Great Basin Naturalist
Volume 53 Number 2 Article 5
6-4-1993
Squirrels as predators
J. R. Callahan University of New Mexico, Albuquerque, New Mexico
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Recommended Citation Callahan, J. R. (1993) "Squirrels as predators," Great Basin Naturalist: Vol. 53 : No. 2 , Article 5. Available at: https://scholarsarchive.byu.edu/gbn/vol53/iss2/5
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 53(2), pp. 137-144
SQUIRRELS AS PREDATORS
J. R. Callahan 1
ABSTHACT-A literature review and field observations indicate that most sciurids are facultative predators on small vertebrates. This behavior is documented for at least 30 sciurid species in 8 genera. The frequency of predation apparently is influenced by various factors including climate, season, gender, reproductive condition, and availability ofplant sources for certain nutrients such as calcium and nitrogen. Although sciurids assimUate as much energy from animal foods as do obligate carnivores, behavior associated with predation appears to be less efficient in sciurids and may rely partly on prey habituation and other adaptive behaviors.
Key words: squirre~ Sciuridae, predator, carnioore, omnioore.
Predators utilize various strategies that maxi sana! communication). There is universal ac mize the probability of successful prey capture ceptance that squirrels eat meat; the question is while minimizing the probability ofinjury. Sci how they obtain it. Squirrels are often seen urid rodents, many of them facultative and op eating carnon on roads but are rarely seen at portunistic predators, are not morphologically tacking live prey. Stomach contents analysis may specialized for this role and should therefore overlookvertebrate flesh and cannot distingUish possess a wide variety of adaptive attack behav live prey from carrion. Thus, each newobserva iors. As discussed below, one ofthese behaviors tion ofa squirrel acting like a predator becomes observed in tree sqUirrels appears similar to a journal note (see Literature Cited). Although insinuation (Curio 1976), a strategy more often most of these notes imply that such behavior is associated with invertebrates than with mam aberrant, collectively they describe a significant mals. component of the sciurid repertoire. The same Predation, as defined here, means the killing feeding adaptations that enable sqUirrels to and eating of active vertebrates (including con crack nuts are sufficient for openingskulls (Lan specifics) or other relatively large, mobile prey dry 1970). by free-living squirrels. This definition excludes TREE SQUIRRELs.-The following field ob the consumption of eggs, nestling birds, small servation (which prompted this review) adds a insects, or any animal that is already dead. Prey species to the hst ofreported sciurid predators. offer some resistance; eating carrion or aphis is On 6 April 1979, I saw a lactating female west similar to broWSing. Predation also excludes kill ern gray squirrel (Sciurus griseus) stalk and at ing that appears unrelated to feeding, as in de tack an adult mountain quail (Oreortyx pictus) fense of the nest (Harris 1985) or as a in mixed-conifer forest at Black Mountain, Riv reproductive strategy (Balfour 1983, Weissen erside County, California, elevation 1800 m. bacher 1987). Finally, behavior of caged squir The quail was standing on a 60-cm stump at the rels is often abnormal and is excluded here as edge of a clearing; five or six other quail were evidence of predation, although it can provide nearby. None of the quail appeared to react as clues to dietary deficiencies. the squirrel crossed the clearing in an odd, Not all biologists accept the idea ofsquirrels crouched posture, rustling the pine needles and as frequent predators. Despite the 70-year lit leaves loudly enough to attract my attention. erature record summarized in Table 1, When it was 20 em from the stump, it leapt up O'Donoghue's recent (1991) finding that squir and pouncedon the quail. After a briefstruggle, rels are the chiefpredator ofjuvenile snowshoe the quail escaped and the covey moved off. In hares elicited general "shock" (c. Krebs, per- six years inthesame area, ontwo otheroccasions
I MUS~\lm ()fSoothwe~tenl Biology, UHiversity of New Mexi~Q. Albuquerque, New Mexico 87131. Mailing <\ddre~s: Box 3140, llernet, G,lifomia \12'546.
