Dewlaps are loose flaps of skin that hang from the necks of some animals, notably certain lizards, birds, and hoofed mammals. These enigmatic ornaments are usually more pronounced in males than in females, suggesting a role in sexual selection. This seems to be the case in lizards and birds, which use their dewlaps in sexual displays. But the role of dewlaps in hoofed mammals is not so well understood.
In a new study, Jakob Bro-Jørgensen of the University of Liverpool explores three hypotheses concerning the function of dewlaps in cows, deer, and antelopes. The first hypothesis posits that dewlaps evolved in hoofed mammals to serve a role in sexual signaling, for example, by indicating age-related fighting ability or making males look bigger to their rivals. The second possibility is that dewlaps make animals appear larger to predators and deter attacks. Alternatively, dewlaps could make it easier for predators to get a hold on their prey, and since only “high quality” animals can develop large dewlaps without getting eaten, the structure signals that an individual will be difficult to kill, thus discouraging attacks. According to the final hypothesis, dewlaps help animals dissipate excess body heat.
Bro-Jørgensen evaluated these hypotheses using a comparative approach, investigating differences between species, and a field study of one species, the common eland. Male common elands develop large dewlaps that can droop more than 40 cm (15.7 inches) beneath their necks.
The results indicated that, unlike in birds and lizards, the dewlap in hoofed mammals does not appear to play a role in sexual selection. And in eland antelopes, large dewlap size was associated with higher, rather than lower, incidence of claw marks, which suggests the structure incurs a predation cost rather than serves as a predator deterrent.
The presence of dewlaps was, however, linked to very large male body size (>400 kg), which supports a thermoregulatory function. Overheating can be a problem for large species in hot climates. Bro-Jørgensen notes that the species he evaluated in which males weigh above 400 kg but do not have dewlaps all live in cold areas or have evolved alternative, behavioral cooling strategies such as wallowing.
The fact that females lack dewlaps is also consistent with the thermoregulation hypothesis. Females are much smaller than males in all species with dewlaps, and thus heat dissipation is less of an issue for them.
Bro-Jørgensen notes that it may be premature to rule out a communication function of the dewlap, and more research is needed to explore this option. But the research so far suggests that hoofed mammals may have evolved dewlaps for purposes other than those used by birds and lizards; namely, to help big males lose heat in hot environments.