Revista Chilena de Historia Natural 71: 65-77, 1998 in : the New World fossorial hystricognaths as study models

Sociabilidad en roedores: los histricofiatos fosoriales del Nuevo Mundo como modelos de estudio

LUIS A. EBENSPERGER

Departamento de Ecologfa, Facultad de Ciencias Biol6gicas, Pontificia Universidad Cat6lica de Chile, Casilla 114-D, Santiago, Chile E-mail: [email protected]

ABSTRACT

Some rodents are social in that multiple adult individuals spend most of their lives in close assoc1atwn with other conspecific individuals, usually sharing a feeding area and a system. The evolution of such sociality has been related to the distribution of food resources, the cost of burrowing, and the risk of . In this article I review these factors in the context of fossorial and semifossorial rodents and show that most of this theory has been formulated considering either North American ground-dwelling sciurids or African mole-rats. I review the behavioral ecology of some selected species of New World hystricognaths and suggest that future studies of species within this group of rodents will broaden our perspective of sociality in and possibly other groups. Such studies will provide the independent tests that current hypotheses explaining the evolution of sociality need. I identify critical predictions of hypotheses and suggest complementary approaches to test them.

Key words: group-living, fossorial rodents, food distribution, risk of predation, cost of digging.

RESUMEN

Algunos roedores son sociales en el sentido de que varios individuos adultos viven asociados estrechamente con otros individuos de Ia misma especie, usualmente compartiendo un área común para alimentaci6n y un sistema de madrigueras. La evoluci6n de esta sociabilidad ha sido atribuida a factores tales como Ia distribuci6n de recursos alimentarios, el costo de cavar, y el riesgo de depredaci6n. En este artfculo, reviso estos factores para el caso de roedores cavadores y muestro que una mayor parte de Ia teorfa asociada ha sido formulada tomando en cuenta casi exclusivamente a roedores siuridos de Norte America o ratas topo de Africa. Reviso Io que se sabe de Ia ecologfa conductual de algunas especies seleccionadas de histricofiatos del Nuevo Mundo y sugiero que estudios futuros que utilicen estas (u otras) especies ampliaran sustancialmente nuestra perspectiva acerca de Ia sociabilidad en mamfferos y posiblemente otros grupos animales. Dichos estudios proporcionaran las pruebas independientes que requien las teorfas actuales que intentan explicar Ia evoluci6n de Ia sociabilidad. Señalo algunas predicciones claves de hip6tesis alternativas y sugiero aproximaciones complementarias para someterlas a prueba.

Palabras clave: vida en grupo, roedores cavadores, distribuci6n del alimento, riesgo de depredaci6n, costo de cavar.

INTRODUCTION of Sociobiology and Behavioral Ecology (Clode 1993, Krebs & Davies 1993, Adults from different animal spectes, Danchin & Wagner 1997, Emlen 1997). including invertebrates and vertebrates, Sociality may result in fitness costs to may spend most of their lives in close group members, including increased association with other conspecific transmission of parasites and diseases, individuals (Wilson 1975, Lott 1991). increased aggression and competition for Understanding the causes of such sociality food, infanticide, or cuckoldry (Hoogland (or group-living) is a major research goal 1979a, 1985, MØller 1987, Davies et al.

(Received 26 September 1997; accepted 13 November 1997; managed by Francisco Bozinovic) 66 EBENSPERGER

