Great Basin Naturalist Great Basin Naturalist

Volume 53 Number 2 Article 5

6-4-1993

Squirrels as predators Squirrels as predators

J. R. Callahan University of New Mexico, Albuquerque, New Mexico

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Great Basin Naturalist 53(2), pp. 137-144

SQUIRRELS AS PREDATORS

J. R. Callahan1

ABSTHACT-A literature review and field observations indicate that most sciurids are facultative predators on smallvertebrates. This behavior is documented for at least 30 sciurid species in 8 genera. The frequency of predation apparentlyis influenced by various factors including climate, season, gender, reproductive condition, and availability of plant sourcesfor certain nutrients such as calcium and nitrogen. Although sciurids assimUate as much energy from animal foods as doobligate carnivores, behavior associated with predation appears to be less efficient in sciurids and may rely partly on preyhabituation and other adaptive behaviors.

Key words: squirre~ Sciuridae, predator, carnioore, omnioore.

Predators utilize various strategies that maxi­mize the probability of successful prey capturewhile minimizing the probability of injury. Sci­urid rodents, many of them facultative and op­portunistic predators, are not morphologicallyspecialized for this role and should thereforepossess a wide variety of adaptive attack behav­iors. As discussed below, one of these behaviorsobserved in tree sqUirrels appears similar toinsinuation (Curio 1976), a strategy more oftenassociated with invertebrates than with mam­mals.

Predation, as defined here, means the killingand eating of active vertebrates (including con­specifics) or other relatively large, mobile preyby free-living squirrels. This definition excludesthe consumption of eggs, nestling birds, smallinsects, or any animal that is already dead. Preyoffer some resistance; eating carrion or aphis issimilar to broWSing. Predation also excludes kill­ing that appears unrelated to feeding, as in de­fense of the nest (Harris 1985) or as areproductive strategy (Balfour 1983, Weissen­bacher 1987). Finally, behavior of caged squir­rels is often abnormal and is excluded here asevidence of predation, although it can provideclues to dietary deficiencies.

Not all biologists accept the idea of squirrelsas frequent predators. Despite the 70-year lit­erature record summarized in Table 1,O'Donoghue's recent (1991) finding that squir­rels are the chiefpredator ofjuvenile snowshoehares elicited general "shock" (c. Krebs, per-

sana! communication). There is universal ac­ceptance that squirrels eat meat; the question ishow they obtain it. Squirrels are often seeneating carnon on roads but are rarely seen at­tacking live prey. Stomach contents analysis mayoverlookvertebrate flesh and cannot distingUishlive prey from carrion. Thus, each newobserva­tion of a squirrel acting like a predator becomesa journal note (see Literature Cited). Althoughmost of these notes imply that such behavior isaberrant, collectively they describe a significantcomponent of the sciurid repertoire. The samefeeding adaptations that enable sqUirrels tocrack nuts are sufficient for opening skulls (Lan­dry 1970).

TREE SQUIRRELs.-The following field ob­servation (which prompted this review) adds aspecies to the hst of reported sciurid predators.On 6 April 1979, I saw a lactating female west­ern gray squirrel (Sciurus griseus) stalk and at­tack an adult mountain quail (Oreortyx pictus)in mixed-conifer forest at Black Mountain, Riv­erside County, California, elevation 1800 m.The quail was standing on a 60-cm stump at theedge of a clearing; five or six other quail werenearby. None of the quail appeared to react asthe squirrel crossed the clearing in an odd,crouched posture, rustling the pine needles andleaves loudly enough to attract my attention.When it was 20 em from the stump, it leapt upand pounced on the quail. After a briefstruggle,the quail escaped and the covey moved off. Insixyears in the same area, on two other occasions

IMUS~\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 of predation by sciurjrk This table is limited to behavior nf free-living squirrel" and induJcs<lnly predation (and prcd,ltioll attempts) as defined in text.

