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Lack of Sex-Biased Maternal Investment in spite of a Skewed Birth Sex Ratio in White-Headed Langurs (Trachypithecus leucocephalus)


Qing Zhao, 1627 1/2 Roosevelt Ave., Toledo, OH 43607, USA.


Numerous hypotheses have been developed to explain sex allocation. In male-dispersing, female cooperatively breeding species, the local resource competition model predicts male-biased birth sex ratio, the local resource enhancement model predicts female-biased birth sex ratio, and the population adjustment model predicts that biased birth sex ratio should not be favored if the two sexes are equally costly to rear. The male quality model predicts that, in polygynous species, females in better physical condition will either produce more sons than daughters or invest more heavily in sons than in daughters. White-headed langurs are a female philopatry and female cooperatively breeding species. During a 11-yr study, a total of 133 births were recorded, among which birth sex ratio (M:F = 73:49) was significantly male-biased. This is consistent with the prediction of the local resource competition model. On the other hand, if mothers balanced their investment between the two sexes, according to Fisher’s population adjustment model, males should be the less-costly-to-rear sex. However, we found no sex difference for infant mortality (12.3% in males and 12.2% in females), and sons induced slightly longer interbirth interval (son: 26.4 ± 1.1 mo, daughter: 24.1 ± 0.6 mo) and lactational period (son: 20.9 ± 1.0 mo, daughters: 19.6 ± 0.5 mo) for their mothers. Thus, the population adjustment model was not supported by this study. The local resource enhancement model was not supported because birth sex ratio did not bias to females who provided more reproductive assistance. On the individual level, probit regression showed no relation between birth sex ratio and group size. Because the group size was considered to be negatively related to female physical condition, our study did not support the male-quality model. We suggested several possibilities to explain these results.