Evolution of the ungulate dewlap: thermoregulation rather than sexual selection or predator deterrence?

Jakob Bro-Jørgensen

doi:10.1186/s12983-016-0165-x

Abstract

BackgroundDewlaps are iconic features of several ungulate species and, although a role in signalling has been postulated, their function remains largely unexplored. We recently failed to find any age-independent link between dewlap size and social status in the common eland (Tragelaphus oryx), pointing to the possibility that sexual selection may not be the primary cause of dewlap evolution in ungulates. Here I use a two-pronged approach to test hypotheses on the function of ungulate dewlaps: an interspecific comparative analysis of bovids and deer, and an intraspecific study of eland antelopes in the wild.ResultsAcross species, the presence of dewlaps in males was not found to be associated with sexual size dimorphism, a commonly used measure of the intensity of sexual selection. The presence of dewlaps was, however, linked to very large male body size (>400 kg), which agrees with a thermoregulatory function as lower surface/volume-ratio counteracts heat dissipation in large-bodied species. In eland antelopes, large dewlap size was associated with higher, rather than lower, incidence of claw-marks (independently of age), a result which speaks against the dewlap as a predator deterrent and rather indicates a predation cost of the structure.ConclusionThe findings suggest that, although an additional function in communication should not be ruled out, the dewlap of ungulates may contrast with that of lizards and birds in thermoregulation being a primary function.

Highlights

Dewlaps are iconic features of several ungulate species and, a role in signalling has been postulated, their function remains largely unexplored
Interspecific study: comparative analysis A dataset was compiled focusing on all extant bovids and cervids including the following variables: presence/ absence of dewlaps in males, mean male body mass, and sexual size dimorphism (SSD), which was calculated as male body mass divided by female body mass
Interspecific study: comparative analysis Among species, the presence of dewlaps in males was predicted by large male body mass, but not by the degree of SSD (Table 1)

Summary

Introduction

Dewlaps are iconic features of several ungulate species and, a role in signalling has been postulated, their function remains largely unexplored. I.e. loose skin flaps hanging from the neck, are a case in point. These striking yet enigmatic structures are found in various vertebrate taxa, notably iguanid and agamid lizards, birds and ungulates. Dewlaps – often referred to as wattles – are present as a diverse set of fleshy excrescences pending from the neck in several taxa (in particular cassowaries and galliformes). Like in lizards, they are more pronounced in males than females and are thought to function as sexual signals of male quality [6]. In spite of some structural similarity with the dewlaps of lizards and birds, Bro-Jørgensen Frontiers in Zoology (2016) 13:33 whether ungulate dewlaps have evolved as a result of the same selective pressures remains an open question

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Conclusion