Updated:08/05/2022 01:34h
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The life history of a species is a cluster of events in which the cost-benefit ratio between growth, survival and reproduction is optimized. Parameters such as the number of offspring, the amount of energy they have to invest in their care and the age at which reproduction occurs are included in the strategy equation.
Those animals that opt for late reproduction can have a greater number of offspring, which are healthier and can provide them with better care. While those who opt for reproduction at an early age are less likely to die without leaving offspring.
In general, short-lived species often reproduce early, while long-lived species will tend to delay their reproduction.
Among the animals that choose to reproduce at an early age are small fish such as ‘guppys‘, which do not grow large enough to fend off predators. For their part, larger fish, such as sharks, use their energy to grow to a size that provides them with greater protection, delaying reproduction, which adds a greater risk of dying before reproducing.
‘R’ strategy
Organisms that have an unstable habitat have a high reproductive rate, producing a high number of offspring, in which there is a high mortality since they lack parental care. This type of reproductive strategy is typical of animals of small size, short age and early reproduction.
This survival strategy is known as ‘r’ and is typical of living beings that live in ponds, bare rocks, polar areas, deserts… where bacteria, protozoa and small animals live. If we focus on fish, we find that swimming pelagic fish such as sardines or anchovies are ‘r’ strategists.
‘K’ strategy
At the other extreme we find the so-called ‘k’ strategy, typical of animals that live in stable environments, with a low reproductive rate and that produce a small number of offspring to which the parents dedicate enormous care, reducing mortality. This strategy may seem, a priori, more successful, although it makes the species more vulnerable to the fate of a small number of individuals.
Among the ‘k’ strategists we find the rhinoceros, the giraffe, the elephants and the human beings, among many others. Generally, species that are in danger of extinction belong to this type of survival strategy. Among fish, cartilaginous (elasmobranchs) such as rays or sharks have a ‘k’ behavior.
Semelparity versus iteroparity
Fecundity is the reproductive capacity of an organism, the number of descendants that it is capable of generating. There are some species that only reproduce once -semelparity- and others that reproduce several times -iteroparity-.
The word semelparity was coined by the biologist Lamont Cole and comes from the Latin ‘semel’ which means ‘once only’ and ‘pario’, to engender. Among the semelparous animals we find many insects, some species of butterflies, arachnids and some molluscs.
Perhaps the best known case is that of Pacific salmon (Oncorhynchus spp) that lives for many years in the ocean before swimming into the freshwater stream for birth, spawning, and death. And it is that after swimming an enormous distance this salmon uses all its energy in reproduction.
For its part, iteroparity comes from the Latin ‘itero’, to repeat. Iteroparous animals include birds, most reptiles and fish, and humans.
Pedro Gargantilla is an internist at El Escorial Hospital (Madrid) and the author of several popular books
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