OIKOS 98: 491–497, 2002

Grouping increases the ability of the social rodent, Octodon degus,to detect predators when using exposed microhabitats

Luis A. Ebensperger and Petra K. Wallem

Ebensperger, L. A. and Wallem, P. K. 2002. Grouping increases the ability of thesocial rodent, Octodon degus, to detect predators when using exposed microhabitats.– Oikos 98: 490–496.

We examined the hypothesis that a main benefit of group-living in the hystricognathrodent, Octodon degus (common degu), is to decrease individual risk of predation.During a first series of field observations, we contrasted group size of degus whenusing covered microhabitats with that of degus using exposed patches. During asecond set of field observations, we assessed how distance to detection and to escapeby degus varied with group size upon the approach of a potential human predator.Degus in exposed patches formed larger groups than degus in covered microhabitats.After excluding the influence of nearest burrow to focal subjects, we found that degusof larger groups detected an approaching human predator at a greater distance thandegus of smaller groups. Likewise, degus of larger groups escaped to nearby burrowsat a greater distance from the approaching predator than degus of smaller groups. Allthese pieces of evidence support the predatory risk hypothesis according to whichgroup-living in degus functions to reduce the risk of predation.

L. A. Ebensperger, P. K. Wallem, Centro de Estudios A�anzados en Ecologıa &Biodi�ersidad, Depto de Ecologıa, P. Uni�. Catolica de Chile, Casilla 114-D, Santiago,Chile (lebenspe@genes.bio.puc.cl).

Understanding the functional aspects of group-living(or sociality) is one main research goal of sociobiologyand behavioral ecology (Krebs and Davies 1993). Toaccomplish this, it is useful to consider that severalfactors may prevent the formation of groups, imposingfitness costs to group members. Such costs may includeincreased transmission of parasites and diseases, in-creased aggression and competition for resources, in-fanticide, and cuckoldry (Krebs and Davies 1993,Alcock 1998). Thus one would predict the existence ofbenefits acting to overcome these inherent disadvan-tages, or constraints, that allow individuals to livesocially. One such benefit is that individuals may live ingroups to reduce their risk of predation (Alexander1974, Treisman 1975, Van Schaik 1983). Reduction ofpredation risk may occur by different mechanisms,including an enhanced ability of grouped individuals todetect predators (i.e. the ‘many eyes effect’), individualslocating themselves such that other group members

become more vulnerable to attacks (the ‘selfish herdeffect’), and grouped individuals repelling predatorsmore efficiently than solitary-living animals (i.e. groupdefense), among others (Hamilton 1971, Pulliam 1973,Romey 1997).

Among rodents (Rodentia), social systems rangefrom solitary-living species to colonial (gregarious) andsocial species (Nowak 1999), in which several individu-als interact frequently, share feeding areas, a territory,and often a den or a burrow system (Rayor 1988,Waterman 1995). The behavior of murid (mice, rats,voles) and sciurid (squirrels, marmots) rodents gener-ally support the generalization that predatory risk fa-vors group-living. For instance, grouped bank voles,Clethrionomys glareolus, and yellow-necked mice,Apodemus fla�icollis, seem to be attacked less often byweasels than solitary individuals, and individual volesand mice are killed less often when in groups(Jedrzejewski et al. 1992). Solitary voles (Microtus

Accepted 6 March 2002

Copyright © OIKOS 2002ISSN 0030-1299

OIKOS 98:3 (2002) 491

epiroticus) are killed faster by kestrels than groupedvoles (Hakkarainen et al. 1992). Among sciurid ro-dents, large-sized groups of black-tailed (Cynomys lu-do�icianus) and white-tailed (Cynomys leucurus) prairiedogs detect simulated predators sooner than prairiedogs of smaller groups (Hoogland 1981), and red-tailedsquirrels (Sciurus granatensis) respond more quickly tohuman predators when foraging in groups than whenforaging solitarily (Heaney and Thorington 1978).

