© 2001 by The Society for Integrative and Comparative Biology
Evolution and Function of Drumming as Communication in Mammals1
1 Department of Biology, San Francisco State University, San Francisco, California 94132
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 SYNOPSIS INTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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An amazing variety of mammals produce seismic vibrations by drumming a part of their body on a substrate. The drumming can communicate multiple messages to conspecifics about territorial ownership, competitive superiority, submission, readiness to mate, or presence of predators. Drumming also functions in interspecific communication when prey animals drum to communicate to predators that they are too alert for a successful ambush. The diversity of mammals that drum in varied contexts suggest independent evolution in different lineages. Footdrumming, as with other signals, probably originated by ritualization of older forms of behavior not associated with communication such as running and digging. Footdrumming patterns are species specific and range from single thumps to individual footdrum signatures. Although mammals communicate above ground with airborne drumming signals, they can also transmit sound seismically into the burrow where the signals become airborne and are received with ears especially adapted to hear low-frequency sound. Footdrumming has been studied the most extensively in kangaroo rats, Dipodomys. A comparison of species of different body mass shows that smaller sized, non-territorial species have no ritualized footdrumming; medium-sized species drum a simple pattern in limited contexts; while larger-sized species communicate territorial ownership with complex patterns. Future studies should examine the mechanics and energy requirements of drumming to test hypotheses about body size limitations on the evolution of footdrumming. Our understanding of drumming as communication is limited until investigators conduct field tests of responses to drumming signals in the contexts in which the signals are generated.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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In 1983, Hubert Markl published a review paper on vibrational communication. In aims for future research, Markl recognized vibrational communication as a fascinating field that had hardly been explored. He called for research to explain vibrations in natural ecological settings and an effort to overcome the formidable problems of observation, recording and experimentation. This paper and others in the symposium demonstrate that obstacles to studying vibrational communication have been overcome and that it is a form of communication as far reaching and important as bird song, visual displays and chemical signals.
Although Markl ignored mammals in his review, probably because little was known about their vibrational communication at the time, recent studies show that vibrational communication is a diverse and important mode of communication in mammals. An amazing number of mammals produce vibrations by drumming a part of their body on a substrate. Footdrumming is the most common means of creating vibrational signals (Randall, 1993, 1994a
), but mammals also communicate with vibrations by drumming with their heads (Rado et al., 1987) and teeth (Giannoni et al., 1997). Most drumming mammals are rodents, but drumming has also been described in carnivores, deer, rabbits, elephant shrews and marsupials. For instance, spotted skunks, Spilogale putorius, stamp their front feet when approached by an unfamiliar person or animal (Crabb, 1948), and white-tailed deer and tammar wallabies stamp their feet when encountering a predator (Caro et al., 1995; Blumstein et al., 2000). Rabbits, Oryctolagus and Sylvilagus, are known to footdrum when alarmed (Orr, 1940; Ewer, 1968), and at least five species of elephant shrew footdrum (Rathbun, 1979; Roeper, 1981; Faurie et al., 1996).
Footdrumming is common in rodents and occurs in both fossorial and semi-fossorial species from all continents except Australia and Antarctica (Randall, 1994a) (Tables 1 and 2). Although many rodents that drum are nocturnal and solitary, the behavior is also seen in highly social rodents such as eusocial mole rats, Cryptomys damarensis, and social gerbils, Rhombomys opimus (Jarvis and Bennett, 1991, 1993; Randall et al., 2000).
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One of the more interesting aspects of footdrumming is the varied context in which it occurs (Tables 1 and 2). Mammals footdrum in at least five different intraspecific contexts: (1) as territorial advertisement, (2) during agonistic interactions in defense of territories and mates, (3) to coordinate mating interactions between males and females, (4) to communicate subordinance and an unwillingness to interact, and (5) to communicate danger from predators to family members. In interspecific communication, prey species footdrum in response to predators, especially snakes, to communicate directly to the predator (Table 3).
