. Effort can be expended in producing offspring or caring for them or both. Different strategies carry different costs and benefits. The reproductive effort is the amount of energy allocated to reproduction (production and care of young). Of the total reproductive effort, the amount remaining after production of the offspring can be allocated to parental care. At one extreme, most invertebrates expend their total reproductive effort in producing eggs and sperm and there is no parental care. At the other extreme, mammals invest heavily in a small number of offspring and the parental care cost is substantial. Between this is a continuum, with some animals adopting alternative strategies, such as brood parasitism. No strategy is necessarily 'better' than any other. They are different solutions to the problem of successful reproduction.

Many eggs or young produced Moderate number of eggs produced

Few eggs or young produced

High mortality No parent al ca r e Low mortality Care b y non par ent s Low mortality P arent al ca r e

Low risk to parents

Low percentage of offspring survive to reproductive age

Little or no parental care

f Large number of offspring produced. f Reproductive effort per offspring is low. f Little of no parental care of offspring. f Reproductive effort is put into producing the offspring, not parental care. f Examples: most fish, amphibians, reptiles, and invertebrates.

Moderate to low risk to parents Higher risk to parents

High percentage of offspring survive to reproductive age

Brood parasites

f Moderate number of offspring. f Reproductive effort per offspring is moderate to low. f Rely on others to raise offspring. f Risk of egg loss is mitigated by distributing eggs amongst a number of hosts. f Examples: some birds, insects, and fish.

High percentage of offspring survive to reproductive age

Parental care

f Few offspring. f Reproductive effort per offspring is high. f Moderate to substantial care of offspring. f Large reproductive effort put into raising offspring to a less vulnerable stage. f Examples: most birds and mammals, some fish.

Broadcast spawning involves no parental investment after the gametes are released. Many invertebrates allocate all their reproductive effort to producing offspring and there is no parental care. Broadcast spawners such as clams and corals (above), release millions of gametes into the water. Very few of the planktonic larvae will survive. This is the most common reproductive strategy in the oceans and is typical of most marine invertebrates and many fish. Many amphibians are also broadcast spawners although there are many exceptions, including New Zealand's native frogs (Leiopelma) in which the males carry the offspring on their backs. Brood parasitism is a strategy adopted by some birds, notably cuckoos and cowbirds. The brood parasite removes an egg from the nest of a host species and lays one of its own in its place. To reduce the risk of eggs being discovered and destroyed, the eggs are spread around a large number of hosts. Most avian brood parasites have short incubation times, so the egg hatches before those of the host and the imposter will eliminate all or most of the host's eggs/nestlings. The host then raises the parasite chick as if it were its own, even when the chick is larger and differs in appearance. The strategy is not without risk - only about half of the parasite's young survive.

A shiny cowbird chick is fed by its host parent, a smaller rufous collared sparrow. Mammals have a high level of investment in offspring before and after birth. Both mammals and birds are well known for their high levels of parental care and mammals also have a high level of prenatal investment. Other vertebrates, such as some amphibians, fish, and reptiles also provide care until the offspring are capable of fending for themselves. Bird parents are required to incubate their eggs in a nest and then feed the chicks until they are independent. Although most mammals give birth to well developed offspring, they are dependent on their mother for nourishment via suckling milk, as well as learning behaviours essential to their survival.

` Producing offspring demands enormous amount of energy and risk. In many vertebrate species, reproduction is almost entirely up to the female (males contributing only sperm) but in other species the male also provides support (e.g. by defending a territory or providing food for the female). ` Because the female is most heavily invested in the offspring, it is important that she has the best possible reproductive outcome each time she mates, and mate choice is critical. In general, females have a limited reproductive outcome and can only produce so many eggs or offspring in a lifetime. For example, a human female produces one egg a month for about 40 years, a maximum of ~480 eggs in a lifetime. Given that gestation and breast feeding (which suppresses ovulation) may take two years, only about 20 children can be raised in the average lifetime (the record is reportedly 69). ` Males, on the other hand, have less invested in offspring. They produce sperm continuously and put no direct energy into the offspring until at least birth or egg laying. Potentially, males could fertilise unlimited numbers of females and so produce far more young without any additional effort. ` These differences in reproductive investment have been important in the evolution of mating systems, e.g. monogamy, with animals adopting strategies that maximise reproductive success in their particular physical and social environment.

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In kaka, both parents are needed to successfully rear the chicks. Monogamy is a common mating system when biparental care is needed for offspring survival.

A New Zealand sea lion male keeps a harem of up to 25 females and their young, which he protects.

1. Describe the different ways in which animals can allocate their total reproductive effort:

2. Animals with parental care protect the investment they have already made in offspring. Explain how factors in the environment (e.g. food resources and risks to young) might influence how much care is provided by each parent:

3. (a) What might be the benefits of a brood parasitism strategy to the brood parasite?

(b) What adaptations of the brood parasite help to maximise the success of its strategy?

4. The shining cuckoo (Chrysococcyx lucidus) is the world's smallest cuckoo. It is a summer migrant to New Zealand, where it is a brood parasite of the much smaller grey warbler (Gerygone igata). It usually parasites second broods, arriving in New Zealand after the grey warblers have already begun breeding. The female shining cuckoo removes one host egg per nest, laying one of her own in its place. After hatching, the cuckoo chick ejects all grey warbler eggs and/or nestlings from the nest and is raised alone.

Explain how the cuckoo's strategy of parasitising the second broods of grey warblers contributes to the continuing success of the cuckoo in its niche:

The shining cuckoo