Sp~ies

THEE sQUlImEI /oj

1';,L>;tcm gr:.ly squirrel(Sdllfll.<; ClImline"sis)

vVcstcm gray squirrel(Sduru.~, grisells)

Ea~tem fox squirrel(Sdurllsniger)

EII)"opcan red squirrel($<;illrw; vulgaris)

Douglas squirrel('I(lIl1j(t.~·ciurnst!oug!({/d)

Pine (= Bed) sCluirrel('famiast.:illnl,\·/uulwmiclI!;)

I.ittle ~"'ecn sqUirrel(AJ!tlwsc.illf1JS T}f)ens~'i)

CI\OU"J) S(,lullllmLs

Whitclail antelope squirrel(AIIIJIJJ.J!>lwmwll!1ilus feIlD'nls )

Nds{1ll antelope sfluirrcl(AmmmJlcnlulpltilus Ilelwmi)

Yellow-!)(dlied manmlt(M(/rnwla fl(lviventrl,~)

California grOlllll! squirrel(SpenTli.IJ,hiLu.\' beer:hcytj

Belding grollnl..1 sqUirrel(S/lemu'1Jhifu'i heldiJl.gi)

Suslik(S,H:nflopllib.,s eitel/liS)

C:nJlllnhian ground squirrel(Srx!nlU'1Jhil1l.s ClJlumb;rJnu.'i)

Fr:ll1klin ground squirrel(Spc.-rmophillls jrtlllklillU)

Golden-mantled squirrel(SptmllO'l'hilllS latemlis)

Mexican ground squirrel(SJu:nnophillls tllexicarllls)

HOllJH!-tai]ed ground sqUirrel(S}lerlJlOphilus tcreticclIIcZ,JS)

T()....'HSentl ground squirrel(,<;;,Jenllophilus tOlV1lsentlli)

13-lin~1 ~rolllld squirrel(SJx,'ruwJ,h ifll:. trilleamJiTlcotu.')

Arctic ground squirrel(SJ,umophilllS uru[uk,tus)

Hot:k squirrel(SpentlAJphirus t.Ylrif:g(fttL~)

IJrev,

Conspecifics, birds

Mountain quail; unidentified smaUmammal (Pcrotll!;'~'Cl1s?)

Dove, blue jay, other birds

Sql1irrel.~, birds

Ground squirrels

Chipmunks, tTtl(;: squirrels, cotton­tail, :.nowshoe Illlre, moumillg dove,other birds

Fwg (h",,?)

Pocket mouse, k;mg:lroo rdt, lizards

J,izanJs

Cnnspecific:~

Ground sqUirrels, rabbits, pocketgophers, ml1ll~s, hirds, lizards

Chipmunks, other small mammals,jlmco, warbler

Field mice, birds, snakes,conspecif'ks

Fish (live?)

Voles, domestic fowl, rlucks, oth~rbirds

Voles, <.leer l1lic.:e, chipmunks, junco,lizards

CottUlltuiJ,

Sparrows

Com~cifks

Rabbits, biros

CAlnspe<:ifics, snowshoe hare,collared lemming

Wild tll1'keys

Source

Holm (1976), Ba~ey(I923)

This paper

Boren (1961), Seton (1929), Shafferand Baker (1991)

GllnJell (1987), Schlogel (l98.5)

Roest (1951)

H,lU (1929), Seton (929). Hamil­ton (19.'14), O·Oonoghue (199[),Nero (1987), Taylor (l9.88)

Emmons (1980)

nrndl~y (1968), Morgalt (1985)

llawbecker (1947)

Armitage et a1. (1979)

Grinnell & Storer (1924), Fitch(1948), .sumner & Dixon (1963),Sundberg and Hanta (l97J), TwHot:t a!. (1986)

Sharsmith (1936), Howell (1938),Sherman & Morton (l9i9).Michener (1982)

Calineseu (1934), IIerzig.Strasdlil(1976)

IloweJ1(1938)

Polder (1955). Johnson (l922),Howell (1938), Sm...-1s (1948),Churomanski & Sargeant (l982)

CHlllemn (1967), Tevis (1953)

P'K:kard (1958)

Mit:hener (1982)

Bridgewater & Penny (1966),Bailey (1923)

O'Donoghue (.1991), Iiolmes(l977), Boonstra et al. (1990),Mi<;hCllcr (1982)

Conk & Henry (1940)

1993J

TABU 1. Continued.

Species

Washington ~und squirrel(Spermophilus washingtoni)

South African ground squirrel(Xems '''''''ris)

African ground squirrel(Xeros rotilus)

Cflli'MUNKS

Cliff chipmunk(Tamw.s dnrsalis)

Merriam's cJupmunk(Tamw.s meniami)

Least chipmunk(Tamias mini.tn.u$)

Asian chipmunk(TamiM sib''''''')

E..stern chipmunk(Tamias striatus)

SQUIRRELS AS PREDATORS

Prey

Consped.fics

Domestic fowl, other birds. turtles,other reptiles

Cob",

Crabs

Lizards, sparrows

Tree swallows

Voles, brrds. liza,ds, frogs

ConspeciJics, voles, SW<l.UoYt'S, star­ling. snakes, frogs, salam<l.nder

139

Source

Alcorn (1940)

