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pairwise comparisons revealed no consistent down-ward trend in variance between estimated indi-vidual statistics of group members and those ofsolitary fish (Table I).

Our model does not contradict the one presentedby Caraco (1981b), but rather extends his idea ofrisk analysis in group foraging. Unlike Caraco'smodel where variance in individual search time isequivalent to variance in group search time, ourmodel includes the assumption that food is encoun-tered individually by each fish and is not sharedamong group members. Therefore the relevantquestion concerns the individual's mean and vari-ance in search time, rather than those of the group.To reaffirm the importance of this distinction, notethat the variance of an individual's search time in aschool is about one order of magnitude greaterthan that of the school itself. For animals thatforage in aquatic systems where food patches arenot shared, there is no advantage to schooling interms of either individual mean or variance in timeto find food.

We are grateful to the many people who helpedus by discussing ideas and reading an earlier draftof the paper. P. Chesson, P. Depuis, L. Gross, W.Fleming, P. Kareiva, J. Kingsolver and J. Waageall made worthwhile contributions. This work wassupported by an NSF grant DEB 80-04282 to J.K.Waage.

JOY M. BERGELSON*

JOHN H. WILLISt;PHILIP E. ROBAKIEWICZ

Division of Biology and Medicine,Box G, Brown University,Providence, R.I. 02912 U.S.A.* Present address: Biology Department, The

University of York, Heslington, York YO1 5DD,England. .

t Present address: Biology Department, TheUniversity of Chicago, Chicago, IL 60637,U.S.A.

t To whom all correspondence should beaddressed.

References

Caraco, T. 1980. On foraging time allocation in astochastic environment. EcolQgy, 61, 119-128.

Caraco, T. 1981a. Energy budgets, risk and foraging indark-eyed juncos (Junco hyemalis). Behav. Ecol. Socio-bioi., 8, 213-217. .

Caraco, T. 1981b. Risk-sensitivity and foraging groups.Ecology, 62, 527-53L

Caraco, T., Martindale, S. & Whittam, T. S.1980. Anempirical demonstration of risk sensitive foragingpreferences. Anim. Behav., 28, 820-830.'

Caraco, T. & Chasin, M. 1984. Foraging preferences:

response to reward skew. Anim. Bel!av., 32, 76-85.I' "

Karlin, S. 1966. A First Course in Stochastic Processes.New York: Academic Press.

Krebs, J. R., MacRoberts, M. H. & Cullen, J. M. 1972.Hocking and feeding in the great tit Parus major: anexperimental study. Ibis, 114, 507-530.

Pitcher, T.J., Magurran, A.E. & Winfield, I.J. 1982. Fishin larger shoals find food faster. Behav. Ecol. Sociobiol.,10, 149-151.

Real, L A. 1981. Uncertainty and pollinator-plantinteractions: the foraging behavior of bees and waspson artificial flowers. Ecology, 62, 20--26.

Real, L, Ott, J. & Silverfine, E. 1982. On the tradeoffbetween the mean and variance in foraging: effect ofspatial distribution and color preference. Ecology, 63,1617-1623.

Thompson, W. A., Vertinsky, 1.& Krebs, J. R. 1974. Thesurvival value of flocking in birds: a simulation model.J. Anim. Behav., 43, 785-820.

(Received 17 September 1984: revised 24 June 1985: MS.number: As-302)

Infanticide in California Ground Squirrels

Infanticide, 'the killing of newly emerged youngwho are still within their natal home range' (Brody& Melcher 1985)has recently received much atten-tion (Sherman 1981; Balfour 1983; McLean 1983;Waterman 1984; Brody & Melcher 1985; Hoog-land, unpublished data). How prevalent this beha-viour is in the Marmotini (ground squirrels, mar-mots, prairie dogs) is not clear, because observedkillings are fairly rare (Michener 1982; Sherman1982), Infanticide by California ground squirrels(Spermophilus beecheyi) has previously only beensuspected (Linsdale 1946). Here, we report the firstdirect field observations of infanticide in thisspecies, and consider five functional hypotheses forthis behaviour.

As part of an ongoing study of their behaviouralecology, we observed a free-living population ofmarked California ground squirrels in Sunol Wil-derness, Alameda County, California. A total of256 h of observation were made from March toNovember 1983 and 150 h for the same period in1984. General observations were made at weeklyintervals in both years and in 1984 these observa-tions were supplemented by 'scan samples' in whichthe activity and location of the visible squirrelswere recorded every 15 min throughout the day.Litters were born from mid-March to early Apriland young (weighing 50-70 g) emerged from earlyto mid-May.

