Reproductive Allocation in Animals

Reproductive allocation is a term used in ecology and evolutionary biology that refers to the proportion of an organism’s energy budget allocated to reproduction at any given time. Reproduction must be balanced (or traded off) against opposing expenditures such as growth, survival, maintenance, and future reproduction. The term also covers division of resources among offspring size and number. Studying reproductive allocation trade-offs is fundamental to the fields of behavioral ecology and physiological ecology, which use evolutionary theory to explain and predict animal behavior and physiology, respectively. More specifically, these trade-offs are central to the field of life history, which studies how growth and reproduction is distributed across animals’ lifetimes. Animals show a vast degree of variation in reproductive allocation. Kiwis, for example, famously lay a single egg that is up to 20 percent of body weight. As if this were nothing, caecilians (amphibians) can bear live litters of offspring that are up to 65 percent of the mother’s body weight. Egg numbers vary enormously and can reach spectacular numbers: tsetse flies bear as few as six live offspring in a lifetime, whereas ghost moths can lay more than 50,000 eggs. Social insects have truly mind-boggling fecundity: driver ants can lay several million eggs per month, and can live for decades; ocean sunfish release about 300 million eggs at a time, more than any other vertebrate. At the other extreme, very many organisms have one offspring at a time. Usually this goes hand in hand with repeated breeding, but perhaps the most puzzling of allocation decisions is found in dung beetles, which have only one ovary; some species lay as few as five to ten eggs. Careful parental care ensures that more than 90 percent of offspring survive, explaining why these species have not become extinct. Clearly, variation of this order of magnitude requires evolutionary explanation. Research into reproductive allocation has progressed from a simple household-economics outlook based on the division of a fixed energy budget toward more sophisticated approaches based on quantitative and mechanistic genetics. Of particular use have been model systems such as Drosophila and Daphnia, where traits of reproductive allocation (body size, egg size, egg number, etc.) have become model traits for modern genetic analyses. Modern approaches to reproductive allocation typically involve elucidating genetic bases for trade-offs expressed across a range of environments. Nevertheless, the classical life history approaches remain relevant, especially in systems in which controlled quantitative genetics are not possible.