1991, MØller & Birkhead 1993, Van Vuren caveat of using this approach is that 1996). Thus one should expect the existence hypotheses (see below) are developed and of benefits acting to overcome these tested using information from the inherent disadvantages. Selective pressures bathyergids themselves. Thus comparative favoring sociality include increased foraging analyses of these species do not provide a efficiency, decreased risk of predation, robust test of potential mechanisms (Lacey increased energy savings through & Sherman 1997), and data on other thermoregulation, or even increased access subterranean groups are needed to provide to mates (Alexander 1974, 1991, Madison such contrasts. 1984, Slobodchikoff 1984, Wrangham & A similar critique can be posed against Rubenstein 1986, Morton et al. 1990, Krebs hypotheses formulated to explain sociality & Davies 1993, Hoi & Hoi-Leitner 1997). among North American sciurids. Species of However, group-living also may occur ground squirrels, prairie dogs, and marmots because solitary living is not a viable option have been ranked according to their to individuals due to ecological (Armitage tendency to live socially, and this 1981, Emlen 1982, 1994, Brown 1987, information has been related to ecological Waser 1988) or phylogenetic and life history and life history characteristics of these constraints (Armitage 1981, Rodman 1988, species (Barash 1974, Armitage 1981, Burda 1990, Van Rhijn 1990). In this case, Michener 1983, Blumstein & Armitage individuals do not gain net benefits from 1998). However, these empirical living with conspecifics. relationships have not been confirmed in Among , many species of rodents other than the North American rodents (Rodentia) are social (Michener sciurids used in this analysis. More 1983, Solomon & Getz 1997) in that recently, Slobodchikoff ( 1984) linked several adults interact frequently (usually sociality to the distribution of food amicably), and share feeding areas and a resources. Although his model was burrow system (Rayor 1988, Waterman intended to be a more general explanation 1995, Lacey et al. 1997). Theory called to of group-living than previous hypotheses, explain sociality in rodents has been experimental and comparative evidence derived mainly from African mole-rats supporting Slobodchikoff (1984)' s model (Bathyergidae; Lovegrove & Wissel 1988, comes mainly from a single species of Burda 1990, Jarvis et al. 1994), and North (Slobodchikoff 1984, Travis & American ground-dwelling squirrels Slobodchikoff 1993), and therefore, (Sciuridae; Barash 1974, Armitage 1981, outgroup (nonsciurid) species are needed to 1988, Hoogland 1981 a, Slobodchikoff test this hypothesis. 1984, Arnold 1990a, 1990b, Blumstein Herein, I argue that New World fossorial 1996). Although such studies have been hystricognaths (Woods 1993) could be important in suggesting potential excellent subjects to provide independent explanations to the causes of sociality in tests of alternative hypotheses of sociality in rodents, the validity of their hypotheses as rodents. Many species within this group are general explanations of group-living in social, fossorial or semifossorial, and other groups of animals or even other occupy both mesic and xeric environments groups remains questionable. For (Kleiman 1974, Redford & Eisenberg 1992). instance, Bathyergidae includes species of In addition, since New World hystricognaths both social and solitary-living individuals are taxonomically and geographically (Jarvis & Bennett 1991), which has been distinct to African mole-rats (Jarvis & used to assess ecological factors favoring Bennett 1990, Woods 1993), any behavioral sociality (Lovegrove & Wissel 1988, Jarvis and ecological similarity may reflect et al. 1994, Faulkes et al. 1997). A potential evolutionary convergence. SOCIALITY IN RODENTS 67

