Sex Ratio at Birth and Age-Reversed Dominance among Female Varecia

Abstract

Analysis of 283 offspring born at the Duke University Primate Center show that young female ruffed lemurs, Varecia variegata, produce significantly more daughters, whereas old mothers produce more sons than expected. Data are compared to 3 hypotheses for sex ratio bias: the Trivers-Willard hypothesis which predicts that dominant females produce more males, the local resource competition (LRC) hypothesis which predicts that subordinate females produce more males, and the local resource enhancement (LRE) extension of the LRC hypothesis that females produce the sex that provides later cooperative benefits. In the case of Varecia, this may include alloparenting or cooperative breeding. The results are more consistent with the LRC model. However, observations of 8 mother-daughter relationships show that female dominance rank in free-ranging Varecia groups is age reversed, with daughters aggressively outranking their mothers. Daughters born into the group were not beneficial as future supporters in within-group intermatriline competition as assumed by the LRE model, or as subordinate alloparents, cooperative breeders or aids in territorial defense. Both sex ratio and ranking observations are consistent, however, with the hypothesis that mothers produce daughters when they are young and able to invade elsewhere and sons as they age and are less able to do so. This is supported by a single observation of a mother dispersing which resulted in her daughter inheriting the natal territory. These data are supportive of the LRE model with female alliances and cooperative breeding among dispersed females.

Sex-Segregated Range Use by Black-and-White Ruffed Lemurs (Varecia variegata) in Ranomafana National Park, Madagascar

“…By contrast, although also determined to be a motheroffspring pair [Baden, 2011], Radio-Blue shared her home range with her adult daughter (Radio-Green). An earlier study suggests that ruffed lemur daughters may inherit their territory from their mother upon eviction or death [White 2009], similar to patterns observed in chimpanzees [Williams et al, 2002;Murray et al, 2008;Langergraber et al 2009Langergraber et al , 2013. While it is not currently possible to assess home range inheritance at this time, Radio-Blue and Radio-Green resided in the same territory for at least three years prior to this study (since 2005), and remained in the territory together until at least 2011, lending anecdotal support for this hypothesis.…”

Does the lack of heritability of human sex ratios require a rethink of sex ratio theory? No: a Comment on Zietsch et al . 2020

“…There is no compelling reason to think that this is true, and there is evidence to indicate that it is not. For example, estimates of the sex ratio at birth vary among primates, but many are statistically similar to the male-bias observed in many human populations or to a 1 : 1 sex ratio at birth [31][32][33]. Similarly, post-birth age-specific mortality rates are lower for females than for males in several primate species, just as in humans [34].…”

“…For example, estimates of the sex ratio at birth vary among primates, but many are statistically similar to the male-bias observed in many human populations or to a 1:1 sex ratio at birth (Sugiyama 2004;White 2009;Bronikowski et al 2016). Similarly, post-birth age-specific mortality rates are lower for females than for males in several primate species, just as in humans (Bronikowski et al 2011).…”