The brightness of the feet decreases with age, so females tend to mate with younger males with brighter feet, who have higher fertility and greater ability to provide paternal care than older males. In a cross-fostering experiment, it was observed that foot color reflects paternal contribution to raising chicks; chicks raised by foster fathers with brighter feet grew faster than chicks raised by foster males with duller feet. Females continuously evaluate their partners' condition based on foot color. In one experiment, males whose partners had laid a first egg in the nest had their feet dulled by make-up. The female partners laid smaller second eggs a few days later. As duller feet usually indicate a decrease in health and possibly genetic quality, it is adaptive for these females to decrease their investment in the second egg. The smaller second eggs contained less yolk concentration, which could influence embryo development, hatching success, and subsequent chick growth and survival. In addition, they contained less yolk androgens. As androgen plays an important role in chick survival, the experiment suggested female blue-footed boobies use the attractiveness and perceived genetic quality of their mates to determine how much resources they should allocate to their eggs. This supports the differential allocation theory, which predicts that parents invest more in the care of their offspring when paired with attractive mates.
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