Data relevant to the predatory risk hypothesis fromspecies other than murid and sciurid rodents could beparticularly informative as it may provide evidence ofbehavioral convergence, and support the hypothesisthat predation is a widespread pressure favoring theevolution of rodent group-living. One group of rodentsthat may provide such data is the New World hys-tricognaths (Branch 1993a, Ebensperger 1998), whichincludes species found in almost every type of habitat,and with social structures ranging from solitary-livingto highly gregarious species (Redford and Eisenberg1992, Nowak 1999). Although less known than theirmurid and sciurid counterparts, evidence revealing thatpredators strongly influence the behavior of hys-tricognath species is not uncommon (Rood 1972,Cassini 1991, Cassini and Galante 1992, Branch 1993a,Marquet et al. 1993, Yaber and Herrera 1994).

One hystricognath species whose behavior is influ-enced by predators is the common degu (Octodon de-gus ; Octodontidae). Degus inhabit the semi-arid areasof matorral in central Chile (Yanez 1976, Contreras etal. 1987). In these habitats, degus live socially wheregroups construct and share an elaborate system ofburrows (Fulk 1976, Yanez 1976, Mann 1978). Prelimi-nary data suggest that degu groups include one to twoadult males and from two to five adult females (Fulk1976, Yanez 1976). More commonly, groups rangefrom two to four individuals (Vasquez 1997, L. A.Ebensperger unpubl.). Members of different groupsgenerally intersperse freely except during breeding timewhen they seem more territorial (Fulk 1976, L. A.Ebensperger unpubl.). While aboveground, degus useboth exposed (between shrub) and covered (undershrub) microhabitats (Lagos et al. 1995a, b) where theyspend most of their time foraging on green leaves ofgrasses, forbs, and shrubs (Meserve et al. 1984, L. A.Ebensperger unpubl.). Therein, degus are frequent preyof local raptors and foxes (Jaksic et al. 1993). Availableevidence supports that shrub habitats provide lowerpredation risk than exposed areas (Meserve et al. 1984,Lagos et al. 1995b, Vasquez et al. 2002).

The purpose of this study was to examine the hy-pothesis that predatory risk influences group-living ofcommon degus. In particular, we hypothesized thatdegus should group more when using exposed, riskiermicrohabitats, and that the ability of degus to detecthuman predators should increase with grouping.

Methods

This study was conducted at the Fundo Rinconada deMaipu, (33°29�S; 70°54�W), a field station of the Uni-versidad de Chile, located 30 km west of Santiago,central Chile. Therein, the study area consisted of twosites (Polvorines, and El Litral), 3–4 km apart fromeach other, both with clear signs of degu activity, asindicated by the relatively high frequency of burrowopenings and from directly observing the animals. Thearea has a Mediterranean climate characterized bywarm dry summers and cold wet winters and is locatedwithin the biogeographical zone known as matorral.Shrub cover in the area, as assessed from nine 200 mlinear and parallel transects, reached 14.5�10.5% (�SD). Dominant shrubs in the study area included Bac-charis spp., Proustia pungens, and Acacia ca�en. Herbswere represented by several species of grasses and forbs,including Clarkia tenella, Erodium spp., Helenium aro-maticum, Madia sati�a, Matricaria chamonilla, Oxalisspp., and Senecio adenotrichius.

During a first period of observation, we assessed thefrequency with which degus perform their activitiesaboveground in solitude and in groups of different size(i.e. grouping behavior), and while in two microhabi-tats: an exposed microhabitat comprising almost exclu-sively bare ground and herbs, and a coveredmicrohabitat comprising medium (Baccharis spp.;height ranged from 0.5–1.0 m) to large shrubs (Proustiapungens, Acacia ca�en ; height ranged from 1.0–3.0 m).Covered microhabitats included patches of 1–3 isolatedshrubs, and more continuous matrixes of severalshrubs. Degu predators such as culpeo foxes (Pseu-dalopex culpaeus), and black-chested buzzard eagles(Geranoaetus melanoleucus) were frequently sighted inthe area. To quantify grouping behavior at these twomicrohabitats, we observed degus from a portableblind-tower (modified version of Rodenhouse and Best1983), and with 10×50 binoculars. Height fromground level to the eyes of an observer was ca. 2.8 m.We located our blind tower such that both exposed andcovered microhabitats were in similar proportionsaround the tower. Distance from the tower to degusubjects ranged from 30–150 m, depending on thelocation and height of surrounding shrubs. We usedscan sampling (Lehner 1996) to quantify grouping bydegus while active aboveground. To do so, we made a360° visual scan at a rate of 24–30 angles per min every15 min. During each scanning episode, we recorded thebehavior of every detected degu and the grouping con-dition it was in. To quantify grouping condition (in-cluding solitary animals), we considered a degu to bepart of a given group if located at a distance of 2 m orless from any other individual. The use of such strin-gent criteria ensured that all putative group memberswere in visual contact of each other (particularly whilein covered patches) and made our results comparable