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EVOLUTION OF FOOTDRUMMING AS COMMUNICATION |
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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Although footdrumming has evolved in many different lineages of mammals (Tables 1 and 2), the path of evolution of this behavior is unknown. Did footdrumming evolve independently in different lineages or was there an ancestor that drummed and passed the trait on? How did the behavior originate? Because drumming costs energy, takes time away from foraging and might subject the drummer to predation risk, it must have some adaptive value for the many mammals that engage in it (Taylor et al., 1990
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The diversity of mammals that footdrum, and the varied contexts in which it is seen, suggest that drumming evolved independently in different lineages (Randall, 1994a). Footdrumming, as with other signals, probably originated by ritualization of older forms of behavior not associated with communication (Tinbergen, 1952; Bursten et al., 2000). For instance, because rodents drum with feet that are also used for digging, footdrumming may have originated as a ritualized form of digging behavior, especially in fossorial mammals (Francescoli and Altuna, 1998). However, although the majority of small mammals that footdrum are either fossorial or semi-fossorial (Randall, 1994a), footdrumming also occurs in species that rarely dig burrows such as elephant-shrews (Rathbun, 1979; Faurie et al., 1996). Also, fossorial mole-rats (Spalax) drum with their heads rather than their feet (Rado et al., 1987), and drumming is not always done with the same feet that are used for digging. The hypothesis that footdrumming is an extension of digging behavior, therefore, does not fit for many species.
Another hypothesis for the evolution of footdrumming is based on the observation that drumming increases when an animal is highly agitated or stressed (Rathbun, 1979; Randall, 1993). Mongolian gerbils, Meriones unguiculatus, footdrum following foot shock (Routtenberg and Kramis, 1967) and in response to novel stimuli (Spatz and Granger, 1970). Anyone with a Mongolian gerbil as a pet or in a laboratory colony is aware of their drumming when disturbed. Kangaroo rats (Dipodomys) also footdrum in cages in the laboratory when disturbed (even in response to the 1989 earthquake in San Francisco), and footdrumming rates are high after particularly intense encounters with a conspecific or predator (Eisenberg, 1963; Randall, 1991; Randall and Matocq, 1997). The act of drumming the feet, therefore, may have originated in some species from responses to stressful situations such as the approach of a competitor or predator. Because mammals footdrum with the same feet that are used for escape, the drumming might be an intention movement of fleeing that has become ritualized (Ewer, 1968).
What stimulates an animal to footdrum when stressed because of a threat? An animal may experience conflicting urges when encountering a threat: either move toward the object of the threat and inspect it to gain more information or run away and escape or hide (Dugatkin and Godin, 1992; Swaisgood et al., 1999). As a result, the animal engages in displacement behavior and drums its feet. Because many desert rodents that footdrum, such as kangaroo rats, inhabit open habitats and tend to forage in areas with little cover, locomotion for escape is well developed (Nikolai and Bramble, 1983). The large hind feet used for bipedal locomotion probably evolved in these desert-dwellers to facilitate escape from predators in open habitats and secondarily became used for sound production.
Kangaroo rats often drum their feet immediately before they run to chase a conspecific, and elephant shrews, Elephantulus rufescens, run the first time they footdrum in an encounter (Roeper, 1981). Footdrumming, therefore, could be an intention movement to indicate a high probability of locomotion to run toward another animal, rather than away from it. A brief drumming bout of 2–4 footdrums sometimes occurs before a chase in smaller-sized species of kangaroo rat (D. merriami and D. ordii) that have not developed ritualized footdrumming for communication (personal observation). The limited drumming in smaller-sized species of kangaroo rats (Eisenberg, 1963) and the progressive development of the behavior for communication as body size increases (Table 2), suggest that footdrumming in kangaroo rats evolved into a communication signal in species that could overcome energetic limitations from size constraints. An additional factor in the evolution of footdrumming in kangaroo rats is that larger species defend territories while smaller species tend to have overlapping home ranges and tolerate the presence of conspecifics (Behrends et al., 1986; Perri and Randall, 1999). Footdrumming in kangaroo rats follows a continuum from no ritualized drumming in small-sized species to a rudimentary drumming pattern in medium-sized species such as D. heermanni to well developed footdrumming signatures in larger sized animals that defend territories (Randall, 1993) (Table 2).