137 138 GREAT BASIN N"rURAUST [Volume 53
T:\fll.1': 1. Puhlish~d r~ptJrts ofpredation by sciurjrk This table is limited to behavior nffree-living squirrel" and induJcs Sp~ies IJrev, Source THEE sQUlImEI /oj 1';,L>;tcm gr:.ly squirrel Conspecifics, birds Holm (1976), Ba~ey(I923) (Sdllfll.<; ClImline"sis) vVcstcm gray squirrel Mountain quail; unidentified smaU This paper (Sduru.~, grisells) mammal (Pcrotll!;'~'Cl1s?) Ea~tem fox squirrel Dove, blue jay, other birds Boren (1961), Seton (1929), Shaffer (Sdurllsniger) and Baker (1991) EII)"opcan red squirrel Sql1irrel.~, birds GllnJell (1987), Schlogel (l98.5) ($<;illrw; vulgaris) Douglas squirrel Ground squirrels Roest (1951) ('I(lIl1j(t.~·ciurnst!oug!({/d) Pine (= Bed) sCluirrel Chipmunks, tTtl(;: squirrels, cotton H,lU (1929), Seton (929). Hamil ('famiast.:illnl,\·/uulwmiclI!;) tail, :.nowshoe Illlre, moumillg dove, ton (19.'14), O·Oonoghue (199[), other birds Nero (1987), Taylor (l9.88) I.ittle ~"'ecn sqUirrel Fwg (h",,?) Emmons (1980) (AJ!tlwsc.illf1JS T}f)ens~'i) CI\OU"J) S(,lullllmLs Whitclail antelope squirrel Pocket mouse, k;mg:lroo rdt, lizards nrndl~y (1968), Morgalt (1985) (AIIIJIJJ.J!>lwmwll!1ilus feIlD' nls) Nds{1ll antelope sfluirrcl J,izanJs llawbecker (1947) (AmmmJlcnlulpltilus Ilelwmi) Yellow-!)(dlied manmlt Cnnspecific:~ Armitage et a1. (1979) (M(/rnwla fl(lviventrl,~) California grOlllll! squirrel Ground sqUirrels, rabbits, pocket Grinnell & Storer (1924), Fitch (SpenTli.IJ,hiLu.\' beer:hcytj gophers, ml1ll~s, hirds, lizards (1948), .sumner & Dixon (1963), Sundberg and Hanta (l97J), TwHo t:t a!. (1986) Belding grollnl..1 sqUirrel Chipmunks, other small mammals, Sharsmith (1936), Howell (1938), (S/lemu'1Jhifu'i heldiJl.gi) jlmco, warbler Sherman & Morton (l9i9). Michener (1982) Suslik Field mice, birds, snakes, Calineseu (1934), IIerzig.Strasdlil (S,H:nflopllib.,s eitel/liS) conspecif'ks (1976) C:nJlllnhian ground squirrel Fish (live?) IloweJ1(1938) (Srx!nlU'1Jhil1l.s ClJlumb;rJnu.'i) Fr:ll1klin ground squirrel Voles, domestic fowl, rlucks, oth~r Polder (1955). Johnson (l922), (Spc.-rmophillls jrtlllklillU) birds Howell (1938), Sm...-1s (1948), Churomanski & Sargeant (l982) Golden-mantled squirrel Voles, <.leer l1lic.:e, chipmunks, junco, CHlllemn (1967), Tevis (1953) (SptmllO'l'hilllS latemlis) lizards Mexican ground squirrel CottUlltuiJ, P'K:kard (1958) (SJu:nnophillls tllexicarllls) HOllJH!-tai]ed ground sqUirrel Sparrows (S}lerlJlOphilus tcreticclIIcZ,JS) T()....'HSentl ground squirrel Com~cifks Mit:hener (1982) (,<;;,Jenllophilus tOlV1lsentlli) 13-lin~1 ~rolllld squirrel Rabbits, biros Bridgewater & Penny (1966), (SJx,'ruwJ,h ifll:. trilleamJiTlcotu.') Bailey (1923) Arctic ground squirrel CAlnspe<:ifics, snowshoe hare, O'Donoghue (.1991), Iiolmes (SJ,umophilllS uru[uk,tus) collared lemming (l977), Boonstra et al. (1990), Mi<;hCllcr (1982) Hot:k squirrel Wild tll1'keys Conk & Henry (1940) (SpentlAJphirus t.Ylrif:g(fttL~) 1993J SQUIRRELS AS PREDATORS 139 TABU 1. Continued. Species Prey Source Washington ~und squirrel Consped.fics Alcorn (1940) (Spermophilus washingtoni) South African ground squirrel Domestic fowl, other birds. turtles, Shortridge (1934), Ry..n (1987) (Xems '''''''ris) other reptiles African ground squirrel Cob", Stiles (19871 (Xeros rotilus) Cflli'MUNKS Cliff chipmunk Crabs Jenkins 1989 (Tamw.s dnrsalis) Merriam's cJupmunk Lizards, sparrows L""on (1986) (Tamw.s meniami) Least chipmunk Tree swallows Ledede et aI. (1985) (Tamias