Sexual segregation and flexible mating patterns in temperate bats

Abstract

Social structure evolves from a trade-off between the costs and benefits of group-living, which are in turn dependent upon the distribution of key resources such as food and shelter. Males and females, or juveniles and adults, may have different priorities when selecting habitat due to differences in physiological or behavioural imperatives, leading to complex patterns in group composition. We studied social structure and mating behaviour in the insectivorous bat Myotis daubentonii along an altitudinal gradient, combining field studies with molecular genetics. With increasing altitude the proportion of males in summer roosts increased and only males were present in the highest roosts. With increasing altitude environmental temperature decreased, nightly variation in temperature increased, and bat foraging activity decreased, supporting the hypothesis that the harsher, high elevation sites cannot support breeding females. We found that offspring in female-dominated lowland roosts had a very high probability of being fathered by bats caught during autumn swarming at hibernation sites, in contrast to those in intermediate roosts, which had a high probability of being fathered by males sharing the nursery roost with the females. Whilst females normally appear to exclude males from nursery colonies, for those in marginal habitats, one explanation for the presence of males is that the thermoregulatory benefits to the females may outweigh disadvantages, such as competition for food, and give some males an opportunity to increase their breeding success. We suggest that the environment, and its effects on resource distribution, thus determine social structure, which in turn determines the mating pattern that has evolved.

Figure 1. Map of the Yorkshire Dales National Park, UK.

Study area with locations of summer roost, foraging and swarming sites.

Figure 2. Change in roost composition along the River Wharfe.

Data from June–August 1996–2007 (upper-elevation n = 175, mid-elevation n = 145, low-elevation n = 356). Numbers are for unique (ringed) bats caught over the period.

Figure 3. Posterior distributions for paternity probabilities at the group level.

Posterior distributions for the probabilities that fathers (at the group level) came from roosts in the (blue) upper-elevation, (yellow) mid-elevation and (green) low-elevation, and from (red) swarming sites. For (A) low-elevation offspring (the inset graph shows the Wharfedale roost posterior distributions in greater detail), and (B) mid-elevation offspring (adapted from [4]).

Figure 4. Posterior distributions for paternity probabilities at the individual level.

Posterior distributions for the probabilities that individual males from the (blue) upper-elevation, (yellow) mid-elevation and (green) low-elevation roosts and (red) swarming sites fathered low-elevation offspring.