492 OIKOS 98:3 (2002)

with previous studies (e.g. Vasquez 1997). We restrictedour field records of grouping behavior to adult degus toavoid potential differences in the perceptual and cogni-tive abilities of degu subjects. Grouping data wereaveraged for each microhabitat for every 15-min scan.Scans with no adult individuals recorded were notincluded. Observations were conducted in 3- to 6-hperiods, between 09:00 to 15:00 h during the southernhemisphere winter and spring, and between 08:00 to11:00 h and from 17:00 to 20:00 h during the summerand autumn, which matches the daily and seasonalactivity periods of degus in central Chile. Data wereobtained within 3–4 days at the beginning of eachseason, from September 1998 to September 1999. Thedegus were not individually marked, but external bodymarks and simultaneous live trapping made duringother ongoing observations suggested that at least 84degus were seen with varying frequency at the two siteswhere observations took place.

During a second period of observation, we studiedthe responses of degus to a potential human predator.In particular, we measured the distance at which degusof different grouping conditions detected and reacted(typically running to the nearest burrow entrance) whenapproached by a human subject while using exposedpatches. We chose the use of human subjects becausethey have been used before during similar studies, andthey elicit responses by the prey that are similar tothose elicited by real predators (Marquet et al. 1993,Bonenfant and Kramer 1996, Schooley et al. 1996).Thirdly, archaeological evidence shows that degus havebeen preyed upon and consumed frequently by humansin the recent past (Simonetti and Cornejo 1991). In-deed, degus utter alarm calls when they see humansapproaching (Fulk 1976), which suggests that humansare perceived as predators by the degus.

During each test, we used a 1.5 m height blind toobserve the degus. Upon sighting a degu group wheretheir members were either foraging or resting (i.e. notmoving rapidly), one of us directly approached thefocal group while the other used 10×50 binoculars totape record the behavior of focal degus during theapproach. While approaching the focal group, our hu-man predator dropped a small (30 mm) plastic capwhenever she noted she was sighted by any member ofthe focal group and stopped her approach completelywhen the first group member ran (usually to enter aburrow). We considered a degu to have sighted theapproaching human predator whenever it interruptedits foraging or resting bout and remained motionlesswith its head raised and pointed directed toward thehuman subject. Upon ending each approach, werecorded the distance to first detection (i.e. the distancebetween the dropped plastic cap and the original loca-tion of focal degus), the distance to escape (i.e. thedistance between the human predator and the focaldegus when the first group member ran), and the total

distance (i.e. the distance between the blind and theoriginal location of focal degus). When distance todetection was not clearly determined during a particu-lar test (due to limitations of our observationalmethod), we considered distance to escape only. Be-cause distance to the nearest burrow has been shown toinfluence alertness and escape initiation distance ofsome semi-fossorial rodents (Leger et al. 1983, Bonen-fant and Kramer 1996), we also recorded the meandistance between each focal degu group and its nearestburrow entrance. Distance from the blind to the focaldegus ranged between 49.2 to 109.0 m, and averaged(�SD) 77.3�14.3 m (n=27). Approaching speed byour human predator averaged (�SD) 0.95�0.25 msec−1 (n=27) and ranged between 0.47 to 1.38 msec−1. All tests were conducted on animals using sparsemicrohabitats, and between 09:30 to 13:00 h, during thesouthern hemisphere autumn (June 1999 and fromMarch to June 2000). Height of herb cover was rela-tively low at this time, which allowed us to distinguishdegus easily. Weather conditions during the experi-ments ranged from sunny to cloudy. During sunnydays, we approached degus such that sunlight wasperpendicular to the trajectory followed by the ap-proaching human predator. To prevent habituation ofdegus to human predators, we limited the number oftests (typically 3–4) per 1-day session, and conductedeach test at different areas of our study site. Whole1-day sessions were conducted once per week. Recordsof grouping at different microhabitats and of distanceto detection/reaction of different sized degu groupswere obtained during alternated sessions at both sites(i.e. Polvorines and El Litral).

