Anint.Behur'.. 1992. U. l -9 Behaviouralmodulation of predationrisk: moonlightavoidance and crepuscular compensationin a nocturnaldesert rodent, Dipodomys merriami MARTIN DALY, PHILIP R, BEHRENDSI, MARGO I. WILSON & LUCIA F. JACOBST Departmentof Psychology,McMaster University,Hamilton, Ontorio L8S4KI, Cunada (Re<'eived9May l99l; initialacceptunce 3 July l99l: fnataccepranu l5 August I99l; MS.number: fi0|2) Abstract,The temporal and spatial distribution of above-groundactivity in Merriam's kangaroo rats, Diyxknnys ntcrriarni, radio-tracked during l0 winters varicd in relation to phaseso[the moon. At thc full nroon,aninra ls wcremore likely to be found in their day burrowsat scheduledhourly radio fixcs,and when thcy cmergcd,they stayedcloscr to home. At partial moon phases,above-ground activity was preferentially allocatcdlo hours when the moon was down. As predictedfrom an extensionof Rosenzweig's(1974, J. llunntul.,55, 193-199)model ofoptimal above-groundactivity, the supprcssionofnocturnal activity at the full moon was pariially offsetby relativelygreat activity at dusk and dawn, and lossesto nocturnal versusdiurnal predatorswere distributeddifferently over the lunar cycle.Gross predation ratesdid not differsignificuntly among moon phases,but the kangaroorats' relatively crepuscular activity pattern at thc full moon both rcducedtheir vulncrabilityto nocturnalpredators, as compared wilh the new moon phase, and raiscdthcir vulncrabilityto diurnal shrikes. I n vanous domains olbehavioural decision making, (Daly et al. 1990),and by which the moon's cffects unimals act as if the risk of predation were a cosl on behaviour can be assessedwithout experimental ro be weighed against expected benefits (Lima & interventions. Dill 1990).r\n example is moonlight avoidance. Kangaroo ftts (Dipodomys, Heteromyidae) arc Nocturnal rodcnts that forage in relatively open prcdominantlygranivorous, burrow-dwelling resi- habitatsrespond to moonlight by reducingactivity dents of western North American arid and grassy outsidethcir nestsor burrows,and by shiftingsuch habitats. Kangaroo rats forage morc in the opcn activitytowards areas ofrelatively dense cover (e.g. spacesbetween perennial shrubs than do sympatric Lockard & Owings I974; Priceet al. 1984;Bowers granivorous rodents such as pocket micc (Lemen & l9E8;Wolfe & Sumnrerlin 1989).Artificial moon- Rosenzweig 1978;Thompson 1982;Pricc & Brown likc illumination elicitssimilar responses(Lockard 1983),and their bipedal locomotion (Nikolai & l9?5; Kotler 1984;Brown et al. 1988). Bramble I 983;Thompson I 985)and hypertrophicd Although thcscbchavioural rcsponses have bccn middle ear cavities (Webster 1962; Webster & assurncdto reduce predation risk, no study of Webster l97l) have been interpreted as adap- moonlight avoidance has incorporated data on tations to a high level of predation risk in thcir actual predation eventsin the wild. Neither have preferred [oraging environmcnt..Crucial decisions rcscarchershitherto assessedlhe rnoon'sinfluence for kangaroo rats include the amount of above- on the spontlneouslruvels of nocturnal rodentsin ground activity to be undertakeneach night and its natural habitats, relying instead on measuresof distribution in time and space (Rosenzweig 1974; activity in enclosuresand on visitsto artificialfood Behrends et al. l986a,b). Daly et al. (1990) sourccs.Radio-tracking is a tcchniquc by which analysed50 predation deaths among 176 radio- natural predation cvenls can be detected and tracked Merriam's kangaroo rats, D. merriani, and rclatcd to thc behaviour ol radio-tracked prey found a strong association between recent surface travelsand the risk olpredation in both sexes,those address:Michael Brandnran Associates. San 'Prcscnt animals who were most mobile being most at risk. I)irgo.CA 921-30,U.S.A. (1974) proposed lPresentaddress: Department of Psychology,University Rosenzwerg thal the benefits ol.Utrh,Salt Lakc City, UT 84112,U.S.A. of above-ground activity are likely to exhibit (X) l.l7219-,0rfi)O1+09 5i @ 1992The Assoortionfor thc Studyof AnimalBehaviour I 1 Animul Behaviour.44.l Daly et al.; Predation risk allocution diminishingrcturns as time allocated to suchactivity (4) Any elcvation of the prcdation rate, or ct al. (1985). We located each radio-implanted tracked in one, two, three or all four moon phases. incrcaseswrthin nights (e.g. as the most casily componcnls thereof,at the full nroon will be dis- animaldailyduring daylight,and on trackingnights The 2386 RAN were distributed among the four collcctcdsccds .rrc dcplctcd und as information on proportionately or solcly imposcd by day-activc (usuallyull thosc othcr than trapping nights),wc moon phasesas follows. (For any pair of moon thc whcrcabou(sand rcproductrvc condition oI prcdatorswho do not