Genetic Substrates for the Initial Evolution of Human Sociality. I. Sex Chromosome Mechanisms

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Abstract
In a species with haplo-diploid sex determination, a female can do more to perpetuate her own genotype by looking after her sisters than her daughters. In a species without haplodiploid sex determination but with chromosomal sex determination, individuals of the homogametic sex can do more to perpetuate their own sex-linked genotype by looking after their same-sex siblings than their own offspring. If sex-chromosome-based deviation from the general genetic coefficient of relationship was operative as a factor favoring the evolution of sociality, then, in comparisons between birds and mammals, some general sex-role reversals in tendencies toward gregariousness or mutual tolerance should be evident. Among mammals females as the homogametic sex are expected to display more social cohesiveness than conspecific males, and they do. Among birds, males are expected to display more social cohesiveness than conspecific females, and they might in at least some species. Since species vary in their mode of sex determination, a wide variety of testable hypotheses can be generated from this proposal.