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Footdrumming in territorial species, therefore, may have been derived from locomotion that originally communicated the drummer's intention to chase. Once footdrumming originated, it became associated with an attack to allow signal receivers to make adaptive behavioral adjustments based on a communication signal and avoid costly interactions (Owings and Morton, 1998
). The drumming becomes an act of self interest to the signal sender because it does not have to spend time and energy and risk injury with a fight in defense of its territory. The savings in time and energy and avoidance of injury become important sources of selection on the communication system to the point that footdrumming becomes a substitute for fighting by successfully communicating fighting ability and resource-holding potential (Parker, 1974; Maynard Smith, 1994). Footdrumming may manipulate behavior of competitors to the drummer's advantage as the meaning of the drumming vibrations progress from signaling "I am going to chase you" to "I want you to leave so I do not have to chase you" (Krebs and Dawkins, 1984; Owings and Morton, 1998).
Footdrumming may have developed in the presence of predators via the same mechanism as when threatened by a conspecific: the drumming was originally a displacement behavior or an intention movement that became ritualized (Ewer, 1968). Kangaroo rats, even those that do not footdrum, tend to approach and investigate novel stimuli (Eisenberg, 1963). Rather than fleeing, kangaroo rats often approach snakes and then jump back if the snake strikes, and the larger-sized species footdrum (Randall and Matocq, 1997; Randall and Boltas King, 2001). Originally, the footdrumming probably conveyed no signal and then became a nonspecific signal of locomotion. Eventually, footdrumming communicated to the snake that the prey was alert and vigilant and not easy prey (Randall and Matocq, 1997). Ultimately, the footdrumming might even convey that the prey "wants the snake to go away."
The path of evolution of footdrumming as a communication signal that I discuss here is speculative and presented as hypotheses for further discussion and testing. These hypotheses certainly are not mutually exclusive, and one or all may apply. Whatever the mechanism for the evolution of footdrumming as a communication signal, the behavior became ritualized for communication because it benefited both the signal sender and the signal receiver and became an effective way for the drummer to convey information or to manipulate behavior of another animal (Krebs and Dawkins, 1984; Owings and Morton, 1998).
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SIGNAL STRUCTURE, TRANSMISSION AND RECEPTION |
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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Species specific footdrumming patterns are documented in gerbils (Daly and Daly, 1975a, b
; Bridelance and Paillette, 1985; Bridelance, 1986), elephant shrews (Faurie et al., 1996) and kangaroo rats (Randall, 1989a, 1997). Patterns differ in the number of drums and how the drums are spaced to create specific rhythms.
The most complex footdrumming patterns are found in the highly territorial banner-tailed kangaroo rat, D. spectabilis. Both sexes drums individually distinct footdrumming signatures which vary in the number of individual footdrums combined together to make a footroll, footrolls in a drumming sequence and drumming rate (Randall, 1989a, 1995). Individual patterns develop in young banner-tails to differ from the footdrumming patterns of close neighbors. The patterns are flexible, and adult kangaroo rats that move territories alter footdrumming signatures to differ from those of their new neighbors (Randall, 1995).
The giant kangaroo rat, D. ingens, is another species with an unusual footdrumming pattern (Randall, 1997). They drum one to two long footrolls at a time, which can consist of hundreds of footdrums, with one foot for a while and then change feet and drum with the other. Footdrumming patterns in other species of kangaroo rats, gerbils and elephant shrews are much less complex and range from single thumps to short footrolls of about 10 footdrums each (Bridelance and Paillette, 1985; Faurie et al., 1996) (Table 2).
Footdrumming signals are unique because they can be transmitted through two channels, the air and the ground. Narins (1990) proposed that footdrumming vibrations might be a side effect of production of airborne sounds with little communication being carried out in the seismic channel. Recent studies strongly suggest that the seismic channel is the more important one for conveying acoustic signals into the burrow. Fossorial mole-rats, Spalax ehrenbergi (Rado et al., 1989, 1998), Georychus capensis (Narins et al., 1992) and semi-fossorial kangaroo rats, Dipodomys spectabilis (Randall and Lewis, 1997) seem to communicate by seismic vibrations in which the auditory system is the primary receiver of the seismic energy. These rodents can use their highly specialized ears to detect the low-frequency acoustic energy generated by the drumming (Lay, 1993; Webster and Plassmann, 1992; Rado et al., 1998). The blind mole rat, S. ehrenbergi, receives seismic vibrations by pressing its lower jaw against the ground to transmit vibrations to the inner ear by bone conduction (Rado et al., 1989). The vibrations are processed by the auditory system (Rado et al., 1998) and by a somatosensory mechanism (Nevo et al., 1991). Other rodents probably process the seismic vibrations when they are converted to airborne sounds in the burrow tunnel or chamber (Randall and Lewis, 1997).