mini.tn.u$) Asian chipmunk Voles, brrds. liza,ds, frogs Ognev (1966) (TamiM sib''''''') E..stern chipmunk ConspeciJics, voles, SW I saw a westem gray squirrel stalk a bird briefly killing a young blue jay, avoided the skeletal butthen retreat without completing apredation muscle and gnawed on the joints and bones near attempt. On 1 April 1993, S. B. Compton (per the surface ofthe skin. This is in agreement with sonal communication) sawa western gray squir other reports of tree squirrels eating bone and rel with a small, live mammal in its mouth, the antlers (Cross 1969, Leach 1977). In othercases, size ofayoung PeromysGUS, beside a road in the however, sciurids have consumed specific prey San Jacinto Mountains (2100 m). organs such as the brain (Hamilton 1934, Elliott Ingles (1947), Cross (1969), Jaeger (1929), 1978) or viscera (Hesterberg 1940), or the flesh and Stienecker and Browning (1970) reviewed of the head (Holm 1976). the food habits ofthe western gray squirrel but Predation by tropical tree squirrels appears reported no predation (although the latter to be rare. Emmons (1980) saw no predation by found feather fragments in one stomach). At any of nine African species, although one least six other tree squirrel species take live prey Aetlwsciurus stomach contained a frog (Table (Table 1), but the frequency ofsuch behavior is 1). Glanz et aI. (1982) wrote that Sciuros unknown. Meat constitutes 2--11 % ofthe diet of granatensis rarely eats animal foods. Paraxerus the eastem gray squirrel, Sciurus carolinensis (Packard 1956, Nixon et aI. 1968); however, cepapi eats eggs, nestlings, and inseds, but ap stomach contents analysis does not reveal how parently no prey as defined here (Shortridge meat was obtained Moreover, squirrels are er 1934); male P cepapi sometimes kill juvenile ratic predators and not all studies are in agree conspecifics, but Weissenbacher (1987) regards ment. Whereas Borell (1961) and O'Donoghue this as a reproductive strategy rather than pre (1991) reported predation by the fox squirrel (S. dation per se. Viljoen (1978) reported no preda niger) and red squirrel (Tamiasciurus luuuiOni tion by Funisciurus congicus. Small arthropods GUS), respectively, Reichard (1976) saw no pre and annelids are theonly known animal foods of dation by either species, Sundasciurus lowii, Lariscus obscuros, Gallo Itis nQt entirely clearwhether a tree squirrel sciurus melanogaster (Whitten 1981), GaUosci attacks live prey to obtain meat perse orcalcium urus erythraem (Setoguchi 1990), and Tamiops and/or phosphorus from the bones. Shaffer and ma:lel/a.ndi (Moore andTate 1965). Borges (1990) Baker (1991) noted that a fox squirrel, after stated that Raittfa is an obligate herbivore. J40 GKEAI' BASIN NAI'U KALISI' lVolume53 The inhequency of sciurid predation in the chipmunk (Tarnias speciosus) is somewhat spe tropics could he an artifact hased on the geo cialized as an arboreal nest predator (Grinnell graphic distribution ofobservers. Cert (1978) studied the "salt drive" phenomenon in to some extent; but neither the apparent low fox squilyels and marmots. Clark (1968) found success rate nor the situation-specific response that the proportion of animal food in Spermo to potential prey is unique to squirrels. Both philus richardsoni stomachs increased from 3% phenomena are reported for many obligate in April to 24% in August. Tevis (1953) reported predators as well (Curio 1976). Moreover, Kara a similar phenomenon for