Infanticide was seen both years in late May andearly June, and only females were observed to kill.In 1984, all observed and suspected infanticidalfemales had litters of their own at the time, and in1983 at least half of these females had litters.

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Although the killing of pups prior to first emer-gence has been reported for other species (Sherman1981;McLean 1983;Hoogland, unpublished data),we have not observed this phenomenon.

The infanticides committed by squirrel 4 (TableI) were videotaped and appeared to be a goal-directed process, occurring over several hours andinvolving repeated approaches by the female to-wards the litter. The young tended to greet female4, rather than becoming alarmed and attractingtheir mother. This infanticidal female made slow,tentative approaches towards the young, conceiv-ably to avoid detection by the mother and tominimize the pups' arousal. Female XI undertookinfanticide in an entirely different way. On oneoccasion she ran towards a pup, chased it andcaught it. The mother, SL, and another female wereattracted by the commotion and the screaming ofthe pup, but were not able to obtain the pup from

Xl. They both chased the infanticidal female, butstopped after several minutes, and X 1took the pupover to her natal burrow to consume it. In everyobserved case, the infantical female sought out thepup rather than accidentally encountering it anddragged the pup back to her own burrow, where thepup, if not already dead, was killed. The dead pupwas always eaten, with the remains sometimesbeing cached for later consumption. Dead pupswere not examined to determine sex. Of the sixmothers who lost young to infanticide, one (SL)moved her litter (by mouth) to another burrowsystem. In one instance, a victimized mother (SC)was later suspected of killing one of the infanticidalfemale's young (Table I).

Relocation of litters to an unoccupied burrowsystem (usually within 30 m of the originalloca-tion) can also be precipitated by encounters withsnakes. Coincident with the infanticidal period is

Table I. Cases of observed and suspected infanticide and interspecific killing in a population ofSpermophilus beecheyi, 1983-1984

0: observed infanticide; S:' suspected infanticide, i.e. the animal in possession of the dead pup issuspected to be the killer; A: attack on a pup, i.e. chase and subsequent physical contact, not resulting inthe immediate death of that j~venile; a: adult animal, > 2 years old; y: yearling, i.e. born the previousspring, reproductively active; f.JfA: not applicable.

Infanticideor

Animal Sex Age prey item Eaten Comments

Infanticide4 F a 0 Yes 2 June 83; mother of young unknown; 4 had a

litter at this time4 F a A NfA 3 June 83; attacked a pup in female SC's area4 F a S ? 10 June 83; dead pup found on 4's burrow; 4 had a

litter at this time4 F a 0 Yes 24 May 84; killed one of SC's pups; 4 had a litter at

this time4 F a S NfA 18 June 84; dead pup from PC's litter found

on 4's burrow; 4 had a litter at this timeXI F Y A NfA 20 May 84; attacked TP'g pupXI F a 0 Yes 22 May 84; killed SL's pup; SL chased XI; XI

had a litter at this timeXI F a A NfA II June 84; injured pup C4TP F y S Yes 22 May 84; seen dragging and eating a freshly

killed pup; TP did not have a litterSC F y S Yes 25 May 84; seen dragging a freshly killed pup

from 4's area; SC had pups at this time? F ? 0 Yes 26 May 83; killed one of 4's pups; this animal was

unmarked; not known whether this animal had,., a litter

Interspecific killingT M a "Lizard No 19 April 83? F ? Mole Yes 12 May 83; unmarked animal4 F a Mole Yes 24 May 83

VS F ? Lizard .Yes 14 July 83

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the emergence of snakes, especially rattlesnakes,which are common predators of ground squirrelpups (Linsdale 1946). The highest frequencies ofrattlesnake sightings were in late May and earlyJune of both years (unpublished data). Of the nineobserved instances of litter relocation in 1983 and1984,eight followed an encounter with a snake; theother followed an infanticidal encounter. Infanti-cide might then be a means of creating open burrowsystems for the relocation of litters.

Four out of 29 (14%) adult females wereobserved or suspected killers in 1984. Infanticidemight thus be idiosyncratic to certain individuals,such as XI who killed a pup in 1984and attackedjuveniles in both years, and squirrel 4 who was anattacker and an observed or suspected killer in bothyears (Table I).

