Anisogamy Overcome: Female Strategies in Snail Kites

Abstract

It has often been suggested that, because of anisogamy, females are likely to have invested more reproductive effort (RE) than males at the moment of fertilization. The energy and risk incurred by snail kite parents from pair formation through egg laying was assessed using time-activity budgets and reproductive bioenergetics. Males contributed significantly more to nest building and to chasing potential predators and conspecifics; they spent significantly more time foraging for their mates and for themselves, providing, on the average, 83% of the female''s food; and they expended nearly twice as much energy as females. Including the cost of clutch production, models of cumulative energy expenditure as estimates of RE based on rapid or slow pair formation indicate that males consistently invested more energy than females through the time of egg laying. RE by female snail kites is lower than that of males because females pursue a strategy of decreasing energy demands during egg laying by decreasing locomotor activity. Withholding energy investments may allow females to avoid energy depletion during egg laying and permit increased iteroparity; this strategy should be highly favored because both nest-failure rates and environmental uncertainty are high. A large initial investment by males before egg laying may contribute to an increased confidence of paternity but is more likely to be a result rather than the cause of females'' overcoming the immediate costs of anisogamy. Snail kites also exemplify a class of exceptions to Trivers'' (1972) theory of sexual differences, whereby the sex competing for mates is predicted to be the one investing less RE.