Capital versus income breeding: an ectothermic perspective

Energy storage is an important component of life-history variation. Some organisms ("income breeders") fuel reproductive expenditure by simultaneous feeding, whereas others ("capital breeders") fuel reproduction from energy gained earlier, and stored prior to use. Most published discussions of this topic have focused primarily on endothermic animals (birds and mammals), and have interpreted the costs and benefits of these alternative breeding tactics in the context of endothermy. The far more diverse array of ectothermic animals has received less attention in this respect. Many features associated with ectothermy preadapt organisms to store energy for long periods prior to use (i.e. to rely on "capital" rather than "income"). For example, birds and mammals experience high costs (in terms of mobility and thermoregulatory efficiency) if they store large body reserves. By contrast, the energetic and demographic costs associated with storage, maintenance, and utilisation of body reserves are low in many ectotherms. Thus, capital breeding (which may also be more efficient energetically in many situations) is extremely common in these low energy systems. Ectotherms comprise the most extreme examples of capital breeders, with a strong tendency towards semelparity where the capital of reserves is massively invested into a single reproductive event. Overall, theoretical and empirical studies of the evolution of capital versus income breeding as alternative strategies of resource use should take account of the important role played by alternative thermoregulatory and metabolic systems. The acquisition of endothermy in avian and mammalian lineages has involved a massive shift towards reliance on "income breeding", and the full spectrum of life-history variation in this respect cannot be appreciated without detailed examination of ectothermic organisms.