Hausfater & Hrdy (1984) reviewed five hypoth-eses concerning the functional value of infanticide.Our data are inconsistent with three of thesehypotheses. The parental-manipulation hypothesisproposes that the parent benefits by killing orallowing relatives to kill its own deformed or'inappropriate-sex' young. We have not observedfemales killing their own or related young. Thesexual-selection hypothesis suggests that femalesgain paternal investment through infanticide. Sucha condition is apparently inapplicable, as we havenot seen male California ground squirrels contri-buting parental investment. The pathologyhypothesis, that infanticide is non-adaptive and aresult of aberrant conditions, does not seem appro-priate here since the following data indicate thatinfanticide in this population may be functionallysignificant.

The resource-exploitation hypothesis suggeststhat the infant is an important food resource for theindividual that kills and eats it. Ttiis hypothesis isconsidered a possible explanation for infanticidewhen ground squirrels are protein deficient(Michener 1983). In the Sunol population, nosignificant difference between infantidical and non-infanticidal female weights (592 g, N=4versus 600g, N = 16) or mean percentage time spent feedingcorrected for time above ground (22,1%, N=4versus 20,7%, N= 15) was found. However, thekilling, consuming and caching of both pups andother vertebrates, especially moles (Scapanus lati-manus) and lizards, by infanticidal females indi-cates the importance of meat as a food source(Table I).

Resource competition, infanticide as a means ofreducing competition for a resource, e.g. burrows,is a possibility in this popukltion and has beensuggested for other colonial ground squirrels (Sher-man 1981; Hoogland, unpublished data). As indi-cated earlier, infanticide may' "be a means of

293

creating open burrow systems by precipitatinglitter relocation.

'In summary, infanticide in California groundsquirrels can partly be explained by the resourcecompetition and resource exploitation hypotheses.As evolutionary hypotheses, however, they over-look the potential functional significance of theontogeny of infanticide (cf. Boggess 1984).

From our preliminary analysis, some bf theproximate conditions that may lead to infanticideare proximity of available pups, frequency of snakeencounters and the availability of open burrowsystems for relocating litters. Infanticide wasaccomplished in different ways by different femalesand so, like hunting, may be a skill which takes timeand energy to perfect. If the likelihood of skilledanimals existing in the above conditions is low,then infanticide in this species might be confined toparticular individuals. Two females (4 and XI)account for the majority of our observations ofconsumption of young and other vertebrates.Thus, under certain conditions, infanticide in Cali-fornia ground squirrels may be an idiosyncraticmeans by which particular females manage theirprotein level and access to open burrow systems.

Given this account, it is unclear why males arenot infanticidal, although males may have diffi-culty recognizing kin (Michener 1983) or may notbe as protein stressed as lactating females withyoung. One might also expect the offspring ofinfanticidal mothers to acquire the necessary skillsmore readily than the offspring of non-infanticidalfemales. However, this was not the case for thosefemales for which we have data. Research with thispopulation is continuing and should help clarifysome of the questions raised by this initial work.

This work was supported by NSF Grant BNS8210065 to D.H.O. and D.F.H.

LYNNE A. TRULIO

W. J. LOUGHRYDA VID F. HENNESSY

DONALD H. OWINGS

Graduate Group in Ecology,Psychology Department.University of California,Davis. CA 95616, U.S.A.

References

Balfour, D. 1983. Infanticide in the Columbian groundsquirrel, Spermophilus columbianus. Anim. Behav.. 31,949.

Boggess. J. 1984. Infant killing and male reproductivestrategies in langurs (Presby tis entellus). In: Infanticide:Comparative and Evolutionary Perspectives (Ed. by G.Hausfater & S. B. Hrdy), pp. 283-315. New York:Aldine.

Brody, A. K. & Melcher, J. 1985. Infanticide in yellow-bellied marmots. Anim. Behav., 33, 673--674.

294 Animal Behaviour. 34. J

Hausfater, G. & Hrdy, S. B. (Eds) 1984. Infanticide:Comparative and Evolutionary Perspectives. New York:Aldine.

Linsdale, J. M. 1946. The California Ground Squirrel.Berkeley: University of California Press.

McLean, I. G. 1983. Paternal behaviour and killing ofyoung in Arctic ground squirrels. Anim. Behat,., 31,32-44.

Michener, G. R. 1982. Infanticide in ground squirrels.Anim. Behal'., 30, 936--938.

Michener, G. R. 1983. Kin identification, matriarchies,and the evolution of sociality in ground-dwellingsciurids. In: Advances in the Study of MammalianBehaL'ior(Ed. by J. F. Eisenberg & D. G. Kleiman), pp.528-572. Shippensburg, Pennsylvania: AmericanSociety of Mammologists.

