THE GENERAL PROTECTED INVASION THEORY - SEX BIASES IN PARENTAL AND ALLOPARENTAL CARE

The biases towards eusociality, female workers and maternal care in ha plodiploid versus diploid insects may result from the relatively low p robabilities that rare mutant, partially dominant alleles promoting th ese behaviours will be lost by genetic drift in haplodiploid populatio ns (Reeve, 1993). A generalization of this 'protected invasion' theory also predicts that parental and alloparental care will tend to be ass ociated with the homogametic sex in diploid populations if the Y chrom osome of the heterogametic sex is absent or largely inert. Sex differe nces in (allo)parental care (i.e. either parental or alloparental care ) should increase with increased asymmetry between the sexes in the fr action of behaviour-influencing loci occurring on their characteristic sex chromosomes. The theory explains the strong predisposition toward s female (allo)parental care in mammals, a contrasting tendency toward s male (allo)parental care in birds, the propensity for joint male and female (allo)parental care in termites, and biases towards female coo peration in social spiders. The theory also explains the apparent rari ty or absence of alloparental care in marsupials, an intriguing conseq uence of preferential paternal X-chromosome inactivation in this taxon . Thus protected invasion theory possibly provides new insights into t he relationship between social structure and the generic system. The t heory does not compete with ecological or kin-selective hypotheses for the advantages of (allo)parental care; indeed, such advantages must e xist for protected-invasion biases to operate.