Seismic signal use by fossorial mammals

Seismic Signal Use by Fossorial Mammals1

SYNOPSIS. The subterranean environment is not favorable for the use of vision or the audition of airborne sounds as means of long-distance sensory perception. However, seismic vibrations have been shown to propagate at least an order of magnitude better than airborne sound between the burrow systems of the molerat Georychus capensis. The use of the seismic channel for communication underground is well documented for other species of bathyergids, as well as the spalacine mole-rat Nannospalax. It has recently been suggested that the golden mole Eremitalpa grand namibensis may also be sensitive to ground vibrations, in this case used in foraging in its desert habitat.

In this paper, the use of seismic signals among these and other fossorial mammals is reviewed from theoretical, behavioral and anatomical standpoints. The question of whether auditory or somatosensory means are used to detect vibratory signals is examined. Attempts to explain the distribution of seismic sensitivity and communication mechanisms among fossorial mammals are considered. The potential influences of different soil type and digging methods are discussed, and it is proposed that digging mechanisms involving the head might preadapt a fossorial mammal towards the development of seismic sensitivity.

INTRODUCTION

There are several families of mammals that contain members that lead a largely subterranean existence (Nevo, 1979). The term fossorial is here applied to those species that possess unusual anatomical or physiological adaptations towards burrowing, and that spend a considerable amount of their lives underground. Among the order Rodentia, the fossorial mammals considered here include the pocket gophers (family Geomyidae) and the ctenomyids (family Ctenomyidae) of the Americas, the bathyergid mole-rats (family Bathyergidae) of Africa and the spalacine mole-rats (family Muridae, subfamily Spalacinae) of Eurasia and North Africa. Among the Insectivora, many of the talpid moles (order Insectivora, family Talpidae) of Eurasia and North America are fossorial. The golden moles (family Chrysochloridae), which also are fossorial, have often been included within the Insectivora, although recent molecular studies group these animals within a clade of endemic African mammals (Springer et al., 1997; Stanhope et al., 1998). The only Australian fossorial mammal is the marsupial mole, Notoryctes typhlops (order Notoryctimorphia, family Notoryctidae). These fossorial mammals are generally small, most weighing well under a kilogram. While the fossorial rodents are herbivores, typically feeding on roots and tubers, the talpid moles, golden moles and marsupial mole are largely insectivorous (see Nowak, 1999).

Most fossorial mammals construct permanent tunnel systems, for shelter and often for feeding, and these tunnels may be very elaborate. The energetic cost of burrowing is considerable (Vleck, 1979, 1981), and therefore a network of tunnels represents a valuable resource. Although intraspecific interactions among the many supposedly "solitary" fossorial mammals may be more common than generally supposed (Lacey, 2000), many species are extremely aggressive in defense of their burrows. In order to avoid costly conflicts, neighboring talpid moles avoid conspecifics by foraging in areas of range overlap at different times of the day (Stone and Gorman, 1985). Alternatively, some form of intraspecific communication may be used to warn intruders, or conversely to indicate that an animal is receptive to potential mates and is unlikely to attack during the breeding season. Some bathyergid mole-rats (Heterocephalus and Cryptomys species) are social and live in colonies of related individuals (Bennett and Jarvis, 1988b; Sherman et al., 1991; Bennett et al., 1994), and in these cases intraspecific communication will take on a different role in mediating the often complex social relationships between animals (Lacey et al., 1991; Pepper et al., 1991). In addition, all fossorial mammals presumably would benefit from a means of detecting approaching predators, and perhaps from a means of detecting food items.

Sensory perception among fossorial mammals will be restricted by the nature of the subterranean environment. Chemical signaling and the sense of smell are probably used extensively in fossorial species (see Burda et al., 1990; Francescoli, 2000), but terrestrial mammals all seem to use at least one other sense to gather information about their surroundings without the delay associated with chemical diffusion or the proximity required for touch or taste. Although some fossorial rodents that spend time on the surface may retain a reasonable degree of visual acuity (see review in Francescoli, 2000), vision is of very limited use in a dark tunnel, and the eyes of many other fossorial species are reduced or vestigial (e.g., Sweet, 1906, 1909; Gubbay, 1956; Eloff, 1958; Quilliam, 1966a, b; Hildebrand, 1985; Burda et al., 1990; Cooper et al., 1993; Hetling et al., 2000). It has been proposed that the nasal tentacles of the starnosed mole Condylura cristata might be used for a form of electroreception underwater (Gould et al., 1993), although this has been disputed (Catania, 1995b). Electrical field detection is highly unlikely in air (Gould et al., 1993), so if present at all, this sensory modality is probably not widespread among fossorial mammals.