Donald A. Jenni

Sex differences in nest construction, incubation, and parental behaviour in the polyandrous American jaçana (Jacana spinosa)

Abstract The polyandrous American jacana shows sex-role reversal. Before egg-laying, males perform most nest-building movements. This behaviour primarily serves a communication rather than a building function. Some males build during incubation. Only males incubate, and incubation constancy is exceptionally low. Incubating males sit on their wings with two eggs between each wing and the breast. Males do almost all brooding; females brood occasionally after 2 or more days of rain. Although adults never feed the chicks, they feed more rapidly when accompanied by the male than when alone. Anti-predator behaviour by both sexes increases at hatching and remains high. The rarity with which females perform parental care suggests that selection has favoured behaviour that maximizes polyandry.

Natal philopatry and breeding area fidelity of long-billed curlews (Numenius americanus): Patterns and evolutionary consequences

SummaryLong-billed curlews (Numenius americanus) appear unique among scolopacid shorebirds so far studied in possessing a significant sex bias in natal philopatry. We resighted 9 curlews at least attempting to breed that were color-banded as chicks; 8 of these were males. Male curlews also cooperate extensively with neighbors in mobbing potential chick predators. This mutualistic behavior may have evolved through kin selection among philopatric males. If so, we would expect such an evolutionary consequence to lead to a similar sex bias in breeding area fidelity. Yet our resightings of colorbanded adults over 4 consecutive years indicate that males and females were equally likely to return to previous nesting territories. Excessive disturbance such as capture and nest loss within a single breeding season was correlated with the likelihood of breeding dispersal by females but not males. This suggests potentially stronger breeding area fidelity of males.

Carrying Behavior in Humans: Analysis of Sex Differences

Behavioral differences between the sexes include methods of carrying books. Females clasp books against their chests; males carry them at their sides. In kindergarten and the first grade, both sexes carry like mature males. Sex-typical carrying appears before adolescence. Behavioral differences seem to be primarily a consequence of morphological differences and social modeling.