Randall and Lewis (1997) demonstrated that seismic signals propagated much better than airborne signals in the burrow of banner-tailed kangaroo rats. Peak amplitude of seismic sounds, as measured with a suspended microphone in a sealed burrow chamber, was approximately 40 dB greater than airborne sound transmitted from the same distance. Airborne sound apparently attenuated because it had to travel from the air outside the burrow through the ground into the air inside the burrow, while ground-borne vibrations passed directly through the ground to become airborne in the burrow. Kangaroo rats inside the burrow responded to playbacks of drumming from both an artificial thumper and a speaker. They did not respond to the airborne footdrumming if loud noise was broadcast at the same time, but they did response to the seismic drumming. Thus, although semi-fossorial rodents can communicate above ground with airborne footdrumming signals, the best way to transmit signals into the burrow chamber is via low-frequency, seismic vibrations.
Predator detection has been proposed for the evolution of low-frequency hearing in rodents (Webster and Webster, 1971; Lay, 1972). Because many rodents with low-frequency hearing are nocturnal and forage in environments with little natural cover, the ability to hear the low-frequency movements of owls and snakes would be highly adaptive. This theory becomes less acceptable, however, if frequency ranges of predator movements are above the peak sensitivity of hearing (Hafner, 1993). An alternative hypothesis is that the specialized ears of rodents function in the perception of low-frequency vibrations for communication. If true, ears of kangaroo rats that footdrum should be more sensitive to low-frequency sounds than ears of kangaroo rats that do not footdrum.
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COMMUNICATING WITH CONSPECIFICS |
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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Footdrumming effectively communicates occupation or ownership of a territory and has replaced long distant chases and fighting among familiar neighbors of at least two species of kangaroo rat (Randall, 1984, 1989b, 1997
). Banner-tailed and giant kangaroo rats generally respond less aggressively to intrusions by neighbors than to intrusions by non-neighbors in what has become known as the "dear enemy" phenomenon (Temeles, 1994). They recognize and discriminate neighbors from non-neighbors by olfactory signals (Randall, 1987b; Murdock and Randall, 2001), and in footdrumming exchanges where they drum back and forth to each other, alternating footdrumming signals with those of neighbors (Randall, 1984). In playback tests in the field, banner-tails discriminated airborne footdrumming signatures of neighbors from non-neighbors and footdrummed at higher rates in response to unfamiliar than to familiar footdrumming signatures (Randall, 1994b). The giant kangaroo rat also drummed in response to airborne playbacks of unfamiliar drumming patterns (Randall, 1997). In contrast, the desert kangaroo rat did not drum in response to footdrumming playbacks and rarely engaged in footdrumming exchanges. They are more aggressive than the other two species and chase and engage in roll-over fights with conspecifics that venture too close to their territory (Randall, 1997; Sullivan, 2000; Randall and Boltas King, 2001). Although they may recognize neighbors, aggression between neighbors did not differ from that between strangers in paired encounters in an arena in the field (Sullivan, 2000). Because of high rates of visits by neighbors in attempts to pilfer seed stores, the relative threat of neighbors is similar to that of strangers. Desert kangaroo rats drum to communicate their presence in their territory, but drumming alone will not deter visits by neighbors so they must follow through with agonistic behavior that often leads to contact and fighting.
Footdrumming plays a different role in regulating social interactions of the medium-sized Heermann's kangaroo rat, D. heermanni (Shier and Yoerg, 1999). Although there are no footdrumming exchanges, agonistic interactions are avoided because footdrumming is done exclusively inside the burrow by the kangaroo rat avoiding contact with the one outside the burrow. Shier and Yoerg (1999) interpreted the behavior as a signal of subordinance to convey the message of "leave me alone," rather than "get away from my home before I chase you." Because the study was done within the space limitations of a laboratory, documentation of similar behavior must also be done in the field.