Since grouping was not normally distributed, we usednonparametric statistics to analyze the influence ofmicrohabitat on this variable. Given that variation ofgrouping with season was not relevant to our hypothe-sis, contrasts between microhabitats at different seasonswere treated independently. To examine the relation-ship between grouping condition and distance to firstdetection, and that between group size and distance toescape, we used MANOVA, with distance to thenearest burrow as a covariate. Statistical analysis wasperformed using Statistica 5.1 for Windows 95 (StatSoftInc., Tulsa, Oklahoma). Data are given as x�SD.

Results

The grouping tendency of degus was significantly influ-enced by the type of microhabitat. At all seasons, degusin exposed patches grouped significantly more thandegus in dense microhabitats (Mann-Whitney U-test:P�0.001 for all comparisons, Table 1).

Distance to the nearest burrow did not covary signifi-cantly with grouping to influence detection (multiple

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Table 1. Grouping (number of degus located at a distance of2 m or less from any other individual) by adult free-rangingdegus when using exposed, and covered microhabitats. Com-parisons are made for different seasons with the use ofMann-Whitney U-tests. Data are x�SD. Sample size (inparentheses) corresponds to the number of scan samplingobservations in which at least one degu was sighted.

P-valueSeason Microhabitat Adjusted Z

Sparse Dense

Autumn �0.0011.26�0.26 1.18�0.38 3.71(n=78) (n=45)

Winter 1.49�0.37 �0.0011.10�0.32 7.61(n=80) (n=67)

Spring 1.78�0.60 1.28�0.43 7.00 �0.001(n=108) (n=90)

Summer 1.48�0.35 1.18�0.34 4.54 �0.001(n=61) (n=49)

Fig. 2. Distance to escape measured to free-ranging degus ofdifferent grouping condition upon the approach of a humanpredator. The arrow is used to indicate an outlier point (seeResults). Distance to escape was successfully measured during27 experiments.

regression analysis, partial R=0.1, t17=0.41, P=0.690) or escape distance by degus to simulated preda-tors (partial R= −0.03, t23=0.12, P=0.903), so weremoved this variable from further analyses. After do-ing so, we found that degus of larger groups detectedan approaching human predator at a greater distancethan degus of smaller groups or in solitude (simpleregression analysis, R=0.64, F1,19=13.36, P=0.007,Fig. 1). Likewise, degus of larger groups escaped tonearby burrows upon the approach of a human preda-tor at a greater distance than degus of smaller groupsor in solitude (simple regression analysis, R=0.45,F1,25=6.29, P=0.019, Fig. 2). Such relationship be-tween distance to escape and group size of degus waseven more statistically significant after removing theoutlier point of Fig. 2 from the analysis (simple regres-sion analysis, R=0.52, F1,24=8.92, P=0.006).

Discussion

Some caveats and assumptions

Since an unknown proportion of our behavioral dataset included repeated observations of same individuals,such data pooling may have biased the results throughinflating degrees of freedom (Leger and Didrichsons1994), particularly in the case of grouping estimates.We can not determine the extent of such potential bias,but some aspects may have decreased it to some extent.First, the study area comprised of two sites that in-cluded different individuals. Indeed, external bodymarks along with simultaneous trapping of the animalsrevealed that at least 84 different individuals were seenin the area during the observation period. Secondly, theanalysis of a limited data base on the distance traveledbetween bushes and the time spent pausing during suchdisplacements by degus (n=8 individuals; R. A.Vasquez unpubl.) revealed that the intra- to between-in-dividual variance ratio ranges from 0.6 to 2.3, whichsuggest that data pooling may not have resulted inbiased results (Leger and Didrichsons 1994). Thirdly,grouping differences between microhabitats were rela-tively small, but the tendency of degus to group morewhen using sparse patches was consistent throughoutyear. Finally, degu groups and behavior are extremelydynamic, so groups form and disrupt within 1–2 min,and animals switch from one behavioral activity toanother within seconds.

