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). BrambleI 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 ( l8 cases)or with- Hourly scheduled radio fixes are independent to risk at these times may also vary with moon Labelled markers at l0-m intervals extend in ull out (ll cases)othcr remainsof the victim, or by data points in the scnsethat the average distancc phase, not becauseof the moon's (minor) effect directions, permitting us to idcntify quickly any abrLrptdisappearancc under circumstancesincon- between successivelixes for a given animal is not upon light levelsat twilight, but becauseof the locus with a prccisionof I m. Elevenhectarcs arc sistent witlr radio failure ( l4 cases).Criteria for lcssthan the averagedistance bctwecn any two fixc: moon's inRuenccon the availablc distribution of thus mappcd,encompassing all positionsat which concluding that predation had occurred are pre- within the samenight (Behrends1984). lndividual low-risk cnlcrgcncc timcs. ln plrticulxr, il'wc wc lrlvc cvcr locatcd radio-intplantcd lrtintlls. scnrcdin grcarcrdcrail in Daly ct al. (1990),Tablc radio tixcs are not indcpendent,howcver, in thr assumcthat phascol'(hc moon has rclativclylittlc Zabriskic ( 1979)dcscribcd thc rcscrvcirnd its llorir ll ol'which providcs l'urthcrtlctails on thc lirst 50 scnsc thlt thcrc arc diflcrcnccsh:twcen unimul' inlluenceon the predationrisk costsofcrepuscular in dctail;our siteis csscntiallylike one hedcscribcd rncidents. (e.9. persistenthome range sizedifferences), diffcr activity as comparcd to its inllucncc on thc pre- nearby,at which 147" of thc soil surlacclay undcr To climinate variability in the times of sunrise encesbetween years (e.g. animals were out of theil dation risk costs of nocturnal activity, we may the canopy of perennial shrubs. Creosote bush, and sunset, portrayal of the influence of moon day burrows more and travelled greater distance. make the lollowing prediction. Lurrea tridcntotzr.is the mos( common shrub, but phasc on activity distribution through the night later in the study after prolonged drought and seer (3) Curtailment of nocturnal activity at the l'ull the mosi substantialarcas ofcover are providcd by (Figs I and 2) is basedon a smallerdata setof 2386 bank depletion), and short-term temporal differ moon should be at leastpartially compcnsatedby a few large Palo Verde lrees, CercidiunrJtoridunr, RAN collectedfrom 128 individuals.These con- ences(e.g. a cold spell may have suppressedactivitl elevatedcrcpuscular activity. thc drooping boughsof which obscureup to about stitutcd all scheduledhourly fixescollected within during a particular moon phase in a particula Suchcrepuscular compensation may beexpcctcd 20 m2 ofsundy wash. 3 wceks of the winter solstice (l December to year).Ideally, we would conlinestatisticalcompari cvcn al somc cost in elcvated prcdation risk by Wc detcrmined radio locations(tixes) to a l-nr I I January). Because the duration of radio- sons (e.9. of activity at the Full Moon versus th, diurnal predators,because the increrncntalvalue of precisionwhile walking through thc studyarea with implantation episodes until predation or radio New Moon at each hour of the night) to within a given amount of crepuscular activity will be radio rcccivcrs(LA- l2 rcccivcr,AVM; or CE- l2 removal was highly variable and often less than animal/within-year contrasts where at least a lcu higher when nocturnal activity levelsare lower. receiver.Custom Electronics,Urbana, lllinois) and a full lunar cycle, and because fieldwork was (say, three) radio fixes had been collccted at eaclr This reasoningalso leads to a further, seemingly hand-held antennlei for a detailcd descriptionol scheduledwithout regard to moon phase,an indi- hour at each moon phase; unforlunately, this paradoxical,prediction. the methodof locatinga signalsourcc,sec '',1r-dison vidual animal in a givgn year could have been would eliminate most of the data from thc analysis Predation risk allocation Animal Behaviour.44. l Dalv et al.;