In addition to territorial defense, rodents footdrum during competitive interactions for mates (Table 2). When the operational sex ratio favors males, male kangaroo rats (D. microps, D. spectabilis, D. ingens) converge at the home area of an estrous female and compete for access to her (Kenagy, 1976; Randall, 1987a, 1991; Hekkala, 1995). Interactions and drumming increase with the number of males present, and the successful competitor footdrums and chases away other males when they venture too close. Footdrumming interactions have also been observed during male competition for estrous females in the great gerbil, Rhombomys (unpublished data). Footdrumming by the dominant male most likely communicates his resource holding potential to minimize the amount of physical contact during the interactions (Maynard Smith, 1974; Parker, 1974).
Both male and female gerbils, M. uguiculaltus and Rhombomys, and kangaroo rats, D. spectabilis and D. deserti, footdrum during mating, probably to coordinate mating interactions and to minimize agonistic behavior by the female (Butterworth, 1961; Burley, 1980; McDermott and Carter, 1980; Randall, 1987a). Male kangaroo rats, as well as male Mongolian gerbils, drum as they approach a female (Swanson, 1974; Randall, 1987a). I have observed male gerbils footdrum softly as they approach a female before mating (unpublished observations). The meaning of the drumming requires more research, but it appears to communicate absence of aggression to potential mates.
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COMMUNICATING WITH AND ABOUT PREDATORS |
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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One of the more interesting aspects of vibrational communication in mammals is seen in the behavior of prey species when they encounter a snake. Instead of running away, they become active participants in an interaction with the predator and approach the snake and footdrum. This behavior is seen in different lineages of both solitary and social species and has been documented in three species of kangaroo rat (Randall and Matocq, 1997
; Randall and Boltas King, 2001), two species of social gerbil (Ågren et al., 1989; Randall et al., 2000), elephant shrews (Rathbun, 1979); and prairie dogs (Owings and Owings, 1979; Loughry, 1987).
Footdrumming in the presence of snakes is best documented in kangaroo rats (Randall and Stevens, 1987; Randall and Matocq, 1997; Randall and Boltas King, 2001). Solitary male and female kangaroo rats approach snakes to within striking distance, jump back and footdrum (Table 3). The banner-tailed kangaroo rat modifies its individual footdrumming signatures by shortening the first footroll and increasing the number of footrolls in a sequence to drum a long series of short footrolls (Randall and Matocq, 1997). Despite the change in footdrumming pattern, the footdrumming does not seem to communicate to other kangaroo rats, as banner-tails show little or no response to the footdrumming of neighbors during an interaction with a snake, and neighbors failed to show any measurable response to airborne broadcasts of the snake drumming pattern during playback tests. Banner-tailed mothers, however, may footdrum to warn their offspring of danger; mothers footdrummed more than non-mothers in the presence of a tethered snake. Because pups were inside the burrow and their response to the drumming could not be observed, it is unknown whether footdrumming by the mother was directed to the pups or to the snake. Drumming in the presence of offspring has not been tested in the other kangaroo rat species, and no other species is known to alter footdrumming patterns in encounters with snakes (Randall and Boltas King, 2001).
The most parsimonious explanation for footdrumming in the presence of snakes by kangaroo rats, with the exception of mothers drumming when they have pups in the burrow, is that they drum in their own defense to cause an attack or pursuit to be abandoned (Hasson, 1991; Caro, 1995). Three hypotheses have been advanced to explain why a prey communicates to a predator: (1) to deter pursuit by informing the predator that it is detected and thus the chances of ambush are thwarted (Woodland et al., 1980), (2) to communicate that the prey is alert and continued pursuit is costly (Caro, 1995; Randall and Matocq, 1997), and (3) to communicate that the prey is healthy and cannot be caught (FitzGibbon and Fanshawe, 1988). Tests of these hypotheses require tests of responses of the predator to the behavior of the prey (Caro, 1995).
