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Iteroparity and Semelparity

Iteroparity and Semelparity

Success for an animal species does not only mean that it survives, but that it survives to reproduce. When a species or population of organisms is unable to reproduce enough offspring to keep the population numbers high, extinction is the result. However, reproduction is probably one of the most energy-depleting events a single organism may endure. Both plants and animals have a wide range of reproductive methods that result in repeated matings, fertilization , production of offspring, and survival of the next generation. However, for many groups, including some plants, all of the energy produced by the organism is directed to successful reproduction.

Within the animal group, invertebrates have a wide variety of reproductive strategies that insure their survival. Many produce so many eggs and sperm that the sheer numbers of offspring make success of a few likely.

Feature Texas South Carolina Ohio
Growth rapid intermed. slow
Adult mortality high intermed. low
Ave. size female 57 63 75
Ave. clutch size 9.5 7.4 11.8
Broods per year 3-4 3 2
Total offspring produced per year 33 22 24
Egg mass per egg 0.22 0.33 0.35
Proportions of births by age of female Texas South Carolina Ohio
1 82 48 0
2 18 30 57
3 0 15 22
4 0 7 21

Vertebrates , however, tend to be more specialized in their reproductive habits. As a consequence, a great deal of energy is spent in assuring the most optimal conditions for survival of the young. Environmental pressures such as bad weather conditions or high predation rates call for species with the highest assurance of success. Two of the most successful types of reproductive styles are called iteroparity and semelparity.

Iteroparity occurs when a parent breeds year after year. This means that the success of the parent's genetic material surviving to another generation is increased with every brood. For most plants and animals this works quite well. If for some reason conditions do not favor the survival of the young one year, there is a repeat chance the next year.

Sea turtles are an example of iteroparity. After mating, the females come out of the water, dig a large nest with their flippers, and deposit several dozen eggs. Should a predator uncover the nest and eat the young, it is not a disaster for the parent since she will return the next year to repeat the egg-laying cycle.

Semelparity is a type of reproduction that occurs less frequently, but is no less driven by the need for reproductive success. A common example of semelparity is found in salmon, a meaty and delicious food source for many animals, including humans, bears, and other water-living predators. Salmon eggs are a nutritious and desirable food source for marine-dwelling organisms. In the face of these facts, natural selection has driven salmon to a very ingenious but costly reproductive strategy.

Juvenile salmon begin the oceanic phase of their lives in massive schools that migrate around the world's major oceans. When they become adults and are ready for breeding, instead of laying their large nutritious eggs in the marine environment, where they are likely to be eaten, the salmon change their entire physiology to survive in freshwater. The breeding adults smell the river in which they were born and begin to swim up to their spawning grounds.

Many films show the intense and difficult trek adult salmon take to get to those grounds. They leap up and over waterfalls and swim against swift and strong currents. Many species actually change their color and the shape of their mouths. They do not feed during this time. Their body tissues are converted to eggs or sperm. At the end of these breeding migrations, salmon look terrible. Their skin is peeling. They are wounded and damaged by accidentally hitting themselves against rocks and they are exhausted. Many fishermen do not eat breeding salmon because the conversion of body tissue to gametes makes the salmon meat soft and undesirable.

Before bear populations decreased, the annual migration of salmon was a source of feasting for them. Many salmon simply died before reaching their breeding grounds or were eaten by predators, including some large birds of prey such as eagles. The survival of adult salmon traveling to individual breeding grounds was often very low. This low adult survivorship is another reason, in addition to the dangers of the marine environment, why semelparity is an advantage to the fish.

The large burst of energy that completely disables the fish ends with the laying and fertilization of the eggs. Afterward, the adult fish all die. However, the adults have provided the next generation a safe place to grow and hatch. When the offspring emerge from the eggs, they have an abundant food supply from aquatic insects and, eventually, minnows. The young have time to grow and develop to a larger size free from most predators. The sacrifice of the parent provides safety to the next generation.

Many spiders, some anguillid lizards, and certain amphibians also undergo semelparity. It is a type of reproduction found among animals whose environmental conditions may be too harsh for the young to survive. On the other hand, iteroparity favors repeated matings. Iteroparous plants and animals live for many years, breeding each season. Often their strategies are to produce high numbers of young. Another approach is found among mammals, who are iteroparous. Most of them give a great deal of adult protection to the young and ensure their survivorship in this manner.

Basically, if the parent is at low risk of death when it is young, its species will be iteroparous. If the mortality rate is high for the young, semelparity might be observed. Both strategies work well as evidenced by the continuation of both reproduction styles.

see also Expenditure Per Progeny; Reproduction, Asexual and Sexual.

Brook Ellen Hall

Internet Resources

The Association for Tropical Biology. <http://atb.botany.ufl.edu/atb>.

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