Echolocation converges under shared environmental conditions in wild bats

Frequency and intensity of animal vocalisations scale with body size resulting in smaller animals generally producing weaker and higher frequency sounds than larger animals. Echolocating bats are very small mammals that not only vocalise to communicate but also produce high frequency calls to perform echo guided foraging in a variety of environments. Thus, in addition to body size, vocal scaling in bats may likely be driven by foraging specialisations and environmental context. To test this hypothesis, we used a microphone array to record the echolocation behaviour of seven wild bat species (6-32 grams) orienting through a narrow tunnel during nightly cave emergence. Under these recording conditions, the bats faced the same sensory scene while solving the same natural behavioural task. As predicted from mammalian vocal scaling, the centroid frequency of calls decreased with body size for both constant frequency (CF) and frequency modulating (FM) bats. However, the frequency scaling slope of FM bats was less negative than predicted, suggesting convergence of call frequency across body size and foraging specialisations. Contrary to general scaling principles, call intensity and call intervals also converged across body size meaning that smaller bats called louder, and larger bats called more often than predicted by their size. This leads us to conclude that environmental context is a very important driver of bat echolocation behaviour and that this deviation from normal scaling principles must be considered when performing comparative sensory studies across bats of different sizes, foraging specialisations and sensory scenes.