chipmunks and gold sov (1982) found that antelope ground squirrels en-mantled sqllirrcls. assimilate energy from animal fi:)ods just as efH A third hypothesis is that predation by squir ciently as do obligate predators. rels is often incidental to killing for some other There is a learned component in predator reason, usually territOlial defense or reproduc recognition and avoidance by birds and mam tive competition. In otherwords, once the other mals (Curio]976, Robinson 1980). It would be animal is dead, it provides an energetic bonus a waste of energy for rabbits to avoid dc~er, for that can he consumed without further risk. This instance, even though one deer ate one rabbit. explanation applies mainly to certain ground But ifsquilTels undergo dietary stress evel)'year squirrel species (Holmes 1977, Michener 1982, and begin eating peculiar things, one might Balfour 1983, Harris 1985). expect prey to catch on. The limited evidence The other question concenlS the advantages available suggests this is not the case. Birds of live prey. Carrion contains protein and other apparently respond to models of squirrels near nutrients, and it does not run away orfight back. their nests (Hobson et a1. 1988)-a not unex Carrion also has disadvantages: nutrient content pected result, since nest predation is a frequent diminishes due to desiccation and removal of sciuridbehavior. Smith (1970) reported that two organs (by the original predator or by earlier cactuswrens (Campylorhynchus brunneicapiU,.,) scavengers); carrion occurs in high-risk situ attacked and injured a Harris antelope squinel ations (near ravens, predator dens, or cars); and (Amnwspernwphilus harrisii) near an old nest. it may contain harmful bacteria. But the worst In other contexts, however, birds and other po thing about carrion is that it may not be available tential prey often seem to ignore squirrels. when needed. Most reports of scavenging by A clue to this blase response may be found free-living squirrels involve road kills, an artifi in the "stalking" behavior occasionally ohserved cially concentrated phenomenon. in tree squinels, including the westem gray "Quasi-prey," such as small frogs and most squirrel as described above. This behavior is not arthropods, are a fairly safo bet when available. associated with the routine operation of nest Nestling birds are somewhat more diHicult be robbing but seems limited to the relatively iu cause oJ nest defense by the parents (Smith frequent attempts au larger prey. Klugh (1927) 1970, Shaffer and Baker 1991). The hardest similarlywrote that red squirrels sometimes ap prey to explain are adult birds, rodents, and pear to stalk grouse or partridges, repeatedly rabbits, since these have defenses sufficient to advancing on the hird and then retreating. This inflict injuryon a squinel. The prevalentview is is similar to my own obselVutions ofwestern gray that squirrels tum to live prey only as an "emer squirrels reported ahove. The squinel engaged gency" food source (Reichard 1976) when other in this near-caricature of a stalking predator is resources are depleted. This view implies that actually more conspicuous them usual (at least to predation is a freakish event that has no real the human observer). It is possible that the hearingon the squilTel's role in the food web. To effect of obvious repeated stalking is to hahitu paraphrase Landry (1970), isolated events of ate potenti,J prey. carnival)' do not a carnivore