Semelparity and iteroparity

[1] Semelparity and iteroparity are not, strictly speaking, alternative strategies, but extremes along a continuum of possible modes of reproduction.

[2][3] The word "semelparity" was coined by evolutionary biologist Lamont Cole,[4] and comes from the Latin semel ('once, a single time') and pario ('to beget').

In amphibians, it is known only among some Hyla frogs including the gladiator frog;[6][full citation needed] in reptiles only a few lizards such as Labord's chameleon of southwestern Madagascar,[7] Sceloporus bicanthalis of the high mountains of Mexico,[8][full citation needed] and some species of Ichnotropis from dry savanna areas of Africa.

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.

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.

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.

A second set of models examines the possibility that iteroparity is a hedge against unpredictable juvenile survivorship (avoiding putting all one's eggs in one basket).

In Lamont Cole's classic 1954 paper, he came to the conclusion that: For an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equivalent to adding one individual to the average litter size.

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.

[18][19] Semelparous species of Dasyuridae are typically small and carnivorous, with the exception of the northern quoll (Dasyurus hallucatus), which is large.

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.

[20] Studies on Antechinus stuartii reveal that male mortality is highly correlated to stress and andrenocortical activity.

Dasyurus hallucatus, the northern quoll, is a large dasyurid and exhibits increased male mortality after the mating season.

Unlike smaller dasyurids, male die-off in D. hallucatus is not due to endocrine system changes, and there was no observed spermatogenic failure after the mating season ended.

[12] Lack of elevated cortisol levels during mating periods in D. hallucatus means that there is no current universal explanation for the mechanism behind increased male mortality in Dasyuridae.

The males also exhibit similar physiological degradation, demonstrated in Antechinus and other semelparous marsupials, such as fur loss and increase of infection from parasites.

[25] Highly elevated cortisol levels mediate the post-spawning death of semelparous Oncorhynchus Pacific salmon by causing tissue degeneration, suppressing the immune system, and impairing various homeostatic mechanisms.

One of the key factors in salmon rapid senescence is that these fish do not feed during reproduction so body weight is extremely reduced.

Under the traditional definition, insects are considered semelparous as a consequence of time scale rather than the distribution of reproductive effort over their adult life span.

In the case of the spongy moth, adults do not possess an active digestive system and cannot feed, but can drink moisture.

Also, female mammals have relatively low reproductive rates compared to invertebrates or fish because they invest a lot of energy in maternal care.

[31] Scientists have hypothesized that natural selection has allowed semelparity to evolve in Dasyuridae and Didelphidae because of certain ecological constraints.

Female mammals ancestral to these groups may have shortened their mating period to coincide with peak prey abundance.

[32] Reproduction is costly for anadromous salmonids, because their life history requires transition from saltwater to freshwater streams, and long migrations, which can be physiologically taxing.

A noticeable difference between semelparous fish and iteroparous salmonids is that egg size varies between the two types of reproductive strategies.

[33] The current hypothesis behind this reason is that iteroparous species reduce the size of their eggs in order to improve the mother's chances of survival, since she invests less energy in gamete formation.

Semelparous species do not expect to live past one mating season, so females invest a lot more energy in gamete formation resulting in large eggs.

The dead bodies of the adult salmon decompose and provide nitrogen and phosphorus for algae to grow in the nutrient-poor water.