Semelparity and iteroparity

Semelparity and iteroparity are two reproductive strategies available to living organisms. A species is semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles. Iteroparity can be further divided into continuous iteroparity and seasonal iteroparity. Some botanists use the parallel terms monocarpy and polycarpy.
In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life. In any iteroparous population there will be some individuals who happen to die after their first and before any second reproductive episode, but unless this is part of a syndrome of programmed death after reproduction, this would not be called "semelparity".
This distinction is also related to the difference between annual and perennial plants: An annual is a plant that completes its life cycle in a single season, and is usually semelparous. Perennials live for more than one season and are usually iteroparous.
Semelparity and iteroparity are not, strictly speaking, alternative strategies, but end points along a continuum of possible modes of reproduction, although such intermediates are rare. Many organisms considered to be semelparous can, under certain conditions, survive their first bout of reproduction to reproduce two or more times.

Overview

Semelparity

The word "semelparity" was coined by evolutionary biologist Lamont Cole, and comes from the Latin semel and pario. This differs from iteroparity in that iteroparous species are able to have multiple reproductive cycles and therefore can mate more than once in their lifetime. Semelparity is also known as "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large as well as fatal. A classic example of a semelparous organism is Pacific salmon, which live for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and dying. Other semelparous animals include many insects, including some species of butterflies, cicadas, and mayflies, many arachnids, and some molluscs such as some species of squid and octopus.
Semelparity also occurs in smelt and capelin, but other than bony fish it is a very rare strategy in vertebrates. In amphibians, it is known only among some Hyla frogs including the gladiator frog;
in reptiles only a few lizards such as Labord's chameleon of southwestern Madagascar,
Sceloporus bicanthalis of the high mountains of Mexico,
and some species of Ichnotropis from dry savanna areas of Africa. Among mammals, it exists only in a few didelphid and dasyurid marsupials.
Annual plants, including all grain crops and most domestic vegetables, are semelparous. Long-lived semelparous plants include century plant, Lobelia telekii, and some species of bamboo.
This form of lifestyle is consistent with r-selected strategies as many offspring are produced and there is low parental input, as one or both parents die after mating. In semelaparous mammals, all of the male's energy is diverting into mating and the immune system is repressed. High levels of corticosteroids are sustained over long periods of time. This triggers immune and inflammatory system failure and gastrointestinal hemorrhage, which eventually leads to death.

Iteroparity

The term iteroparity comes from the Latin itero, to repeat, and pario, to beget. An example of an iteroparous organism is a human—humans are biologically capable of having offspring many times over the course of their lives.
Iteroparous vertebrates include all birds, most reptiles, virtually all mammals, and most fish. Among invertebrates, most mollusca and many insects are iteroparous. Most perennial plants are iteroparous.

Models

Trade-offs

It is a biological precept that within its lifetime an organism has a limited amount of energy/resources available to it, and must always partition it among various functions such as collecting food and finding a mate. Of relevance here is the trade-off between fecundity, growth, and survivorship in its life history strategy. These trade-offs come into play in the evolution of iteroparity and semelparity. It has been repeatedly demonstrated that semelparous species produce more offspring in their single fatal reproductive episode than do closely related iteroparous species in any one of theirs. However, the opportunity to reproduce more than once in a lifetime, and possibly with greater care for the development of offspring produced, can offset this strictly numerical benefit.

Models based on non-linear trade-offs

One class of models that tries to explain the differential evolution of semelparity and iteroparity examines the shape of the trade-off between offspring produced and offspring forgone. In economic terms, offspring produced is equivalent to a benefit function, while offspring forgone is comparable to a cost function. The reproductive effort of an organism—the proportion of energy that it puts into reproducing, as opposed to growth or survivorship—occurs at the point where the distance between offspring produced and offspring forgone is the greatest.
In some situations, the marginal cost of offspring produced decreases over time and the marginal cost of offspring forgone increases. In these cases, the organism only devotes a portion of its resources to reproduction and uses the rest for growth and survivorship so that it can reproduce again in the future.
In other situations, the marginal cost of offspring produced increases while the marginal cost of offspring forgone decreases. When this is the case, it is favorable for the organism to reproduce a single time. The individual devotes all of its resources to that one episode of reproduction, then dies as it has not reserved enough resources to meet its own ongoing survival needs.
Empirical, quantitative support for this mathematical model is limited.

Bet-hedging models

A second set of models examines the possibility that iteroparity is a hedge against unpredictable juvenile survivorship. Again, mathematical models have not found empirical support from real-world systems. In fact, many semelparous species live in habitats characterized by high environmental unpredictability, such as deserts and early successional habitats.

Cole's paradox and demographic models

The models that have the strongest support from living systems are demographic. In Lamont Cole's classic 1954 paper, he came to the conclusion that:
For example, imagine two species—an iteroparous species that has annual litters averaging three offspring each, and a semelparous species that has one litter of four, and then dies. These two species have the same rate of population growth, which suggests that even a tiny fecundity advantage of one additional offspring would favor the evolution of semelparity. This is known as Cole's paradox.
In his analysis, Cole assumed that there was no mortality of individuals of the iteroparous species, even seedlings. Twenty years later, Charnov and Schaffer showed that reasonable differences in adult and juvenile mortality yield much more reasonable costs of semelparity, essentially solving Cole's paradox. An even more general demographic model was produced by Young.
These demographic models have been more successful than the other models when tested with real-world systems. It has been shown that semelparous species have higher expected adult mortality, making it more economical to put all reproductive effort into the first reproductive episode.

Semelparity

Semelparity in mammals

In Dasyuridae

Semelparous species of Dasyuridae are typically small and carnivorous, with the exception of the northern quoll, which is large. Species with this reproductive strategy include members of the genus Antechinus, Phascogale tapoatafa and Phascogale culura. The males of all three groups exhibit similar characteristics that classify them as semelparous: First, all of the males of each species disappear immediately after the mating season. Also, males that are captured and isolated from others live for 2 to 3 years.
If these captured males are allowed to mate, they die immediately after the mating season, like those in the wild. Their behaviour also changes drastically before and after the mating season. Before mating, males are extremely aggressive and will fight with other males if placed close together. Males that are captured before they are allowed to mate remain aggressive through the winter months. After the mating season, if allowed to mate, males become extremely lethargic and never regain their aggressiveness even if they survive to the next mating season. Other changes that occur post-mating include fur degradation and testicular degeneration. During adolescence, male fur is thick and becomes dull and thin after mating, but regains its original condition if the individual manages to survive past the mating season. The fur on the scrotum completely falls off and does not grow back, even if the male survives months after the first mating season. As the marsupial ages, its testicles grow until they reach a peak size and weight at the beginning of the mating season. After the individual mates, the weight and size of the testes and scrotum decrease. They remain small and do not produce spermatozoa later in life, if maintained in a laboratory. The 1966 Woolley study on Antechinus spp. noticed that males were only able to be maintained past mating in the laboratory, and no senile males were found in the wild, suggesting that all males die shortly after mating.