Sex allocation within broods: the intrabrood sharing-out hypothesis

Abstract

Selection is expected to cause parents to adjust the sex of their offspring when the environment is predictable during development, and it is expected to affect each sex differently. When several offspring compete for limited resources, the environmental conditions acting on the brood are not a good predictor of the conditions affecting individual offspring. There is evidence for some species that, regardless of any bias in brood sex ratio, the sex of individual offspring within a brood may be related to its position in the hatching/birth/weight rank, in ways that might correlate with the expected share of available resources. Here I propose that parents may be selected to adjust offspring sex within the brood, provided that some depreciable environmental quality is unequally distributed among siblings in a predictable manner. I call this the ''intrabrood sharing-out'' hypothesis and present a graphical model to derive predictions about the relationship between offspring sex and positions within the brood. The model considers that sibling competition not only produces differences in the mean share of resources among siblings, but it also increases the predictability of the share obtained by high-ranking sibs and decreases the predictability of the share for low-ranking ones. Consequently, parents should be selected to deal with such a distribution by promoting the conditions to make it more predictable and then adaptively adjust the sex of particular siblings, especially in high-ranking positions within the brood, rather than to modify the sex ratio of the brood as a whole.

Sex‐specific sibling interactions and offspring fitness in vertebrates: patterns and implications for maternal sex ratios

“…However, as argued in the present paper, intrabrood sex ratio adjustment could also evolve in response to sex-specific sibling interactions (see also Young & Badyaev, 2004). Most of the work cited above suggests that the consequences of sexspecific sibling interactions are probably more important in sexually dimorphic species (see also Mock & Parker, 1997;Carranza, 2004). Thus, in general, we would expect higher Table 2.…”

“…within-brood sex ratio bias in species where the sexes differ in, for example, size or growth rate. A similar prediction can be made from the model of Carranza (2004). However, the selection pressures may act in different directions.…”

“…Carranza (2004) argued that within-brood sex ratio bias was due to resource-related sex-specific fitness returns (the intrabrood sharing-out hypothesis). In this scenario, selection favours production of the sex that benefits most from position-specific resources within the brood.…”

Mother traits and offspring sex in two threatened gazelle species in captivity

“…For Cuvier's gazelle, where twins are common, litter size very likely affect offspring sex. Moreover, the sex of the littermates may not be independent as it may be more likely for a calf to be male if singleton or with a daughter than with a brother (see Cassinello and Gomendio, 1996;Carranza, 2004). As a way of including this likely influence in the analyses, we have built a factor that captures the investment of the mother when producing the focal offspring.…”

Sex, melanic coloration, and sibling competition during the postfledging dependence period

“…Usually, smaller and/or younger individuals show a competitive disadvantage in sibling rivalry. In birds, hatching asynchrony, sexual size dimorphism, brood sex ratio (in sexually size dimorphic species), and within-brood ranking position are factors modulating sibling competition for limited resources in the nest (Clutton-Brock et al 1985;Arroyo 2002;Fargallo et al 2003Fargallo et al , 2006Carranza 2004;Kalmbach and Becker 2005;Müller et al 2005). However, sibling competition does not stop at the nestling stage but continues during the fledging and postfledging periods if siblings continue competing with each other for limited resources (Mock and Parker 1997).…”