Sexual dimorphism in bat wing morphology — variation among foraging styles

Sexual dimorphism can lead to differences in foraging style among conspecifics due to morphological differences. Within bats, maneuverability and speed of flight are influenced by wing shape and size, which may differ between sexes. Female bats gain about 30% of their body mass during pregnancy, affecting their agility and flight efficiency. To fill the same foraging niche as males, pregnant female bats would require wing size and (or) shape modifications to maintain maneuverability. We investigated sexual dimorphism in bat wing morphology and how it varies among foraging guilds. Wing photos of male and female adult bats (19 species) in Canada, Belize, and Dominica were analyzed using two-dimensional geometric morphometrics, wing loading, and aspect ratios. Nonpregnant female bats had higher wing loading than males, suggesting that they are less maneuverable than males. Additionally, mass increases during pregnancy may not permit female bats to forage as male conspecifics do. Wing shape differed minimally among foraging guilds with only frugivores differing significantly from all other guilds. Further studies should investigate how female bats forage during their reproductive cycle and determine how frugivore wings differ and whether there are individual differences in wing shape that are not consistent among bat species.

The influence of reproductive condition and concurrent environmental factors on torpor and foraging patterns in female big brown bats (Eptesicus fuscus)

Unlike many other mammals, bats in temperate regions employ short bouts of torpor throughout the reproductive period to maintain a positive energy balance. In addition to decreasing energy expenditure during the day, they typically alter foraging patterns as well. It is well known that various environmental conditions influence both torpor and foraging patterns, but studies of these factors often have focussed on one element in isolation thus it is not known how the two behaviours are collectively influencing temperate bats. The objective of our study was to assess how reproductive condition and environmental factors concurrently affect energy balance in female big brown bats (Eptesicus fuscus). We equipped pregnant and lactating bats in southwest Saskatchewan, Canada with temperature-sensitive radio-transmitters. While transmitters were active, skin temperature data were collected and foraging patterns were determined using triangulation. Of the various environmental and physiological parameters used to model torpor characteristics, roost type was the most important factor. Bats roosting in trees used deeper and longer torpor bouts than those roosting in buildings. Lactating bats had a tendency to forage for longer durations than pregnant bats, and often made more foraging trips. When taken together, we found that foraging duration and torpor duration were not directly related during pregnancy, but exhibited an inverse relationship during lactation. This provides support for the hypothesis that there are physiological trade-offs for reproductive bats and suggests that how bats compensate is not entirely predictable based on current environmental conditions.

Thermoregulation in two free-ranging subtropical insectivorous bat species: Scotophilus species (Vespertilionidae)

Little is known about the thermal regulatory behaviour of free-ranging subtropical bats. We studied torpor use in two free-ranging subtropical sibling species of insectivorous bat, Scotophilus dinganii (A. Smith, 1833) and Scotophilus mhlanganii , during the austral autumn. All S. dinganii (mass 28.9 ± 2.6 g (mean ± SD)) roosted in buildings, whereas all S. mhlanganii (28.0 ± 0.4 g) roosted in the foliage of trees or in a tree cavity. Contrary to what has been seen in other subtropical species, both Scotophilus species used only one bout of torpor per day. Bats entered torpor 2 h after returning from foraging and aroused passively as the roost was heated by the sun the following day. There was a negative correlation between duration of foraging and duration of torpor, probably because bats that foraged longer had less time for torpor. Despite physical differences between roosts, foliage and building roosts appeared equally thermally labile, resulting in torpor bouts of similar depth and duration between individuals of the two species that used such roosts. Cavity-roosting female S. mhlanganii, on the other hand, used torpor bouts that were longer and shallower than those used by its female conspecifics in foliage roosts and female S. dinganii in buildings. Thus, thermal regulatory behaviour was determined more by the type of roost used than by interspecific differences in physiology.