The bigger they are, the higher they go: Australian insectivorous bats confirm Bergmann’s 175-year-old prediction

Context

Some insectivorous bats are some of the smallest flying endotherm. They have a high energy demand to maintain body temperature. Therefore, one can expect that larger animals of a species and larger species occur in colder environments as a result of improved energy conservation related to reduced surface to volume ratio in larger endotherm animals. Evidence of this general rule is scarce in bats, although Bergmann predicted this some 175 years ago for closely related species.

Aims

In this work, I investigated whether bat body size increases with above-sea-level elevation-related temperature decrease for three closely related Australian bat species of the genus Vespadelus. The purpose of this was two-fold. First, to investigate whether there is a relationship between bat size and elevation by using more recent computational techniques of Bayesian multilevel modelling (BMM). Second, to provide an example of applying recent advances in BMMs to wildlife research and to predict potential consequences of climate warming for these bats.

Methods

I investigated whether bat size relates to elevations of bat-capture locations. I included measurement errors for elevation and forearm length measurements by using a BMM in an high-performance computing environment. This model uses measurements of 775 bats from locations in the western slopes of the Australian Alps.

Key results

The BMM analysis showed that bat forearm length increased 0.11 mm for every 100 m elevation, with a low standard error of 0.01 mm, indicating a high precision. The standard deviations of the variables species and sex within species were large. This means that they did not provide sufficient explantory power for the overall model and predictions to warrant inclusion.

Conclusions

This study showed that there is a linear increase of bat size with elevation. This is the first study to show that bat size is related to elevation (and associated temperature decline) in three sympatric, closely related species of the same genus and it confirmed what Bergmann predicted over 175 years ago.

Implications

Under a warming climate, the results predict that bats become smaller on average. When incorporating average temperature-lapse rate to calculate elevations that assume a 1.5 and 3°C change in future average climate, the study coarsely quantified reduction in suitable habitat for the largest of the three species, V. darlingtoni, of up to 3%.