Zebra finch nestlings, rather than parents, suffer from raising broods under low nutritional conditions

Abstract

In sexually reproducing species, parents and offspring have different optima in terms of the amount of parental investment. For offspring, higher investment than the parental optimum generally increases their fitness. However, such higher investment will, in theory, result in net parental fitness loss because increased benefits from current offspring will be more than offset by a potential decrease in parental survival or future reproduction. Whether and how parents respond to a shortfall in resources might therefore have important fitness consequences. Here, we manipulate the nutritional condition of captive zebra finch families to investigate how variation in environmental conditions influences parental resource allocation between themselves and their offspring. By allowing the same zebra finch pairs to raise one brood under high and another brood under low nutritional conditions, we found that parents lost more body mass and their offspring grew at slower rates under low nutritional conditions. Therefore, both parents and their offspring were affected by the nutritional treatments. Offspring incurred greater costs, because slower growth rates under low nutritional conditions resulted in an overall lower body mass of nestlings after reaching independence. Nutritional treatments had sex-specific effects on parental body mass, because, in contrast to fathers, only mothers recouped their losses under high nutritional conditions. Furthermore, both parents spent less time brooding their nestlings under low nutritional conditions. Parental strategies hence vary with the nutritional quality of their food, even in captivity, supporting the idea that parental resource allocation may be driven by the expected costs and benefits from current offspring. Parental care is among the most influential traits affecting fitness in many sexually reproducing animals. The more care offspring receive, the higher their chances of survival and reproduction. However, caring for their young does not only provide benefits for parents: all costs of parental care are paid by them. Parental resource allocation is therefore expected to be optimised. But how should parents respond to changing environmental conditions when they are already breeding? By manipulating nutritional conditions in a captive population of zebra finches Taeniopygia guttata, we show that both parents and their offspring lose body mass under low nutritional conditions. The effect on offspring was more pronounced and long lasting, and parents also decreased their parental effort. Our results show plasticity in parental investment of zebra finches in response to nutritional conditions and reproductive value of their offspring.