Body Size Predicts Echolocation Call Peak Frequency Better than Gape Height in Vespertilionid Bats

Jeneni Thiagavel,Sharlene E Santana,John M Ratcliffe

doi:10.1038/s41598-017-00959-2

Jeneni Thiagavel, Sharlene E Santana + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-00959-2

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Abstract

In most vocalizing vertebrates, lighter animals tend to produce acoustic signals of higher frequency than heavier animals. Two hypotheses propose to explain this negative relationship in vespertilionid bats: (i) mass-signal frequency allometry and (ii) emitter-limited (maximum gape) signal directionality. The first hypothesis, that lighter bats with smaller larynges are constrained to calls with higher frequencies, is supported at the species level. The second hypothesis proposes that in open space, small bats use higher frequencies to achieve narrow sonar beams, as beam directionality increases with both emitter size (maximum gape) and signal frequency. This hypothesis is supported within a comparative context but remains untested beyond a few species. We analyzed gape, body mass, and echolocation data under a phylogenetic comparative framework to test these hypotheses, and considered forearm length as both a proxy for wing design and an alternative measure of bat size. Controlling for mass, we found no support for the directionality hypothesis. Body mass and relative forearm length were negatively related to open space echolocation call peak frequency, reflecting species-specific size differences, but also the influence of wing design and preferred foraging habitat on size-independent species-specific differences in echolocation call design.

Highlights

An alternative hypothesis, sonar beam directionality, has recently gained attention as an overlooked explanation for (i) the production and use of high frequency sounds and (ii) the negative relationship between size and call frequency in vespertilionid bats[7, 8]
We found significant phylogenetic signal in all variables, and a lambda model provided the best fit for the data (Table 1)
Using phylogenetic generalized least squares (PGLS) by restricted maximum likelihood (REML), we found that peak frequency (PF) scaled negatively with mass, forearm length, and gape height

Summary

Introduction

Sonar beam directionality, has recently gained attention as an overlooked explanation for (i) the production and use of high frequency sounds and (ii) the negative relationship between size and call frequency in vespertilionid bats[7, 8]. Vespers unable to produce wide gapes (i.e. those with small mouths and/or short snouts), which would otherwise produce low directionality calls, have been argued to use very high frequency calls to achieve highly directional sonar beams while hunting in open space[8]. It has been suggested that maximum gape may better predict species-specific PF than body mass[8]. Neither of these proposed morphological correlates of echolocation call frequency has been assessed independently of the other, nor has either been considered within a phylogenetic comparative framework. We predicted that the two proxies of body size (i.e. mass and forearm length) would be strong, independent predictors of PF due to size-signal allometry (as observed in non-echolocating vocalizing vertebrates) but that gape height (when corrected for mass) would not influence PF as strongly

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