actuallycxploit moonlight to typically located each anrmal hourly, for any- phases,the particular animals trackedconstituted conspecificsis updatcd),whilc thc costsol'cxposure hunt. whcrc from 6 to | 4 consecutivchours. These hourly highly over-lappingbut non-identicalsets, and it is (prcdationrisk, eold strcssand cvxporatlvclosses) Thc prcscnt study tcsts thcsc four prcdictions fixcs provide the data base for analysespresented only by chancetha( the numbers of animals con. are likely to continuc to risc with positivcacccler- from radio-trackingdata. nerc. tributing data were identical for full and new moon ation bccauscanirnals c:rn allocatcilctivity prcfcr- Altogether, 179 individual kangaroo rats bore phasesand for waning and waxing phases.) js entiallyto lhe lowcst-costtjnres. llcncc, thcre an radios for 6516radio-animal-nights (RAN), which ( l) Full Moon: 3252fixes from I06 individualsin optin)al anlount of tirnc allocatcdto sucliactivity. wcrc categorizcdinto four moon phasesas follows. 56I RAN. M ETHODS With rcasonablcassurnptions, that optirnunr is ( l) f:ull Moon. Thc 7-day pcriod centredon the (2)Waning Moon: 29l0fixesfrom l0l individuals likcly to dcclinc if the cost curvc riscs whilc thc date ol'llll moon ( I 570 RAN). in 529 RAN. bcnefi(curvc rcnrainsconstant (sec Rosenzweig's We radio-trackedMcrriam's kangaroo ratsduring (2) Waning Moon. Dates betweenFull and New (3) New Moon: 3889fixes from 106individuals in Fig. l). lf moonlight has littlc inlluencc on thc cachol' l0 consecutrvewintors from DecembcrI980 phases( 1679RAN). 673 RAN. cxpr-ctcdbencfit curvc, but substantiallyraiscs thc to March 1990at thc Boyd Decp Canyon Dcscrt (3) New Moon. The 7-day period centredon the (4)WaxingMoon: 4043fixes from l0l individuals predation risk cost of nocturnal activity, we may ResearchCenter near Palm Desert. California. date ol-newmoon ( 1592RAN). in 623 RAN. predictthe lbllowing. U.S.A. We trappcd kangaroo rats and othcr (4) Waxing Moon. Dates betweenNew and Full For theseanalyses, the critical data were whether ( l) Animals will reduccabove-ground activity at rodents in Sherman live traps (8 x 9 x 23 cm until phases(1675RAN). the animal wasaway from its day burrow at a given thc lull nroonas comparcd with thc ncw moon. 1986;8 x 9 x 30cm since 1987)baitcd with rollcd During thc Waning Moon phase,the night was scheduledradio lix, and, ifso, how faraway. (When (2) Animals will prcfercntiallyallocate above- oats. We markcd all kangaroo rats individually, darl: soon after sunset,with the moon rising from approached for radio location, kangaroo rats may ground actrvityto moonlcsstimes ol'the niBht. by toc clipping until 1988 and by subcutaneous bchind nearby hills between2100 and 0330 hours. skulk under a shrub or in a nearby refuge, so fixes Both of thesc predictionsare upheld by dala injcction oI PassiveIntcgratcd Transponder tags During the Waxing Moon phase, the night was do not provide a direct indcx of the time spent in fiom captiveenclosures (Wolfe & Summerlin 1989) (Biosonics,Seattle, Washington) since 1989. We initially bright until moonset, which occurred exposed sites.) Merriam's kangaroo rats on our and visits to artilicial food sourcesin thc ficld implantcd SM-l mousc-stylc radio transmittcrs almostan hour later than moonriseon correspond- study site virtually never changed locales during (Lockard & Owings 1974). (AVM Inslrument, Dublin, California) sub- ing dates of the Waning phase becausethe western the day, but there was a probability of about 0'22 Much lessattention has beenpard to the issueof cutancously,undcr Kctasctanacsthesia in thc lab- horizon was relativelylow. that the burrows occupied on successivedays variationsin predationrisk in relationto changing oratory. Trapping schedulesand our proccdurcs Analyscsof predation risk in relation to moon would be different (Behrendset al. I 986a).On such light lcvelsat dawn and dusk (Lima 1988).Many for handling and radio-implanting animals are phase are based on the full data set. Predation occasions,the shift of day burrow was considered nocturnulrodcnts emergc from thcir burrowswhcn describedin grcatcrdetail in Daly ct al. ( 1990). cvents wcre detcc(ed by direct observation of (he (o have occurred at that point during the night light levelsare still adequate(o supporrhuntrng by The study sitc,ccntred on a I-ha grid ol'100 trap predator still bearingthe kangaroo rat's transmit- which would minimize the sum of the distances such predatory birds as shrikes,roadrunners and siationsin a l0 x l0 array at l0-m intervals,is situ- tcr cmilting a radio signal(8 cases),by the discovery from home for that night's radio fixes. various hawks and falcons.Self-regulatcd exposure atcd on an alluvial plain at about 250 m elcvation. of the discardedtransmiiter with