We assumed a priori that exposed areas were morerisky than shrubby, covered microhabitats (Kleiman1974, Dunbar 1989). This contention is supported bythe observation that O. degus use shrub and exposedhabitats in similar proportions when overall shrubcover is intermediate and when predators have been

Fig. 1. Distance to detection by free-ranging degus of differentgrouping condition upon the approach of a human predator.Distance to detection was successfully measured during 21experiments.

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experimentally excluded, but show the more traditionalshrub bias when shrub cover is low and predators arepresent (Jaksic et al. 1979, Meserve et al. 1984, Lagos etal. 1995b). In addition, degus in exposed microhabitatsdevote a greater percentage of their aboveground activ-ity time to vigilance than degus in shrub microhabitats(Vasquez et al. 2002). Our data suggest that degusadjust their grouping behavior in response to habitatconditions. The observation that collective vigilanceincreases with grouping (Vasquez 1997), and that degusgroup more when in exposed as compared with coveredpatches are consistent with the assumption that exposedareas are riskier to degus.

Although vegetation may obstruct locomotion andvisual detection of predators by rodent prey (Schooleyet al. 1996), or increase prey vulnerability to certaintypes of predators (Kotler et al. 1992, Korpimaki et al.1996), several aspects of behavior and ecology of ro-dents generally support an inverse relationship betweenthe amount of plant cover and predatory risk(Ebensperger 2001a). For instance, rodents generallyseek shrub cover upon the approach of potential (ter-restrial or aerial) predators (Rood 1972, Hanson andCoss 1997), and some species are less vulnerable tomammalian predators when in patches with greaterdensity of cover than in patches with less cover (Kotleret al. 1991, 1992, Longland and Price 1991). In addi-tion, rodents limit their activity to safer shrub micro-habitat when direct risk from aerial predators isincreased, and they switch to using exposed patcheswhen such predation risk decreases (Abramsky et al.1996).

Benefits of degu group-living

Our observations revealed that adult degus group morewhen using exposed, riskier microhabitats. Our dataalso confirmed that, when in exposed microhabitats,degus in larger groups are capable of detecting anapproaching predator at a greater distance than degusin smaller groups, most likely because collective vigi-lance increases with degu grouping (Vasquez 1997).Taken together, these observations are consistent withthe hypothesis that grouping decreases the predatoryrisk of degus. This conclusion adds to previous evidenceshowing that other aspects of degu behavior such asspace use and movement are influenced by predatoryrisk (Jaksic et al. 1979, Meserve et al. 1984, Lagos et al.1995b, Vasquez et al. 2002).

Is there any benefit to group-living by degus otherthan decreasing predatory risk? Several, non-mutuallyexclusive alternatives need to be considered. First, thefact that degus do not commit infanticide (Ebensperger2001b) rules out the possibility that degus live sociallyto reduce the risk of infanticide by non-group membersas it may occur in some ground squirrels and mice

(Sherman 1980, Manning et al. 1995). Secondly, group-living degus may become more efficient at defendingfood resources than solitary-living conspecifics (Wrang-ham 1980). If so, sociality of degus should increase withthe abundance and patchiness of resources (Slobod-chikoff 1984). The observation that the abundance andquality of food resources preferred by degus vary sea-sonally (L. A. Ebensperger unpubl.) and spatially (Jak-sic and Fuentes 1980, Holmgren et al. 2000) suggeststhat the resource-defense hypothesis might apply todegu sociality. However, we lack data on variation inthe number of degus that are actually sharing a sameterritory or den that are needed to test this hypothesis.

A third possibility might be that degus derive ther-moregulatory benefits from grouping (Madison 1984).The fact that individual degus reduce their energy ex-penditure through huddling with conspecifics (Canals etal. 1989) suggests that thermoregulatory considerationsare influencing the tendency of degus to live socially.Indeed, the social thermoregulation hypothesis is con-sistent with the observation that when aboveground(i.e. out of their burrows) degus carry out their activi-ties in relatively small groups or in solitude. However,thermoregulatory benefits are unlikely to be critical attimes other than during winter-early spring when rela-tively cold temperatures occur.

