The economics of harem maintenance in the sac-winged bat, Saccopteryx bilineata (Emballonuridae)

Saccopteryx bilineata has a polygynous mating system in which males defend females in a harem territory. Harem defense and courtship include energetic ally costly flight maneuvers and hovering displays. We tested if(l) harem m ales have a greater field metabolic rate than non-harem males or females an d if (2) the field metabolic rate of harem males is correlated with the num ber of females in a harem territory. We measured the energy budget in 32 S. bilineata with the doubly labeled water method and compared these estimate s with behavioral observations in the daytime roost. Among adult bats, fiel d metabolic rate varied with body mass by an exponent of approximately two. We found no significant difference in field metabolic rate or mass-specifi c field metabolic rate between harem and non-harem males. The mass-specific field metabolic rate of harem-males increased with harem size. The latter finding supports the hypothesis that the energy costs of courtship display and territorial defense influence the energy budget of harem males. Overall , field metabolic rates of S. bilineata were lower than those of similarly sized bats of the temperate zone and only 2.3 times above the basal metabol ic rate recorded for this species. We suggest that male S. bilineata did no t take advantage of their metabolic capacity because a prudent allocation o f energy to activities of harem maintenance is an adaptive strategy for mal es in this mating system.