Chapter 11

Parental Care

1. Not all offspring are created equal or have the same reproductive value. Offspring with high reproductive value are likely to go on to be successful breeders themselves, whereas those with low reproductive value are unlikely to reproduce much, if it all, during their lifetimes. An individual’s reproductive value influences how much care a parent is willing to invest for that individual. To help communicate their value to parents and ensure that they are fed, offspring have evolved complex signaling systems that advertise their need or quality. Parents not only favor some offspring in terms of care, but they also can manipulate the sex of the progeny they produce. Maternal condition, as well as the local social and ecological conditions, can influence the relative value of each sex and which one parents produce or invest in.

2. The parents of some species show complete indifference to lethal aggression among their young offspring. This may be explained as part of a parental strategy to let the offspring themselves identify which individuals are most likely to provide an eventual genetic payoff to the parents that produced them. The more general principle is that selection rarely favors completely even treatment of offspring, because some young are more likely than others to survive to reproduce.

3. Many species lack biparental care even though offspring that receive parental help are generally more likely to survive to reproduce. One explanation for this is that the time, energy, and resources that parents devote to their offspring have costs, including reduced fecundity in the future and fewer opportunities to mate in the present. Moreover, because the costs of paternal care are often greater than the costs of maternal care, particularly in polygynous species where males forego additional mating opportunities by caring for young, paternal care is generally less common in most taxonomic groups than is maternal care.

4. Examining cases of sex role reversal can be instructive for our understanding of how parental care systems evolve. Thus, uniparental care by male fishes may have evolved because males caring for eggs laid in the nests they guard can be more attractive to potential mates than if the males lacked eggs to protect. In contrast, the costs of maternal care to female fishes may include large reductions in growth rate and consequent losses of fecundity. In frogs, paternal-only care has evolved in species where males guard territories that house oviposition sites, and as a way to ensure paternity certainty. Fecundity losses for females may also be involved in the evolution of exclusive paternal care by some water bugs.

5. An evolutionary approach to parental care yields the expectation that when the risk of investing in genetic strangers is high, parents will be able to identify their own offspring. As predicted, offspring recognition is widespread, particularly in colonial species in which adults have many opportunities to misdirect their care to foreign offspring. However, adults do sometimes adopt nongenetic offspring, including those of specialist brood parasites, with consequent losses of fitness. Multiple hypotheses exist to account for these puzzling cases, including the possibility that highly discriminating host adults could lose fitness by sometimes erroneously rejecting their own offspring. Host–parasite interactions lead to coevolutionary arms races as hosts evolve ways to discriminate between their own offspring and those of the parasite, and parasites evolve ways to trick hosts into accepting their parasitic eggs and chicks.