Viability of Booby Offspring is Maximized by Having One Young Parent and One Old Parent

Abstract

It is widely expected that the quality of offspring will vary with the age of their parents and that this variation should influence animals’ choice of mates. However, theoretical predictions for age effects are contradictory and, to our knowledge, we do not know for any wild animal how the quality of offspring is affected by both parents’ ages across their lifespans, or whether mothers’ and fathers’ ages interact. We tackled this question using long-term data on a highly philopatric, insular population of the blue-footed booby (Sula nebouxii). In this species extra-pair paternity is most common in pairs of two young parents or two old parents, implying that these age combinations might prejudice offspring quality. Analysis of the viability of 3,361 offspring of parents up to 21 years old revealed that fledglings with two young parents or two old parents were least likely to become breeders, whereas fledglings with one young parent and one old parent were most likely to do so. For young parents of either sex, offspring viability increased with age of the other parent; for very old parents, it decreased. These effects could be mediated by parents flexibly modifying their investment in offspring in response to their own and their partners´ ages, but evidence for this was lacking. In 5,343 breeding attempts, although mothers’ and fathers’ ages independently affected four heavily care-dependent breeding traits at the clutch and nestling stages, their interaction did not affect any trait. The effects of parental age combinations on viability could also be mediated by genes: fledglings with one young parent and one old parent could benefit from greater heterozygosity or better genes.

Ectoparasite burden of Blue‐footed Booby chicks depends on combined parental ages

“…The pattern mirrors the interacting effect of parental ages on fledgling viability in figure a of Drummond and Rodríguez () and is partially consistent with the effects of parental ages on immune responses of House Wrens shown by Bowers et al . (: S1a), implying that the two phenomena may be linked.…”

“…) or by occupying heavily infested nest‐sites. By themselves, such age effects cannot explain minimal tick infestation occurring in offspring of different‐aged parents, but such an effect could arise if parents take their own age into account when calibrating investment in offspring to their partner's age (Drummond & Rodríguez ).…”

“…Parental age combinations affect tick abundance and nestling body‐condition of Blue‐footed Boobies in a complex pattern that mirrors their effects on fledglings’ prospects for recruiting into the breeding population several years later (Drummond & Rodríguez ). This provocative pattern of correlations deserves further study.…”

“…In the long‐lived Blue‐footed Booby Sula nebouxii , it was recently found that the probability of fledglings surviving to become reproductive adults is heavily dependent on a curious parental age interaction. Regardless of their body‐condition, fledglings with one old parent (of either sex) and one young parent are the most likely to recruit into the breeding population, whereas those with two old parents or two young parents are the least likely (Drummond & Rodríguez ). This result confounds expectations because in this and other avian species, reproductive performance of both males and females, measured by number of fledglings produced, tends to peak in middle age (Velando et al .…”

Interactive effects of parental age on offspring fitness and age‐assortative mating in a wild bird

“…For example, offspring of older parents were recently found to be less fit than those of younger parents in some avian species, including the common tern ( Sterna hirundo ; Bouwhuis, Vedder, & Becker, ) and the house sparrow ( Passer domesticus ; Schroeder, Nakagawa, Rees, Mannarelli, & Burke, ). Moreover, results of previous studies suggest that parental ages can interact to affect offspring in humans (Fisch et al., ) and birds (Drummond & Rodríguez, ), whereby the effect of the age of one parent depends on the age of the other (but see Richard, Lecomte, De Fraipont, & Clobert, for an exception in common lizards, Lacerta vivipara ). It is unclear whether parental age affects breeding performance over just a few years, as may be the case for shorter‐lived species that might not experience significant senescence, and whether parental ages interact to influence offspring fitness in such species.…”