Shortridge (1934), Ry..n (1987)

Stiles (19871

Jenkins 1989

L""on (1986)

Ledede et aI. (1985)

Ognev (1966)

Krull (1969). Seton (L929), Cinev"n(1970, Hesterberg (1940), Harriot(1940). Shackleforo (19661.1bn-es(1937)

I saw a westem gray squirrel stalk a bird brieflybut then retreat without completing a predationattempt. On 1 April 1993, S. B. Compton (per­sonal communication) saw a western gray squir­rel with a small, live mammal in its mouth, thesize ofa young PeromysGUS, beside a road in theSan Jacinto Mountains (2100 m).

Ingles (1947), Cross (1969), Jaeger (1929),and Stienecker and Browning (1970) reviewedthe food habits of the western gray squirrel butreported no predation (although the latterfound feather fragments in one stomach). Atleast six other tree squirrel species take live prey(Table 1), but the frequency ofsuch behavior isunknown. Meat constitutes 2--11% ofthe diet ofthe eastem gray squirrel, Sciurus carolinensis(Packard 1956, Nixon et aI. 1968); however,stomach contents analysis does not reveal howmeat was obtained Moreover, squirrels are er­ratic predators and not all studies are in agree­ment. Whereas Borell (1961) and O'Donoghue(1991) reported predation by the fox squirrel (S.niger) and red squirrel (Tamiasciurus luuuiOni­GUS), respectively, Reichard (1976) saw no pre­dation by either species,

It is nQt entirely clear whether a tree squirrelattacks live prey to obtain meat perse orcalciumand/or phosphorus from the bones. Shaffer andBaker (1991) noted that a fox squirrel, after

killing a young blue jay, avoided the skeletalmuscle and gnawed on the joints and bones nearthe surface ofthe skin. This is in agreement withother reports of tree squirrels eating bone andantlers (Cross 1969, Leach 1977). In othercases,however, sciurids have consumed specific preyorgans such as the brain (Hamilton 1934, Elliott1978) or viscera (Hesterberg 1940), or the fleshof the head (Holm 1976).

Predation by tropical tree squirrels appearsto be rare. Emmons (1980) saw no predation byany of nine African species, although oneAetlwsciurus stomach contained a frog (Table1). Glanz et aI. (1982) wrote that Sciurosgranatensis rarely eats animal foods. Paraxeruscepapi eats eggs, nestlings, and inseds, but ap­parently no prey as defined here (Shortridge1934); male P cepapi sometimes kill juvenileconspecifics, but Weissenbacher (1987) regardsthis as a reproductive strategy rather than pre­dation per se. Viljoen (1978) reported no preda­tion by Funisciurus congicus. Small arthropodsand annelids are the only known animal foods ofSundasciurus lowii, Lariscus obscuros, Gallo­sciurus melanogaster (Whitten 1981), GaUosci­urus erythraem (Setoguchi 1990), and Tamiopsma:lel/a.ndi (MooreandTate 1965). Borges (1990)stated that Raittfa is an obligate herbivore.

J40 GKEAI' BASIN NAI'U KALISI' lVolume53

The inhequency of sciurid predation in thetropics could he an artifact hased on the geo­graphic distribution of observers. Cert<un othermammals, however, arc facultative predators inonly palt of their geographic range (e.g., thechimpanzee; Curio 1976). For sciurids, the mostlikely explanations include the following: (1)certain tropical plants and tree harks are rich incalcium and other nutrients (see Borges 1990);(2) carnivory in the tropics may be associatedvvith increased numbers of stomach nematodes(Emmons 1980); (3) colder climate necessitatesa high-tat diet; or (4) facultative predation ispaltly a learned behavior that can spreadthrough a Jocal population, but need not occurover the entire range of a species. The firsthypothesis is supported by the faet that tropicaltree squirrels tum to predation when caged anddeprived of a normal diet (e.g., Kcshava Bhat1980).

GHOUND SQUIHHELs.-Meat (live prey andcarrion) is a major food source for ground squir­reJs worldwide, although fewer data are avail­able regarding Old World species. Table 1 listsreports of predation by ] 8 species of Sperrrw­phill1S, Amrrwsperrrwphilus, Mannota, andXenl-s. A possihle exception is the Mohaveground squirreJ (Spemwphil11s mohaven.sis),which has heen studied intensively (Leitner eta1. 1991) hut is not known to take live vertehrateprey. The cheek poucbes and stomacbs of fourSdurotamias davidianus specimens containedonly pJant material (Callahan and Davis 1982),but no fleld data on this endemic Chinese genusare available.