Randall and Matocq (1997) showed that gopher snakes (Pituophis melanoleucus) respond to footdrumming of the banner-tailed kangaroo rat. Non-hungry snakes avoided playbacks of footdrumming from a mechanical thumper in field tests. In encounters in the laboratory, the snakes decreased stalking rates as footdrumming by kangaroo rats increased. Kangaroo rats probably footdrum to inform the snake that they are alert and not easy prey, and the footdrumming of a long series of short footrolls may be more effective in keeping a snake's attention than drumming individual signatures (Caro, 1995; Randall and Matocq, 1997). The kangaroo rats do not footdrum to inform the snake that it has been detected, because footdrumming always occurs after an encounter with the snake, not at first sighting (Randall and Stevens, 1987; Randall and Matocq, 1997; Randall and Boltas King, 2001). All three species that footdrum at snakes approach the snake first, retreat and usually footdrum until the snake is removed at the end of an experiment or leaves on its own.
Social rodents also interact with snakes. Because predation by snakes is widespread, it is expected that approach and confrontation of snakes by rodents would be widespread and not restricted to solitary species. Social ground squirrels (Spermophilus) and prairie dogs (Cynomys ludovicianus) do indeed interact with snakes and use some of the same snake-directed behavior as the solitary kangaroo rats (Owings and Owings, 1979; Swaisgood et al., 1999). Footdrumming, however, seems uncommon in sciurids, and has been reported, but not systematically studied, in prairie dogs (Owings and Owings, 1979; Hoogland, 1995).
The great gerbil, Rhombomys opimus, is a highly social rodent that footdrums in conjunction with vocal alarms to signal danger to family members (Randall et al., 2000). Adult and juvenile gerbils of both sexes give alarm calls and footdrum in the presence of an assortment of terrestrial predators: snakes, monitor lizards, foxes, polecats, weasels, humans and large ungulates. Because group membership is necessary for the expression of the alarm behavior, and there is a higher frequency of footdrumming and vocalizing in family groups with newly emergent pups than in family groups without pups, the alarm is most likely intended to inform offspring of danger as well as adults (Randall et al., 2000). Footdrumming could communicate danger to family members inside the burrow, while vocalizations transmit the alarm above ground.
Different responses to different terrestrial predators suggests that a predator's hunting behavior influences the footdrumming responses of great gerbils. In an experiment with three terrestrial predators (monitor lizard, snake and hunting dog) tethered in activity areas of family groups, gerbils footdrummed at different rates and locations in response to the different predators. They drummed at higher frequencies on the surface of the burrow near a snake that could enter the burrow than when they drummed inside the burrow. In contrast, they drummed at lower frequencies on the surface and higher frequencies inside the burrow in response to a large hunting dog that could not enter the burrow. Because the most prevalent terrestrial predators are those that can enter the burrow (monitor lizards, Varanua griseus; sand boas, Erix miliaris, and polecats, Vormela peregusna (Randall et al., 2000 and personal observation), footdrumming may function to convey information about potential danger in the burrow to family members that have taken refuge there. An alternative hypothesis is that the gerbils approach snakes and footdrum to communicate directly to them.
Recently my students and I discovered that the giant kangaroo rat footdrums in the presence of a terrestrial mammal, the kit fox (Vulpes macrotus). Kit foxes are plentiful in the habitat of this species, and we have seen six natural encounters between kangaroo rats and kit foxes in which the kangaroo rat footdrummed and the kit fox moved away. In an experiment conducted by Maria DeAngelo with a stuffed kit fox, the kangaroo rats ran to a burrow entrance, stopped and began to footdrum while oriented in the direction of the model. We are continuing our tests of this behavior to determine whether the kangaroo rats respond specifically to the kit fox as a predator or to any small four legged animal in their territories.
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The evolution and function of communication signals created by drumming are rich topics for future research. Footdrumming is an effective means of communication and will no doubt be found in many more species than those described in this paper. Communication by mechanical vibrations in mammals is widespread, but only a few species have been studied in any detail. There are many possibilities, especially for comparative research in mole rats, agouti, gerbils and elephant shrews (Table 1). Because these mammals comprise very different lineages from diverse habitats in different continents, they provide excellent opportunities for study of the diversity of vibrational communication in mammals. As environments change, the factors that shape social structure also change, which in turn affect the communication context. Comparative research must go beyond descriptions of footdrumming patterns and include behavioral observations and tests of responses to the signal in the natural context in which the signal is generated (Bridelance and Paillette, 1985
; Bridelance, 1986; Faurie et al., 1996).
