Sexual differences in the annual lipid cycle of the big brown bat Eptesicus fuscus

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Abstract
The annual lipid cycle in male and female big brown bats, Eptesicus fuscus, was studied in relation to the reproductive patterns for each sex. The basic pattern of lipid deposition and use was similar in males and females; however, there were differences between the sexes in the timing of these activities. Females began rapid fat deposition 1 month earlier than males. The beginning of rapid fat deposition in females is associated with the end of lactation and in males with the end of spermatogenesis and the peak mating season. Rates of fat use were similar between the sexes for the entire hibernation period, but the rate in females was more consistent than the rate in males. The caloric content of male bats did not differ throughout the year, but females had higher values during pregnancy and lactation than at other times of the year. Differences between the sexes in the timing of specific events within the annual lipid cycle appear to be adaptations for optimizing reproductive success.