Most organisms have one of two basic, genetically programmed life histories. Some can (potentially) reproduce several times during their lives; they are said to be iteroparous — capable of reproducing repeatedly. Others can reproduce just once in a lifetime; they are called semelparous.
The label “iteroparous” comes from Greek words meaning “repeat” and “birth,” but the first part of the label “semelparous” probably comes from mythology. A human woman, Semele, was the mistress of the head god, Zeus, and became pregnant. Perhaps goaded by Zeus’ chief wife, Hera, she foolishly wanted to see and hear Zeus in all his splendor; merely getting pregnant by him was not enough, apparently. He obliged, the experience was too much for her, and she died. The unborn baby was taken by Zeus and reared in his thigh. Marvellous (and bizarre) things happen in myths! Semele’s name was applied by scholars to the concept of one-off reproduction.
Semelparity is found in all sorts of plants and animals. For decades, scientists have puzzled over how it evolved. A fundamental question is Why die after reproducing only once? Two kinds of conditions might favor that. Adult mortality might be very high, reducing the probability of living long enough to try again, so it might be adaptive just to go for broke the first time, expending all the resources on reproducing NOW, even if it means dying as a result. Some possibly-supportive correlations including cross-species comparisons are known. For example, semelparous African Lobelia species occupy the harshest environments, where adult survival is low even before attempts at reproduction, whereas iteroparous species live in milder areas.
Annual plants, including most pansies, have been deliberately selected to have high reproductive effort and short lives (Photo by S.P. Stanway)
Very high juvenile mortality could also lead to the evolution of semelparity. Juvenile mortality in many species is often quite high, but if almost all of the offspring die before they can reproduce, it may be advantageous to go all-out, making as many as possible, to increase the probability that at least some will survive, even if the adults exhaust themselves and die. Bamboos and cicadas have long non-reproductive periods before maturation, after which they embark on a single huge expenditure of reproductive effort, producing lots of offspring. In those species, the effect of numerous offspring is multiplied many-fold by all members of a population reproducing in one short time period. Researchers have suggested that the resulting stupendous production of offspring may keep predators from demolishing all of them, calling it “predator satiation,” and the same concept could apply in other cases. In our domestic plants, many of our annuals and biennials have been deliberately selected for high seed production, large flowers, or big inflorescences, at the cost of longer lives and future reproduction.
Well-known local examples of semelparity include the five species of anadromous Pacific salmon, which rear in fresh water, go to sea and grow, engage in long oceanic migrations, migrate back to fresh water to breed and die (feeding the multitudes with their carcasses). The hazards of long oceanic migrations are likely to be large, with high risks of predation or accidents. Furthermore, Pacific salmon in many populations have long, energy-demanding and dangerous up-river runs to reach their spawning grounds, and adult survival might be even lower if they attempted to go back out to sea after an arduous spawning run. Cross-species comparisons are supportive: most relatives of salmon migrate relatively short distances are non-migratory and iteroparous. Atlantic salmon migrate too, but commonly for shorter distances than the Pacific species; they are more iteroparous than Pacific ones.
The suggested importance of the costs of migration begs the question of why the juveniles don’t stay in fresh water, as happens for some salmon relatives. Why go to sea at all? Ocean-going salmon probably have an opportunity to feed well and grow larger; this may have enabled the observed production of more and larger eggs by females and better competitive ability of males. Some salmonid ancestor may have found that to be a reproductive advantage, allowing the production and survival of more offspring, and the ocean-going custom became ultimately established genetically in some populations — despite the risks and the costs of migration. That leaves the question of why go back to fresh water to spawn — maybe it’s safer for offspring than the ocean, where myriad consumers of eggs and small fish could make survival difficult.
Some species do not fall clearly into the simple dichotomy of semelparous vs iteroparous because individuals of one sex are semelparous but individuals of the other sex are iteroparous. This can happen when one sex has much higher costs of reproduction than the other: for instance, there may be extreme competition for mates, when adults compete so intensely for mates that they use up all their energy and die. Male Antechinus marsupials in Australia reproduce just once in their lifetime but the females of some species may reproduce in two or three times per lifetime. And in some populations of Labord’s chameleon in Madacascar, the males exhaust themselves in fierce mating competitions and die after one season, while the females are iteroparous. On the other hand, females of certain European vipers are commonly semelparous, and females of some mites and spiders die after reproducing once, having exhausted themselves in the process.
It is clear from these comparisons and from ecological modelling that patterns of reproduction can be associated with patterns of mortality. However, what scenario initiated a given life history for any particular species is not necessarily easy to determine. Furthermore, correlation is not the same as causation. Nevertheless, there is a lot to think about.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