Studies of responses to drumming vibrations should be conducted in the field in a natural habitat, because drumming is often very context-specific and the behavior is difficult to interpret when done in an artificial setting in the laboratory. The use of geophones, artificial thumpers, night vision equipment and other techniques for observing and conducting behavioral tests of nocturnal mammals now provide essential tools for testing responses to drumming in the field (Lewis and Narins, 1985; Narins et al., 1992; Randall and Lewis, 1997; Randall and Matocq, 1997).
The only in-depth comparative research on footdrumming has been done in the genus Dipodomys in which a continuum in the development of footdrumming behavior has been found (Table 2). Footdrumming evolved in the larger-sized species that defend territories, is rudimentary in medium-sized species, and absent in the smaller-sized species. The question of whether body mass limits ability to generate footdrumming patterns in kangaroo rats is an interesting one, and studies of the biomechanics of footdrumming could be a rich area for future research.
Signal transmission and reception in different habitats is another rich area for research. As properties of the habitat change (e.g., soil type, vegetation, amount of moisture, complexity of burrow system) properties of signal transmission also change. There is little information on how transmission of either air-borne or seismic drumming signals varies with changes in properties of the habitat. Because drumming vibrations can be transmitted in two modalities instead of one, the animals that drum have more options for transmission of information than those that only vocalize. We know a great deal more about how animals optimize foraging choices (See reviews by Krebs and Kacelnik, 1991; Lima, 1998), than we know about the choices they make to optimize communication. The dual channels of transmission of footdrumming vibrations provide a model for testing how mammals make choices for signal transmission under varying environmental conditions. For instance, under what conditions might a semi-fossorial rodent switch from transmission of air-borne signals to ground-transmitted signals? Do social rodents communicate to relatives inside the burrow?
Because much of what is known about vibrational communication in mammals is from solitary species, more research needs to be done on how footdrumming functions in a social context. Compared with solitary kangaroo rats that drum in multiple contexts, footdrumming in the social gerbils is mainly limited to interactions with predators (Randall et al., 2000). As in ground-dwelling squirrels, factors other than social complexity may determine complexity of communication (Blumstein and Armitage, 1997). Because the social gerbils are diurnal, they depend much more on visual signals and vocalizations than the nocturnal kangaroo rats. Footdrumming is usually done above ground in conjunction with an alarm call and below ground in the burrow. Why the gerbils footdrum is still unknown, but the best hypothesis, and one that is currently being tested, is that footdrumming informs family members in the burrow of the possibility that a predator can enter the burrow. If data support the hypothesis that social rodents footdrum to communicate impending danger to relatives in the burrow, one more use of drumming for communication will have been found and our concept of social communication expanded.
Markl's (1983) plea for research to explain vibrations in natural ecological settings is still highly relevant to research in mammals. Now that the formidable problems of observation, recording and experimentation have been overcome, it is my hope that questions about how mammals communicate non-verbally with mechanical vibrations will be the source of much more research in the future.
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ACKNOWLEDGMENTS |
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I wish to thank Peggy Hill for the invitation to participate in the symposium and the National Science Foundation for travel support. I also wish to thank the students in my laboratory for a lively discussion on the evolution of footdrumming, and I extend a special thank you to all the students who participated in the research over the years as research assistants and graduate students. My research on the great gerbil could not have been done without my collaborator, Konstantin Rogovin. I am grateful to the National Science Foundation, the National Geographic Society and the Civilian Research and Development Foundation for grant support.
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FOOTNOTES |
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1 From the Symposium Vibration as a Communication Channel presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3–7 January 2001, at Chicago, Illinois.
2 E-mail: jrandall{at}sfsu.edu
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SYNOPSISINTRODUCTIONEVOLUTION OF FOOTDRUMMING AS...SIGNAL STRUCTURE, TRANSMISSION...COMMUNICATING WITH CONSPECIFICSCOMMUNICATING WITH AND ABOUT...CONCLUSIONS AND FUTURE RESEARCHReferences |
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