Expenditure Per Progeny

The goal of each living thing is to pass on its own genetic information. The limited resources of the world have created a complex, biological community under intense selective pressure. In order to ensure survival of oneself and one's genes, an individual must utilize a competitive reproduction strategy. Genetic adaptations occur over time to make organisms run faster, grow more efficiently, or better detect their food. These evolutionary "decisions" are fairly easy to make: the tradeoff of spending energy on stronger muscles is easily worth it if stronger muscles keep their owner alive.

Numerous reproduction strategies and compromises are evident in nature, and certain constraints favor one or another. If you were given power to design a completely new organism, you would be forced to make a number of decisions regarding its production. Eggs or live young? How many eggs? How much time invested in raising them? Do the males care for the young? The females? Both? Neither? Organisms evolve to take the best strategy under the given circumstances, and one "choice" they have to make is how to allocate energy towards reproduction.

One variable is how many offspring to have. For example, long-term studies of the great tit, a bird in Britain, show that the optimal number of eggs for a great tit is about eight. If a pair of birds raises a larger brood, chicks in that brood get fed less often. They eat fewer caterpillars and weigh less when leaving the nest. However, heavier chicks tend to live longer and healthier lives.

So if a parent wants to maximize the number of healthy young it can have, and maximize the chance that all of those young will live to be old enough to reproduce and generate healthy offspring of their own, it cannot raise too many. If it raises fewer than the optimal number, its chicks might well turn out healthy, but there will be less of them to pass on the family genes.

Christopher M. Perrins showed in 1979 that if the size of broods are artificially manipulated by sneaking into nests and adding or removing eggs, the optimum number is between eight and twelve eggs. We know this because Perrins tried to recapture chicks from the nests later on; he had the best luck recapturing chicks from the nests with eight to twelve eggs. More eggs meant the chicks were less healthy overall; fewer eggs simply meant there were fewer chicks to start with.

This assumes that parents will expend about the same amount of energy on a brood of chicks regardless of brood size. Raising chicks is an enormously expensive endeavor: parents must expend energy in making the eggs, making the nest, sitting on the nest and protecting it, and getting food for the brood once it hatches. Great tits bring back an item of food every thirty seconds as long as there is daylight. Plus there is the matter of multiple breeding seasons. Is it better to work like a horse for one season on one brood and be tired and inefficient for subsequent seasons, or more slowly expend oneself? In the end, each chick receives a certain amount of energy in metabolic production, feeding, and raising. This amount is known as the expenditure per progeny.

Predictably, differing species exhibit a wide range of "decisions" regarding expenditure per progeny. An organism as physiologically and socially complex as a human being requires a lot of energy to develop inside and outside of the womb, so any one individual offspring represents an enormous investment. The evolutionary "hope" is that an incredibly complex, adaptable individual will be able to reproduce, and thus only a few are made. This is a good thing, because human beings are so energetically expensive!

Contrast this reproductive strategy with that of a spider. Spiders have no social development and require little to no care after birth. They are also small and comparatively easy to make. Thus, a reproducing spider will invest all of her energy into making thousands and thousands of eggs. While baby spiders possess negligible learning capacity and are not as equipped as human beings to deal with complex, new situations, it is reasonable to assume that if a parent generates thousands of them, at least a few of them will make it to reproductive age.

Making energetic decisions like these, or tradeoffs, is part of the process of evolutionary development. As changing environmental circumstances dictate what might be the most effective reproductive strategy, parents must allocate their energies accordingly.

see also Reproduction, Asexual and Sexual.

Ian Quigley

Bibliography

Davies, Nicholas B., and John R. Krebs, eds. An Introduction to Behavioural Ecology. Cambridge, MA: Blackwell Science, 1993.

Dawkins, Richard. The Selfish Gene. Oxford, U.K.: Oxford University Press, 1989.