This article is not intended to consider tubers, which can be reached only via every hypothesis that has ever been extensive burrowing (Jarvis & Bennett formulated as a possible explanation for 1990, 1993, Honeycutt 1992, Jarvis et al. group-living. Instead, I concentrate my 1998). Although both species occur in attention on the potential influence of food different types of soil, the substrates used distribution and its interplay with the energy by these two mole-rats can be efficiently cost of digging, as well as that of predation excavated only when recently softened by risk. I first review the empirical evidence rain (Jarvis et al. 1994, Jarvis et al. 1998). that supports a role for these factors on the Thus, the patchy distribution of food, along evolution of rodent sociality. After doing so, with the excessive cost of burrowing to I use this framework to review potentially locate these food patches, and the brief relevant aspects of the behavioral biology periods of time during which tunnel and natural history of some selected species excavation is possible (due the low of hystricognath rodents that might suggest abundance and unpredictability of rains) a role for either the distribution of food determine that solitary mole-rats would be resources or predation risk on the tendency unlikely to locate enough food to sustain of these species to live socially. When doing themselves through long and unpredictable so, I further restrict my analysis to those dry periods (Lovegrove & Wissel 1988, species that are fossorial or semifossorial, Jarvis et al. 1994, Lacey & Sherman 1997, which often live in groups. I identify Jarvis et al. 1998). As a consequence, specific predictions to the hypotheses, and individuals are forced to remain in their suggest comparative and manipulative natal group, and group-living becomes a observations that could help testing these necessity to cope with the high cost of predictions. Throughout the text, I follow foraging (the aridity-food distribution Wilson & Reeder (1993) for rodent hypothesis). Foraging cost includes the . energy needed to dig tunnels and the risk of unproductive foraging. Some support to this hypothesis comes from the observation MAIN CURRENT EXPLANATIONS OF that both solitary and social Bathyergids SOCIALITY IN RODENTS: EVIDENCE FROM occur in mesic but only social BA THYERGIDAE AND SCIURIDAE species are abundant in xeric habitats (Jarvis et al. 1994, Faulkes et al. 1997). Distribution offood resources Thus solitary species are precluded from xeric and arid areas. Within Bathyergidae, the most The aridity-food distribution hypothesis comprehensive explanation of sociality is more likely to apply to truly subterranean corresponds to the aridity-food distribution species in which foraging takes place hypothesis (Lovegrove & Wissel 1988, underground through digging than to Lovegrove 1991, Jarvis et al. 1994). burrowing species in which individuals According to this model, group-living is regularly feed aboveground. To such necessary to cope with unpredictable and semifossorial species, digging is not patchily distributed food resources in arid essential to forage, which may facilitate and semiarid environments. Damaraland solitary individuals to locate and obtain (Cryptomys damarensis) and naked enough amounts of food. Nonetheless, the ( Heterocephalus glaber) mole-rats are distribution of resources still may promote fossorial and social rodents that inhabit the group living in animals that forage hot, dry regions of eastern and southern aboveground. One potential mechanism to Africa, respectively, and they both this is that individuals in groups could be consume patchily distributed bulbs and more efficient in defending food resources 68 EBENSPERGER than solitary individuals. According to because by doing so they decrease the risk Slobodchikoff (1984 ), more efficient of being attacked by a predator compared defense of food will occur when food is with solitary animals. Among other abundant and patchily distributed, which mechanisms (Krebs & Davies 1993 ), will promote sociality. In contrast, when decreased predation may occur because food is scarce and uniformly distributed, animals in groups detect predators sooner defense of food, or food territoriality, is than solitary individuals, or because precluded and solitary-living is favored animals locate themselves such that other (Slobodchikoff 1984). Observed variations group members become more vulnerable in the social structure of the semifossorial to attacks (i.e. selfish-herd effects; and social Gunnison's prairie dogs Hamilton 1971 ). Among semifossorial (Cynomys gunnisoni) after manipulations rodents, decreased predation risk is a of the abundance and distribution of their potential benefit of group-living black- food offer empirical support to this tailed prairie dogs (Hoogland 1995). hypothesis. Thus when the abundance and Prairie dogs in large groups detect patchiness of food are increased (by adding potential predators more quickly than do seeds), the feeding territory of each group prairie dogs in smaller groups (Hoogland of prairie dogs contracts and the number of 1981 a). Selfish-herd effects also could group members increases (Slobodchikoff influence the social structure of prairie 1984). When the abundance of food is dogs and other sciurids. In black-tailed decreased and its distribution made more prairie dogs and yellow-bellied marmots uniform (by removal of plants), the size of (Marmota flaviventris) individuals located feeding territories increases and the number in the periphery of a group devote more of animals per group decreases time to scan for predators than individuals (Slobodchikoff 1984). More recently, a in more central positions (Armitage 1962, comparison between two populations of Hoogland 1979b, 1981a). Presumably, the Gunnison's prairie-dogs in habitats with risk of predation is higher in peripheral as different distributions of food resources has compared with central locations. provided additional support to a functional Any of the above provide mechanisms association between food distribution and by which predation can have a direct the size of prairie-dog groups (Travis & influence on sociality. However, the Slobodchikoff 1993). Further evidence influence of predation could be more supporting this hypothesis comes from indirect. Fossorial and semifossorial comparisons between the social structure of rodents construct subterranean as a white-tailed prairie dogs (C. leucurus) and major strategy to avoid predators (Waser that of the more social black-tailed prairie 1988, Jarvis & Bennett 1990, Jarvis et al. dogs (C. ludovicianus). Black-tails are 1994 ). Therefore, constructing a burrow found in places where quality of food system could be essential for individuals to patches is highly variable, whereas the less use open, more exposed, patches where the social white-tailed prairie dog seems to live risk of predation from visually-oriented in sites of fairly uniform, low-density food predators is probably high. If so, animals patches (Slobodchikoff 1984). may live in groups and benefit from cooperation when building these Risk of predation subterranean refuges (Hoogland 1981 a, 1981 b, Jarvis & Bennett 1990, Powell & Besides the abundance and distribution of Fried 1992). Individuals in groups may food resources, predation risk also may spend less time and energy digging influence sociality (Alexander 1974, burrows than individuals living without 1991). Animals may live in groups conspecifics. SOCIALITY IN RODENTS 69

