Determinants of offspring sex in kangaroos: a test of multiple hypotheses

Abstract

When the fitness costs and benefits of sons and daughters differ, offspring sex ratio manipulation could be an important reproductive tactic. We explored the effects of environment and maternal caring ability on offspring sex to test four adaptive sex ratio modification hypotheses: the extrinsic modification hypothesis (EMH), carrying capacity hypothesis (CCH), Trivers-Willard hypothesis (TWH), and cost-of-reproduction hypothesis (CRH). The EMH and CCH propose that environmental conditions shape offspring sex ratios, directly or in interaction with maternal condition. The TWH and CRH predict a positive relationship between maternal condition and production of the costlier sex. The TWH predicts that mothers with superior caring ability should produce more of the sex that can provide the greatest fitness returns from additional maternal allocation, and the CRH proposes that females with limited caring ability should reduce fitness costs by producing the cheaper sex. Repeated measures on 83 known-age eastern gray kangaroos, polygynous marsupials with strong sexual dimorphism, revealed that offspring sex ratio was independent of per capita forage, supporting neither the EMH nor CCH, but was dependent on maternal mass, consistent with the TWH and CCH. Our results, however, cannot clearly identify the ultimate cause of the relationship between maternal mass and greater production of sons. One of the three assumptions of the TWH could not be verified, and mothers of sons suffered only marginal additional fitness costs. Sex ratios in higher vertebrates are likely not solely explained by factors dependent on maternal control.

Conservation Biology and Reproduction in a Time of Developmental Plasticity

“…Conversely, a significantly male-biased sex ratio of Asian elephants born in European zoos between 1962 and 2006 (ratio: 0.61, p = 0.044) was reported in 2009 [ 46 ], as well as an even more striking male bias in elephant births following artificial insemination (0.83, p = 0.003). It has clearly been very difficult for researchers to find a unifying theory that explains the differing reports of biased mammalian sex ratios at birth in diverse wild and captive species (see, for example [ 47 , 48 , 49 , 50 , 51 , 52 , 53 ]), in relation to environmental conditions. However, that such mechanisms exist and may sometimes, but not necessarily always, be adaptive, is now beyond dispute.…”

Paternal influences over offspring sex ratio in mammals – tested hypotheses and potential mechanisms

“…1999), concentrate on environmental factors affecting SR. An overview of these hypotheses could be found in excellent papers by Douhard and Geffroy (2021) and Toni et al. (2021).…”

“…Adaptive hypotheses of biased SRs concern particular interactions between sexes and could be tested according to local resource competition (LRC; Silk 1984), local resource enhancement (Clutton-Brock & Iason 1986) and parental condition hypothesis (Trivers & Willard 1973). Other hypotheses, the extrinsic modification hypothesis (Post et al 1999) and carrying capacity hypothesis (Kruuk et al 1999), concentrate on environmental factors affecting SR. An overview of these hypotheses could be found in excellent papers by Douhard and Geffroy (2021) and Toni et al (2021).…”

Parental investment in the Columbian ground squirrel: empirical tests of sex allocation models

“…2015, Toni et al. 2021), support for the prediction of a positive association of maternal condition and offspring sex ratio remains equivocal.…”

“…This hypothesis assumes that the relative cost of producing male and female offspring differs, that there is a positive association of maternal body condition and the production of males, that early investment in males should persist into adulthood, and that such males should have greater fitness as adults. Despite considerable empirical and theoretical study of offspring sex ratio in mammals (Clutton-Brock and Iason 1986, Festa-Bianchet 1996, Kojola 1998, Hewison and Gaillard 1999, Brown 2001, Cameron 2004, Cameron et al 2008, Schindler et al 2015, Toni et al 2021, support for the prediction of a positive association of maternal condition and offspring sex ratio remains equivocal. Clark (1978) developed the idea of local competition among relatives as an influence on offspring sex ratios that deviate from 50:50 to explain offspring sex ratios that typically vary between 60% and 75% male in species such as galagos (Galago sp.)…”