Reproductive Systems

The other hormones and the glands that secrete them include the following:

• Pineal gland—located in the brain and makes melatonin in response to darkness so that it helps to regulate the sleep-wake cycle.

• Thyroid gland—this is located in the neck and secretes two hormones (thyroxine and triiodothyronine) that regulate metabolism on a cellular level.

• Parathyroid glands—these are located behind the thyroid gland and make parathyroid hormone that increases the calcium level in the bloodstream.

• Adrenal glands—the adrenal glands are located above the kidneys. They make mineralocorticoids that regulate salt excretion and blood pressure, sex hormones, and cortisol, which has many effects on the immune system and on blood sugar values.

• Pancreas—this is located in the abdomen and regulates blood sugar by secreting insulin to lower blood sugar and glucagon to raise blood sugar.

• Ovaries—these are located in the female pelvis and are responsible for producing estrogen and progesterone under the influence of the pituitary gland.

• Testes—these are located in the scrotum of males. They secrete testosterone, which is the major male hormone.

REPRODUCTIVE SYSTEMS

There is both sexual and asexual reproduction in the animal kingdom. There are advantages and disadvantages to both methods of reproduction. Asexual reproduction has the capacity to release large numbers of offspring that are genetically identical to the parent organism. It leads to highly adapted offspring during situations where the environment is stable. If the environment is unstable; however, the offspring might not be able to adapt to the changing environment.

Sexual reproduction produces an organism that requires two parents but that creates a genetically unique organism. This creates diversity in the environment, which is potentially helpful in ensuring survival of the population. Usually, this involves the

presence of a female (offspring-bearing organism) and a male (which donates only the genetic material to make the offspring). This type of system leads to fewer offspring, which might be a reproductive disadvantage over asexual reproduction.

We’ve talked a little about asexual reproduction, which occurs in prokaryotes and many single-celled eukaryotic organisms. Some animals can reproduce asexually and there are those that can do both asexual and sexual reproduction.

Binary fission does not generally occur in animals in the way we’ve discussed in the past. With animal binary fission, what’s meant by that is that the organism can regenerate a missing part. This can happen to certain types of flatworms (called planarians), sea anemones, and sea cucumbers, which can be split in half longitudinally in order to regenerate another half.

Budding can happen in which an outgrowth of the main body will separate from the organism to grow a smaller version of itself. This can happen in invertebrate animals, such as hydras and corals but is otherwise a rare phenomenon in animals.

Fragmentation involves the breaking of an individual animal, after which it regenerates the missing parts. If the part broken off is big enough, it can grow a separate individual in each part. Reproduction using this method can be seen in echinoderms, annelids, sponges, some turbellarians, and some cnidarians. Sea stars can regenerate a whole new sea star with just one arm and a portion of the central disc remaining.

Parthenogenesis involves the development of an individual organism from an unfertilized egg. This can result in a diploid or haploid individual, depending on the species. This can occur in several types of insects, including aphids, stick insects, water fleas, wasps, bees, and ants. Ants, wasps, and bees will use parthenogenesis to create a haploid drone insect (which is always male). Female workers and queen bees are diploid and come directly from a fertilized egg.

Larger vertebrate animals can also reproduce via parthenogenesis, including fish, amphibians, and reptiles. It can happen when females of certain species are isolated from males, and has happened in sharks and Komodo dragons.

Sexual reproduction is the combination of reproductive cells from two organisms of the same species to create a unique offspring organism. There will be two sexes in one individual (called hermaphroditism) or two separate sexes.

Some animals are hermaphrodites, having male and female reproductive systems. This is the case in slugs, tapeworms, and snails. They may self-fertilize but will usually mate with another. Self-fertilization can happen in organisms that do not move, such as clams and barnacles. Self-breeding, however, does not confer an ecological advantage as it causes inbreeding and offspring that are potentially not as fit as with true sexual reproduction.

In mammals, there are two sexes. The female sex has two X chromosomes, while the male sex has an X and a Y chromosome. This system is seen in plants and in insects but is not seen in all animal species. In birds, there is a ZW system, in which the ZZ genotype results in a male and the ZW genotype results in a female. Insects, reptiles, crustaceans, and fish (some species) also follow the ZW system. There can be more complicated three-chromosome systems as is seen in swordtail fish, which have three sex chromosomes instead of two.

Some species will have the determination of sex not by the chromosome situation but by some environmental factor. Alligators, tuataras, and some turtles will have a sex determination based on the temperature of the environment during incubation. Cool temperatures in some turtle species will produce males while warm temperatures will produce females. The reverse can be true in other turtle species. In some crocodiles, moderate temperatures favor both genders being produced while extremes in temperatures produce mainly females.

Some species will change gender during their lifetime. Protogyny means the animal is female first and then male, while protandry means the animal is male first and then female. Oysters are all born male and then grow to become females that lay eggs. Certain reef fishes are also considered “sequential hermaphrodites” and will switch gender. Some fish will have a group of females and a single male of the species but, if the male dies, one of the females will grow and develop into a male.

Fertilization involves the fusion of a sperm and egg. This process can be internal (happening within the female) or external (happening outside the female). Humans and most mammals have internal fertilization. The frog and the duck billed platypus will lay eggs that will be fertilized outside of the female body.

External fertilization will often happen in aquatic environments in which the eggs and sperm are released in the water. This is what happens when animals spawn. In spawning, there is the release of the eggs in large numbers as well as the release of sperm in the same vicinity. This is what happens in fish, crabs, oysters, shrimp, squid, sea urchins, sea cucumbers, frogs, mosquitos, corals, and mayflies.

Internal fertilization happens in many terrestrial animals, although it can be seen in aquatic animals. The male may deposit sperm in the female directly or can happen with sperm deposited into the environment, which the female later picks up and deposits.

After internal fertilization, the offspring can be produced in several ways. In oviparity, the fertilized eggs are laid and develop outside of the body (as in birds, a few mammals, some fish, some reptiles, some amphibians, and some cartilaginous fish). Most reptiles and insects produce eggs with a leathery cover, while birds and turtles have hard-shelled eggs. The duck-billed platypus lays leathery eggs.

In ovo-oviparity, fertilized eggs are retained in the female and are hatched inside of her or are laid just prior to hatching. This happens in some bony fish, some snakes, some sharks, lizards, some vipers, and in certain invertebrate animals. In viviparity, the young are born alive. This is seen in some cartilaginous fish, some reptiles, and almost all mammals.