SOCIALITY IN OTHER RODENT GROUPS: unpredictable within and among years in HYSTRICOGNATHS AS STUDY MODELS arid coastal central Chile (i.e. mid to northern distribution of S. cyanus), with the Distribution offood resources common occurrence of several years of drought (Armesto et al. 1993). Fossorial species Although most ctenomyids (tuco-tucos) are solitary subterranean rodents (Nevo Among octodontids, Spalacopus cyanus, or 1979, Reig et al. 1990), the recently cururo, has been signaled as potential described Ctenomys sociabilis, endemic to "target" to provide important clues to the the western Limay Valley (Neuquen, evolution of group-living (Lacey & Argentina), is social (Pearson & Christie Sherman 1997). Cururos are subterranean 1985, Lacey et al. 1997). Groups of C. rodents endemic to Chile, where they occur sociabilis may include as many as five in both arid and more mesic regions of adults sharing the same nest site and coastal central Chile (Contreras et al. 1987, burrow system (Pearson & Christie 1985, Redford & Eisenberg 1992). Although the Lacey et al. 1997). Thus, C. sociabilis behavioral ecology of cururos is poorly offers a good opportunity to perform known, available information suggests a comparative studies with other solitary number of intriguing parallels with social ctenomyids (e.g. C. haigi; Pearson & African mole-rats. First, S. cyanus seems to Christie 1985), and use this behavioral be social. Groups may include from 6 to 15 differences among closely related taxa to animals (including several adults), assess ecological correlates of group-living belonging to three or more generations, and (see below). Besides C. sociabilis, some using a single burrow system (Reig 1970, scattered reports of multiple adults sharing Torres-Mura 1990). Second, the ecology of the same burrow system includes C. cururos resembles that of the social peruanus (Pearson 1959) and C. azarae bathyergids. Cururos are found in arid and (Contreras & Maceiras 1970). In fact, the semi-arid environments where woody possibility of intraspecific variation in the plants cover is less than 60% of ground tendency to live in groups also has been surface (Contreras et al. 1987). In these reported in tuco-tucos (Reig et al. 1990). areas, cururos feed on shoots of grasses and forbs when available, but mainly on storage Semifossorial species organs of several geophytes to which cururos gain access by way of their The distribution of food resources also may burrows (Reig 1970, Castillo et al. 1978, influence the tendency to live in groups of Mann 1978, Torres-Mura 1990, Contreras semifossorial species that construct & Gutierrez 1991, Contreras et al. 1993). underground burrow systems but that Geophytes are particularly consumed usually feed aboveground. Thus when food during the dry months (January and is abundant but patchily distributed, February) and in dry years (Cox et al. individuals in groups could be more 1995). Although these food resources efficient in defending such food patches (geophytes) may be abundant and constant than solitary animals (Slobodchikoff 1984; through time, they are patchily distributed Travis & Slobodchikoff 1993). (Contreras et al. 1993). Third, digging Among semifossorial species, voles and activity of cururos occurs mainly during the lemmings (Muridae; Arvicolinae) have been rainy season and when soil humidity is considered as potentially useful models to relatively high (Torres-Mura 1990; perform comparative studies with although see Contreras et al. 1993). Finally, bathyergids (Solomon & Getz 1997). I precipitation is relatively infrequent and propose octodontids as an attractive 70 EBENSPERGER