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o 17 18 19 ?O ?l 2? ?3MiO | ? 3 4 5 6 t7 18 19 20 ?l ?2 ?3Mtd | ? 3 4 5 6 T imeof nrght Ttme ol ntqhl l'igure2.Above-groundactiviryduringWaning(O)andWaxing(O)Moonphases; formatasinFig l.Duringthe anintuls'locutions at schcduledhourly rudio lixes' during Full Moon t'igure t. Ab6ve-grounr! activity as rcverled by Waning Moon p-hase,moonriic ou:uired b€tweenabout 2100and 0330 hours;during the Waxing Moon phase, dateswithin J weeksol'the wintcr solstice;sunsct occurred withrn thc 1ii1 and New Mo-on( O ) phases.AII data arc frbnr nroonsetoccurred betwecn about 2200 and 0430 hours hour bclbrr thc I ?00-hradio l'rxand sunrisewithin thc hour alicr rhc 0600-h ratlio lix. (a) The mcan dislnncetllanimals' radio locarionsfronl rhcir day burrows, (b) thc probability ofbcingoutsidc onc'sday burrow and (c) thc n)candistancc in the away fronr it whcn out. at the Ncw Moon (P < 0'001by Mann-Whitney test phase, activity was strongly concentratcd in cach case;Fig. la). There was also a downward early (dark) part of the night. ln the Waxing Moon trcnd in activity over the night. phase,activity was more evenly distributed through J'hus,to evaluatethc differcnccsbetwcen points in RESU LTS Two potentiallyindepcndent components of this thc night, with some suppressionearly under Figs I and 2.we presentMann -Whitney U-statistics nrcundistance, the probabiliry of beingout at each moonlight. The pattern is one of preferentialallo- of tnhibilionof Above-groundActivity under rs if crch radio tix wcrc indcpcndent.Violarions periods, in accordance Moonlight hour ol thc night and the mcan distancefrom home cation of activity to dark this independenceassumption are unlikely to have il'out, contribute,in similar fashion,to the effectof with prediction (2), superimposed on a Seneral produceda spuriousappearance ofpatlcrn in the Abovc-ground activity was inhibitcd whcn thc rnoon phase( Fig. I b, c), downward trend over the night. ilara.sincc ltiyearsof dataandover l00animals moonwasl'ull andhigh.inaccordanccwithprcdic' figure,antl sincethe patterns tion (l). Figure I contrastslixcs at the Full Moon contributcd to each Allocation of Activity to MoonlessTimes of Night CrepuscularCompensation drc sntoothand meaningful;neverthless, statistical vcrsus Ncw Moon phascsovcr the course of thc Figurc 2 portrays the sameinformation as Fig. I In accordancewith prediction (3), the suppression cautioLrslyin night. For eachof thc nine hourly fixesC:signated rcstsol' thesedata must be evaluated under the Full Moon was - -n '1 for c:'zrrtcr moon p\ases. In thc Waning Moon of nocturnal activity lighr of their complex muitiple non-i n

'l eblr l. Thc distri bul ion acrossnroon phasesof incidcntsof prcdation upon radio-implanted kangaroo rats by di urna I rnoonlitpt:riods, confirming prcdictions ( l) and (2). have been observed to prey upon kangaroo rats vcrsus preoators noclurnal Prcfcrcntialallocation ol'activity to the moonless nearby(J. Foott, personalcommunication). part of the night was superimposedupon a down- Prediction (4) was confirmed in that diurnal Known diurnal Known nocturnal Unknown ward trcnd in activity through the night, such that prcdators bcnefitedfrom the Full Moon morc than predators(shrikcs) predators preda(ors il'thc Waxing Moon phasewere considered alone, nocturnal predators. However, our theorilng did activiiy would appearto be little affectedby moon- predict Moon phasc RAYs' Deathsper RAY /vt Deathspcr RAY N1 Deathsper RAY not that nocturnal predation would actually set (F'ig. 2); it is the contrast with the Waning reversethe moon phascpattern ofdiurnal predation. Moon phase that makes moonlight's relevance Even if kangaroo rats are lessactive under moon- ljull Mrxrn 4t0 4 091 0 000 7 | .63 clcur. The dcclinc rn ac(ivi(y (hrough the night light, thcir increascdvisibility might have kept Wrnrtg or Waxing 9l8 .] 