FLYING SQUIlUlELs.-The southem flyingsquilTel (Glaucomys volans) eats eggs, nest­lings, and carrion (Bailey J923, Landry 1970),but not consistently (Harlow wld Doyle 1990).I have found no record of predation as definedhere.

CIlll'MUNKS.-The eastem chipmunk (1hrniflsstriatl1s) and the Asian chipmunk (T. sihir1t;us')take a variety of prey (Table 1). Lederle et al.(198.5) reported that least chipmunks (Tami"srninimus) prey upon adult tree swallows, a..'l wellas eggs and young. Jenkins (1989) obselved clill'chipmunks (1hrnias dDrsalis) in coastal Sonora,Mexico, eating crabs and other marine inverte­brates in tide pools; because the exact size of thecrabs was not documented, this is a borderlinemse of "predation." Larson (1986) repolted thatMeniam's chipmunk (Tamias merriami) occa­sionally eats lizards and birds. The lodgepole

chipmunk (Tarnias speciosus) is somewhat spe­cialized as an arboreal nest predator (Grinnell1908, Grinnell and Storer 1924), but it has beenreported to eat eggs rather than adult birds.Similar behavior is reported for the Uinta chip­munk (Tamias wnbrinus) (Smith and Anderson1982).

SIGNIFIGANGF:.-There are really two ques­tions here: (1) Why eat meat? (2) Why catch itwhile it is still alive?

Afrequent answer to the first question is thatsquirrels, especially reproductive females, mayneed a concentrated source of protein and/orcertain minerals. This view is supported by stud­ies of calcium self-selection by male and femaleMalabar giant SqUilTelS (&t11fa indica) (Borges1990). Smith (1968) and Carlson (1940) re­pOited that only pregnant and lactating treesquirrels regularly eat animal food. Studies citedby Guruell (1987) showed that female treesquiITels cannot always obtain enough calcium,phosphorus, sodium, or nitrogen from a diet ofseeds. Goodmm (1940) speculated that femalesquirrels may need meat to reproduce success­fully. Keyrner and Hime (1977) reported a wildEuropean red squirrel (Sciurus vulgaris) withnutIitional osteodystrophy, suggesting that die­tary calcium may be a limiting factor in thedistribution of certain species.

A second viewpoint is that the seasonal in­crease in meat consumption, whether of liveprey or carrion, compensates for a seasonal de­cline in the quality of plant food (especiallyprotein content) and is not specifically relatedto reproduction. Nutrient density and watercontent of plants eaten by squinels decline inspring and summCf (Bintz 1984). ]n the MojaveDesert, plant foods evidently contain sufficientcalcium, hut nitrogen is likely to he limiting fordesert ground squirrels that are active year­round (Karasov ] 985); mnversely, tree squirrelsin more mesic environments have fungi avail­able a..'l a source of nitrogen but are more likelyto need calcium seasonally (Carlson 1940,Coventry 1940, Keymer and Hime] 977). Phos­phorus also may be a factor in food selection;Cano and Colome (1986) attribute the con­sumption of carrion by cattle in parts of SouthAfiica to phosphorus-delicient soils. When kepton a herbivorous diet, Belding's ground squir­rels select plant palts highest in protein andwater (Eshelman wld Jenkins 1989). GurneJl(1987) wrote that tree squirrels use animal foodmainly in the summer. Weeks and Kirkpatrick

1993] SQUTHRELI AS PREDATORS 141

(1978) studied the "salt drive" phenomenon infox squilyels and marmots. Clark (1968) foundthat the proportion of animal food in Spermo­philus richardsoni stomachs increased from 3%in April to 24% in August. Tevis (1953) reporteda similar phenomenon for chipmunks and gold­en-mantled sqllirrcls.

A third hypothesis is that predation by squir­rels is often incidental to killing for some otherreason, usually territOlial defense or reproduc­tive competition. In other words, once the otheranimal is dead, it provides an energetic bonusthat can he consumed without further risk. Thisexplanation applies mainly to certain groundsquirrel species (Holmes 1977, Michener 1982,Balfour 1983, Harris 1985).