possibility as well. In particular, the degu, and high shrub cover. If so, corresponding degus (Octodontidae), seems an differences in the extent of sociality appropriate subject to test the food-defense between degus and 0. lunatus and 0. hypothesis. Degus are medium sized (ca. bridgesi should be observed. Unfortunately, 180 g, Jaksic & Yafiez 1979) rodents that the social structure of 0. bridgesi and 0. inhabit areas of central Chile with a low lunatus is virtually unknown. Only Ipinza cover of shrubs (Fulk 1975, Glanz 1977, Le et al. (1971) have suggested that 0. Boulenge & Fuentes 1978, Jaksic et al. bridgesi might form family groups. Studies 1981 ). Degus are diurnal and social (Yafiez assessing the extent of sociality in these 1976, Le Boulenge & Fuentes 1978, Mann octodontids are strongly needed. 1978, Iriarte et al. 1989). Individual groups include 2 to 5 females and 1 to 2 males that Risk of predation share the same burrow system (Fulk 1976, Yafiez 1976). Degus mainly consume young The observations that degus give alarm leaves of herbs and shrubs along with seeds calls in the presence of potential predators (Bustos et al. 1977, Fuentes & Le Boulenge (Fulk 197 6, Yafiez 197 6) and that nearby 1977, Meserve et al. 1983, 1984). Thus noncallers respond accordingly to these digging is not directly required for degus to calls (Vasquez 1997) suggest that group- obtain food. Seasonally, degus prefer young living in degus may enhance survival rather than mature leaves of herbs and through increased detection of (and shrubs, and parturition coincides with the warning from) predators. In fact, although time when young foliage is more abundant degus foraging in groups decrease their per (winter and spring; Meserve et al. 1983, capita time spent vigilant compared to 1984). Since group members are aggressive solitary-foraging degus, total vigilance (and probably territorial) against members when in groups increases with group size of other such groups during this time of the (Vasquez in 1997). Thus social degus seem year (Fulk 1976), sociality in degus may to benefit from enhanced alertness to well be a strategy to defend patches of potential predators, which might have temporarily abundant young leaves. influenced their tendency to live socially. Although young leaves are temporarily In addition, the influence of predation patchy (i.e. they are mostly available during risk on group-living by degus could be winter-spring but not during summer-fall), more indirect. Burrow systems dug by the assumption that they also are spatially degus are extensive and elaborate, with clumped needs to be assessed. several entrances, subterranean nests and Contrasting the social structure of degus tunnels (Fulk 1976, Yafiez 1976, Mann to that of other species within Octodon 1978). Degus are usually found in areas of could be informative in assessing the role relatively low shrub cover. In these places, of food distribution on the evolution of burrows with multiple openings in open group-living. As so 0. degus, coastal degus areas between shrubs seem to function as (0. lunatus) and Bridge's degus (0. refuges to escape from potential predators bridgesi) are folivorous rodents (lpinza et (Yafiez 1976, Yafiez & Jaksic 1978). If so, al. 1971, Meserve & Glanz 1978, Mufioz & extensive burrow systems could be Murúa 1987). However, 0. lunatus and 0. necessary for degus to use open habitats bridgesi prefer areas of higher shrub cover where the risk of predation is presumably than degus to construct their nests (Glanz higher compared with denser habitats 1977, Tamayo & Frassinetti 1980, Mufioz (Jaksic 1986, Meserve et al. 1984, Lagos et & Murúa 1987). One might expect the al. 1995). As a result, degus may be social distribution of leaves of shrubs and grasses to share the cost of constructing and to differ between areas of medium-to-low maintaining these underground systems. SOCIALITY IN RODENTS 71