Sherman, P. W. 1981. Reproductive competition andinfanticide in Belding's ground squirrels and otheranimals. In: Natural Selection and Social Behavior (Ed.by R. D. Alexander & D. W. Tinkle),pp. 31 1-331. NewYork: Chiron Press.

Sherman, P. W. 1982. Infanticide in ground squirrels.Anim. Behav., 30, 938-939.

Waterman, J. M. 1984. Infanticide in the Columbianground squirrel, Spermophilus columbianus. J. Mam-mal., 65, 137-138.

(Received I I April 1985; revised 20 JI/lU! 1985; MS.number: As-333)

Egg Moving by Two Species of Corvid

The phenomenon of passerine birds moving eggs israre, as most of these birds probably do not havethe ability to carry eggs in their beak. The familyCorvidae though, is well known for preying uponeggs of other species (Goodwin 1976), usually byremoving an egg from a nest and eating it else-where. However, corvids removing their own eggsand placing them in a nest of a conspecific has notbeen reported. Here, we present evidence of eggmoving by black-billed magpies (Pica pica) andpinon jays (Gymnorhinus cyanocephaJus) nearPocatello, Idaho. Trees containing magpie nestswere climbed almost daily throughout the nestingcycle. It took approximately 5 min to climb to andfrom a nest and frequently one or both parentswould mob. In 10 nests in 1979 and in 39 nests in1981, magpie eggs were I1,ijmberedwith indelibleink each day they were laid. The code on each egg'indicated the number of its nest, and in 1981, theorder in which it was laid. If any discrepancies werenoted in the clutch size during later examinations ofthe nest, the clutch was counted twice to confirmthe observation.

Two specific instances of .egg movement bymagpies were recorded. On 25 April 1979,nest 916had nine marked eggs in incubation. The nest wasnot checked on 26 April. On 27' April., eight eggswere present. When the nest was checked on 28 and

. 29 April, one newly-hatched young and four eggswere present. On 30 April, the nest contained twonestlings and five eggs marked with the correct nestnumber 916. Thus, during incubation four eggswere removed from the nest and two were ultima-tely returned. We do not know where these eggswere stored.

In 1981, another episode with two nests fur-nished more evidence for egg moving by magpies.Nests 206 and 921 were 70 m apart. The nests wereexamined daily and all eggs were marked insequence. Both nests had one egg on 20 March. On28 March, nest 206 had nine marked eggs and oneof these was the third egg from nest 921 (921-3).Also on this date, nest 921 had six eggs; 921-3 wasmissing. Apparently, the egg was moved from itsparental nest to nest 206 on 27 March. On 12April,nest 206 still had nine eggs. On 13April, it had onenestling and seven eggs. The eggs missing were 206-5 and 921-3. The hatchling came from one of theseeggs, but we do not know which. The fate of theother egg was uncertain.

Additional, albeit indirect, evidence of egg mov-ing by another corvid was provided by one episodewith pinon jays in 1978. The typical clutch size forpinon jays is three to four eggs (Goodwin 1976). Anest containing three newly-hatched young andone egg was discovered on 19 April. When it wasnext visited (26 April), it contained five young. Oneof the new young probably hatched from the eggcounted the previous week, but one week is tooshort a time for a newly laid egg to hatch. Thus,another well-incubated egg must have been depo-sited in the nest during the interval. Four of the fivejay nestlings were old enough to band on 30 April.The remaining nestling was too small to be bandeduntil 6 May. We assume this smallest nestling wasthe immigrant.

Our additional records of egg movement aremore speculative. We commonly observed thatsingle eggs disappeared from magpie nests duringthe laying and incubation periods. Of the 39 nestsexamined daily in 1981, 15(38,4%) had one to threeeggs (X = I.5) disappear between laying and thefirst day of hatching. Three nests (7.7%) had anunmarked egg appear toward the end of incuba-tion. Six of the 39 nests (15'3%) had the samenumber of eggs for two consecutive days during thelaying period, with the females then finishing theirclutches. Females typically laid one egg everymorning until their clutch was complete. The meanclutch size for magpies was 6,3.

Predation may have been a cause in the 15nestswhere eggs disappeared, but there was little evi-dence to support this. Mammalian predatorsusually take more than one egg (Rothstein 1975),leaving the remains of eggshells and/or a ruffled