The other question concenlS the advantagesof live prey. Carrion contains protein and othernutrients, and it does not run away or fight back.Carrion also has disadvantages: nutrient contentdiminishes due to desiccation and removal oforgans (by the original predator or by earlierscavengers); carrion occurs in high-risk situ­ations (near ravens, predator dens, or cars); andit may contain harmful bacteria. But the worstthing about carrion is that it may not be availablewhen needed. Most reports of scavenging byfree-living squirrels involve road kills, an artifi­cially concentrated phenomenon.

"Quasi-prey," such as small frogs and mostarthropods, are a fairly safo bet when available.Nestling birds are somewhat more diHicult be­cause oJ nest defense by the parents (Smith1970, Shaffer and Baker 1991). The hardestprey to explain are adult birds, rodents, andrabbits, since these have defenses sufficient toinflict injury on a squinel. The prevalent view isthat squirrels tum to live prey only as an "emer­gency" food source (Reichard 1976) when otherresources are depleted. This view implies thatpredation is a freakish event that has no realhearing on the squilTel's role in the food web. Toparaphrase Landry (1970), isolated events ofcarnival)' do not a carnivore make. (To supportthis statement, Landry cites an ohservation of adeer eating a rabbit.)

Again, however, a growing body of evidencesuggests that predation is a nonnal componentof the feeding repertoire for most scimids, atleast outside the tropics. This does not implythat squiD'els are flmdamental predators, butsimply that they are opportunistic. One re­viewer of this paper commented that squilTelsare "lousy predators." Undoubtedly this is true

to some extent; but neither the apparent lowsuccess rate nor the situation-specific responseto potential prey is unique to squirrels. Bothphenomena are reported for many obligatepredators as well (Curio 1976). Moreover, Kara­sov (1982) found that antelope ground squirrelsassimilate energy from animal fi:)ods just as efH­ciently as do obligate predators.

There is a learned component in predatorrecognition and avoidance by birds and mam­mals (Curio] 976, Robinson 1980). It would bea waste of energy for rabbits to avoid dc~er, forinstance, even though one deer ate one rabbit.But if squilTels undergo dietary stress evel)'yearand begin eating peculiar things, one mightexpect prey to catch on. The limited evidenceavailable suggests this is not the case. Birdsapparently respond to models of squirrels neartheir nests (Hobson et a1. 1988)-a not unex­pected result, since nest predation is a frequentsciurid behavior. Smith (1970) reported that twocactus wrens (Campylorhynchus brunneicapiU,.,)attacked and injured a Harris antelope squinel(Amnwspernwphilus harrisii) near an old nest.In other contexts, however, birds and other po­tential prey often seem to ignore squirrels.

A clue to this blase response may be foundin the "stalking" behavior occasionally ohservedin tree squinels, including the westem graysquirrel as described above. This behavior is notassociated with the routine operation of nestrobbing but seems limited to the relatively iu­frequent attempts au larger prey. Klugh (1927)similarly wrote that red squirrels sometimes ap­pear to stalk grouse or partridges, repeatedlyadvancing on the hird and then retreating. Thisis similar to my own obselVutions ofwestern graysquirrels reported ahove. The squinel engagedin this near-caricature of a stalking predator isactually more conspicuous them usual (at least tothe human observer). It is possible that theeffect of obvious repeated stalking is to hahitu­ate potenti,J prey.

Most published reports of sciurid predationare brief notes, since it is diffIcult to conduct aqu:mtitative study of any rare phenomenon.However, some tentative inferences can bedrawn. The proximate significcmce of stalkingmay be that the squirrel is showing conflictbehavior, advancing and then retreating if thebird or other prey appears in a position to de­fend itself. In the ultimate sense, the squirrelshould benefit from this behavior if its effect isto condition the local prey population to disregard

142 GREAT BASIN NATUMLIST [Volume 53

skulking squirrels. If most squirrels acting likepredators do not follow through, then preyshould learn not to respond. This behavior isanalogous to the hoarding of nuts, in that thesquirrel is hoarding prey confidence. Later,when normal food items are in short supply, thesquirrel can exploit this conditioning. In thelonger term, selection for such behavior poten­tially represents an entry point to a new feedingniche, particularly in marginal habitats wheretree squirrels may be more likely to resort topredation.

ACKNOWLEDGMENTS

This paper was begun with the assistance ofan NSF grant (BNS 78-17469) to the Universityof Georgia, and was resumed under a 1990-91Theodore Roosevelt Memorial Grant to tbeauthor.

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1993] SQUIRRELS AS PREDATORS 143

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Her-ei1XX] J Decemher 1992AC("A.->pted 7 Decemher 1992