Comparing the social structure of degus Although active at night (Branch to that of 0. lunatus and 0. bridgesi also 1993a), plains vizcachas (Langostomus could be rewarding to assess the role of maximus; Chinchillidae) may face predation risk on the evolution of group- ecological problems similar to those of living. In contrast to 0. degus, the degus. Plains vizcachas are social and live congeneric species 0. lunatus and 0. in open scrub areas (Weir 1974, Mares et bridgesi are nocturnal and do not burrow as al. 1989). In this , group-living much as degus and they establish their vizcachas are heavily preyed by mountain nests in denser shrub areas than that where lions (Branch 1993a, Branch et al. 1996). de gus are found (Greer 1965, Ipinza et al. As expected, vizcachas give at least two 1971, Glanz 1977, Mann 1978, Tamayo & types of alarm calls which seem to signal Frassinetti 1980, Muñoz & Munia 1987). different amounts of risk (Branch 1993a). Since nocturnal activity and the use of When alarm calls are given in response to areas with more dense vegetation (i.e. with mountain lions (and dogs), neighboring more cover) presumably provides a lower vizcachas in fact run toward their principal risk of predation from aerial raptors (the burrows (Branch 1993a). In addition, most common predators in areas of Chile foraging distances of vizcachas from their inhabited by these octodontid species; central burrows seem related to the risk of Meserve et al. 1984, J aksic 1986), 0. predation (Branch & Sosa 1994). bridgesi and 0. lunatus are expected to be On the other hand, plains vizcachas also less social than 0. degus. may excavate extensive burrow systems in Comparative studies between 0. degus open scrub areas (Weir 1974, Mares et al. and more distantly related species of 1989), which might be a response to escape semifossorial rodents facing similar from predators. Each group, normally ecological problems also should be composed of one or more males, several informative. The social structure of degus females and immatures, uses a burrow resemble that of North American prairie system communally (Branch 1993a, dogs (Sciuridae; Hoogland 1981 a) in that 1993b). Interestingly, all group members degus are aggressive (and possibly (independently of age and sex) participate territorial) against members of other groups equally in digging to maintain the during the breeding season (Fulk 1976). communal burrow (Branch 1993a), which Further, social prairie dogs are usually suggests that cooperation associated with found in open sites with low vegetational digging of burrows is a benefit of group- cover of shrubs, and predation seems to living in these rodents. have played an important role in the Besides degus and plains vizcachas, evolution of sociality in these rodents predation may influence the social structure (Hoogland 198Ia, 1995). In fact, of small cavies (Microcavia australis; interspecific differences in the degree of Caviidae). Small cavies are diurnal and sociality between black-tailed and white- social (Rood 1970, 1972); they are tailed prairie dogs seem linked to herbivorous, feeding above-ground, and differences in protective cover (Hoogland they can excavate complex burrow systems 1981a). White-tailed prairie dogs live in (Contreras & Roig 1978). Within a group, smaller, less densely populated groups than animals seem to associate in smaller do black-tails, and unlike black tails, white- "family" groups whose members use the tail groups are located in places with more same burrow system (Contreras & Roig protective cover (Hoogland 1981a, 1995). 1978). Although individuals do not use the Comparative studies within Octodon could burrows of other family groups, provide an independent test of this interactions between members of different hypothesis. family groups are rarely aggressive (Rood 72 EBENSPERGER

