The bright incubate at night: sexual dichromatism and adaptive incubation division in an open-nesting shorebird

Abstract

Ornamentation of parents poses a high risk for offspring because it reduces cryptic nest defence. Over a century ago, Wallace proposed that sexual dichromatism enhances crypsis of open-nesting females although subsequent studies found that dichromatism per se is not necessarily adaptive. We tested whether reduced female ornamentation in a sexually dichromatic species reduces the risk of clutch depredation and leads to adaptive parental roles in the red-capped plover Charadrius ruficapillus, a species with biparental incubation. Males had significantly brighter and redder head coloration than females. During daytime, when visually foraging predators are active, colour-matched model males incurred a higher risk of clutch depredation than females, whereas at night there was no difference in depredation risk between sexes. In turn, red-capped plovers maintained a strongly diurnal/nocturnal division of parental care during incubation, with males attending the nest largely at night when visual predators were inactive and females incubating during the day. We found support for Wallace's conclusion that reduced female ornamentation provides a selective advantage when reproductive success is threatened by visually foraging predators. We conclude that predators may alter their prey's parental care patterns and therefore may affect parental cooperation during care.

Broad‐scale variation in sexual dichromatism in songbirds is not explained by sex differences in exposure to predators during incubation

“…Moreover, to be sure that our analyses were not affected by phylogenetic uncertainty, we repeated all analyses across 100 phylogenetic trees from birdtree.org with their original branch lengths. While these effects may apply in individual species (Ekanayake et al 2015), our broad-scale analysis does not support Wallace's hypothesis as a general explanation for the marked broad-scale interspecific differences in sexual dichromatism in passerines, because none of these three predictions was supported. Table 1.…”

“…Moreover, nests attended by experimental models designed to look like males were more vulnerable to depredation than female models during the day (Ekanayake et al 2015). The strongest support comes from a study of the red-capped plover Charadrius ruficapillus, where brightly-coloured males incubate at night and more drab females incubate during the day.…”

Sharing the Load: Role Equity in the Incubation of a Monomorphic Shorebird, the Masked Lapwing (Vanellus miles)

“…These represent a gradient from no sex role specialization to clear "shift work" of the sexes (Wallander 2003;St Clair et al 2010a, b). Evidence exists that in sexually dimorphic species, the more ornamented sex may be more detectable to visually foraging predators and so may contribute relatively less to daylight incubation (Ekanayake et al 2015). Thus, there may be a positive association between the degree of sex role specialization in incubation and the degree of sexual dimorphism.…”

“…Neither of these subtle effects influenced our measure of overall contributions to incubation by the sexes, but they may reflect slightly different investment in incubation, role in clutch defense, or even non-visual differences in detectability (e.g., vocalizations) by the sexes during incubation. More comparative studies are required across a range of species exhibiting a variety of dimorphism, and across gradients of predator environments, to further support or refute the incubator conspicuousness hypothesis (Ekanayake et al 2015). In addition to phylogeny, such studies should account for other factors which influence incubation, such as different mating systems, migration, and environmental influences on incubation (Reynolds and Székely 1997).…”

Parental cooperation in a changing climate: fluctuating environments predict shifts in care division

“…In plovers, females usually incubate during the day, while males are responsible for night‐time incubation when conditions are often more benign (Vincze et al ., ; Ekanayake et al . ). Consequently, we expect males’ share relative to females’ to increase under harsh ambient conditions, such as high or low average temperatures or high inter‐annual fluctuations of temperature (stochasticity).…”