Finally, degus might live in groups to reduce theenergetic cost of burrow construction. Degu burrowsprovide refuge against predators and extreme tempera-tures; they are energetically costly to build, and areshared by group members (Fulk 1976, Yanez 1976,Lagos et al. 1995a, Ebensperger and Bozinovic 2000a).Although degus in groups do not reduce their burrow-ing time in the short term as compared with solitarydiggers, grouped individuals coordinate their diggingand remove more soil per capita than solitary diggers,which suggests that social burrowing may reduce thecost of burrow construction in the long term(Ebensperger and Bozinovic 2000b).

Predatory risk and rodent group-living

Behavior of rodents other than the New World hys-tricognaths generally supports the hypothesis that therisk of predation (including that due to conspecificpredators) favors group-living (Ebensperger 2001a).Among murid rodents, grouped individuals are at-tacked or killed less frequently by predators than soli-tary animals (Hakkarainen et al. 1992, Jedrzejewski etal. 1992). Among sciurid rodents (squirrels, marmots)individuals of large-sized groups detect predatorssooner than rodents of smaller groups (Hoogland1981), and they respond more quickly to human preda-tors when foraging in groups than when foraging soli-tarily (Heaney and Thorington 1978).

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Regarding the New World hystricognaths, wildguinea pigs (Ca�ia aperea), capybaras (Hydrochaerishydrochaeris), and possibly plains vizcachas (Lagos-tomus maximus) seem to decrease their predatory riskthrough grouping. Thus, wild guinea pigs have beenshown to increase their foraging efficiency when ingroups through decreasing individual time devoted toalertness (Cassini 1991), and aggregate more when inpatches of low vegetation (Cassini and Galante 1992).On the other hand, individual capybaras located at theperiphery of a group devote more time to vigilance thanindividuals at more central positions (Yaber and Her-rera 1994), and grouped capybaras coordinate them-selves to protect juveniles from the attack of feral dogs(Macdonald 1981). These observations suggest that so-cial capybaras locate themselves such that other groupmembers become more vulnerable to attacks (i.e.‘selfish herd effect’), and use group defense to repelpredators more efficiently. Regarding plains vizcachas,these rodents live in groups, and use relatively exposedhabitats where they construct a communal system ofunderground burrows (Branch 1993a, b). According toBranch (1993a), the observation that vizcachas areheavily preyed upon by terrestrial predators, and thatthey extensively use warning calls (Branch 1993b)would support that predator pressure is one main causeof vizcacha group-living. Nonetheless, more critical evi-dence is needed to support this contention.

The above evidence suggests that predatory risk haspromoted group-living of some species of New Worldhystricognaths, including degus. However, the roleplayed by predators may not be so widespread acrossthese rodents. An across species examination of somecorrelates of group-living by the New World hys-tricognaths did not support the predatory risk hypothe-sis (Ebensperger and Cofre 2001). Species that formrelatively large social groups do not use particularlyexposed, riskier habitats as might be expected under thepredatory risk hypothesis (Ebensperger and Cofre2001). Such a discrepancy between comparative ap-proaches and individual species analyses may not beunique to the New World hystricognaths, as a compar-ative examination of predatory risk and its influence ongroup-living across murid and sciurid rodents is needed.

Acknowledgements – We are greatly indebted to F. Bozinovicfor sponsoring the first author during a post-doctorate resi-dence at P. Universidad Catolica de Chile. Our thanks to theUniversidad de Chile, particularly to Danilo Araya and JoseDaniel Garcıa (former and current Field Station administra-tor, respectively) for providing the facilities during field work.Claudio Veloso helped to estimate plant cover, and A. Caiozziprovided further logistical assistance. We thank R. Vasquezfor granting us the access to his data set on degu movement.Comments and suggestions made by D. Leger and two anony-mous reviewers to an early version of this article are greatlyappreciated. Funding was provided by a Post-doctoralFONDECYT grant 3970028.

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