0ll 0.44 t9 201 the Ncw Moon 4.36 U 00u 6 l.l8 6 | .84 roughly parallcleddeclining average tempcratures. predation rate at the Full Moon as high as at the Nightly lows near the winter solsticctypically fall New Moon, or higher. Such was not the case,how- bclow l0"C and often below 5"C, and comparisons evcr. Despitethe depredationsofshrikes, the overall 'RAY: radio-animal'ycar.One radio-animal-year:165 25 aninral nights ol radio-bearing tNumbcr ofdeaths. acrossvariably cold nightssuggesl lhat it was thcsc risk of predation, although not signihcantly relatcd cold temperatures that inhibited above-ground to moon phase, was actually maximal at the New partially offsetby increasedabove-ground activity l0 years.These cases were identilied on the basisof activrtyas the night progrcssed:the effectsof time Moon and minimal at the Full Moon, just the at dusk and dawn. This compensationis evidentin the shrike'scharacteristic treatment of the victim, of night upon mean distancemoved betweensuc- opposite of what might be supposcd from simple l-ig. I, in whjch the gcnerallylower activity at the uhosecranium and upperchest cavity werepecked cessivefixcs were essentiallyaccounted for by the considerationof theIikely influence of moonlighton Full Moon phascis rcversedat thc first two and last openand brarnand visceraremoved. (One was thcn cflcctsof tcmpcrature(Behrends 1984). detectability.Webster & Webster(1971) followed two fixesol'the night.This doublecross-over of the impaled on a cactusspine.) No other avian prcd- Kangaroo rirts increased their crepuscular the survival of hearing-impaired D. merriami and Full Moon and New Moon curvescharactenzes ator in the region is known to proccssprcy in this activity at thc Full Moon, when the dangerof such two control groups by frequent retrapping, and both thc probability of being out and the distance way. Ten other kangaroo rats were killed by a irctivily relativeto that of nocturnal activity was noted that almost all of | 3 disappearancesoccurred l'ronrhome if out. varietyof predominantlynocturnal huntcrs:three prcsumablylowest, confirming prcdiction (3). The near the new moon. Priceet al. (1984)cite this result The first cvcning radio fix (bctwcen 1700 and by great horned owls, Eubo tirgianus, thrcc by com- ubsolutedanger of crepuscularactivity, however, as an anomaly: 'Contrary to what one would expect, 180()hours) wts rccordcdin twilight: altcr sunsct mon whipsnakcs,Masti:phis .flugellum,and onc is unlikcly to be much afl'ectedby moon phase, their results suggest that kangaroo rats are less but bclbrc fulldarkncss.Mcln distanccfrorn home eachby a rcd rattlesnake,Crotulus ruber,a spcckled and thc predictable consequencewas that lhe vulnerablc during moonlit pcriods, for rcasons at thc first fix was significantlygreater dt (he Full ra(tfesnake, C. mitt'helli, a sidewindcr, Crotalus ratc of predation by shrikes exhibited the same unknown'. However, b€causeWebs(er & Webster Moon (27 1m) than at Waning (19'6m), New cerastes,and a coyote, ('unis lulruns. puttcrn as did crcpuscularcxposure: Full Moon > (19?l) releasedall their experimentaland control (229) or Waxing (22'2) Moon phases(P<0 01, Talle I presentsthc frequenciesand ratesofprc- Waxing or Waning > New Moon. Nocturnal animals during the waning and new moon phases, Mann Whitney U-testfor eachcomparison). dation by known diurnal predators(shrikes) and cxposure exhibited the reverse pattern (New and becausethe study lastedless than a month, dis- Thc last morning fix was recordedundcr a light nocturnal prcdators,as a function of moon phase. Moorr>Waxing or Waning>Full Moon), con- appearanceat lhe new moon was essentiallysynony- sky shortlytr,cfore sunrisc, and laintersigns ol'dawn Shrikescapturcd kangaroo rats mainly in the Full tirrningprediction ( I). Prcdationby known noctur- mous with almost immediatedisappearance, and the were typically evident for the preceding lix Moon phaseand never in the Ncw Moon phase, nal predators was similarly maximal at the New apparentrelevance ofmoon phas€may havebcen an (betwccn0500 and 0600 hours) as well. At both with an intermediaterate of predation in the inter- Moon and minimal at the Full Moon. artefact.Nevertheless, our grosspredation ratedata {iles,mcan distancescores at Full Moon ( l2'6 and mediate(Waxing and Waning) phases.Noclurnal Thc fact that differentialinfluence ofmoon phase arc weakly in the samedirection. 