Kangaroo rats drum their foot on the ground upon encountering a snake. Why? Read on for this and many other fascinating examples of vibrational communication in mammals…

Vibrational signalling is an important and diverse, albeit not too well studied, mode of communication in mammals, where it has evolved several times independently. It is probably best known in the form of footdrumming, a behaviour that is employed by a wide range of smaller and mainly solitary species and is particularly well developed in kangaroo rats. Unsurprisingly, seismic communication plays a major role in the life of subterranean mammals that are adapted to a life underground, such as blind mole rats. But there is growing evidence for the highly social elephants also relying on seismic signals, which they probably receive through their feet, for long-distance communication with conspecifics.

Elephants

African (Loxodonta africana) and Asian (Elephas maximus) elephants are well known for their low-frequency (about 20 Hz), high-amplitude (more than 100 dB) vocalisations that propagate through the air over considerable distance. Simultaneously, these rumbles travel through the substrate as seismic waves, necessarily at a different velocity and wavelength. Due to their large size, elephants also produce vibrations in the ground when moving. Evidence is mounting that elephants employ these far-ranging seismic signals (they are assumed to propagate up to 16 km, depending on the properties of the soil) in intraspecific communication. African elephants were shown to become more vigilant in response to the substrate-borne component of alarm calls and reacted more strongly to alarm calls of familiar as compared to unfamiliar individuals. They also seem to respond to environmentally generated seismic activity, such as thunder or earthquakes. Seismic communication could extend the elephants’ communication range when seismic conditions are beneficial, replace vocalisations in conditions of poor air-borne transmission (e.g. in dense forests, where calls are attenuated by vegetation) or complement them in a multimodal communication system. Asian elephants possess the largest cerebral cortex of all terrestrial animals, which in principle would allow for the integration of such complex multimodal signals.

It is likely that elephants use their feet to receive seismic signals. When ground-borne stimuli are greatest, elephants have been observed to adopt particular foot postures, such as shifting weight to the forefeet, thus aligning the ears with the feet and legs. Whether they employ bone conduction (where vibrations travel through feet, legs and shoulders to the middle ear), somatosensory reception (relying on vibration-sensitive mechanoreceptors in the skin) or both is still being investigated. The enlarged malleus in the elephants’ middle ear as well as their foot postures provide some support for bone conduction, which could be further enhanced by cartilaginous fat pads in the feet. These cushions are reminiscent of “acoustic fat” in other species (e.g. the fatty “melon” that is involved in echolocation in toothed whales) and might provide an impedance-matching mechanism at the air-ground interface. Furthermore, elephants can occlude their ear canal with a sphincter-like muscle, which by dampening air-borne sound could improve the reception of seismic signals. However, there is also evidence for a somatosensory mechanism of seismic detection in these animals. They possess large clusters of mechanoreceptive Pacinian corpuscles in the dermis of their fore and hind feet, the distribution of which reflects their seismic detection postures.

Other large mammals, such as lions and rhinoceroses, were also shown to generate seismic vibrations, but whether they use them in communication is still unknown.

Drumming

Many mammals (at least 32 species in 11 species) communicate by drumming a body part, often a foot, but sometimes the head or even the teeth, on a substrate. The resulting low-frequency signals consist of an air-borne as well as a ground-borne component, with the latter being more important for transmitting sound into the burrow. Footdrumming patterns are species-specific, but differ in complexity, ranging from simple thumps to complicated individual signatures. Most of the mammals that footdrum are semi-fossorial or fossorial rodents, but this behaviour has also been observed in carnivores (e.g. spotted skunks, Spilogale putorius), deer (e.g. white-tailed deer, Odocoileus virginianus), marsupials (e.g. tammar wallabies, Macropus eugenii), rabbits (e.g. European rabbits, Oryctolagus cuniculus) and elephant shrews (Macroscelididae). Within species, footdrumming can be involved in mating or competitive interactions as well as signal alarm or territorial ownership, but often it also plays a role in interspecific communication with predators. Like other signals, drumming has probably evolved by ritualisation of a behaviour not originally involved in communication (such as running in order to flee from or chase another animal), but not much is known about its evolutionary path. However, “the diversity of mammals that footdrum, and the varied contexts in which it is seen, suggest that drumming evolved independently in different lineages” (Randall 2001, American Zoologist, vol. 41, p. 1144).

Kangaroo rats

Footdrumming is best studied in the nocturnal, solitary kangaroo rats (genus Dipodomys) of North America, particularly the banner-tailed kangaroo rat (D. spectabilis). This species produces highly complex drumming patterns in a number of different contexts. First, footdrumming plays a role in male-male competition and mating, probably coordinating interactions between mates and reducing female aggressiveness. Second, it is involved in territorial behaviour, where it has rendered long-distance chases and fights between familiar neighbours unnecessary. Individuals communicate ownership of a mound to neighbours by a distinct footdrumming pattern and are able to discriminate the signals of familiar neighbours from those of unfamiliar strangers. Finally, banner-tails footdrum in the presence of a snake, producing signals different from those used in interactions with conspecifics. As drumming occurs only after an encounter with a snake, its function is not to inform the predator of its detection. Neither does it startle or disturb the snake. Most likely, footdrumming conveys that the rat is too alert for a successful ambush, thus preventing the snake’s pursuit. Furthermore, mothers might signal danger to vulnerable offspring (they footdrum more intensely in response to a snake than non-mothers), but drumming is not employed to warn adult conspecifics. However, snakes might eavesdrop on territorial footdrumming of kangaroo rats – while non-hungry individuals moved away from footdrumming, hungry snakes were found to actually approach the signal.

Other kangaroo rats (e.g. D. ingens and D. deserti) also employ footdrumming, but their signals are less complex than those of D. spectabilis. Footdrumming generally seems to be restricted to larger, territorial species, while it is absent in smaller, non-territorial species and rudimentary in species of medium size. The question as to what extent body size might limit footdrumming in this genus has, however, not yet been studied in depth.

Other rodents

Not only solitary, but also social rodents employ footdrumming. The highly social great gerbil (Rhombomys opimus), for example, is diurnal and relies more strongly on visual signals than the nocturnal kangaroo rats. However, it too footdrums in response to terrestrial predators, using different signals for different predators. Here, the footdrumming is most likely not directed to the predator, but serves to warn offspring as well as adult family members. Furthermore, footdrumming has been reported to be involved in mating and male-male competition, where the dominant male might indicate its resource holding potential by drumming, thus minimising physical contact with rivals.

Ehrenberg’s mole rat (Spalax ehrenbergi) was the first mammal, for which seismic communication was documented. These blind fossorial rodents bang their head against the walls of their underground tunnels, which was initially interpreted as part of their tunnel building behaviour. In fact, they generate temporally patterned seismic signals for long-distance communication with neighbouring territory owners. Intriguingly, there has been some experimental evidence that mole rats might use reflected self-generated seismic waves to detect and bypass underground obstacles in a form of “seismic echolocation”. To receive such signals, they seem to use a somatsosensory mechanism that transmits vibrational signals detected by mechanoreceptors located mainly in the paws directly to the brain (however, there is also some support for bone conduction through the lower jaw). The related South African cape mole rat (Georychus capensis) produces seismic signals by footdrumming, which seem to propagate at least an order of magnitude beyond air-borne sound.