make. (To support Most published reports of sciurid predation this statement, Landry cites an ohservation ofa are briefnotes, since it is diffIcult to conduct a deer eating a rabbit.) qu:mtitative study of any rare phenomenon. Again, however, a growing body ofevidence However, some tentative inferences can be suggests that predation is a nonnal component drawn. The proximate significcmce of stalking of the feeding repertoire for most scimids, at may be that the squirrel is showing conflict least outside the tropics. This does not imply behavior, advancing and then retreating if the that squiD'els are flmdamental predators, but bird or other prey appears in a position to de simply that they are opportunistic. One re fend itself. In the ultimate sense, the squirrel viewer of this paper commented that squilTels should benefit from this behavior ifits effect is are "lousy predators." Undoubtedly this is true to condition the local preypopulation to disregard 142 GREAT BASIN NATUMLIST [Volume 53 skulking squirrels. If most squirrels acting like CANO, R. J., AND J. S. COLOME. 1986, Page 665 in Microbi predators do not follow through, then prey ology. \Vest Publishing Co., St. Paul, Minnesota. CAHLSO:'-l, A. J. 1940. Eating of bone by the pregnant and should learn not to respond. This behavior is lactating gray squirrel. Science 91: 573. analogous to the hoarding of nuts, in that the CnOH.OMANSKI. J., AND A. B. SAI\GF:ANT. 1982. Gray go squirrel is hoarding prey confidence. Later, phers and prairie ducks. North Dakota Outdoors 4.5: when normal food items are in short supply, the 6-9. CU.. H.K, T. W. 1968. Food uses of the Hichardson ground squirrel can exploit this conditioning. In the squirrel (Spermophilus richardsonii elegans) in the longer term, selection for such behavior poten Laramie Basin of Wyoming. Southwestern Naturalist tially represents an entry point to a newfeeding 13, 248-249. niche, particularly in marginal habitats where COOK, A. H" A.' HAWllF.CKEH, A. C. 1947. Food and moisture requirements MOll.GAl\'C J. H. 1985. Carnivorous hehavior by a white of the Nelson antelope ground squirrel. Journal of tailed antelope ground squirrel, Amlfwspennophillls MammalogyrZ,s: 115-125. lCllnmls. Southwestern Naturalist 30: 304--305. HEHZIG-STHASClllL, B, 1976. Nahrung und Nahrung NEl\O, R. W. 1987. House sparrow killed by red squirrel. serwerh des Ziesels. Acta Theriologica 21(7): 131-139, Blue Jay 45(3): 180-Un. HESTEHBRHC, G. A. 1940. Chipmunk eats frog. Journal of NIXON, C. M., D, M. WAHLEY, AND M, W. MCCLAIN. 1968. Mammalogy 31: 350--35l. Food habits of squirrels in southeast Ohio. Journal of HOBSON, K. A., M. L ROUCllAllT, AND S. G. SEALY. 1988. Wildlife Management 32: 294--305. Responses of naive yellow warblers to a novel nest O'DoNocnuE, M. 19tH. Reproduction, juvenile survival predator. Animal Behaviour ,'36: 1823--1830. and movements of snowshoe hares at a cyclic popula HOLM, H. F. 1976. ObselVations on a cannibilistic grey tion peak. Unpublished masters the.sis, University of squirrel. Natural History Miscellany (Chicago Acad British Colmnbia, Vancouver. emy of Sciences) 197: 1-2. OGNEV, S. I. 1966, Mammals of the USSR and adjacent HOLMES, W. G. 1977. Cannibalism in the Arctic ground countries. VoL IV Rodents. Israel Program for Scien tific Translations, Jerusalem. squirrel (Spennophilus pafryi). .l0uma] ofMammalogy ,58,437-438. l'ACKAHD, H. L. 1956. 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