3 8 m) significantlyexcceded corrcsponding scores predatorscxhibitcd prcciselythe oppositepattern. on diurnal shrrkesversus predominantly nocturnal But would maximal predation risk at the new Ior Waning(5 4 and l 2 m) and New (? l and l l m) Moses ( 1986, pagc 417) providcs a r-(cst prcdatorswas highly significant,on thc basisofjust moon, ifconfirmed, actuallybc contrary to cxpec- Moon phascs (P<0'01, Mann-Whitncy U-tcst approach to the comparison of counted duta in l7 casesinvolving known predators,implies that tation? lf a kangaroo rat had a small requisite lbr each comparison), but wcre not siSnificanlly ordcred catcgorics.By this tcst. with moon phasc thc genuinc ellect of moon phase is substantial. amount of above-ground activity to eng,ag,ein grca(crthan scoresat thc Waxing Moon (| l 7 and orderedFull> (Waxing= y/nnlng)> New, thc lunar Indced,thcre were no known shrikc predationsin monthly, and wcre free to allocatc it to thc lowest- 32m). distributionsof predationby shrikcsvcrsus known thc Ncw Moon phascand no known predationsby risk trmcs,it might be optimal (o allocateit nll to nocturnal prcdators were significantly diffcrcnt prcdominantlynocturnalanimals in the Full Moon dark nights;the consequencecould be that all prc- Predationin Relationto Moon Phase (t:4.54, ll :15, P<0'001). Moreovcr, thc lunar phase. Wc doubt that such an extreme contrast dation occursat the new moon and none at the full Gross prcdation ratcswere not significuntlydif- distribution ol'the scvcn known shrikc predations would persistifthc 34 victinrsofunknown preda- moon, even though moonlit nights are riskicr per lerentbetween moon phases.Eleven kangaroo rats diffcrcd from that ol'thc remaining44 prcdaticrns torscould be classified,but they cannot. We might unit of time of exposure.Conversely, if there are wcrc killcd rn thc Full Moon phasc (2'56 dcaths 1r:2.70, 4 =Ca, P <0 005). These comparisons assumethat thc | 4 animalst hat disappcared a bruptly new benefits to bc garnered from above-ground per Radio-Animal-Year),l3 in the Wtning phase confirm prcdiction (4), in thut it was diurnal prcd- rnust havc bccn transported bcyond dctection rangc activity each night, and such b€nefits reach an (2 83), l4 in the New Moon phuse(3'21), and l3 in ators thdt werc especiallysucccsslul at the Full and hencefurther than a shrike could carry them, asymptote that is affccted only by the prescnl thc Waxing phasc(2 83). Moon phasc. but wc could not then assumethat they fell victim to night'sexcursions and not thoseofprevious nights, Predatorswere identified in l7 of thc 5l pre- predominantly nocturnal predators,since there is at it could be optimal to budget surface time in such a DISCUSSION dation incidents.Seven kangaroo rats were killed leastone other diurnal/crepuscularpredator capablc way that predation became maximal at lhe full pre. by a diurnal predrtor, the loggerhead shrike, Mcrriam's kangaroorats modulatedpredation risk of such transpon: greater roadrunners, Geococcyx moon. Thus, no expecteddistribution of total Lanius ludovicianus,observed at the study sitc in all by allocating at'ove-ground activity uwa* from rulifornianus, are frequently seen on our site and dation over the lunar cyclecan be specifiedwithout ,4nimulBehaviour. 44. I Dalv et ol.: Predation risk allocation

somecmpirical or thcorcticalspecification ol'how Bchrends,I'. R., Daly, M. & Wilson, M. L l986b. Wcbstcr,D. B. & Webstcr,M. 1971.Adaptive value of lunarlighronnocturnalactivityoftheold-fieldmouse. Abovegroundactivity of Merriam'skangaroo rats hearing and vision in kangaroo ra t predatoravoidance. Anim.Eehav-,31,410 -414. the expectedbenefits of above-groundactivi(y arc (Dipo