1970, Contreras & Roig 1978). The fact have been noticed to give alarm calls in that foraging above-ground occurs with a response to potential predators (Pearson continuous mixing up of individuals from 1959, Fulk 1976). These observations different groups (Contreras & Roig 1978) suggest that the potential role of predation suggests that defending food resources is on the tendency of cururos and tuco-tucos, not a benefit of social cavies. In contrast, as well as in other fossorial rodents to live predation could be a more important factor. socially needs to be assessed. Small cavies give alarm calls in response to potential predators (Rood 1970, 1972), and burrow systems are typically built in TESTING HYPOTHESES OF SOCIALITY association with shrubs with overhanging branches (Rood 1970, Tognelli et al. 1995). Both intraspecific and interspecific Since these shrubs usually are not comparisons could be used to assess the consumed by cavies, protection from aridity-food distribution hypothesis in New predators (rather than food) could be the World hystricognaths. All else being equal, cause of such selection of microhabitat the aridity-food distribution hypothesis (Tognelli et al. 1995). predicts that (I) the size of groups should The cavy, Microcavia niata, is an increase with the patchiness of food herbivorous rodent that lives in bog resources, so more group members are communities of the High Andean Plateau. needed to locate more widely dispersed In these areas, M. niata lives in groups of 4 food patches. Thus (2) the probability of to 17 individuals including multiple adults locating patches of food resources should of both sexes which use the abandoned increase with the number of diggers. burrows of ctenomyids (Marquet et al. Whereas checking for intraspecific 1993). The fact that these social rodents differences in the social structure of use the abandoned burrows of other (truly animals living in sites with different fossorial) rodents, makes cooperation to distributions of food resources will help burrow unlikely to be a benefit of group- assessing the first prediction, observations living in this case. However, they can be of the animals under more controlled useful models to test the influence of conditions could be necessary to test the predation risk. Microcavia niata gives second expectation. Regarding interspecific alarm calls elicited by potential human comparisons, the patchiness of food predators, and other group members react resources in the habitat used by social to these calls by running into their burrows species should be higher than the (Marquet et al. 1993). patchiness of food resources observed in The wild guinea pig (Cavia aperea; the habitat of solitary-living species. Caviidae) also seem to obtain benefits from If animals live in groups to exclude group foraging under predation risk. conspecifics from patches of food, the size Individual guinea pigs spend less time of communal territories should increase vigilant when foraging in groups than when with the number of group members, and the foraging singly, and such time savings are intensity of group territoriality is expected allocated to foraging (Cassini 1991). to increase with the patchiness and Finally, truly fossorial rodents also are abundance of food resources. vulnerable to predation when they dispose The hypothesis that sociality is a excavated soil above ground (Jarvis & response to avoid predation will be Bennett 1990, Jarvis et al. 1994). Cururos supported by observations showing that the and tuco-tucos can be preyed upon by individual risk of predation decreases with raptors (Pearson et al. 1968, Castillo et al. an increase in the size of groups. 1978, Mann 1978, J aksic 1986), and they Manipulations that decrease the risk of SOCIALITY IN RODENTS 73 predation (e.g. removing or excluding Assessing the influence of any of the predators) should result in smaller numbers above factors may require the consideration of individuals per group. Among species, the of different spatial scales, and the predation hypothesis will be supported by appropriate scale of analysis will depend on showing that the risk of predation is higher in the spatial pattern of variation of that habitats used by social species compared with particular factor. Thus, in the context of the those used by solitary-living species. If any aridity-food distribution hypothesis, food of these effects are demonstrated, additional distribution and aridity might be expected observations will be needed to assess the to change little across the feeding area used particular mechanisms by which individuals by neighboring groups but greatly between decrease their risk of predation when in nonneighboring groups pertaining to groups as compared with solitary-living spatially separated populations. On the animals. Thus, if increased vigilance is other hand, selfish-herd effects could be