Recent work on Daubenton’s bats in Upper Wharfedale funded by The National Trust and The Environment Agency has revealed an unusual population structure. There appears to be a marked sexual segregation, with females occupying the lower reaches of the river, while the upper reaches are populated exclusively by males. Daubenton’s bat is heavily reliant on river systems for foraging and the question is why females do not use the upper reaches, since as male density is high, prey is expected to be abundant? The hypothesis is that the mean prey abundance at higher altitudes is the same as that at lower alititudes, but that due to more marked temperature variations, prey are temporally clustered. Males, which can use torpor (a form of temporary hibernation) during periods of low prey availability, can exploit this resource, while pregnant, or lactating females are effectively excluded since the use of torpor would slow foetus or offspring development.