important, the time required to detect (and expected either at level of an entire colony then react to) a predator should decrease as -a group composed by all those individuals group size increases. If protection occurs due that share a burrow system (Waterman to a selfish herd effect, individuals using 1995, Lacey et al. 1997)- or during the more peripheral locations within a group formation of more occasional groups within should more likely be attacked (and killed) the colony (e.g. when feeding). Thus than animals in more central locations. If predation risk might vary with the location protection against predators is related to the of an animal's burrow within a colony, or quality of burrows, burrow systems of larger according to the location of an individual groups should offer more protection than within a feeding group. burrows constructed by smaller groups. Independently from whether burrow systems are needed to locate food or to CONCLUDING REMARKS construct refuges against predators, the influence of the energetic cost of digging Despite some efforts to highlight the on the tendency of individuals to live ecological factors influencing sociality in socially will be supported by showing that different groups of rodents (bathyergids: groups using patches whose soils are more Lovegrove & Wissel 1988, Burda 1990, difficult to dig (e.g. hard soils) are more Jarvis et al. 1994; murids: Madison 1984, numerous than groups living in less hard McGuire & Getz 1995, Berteaux et al. soils. Alternatively, the time that each 1996; sciurids: Armitage 1981, 1988, individual allocates to digging activities Hoogland 1981a, Slobodchikoff 1984, (presumably a cost of digging) will be Arnold 1990a, 1990b, Blumstein 1996), its lower in more numerous groups compared causes still remain poorly understood. I with smaller groups. Comparing group size suggest that future studies with New World and digging activity of animals living under hystricognaths will provide raw material to different soil conditions (e.g. hardness) will test hypotheses by means of comparisons be necessary to test the above prediction. with previously known groups, and Alternatively, digging costs can be assessed improve our understanding of group-living in the laboratory to animals maintained on in rodents. different types of soils and number of Studies on the behavioral ecology of New cagemates. In addition, soils of habitats World hystricognaths will provide insights preferred by solitary digging species are into other closely related aspects of group- expected to convey a lower energy living. In particular, several species of New expenditure to diggers than soils of habitats World hystricognaths are known to used by their social counterparts. communally nurse their litters and to give 74 EBENSPERGER alarm calls to warn others from predators LITERATURE CITED (Pearson 1948, Rood 1972, Macdonald 1981, Branch 1993a, Herrera & Macdonald 1993, Kiinkele & Hoeck 1995). Such behaviors ALEXANDER RD (1974) The evolution of social behavior. Annual Review of Ecology and suggest the occurrence of benefits that are not Systematics 5: 325-383. simply due to group size effects (sensu ALEXANDER RD ( 1991) Some unanswered questions about naked mole-rats. In: Sherman PW, Jarvis Jennions & Macdonald 1994), but of more JUM & Alexander RD (eds) The biology of the elaborate ways of cooperation such as naked mole-rat: 446-465. Princeton University Press, Princeton, New Jersey. 518 pp. communal breeding and anti-predator signals ARMESTO JJ, PE VIDIELLA & JR GUTIÉRREZ (1993) (Dugatkin 1997). The evolution of cooperative Plant communities of the fog-free coastal desert of breeding and alarm signals are particularly Chile: plant strategies in a fluctuating environment. Revista Chilena de Historia Natural66: 271-282. puzzling behaviors to explain as individuals ARMITAGE KB (1962) Social behaviour of a colony of exhibiting such comportments may incur in the yellow-bellied marmot (Marmora jlaviventris). Animal Behaviour 10: 319-331. costs (Sherman 1985, Solomon & Getz 1997). ARMITAGE KB (1981) Sociality as a life-history tactic of Moreover New World hystricognaths ground squirrels. Oecologia (Berlin) 48: 36-49. ARMITAGE KB ( 1988) Resources and social organization may provide models of cooperation among of ground-dwelling squirrels. In: Slobodchikoff CN individuals of different species. Thus 0. (ed) The ecology of social behavior: 131-155. 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Medio Ambiente (Chile) 3: 70-73. the time in which this article was written, I CASSINI MH (1991) Foraging under predation risk in the wild guinea pig Cavia ape rea. Oikos 62: 20-24. was supported by a postdoctoral CASTILLO H, TORRES D & M TAMAYO (1978) Los FONDECYT grant 3970028. roedores chilenos y sus relaciones tr6ficas. Museo SOCIALITY IN RODENTS 75

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