Altruism and recognition in the antipredator defence of deer: 2. Why mule deer help nonoffspring fawnsMethodsStudy Site and SubjectsPreparation of Call StimuliPlayback ExperimentsSpecies discriminationIndividual discriminationResponses of females with reliable information that their own fawns were safeReproductive state and the response of mule deer femalesResponses of malesScoring Deer ResponsesData AnalysisResultsSpecies DiscriminationIndividual DiscriminationResponses of Females with Reliable Information That Their Own Fawns Were SafeReproductive State and the Responses of Mule Deer FemalesResponses of MalesDiscussionTraditional Explanations for Altruism: By-product of Parental Care, Kin Selection and Reciprocal AltruismWhy Do Mule Deer Females Defend Fawns Indiscriminately?AcknowledgmentsReferencesAppendixAltruism and recognition in the antipredator defence of deer:2. Why mule deer help nonoffspring fawnsSUSAN LINGLE*†, DREW RENDALL *, W. FINBARR WILSON*, RANDY W. DEYOUNG‡ & SERGIO M. PELLIS§*Department of Psychology, University of LethbridgeyDepartment of Biological Sciences, University of AlbertazCaesar Kleberg Wildlife Research Institute, Texas A & M UniversityxDepartment of Neuroscience, University of Lethbridge(Received 9 July 2006; initial acceptance 12 September 2006;final acceptance 1 November 2006; published online 19 April 2007; MS. number: A10506)Both white-tailed deer, Odocoileus virginianus, and mule deer, O. hemionus, females defend fawns againstcoyotes, Canis latrans, but only mule deer defend nonoffspring conspecific and heterospecific fawns. Dur-ing a predator attack, females may have to decide whether to defend a fawn while having imperfect infor-mation on its identity obtained from hearing a few distress calls. Although imperfect recognition caninfluence altruistic behaviour, few empirical studies have considered this point when testing functionalexplanations for altruism. We designed a series of playback experiments with fawn distress calls to testalternative hypotheses (by-product of parental care, kin selection, reciprocal altruism) for the mule deer’sdefence of nonoffspring, specifically allowing for the possibility that females mistake these fawns for theirown. White-tailed deer females approached the speaker only when distress calls of white-tailed deer fawnswere played and when their own fawn was hidden, suggesting that fawn defence was strictly a matter ofparental care in this species. In contrast, mule deer females responded similarly and strongly, regardless ofthe caller’s identity, the female’s reproductive state (mother or nonmother) or the presence of their ownoffspring. The failure of mule deer females to adjust their responses to these conditions suggests thatthey do not defend nonoffspring because they mistake them for their own fawns. The lack of behaviouraldiscrimination also suggests that kin selection, reciprocal altruism and defence of the offspring’s area areunlikely to explain the mule deer’s defence of nonoffspring. We identify causal and functional questionsthat still need to be addressed to understand why mule deer defend fawns so indiscriminately.! 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.Keywords: aggressive defence; altruism; behavioural discrimination; cooperation; motivational constraint; mule deer;Odocoileus hemionus; Odocoileus virginianus; recognition error; white-tailed deerLike parents of many species, both mule deer, Odocoileushemionus, and white-tailed deer, O. virginianus, mothersaggressively defend their young from predators (reviewedin: Montgomerie & Weatherhead 1988; Caro 2005). Muledeer females, however, take the defence of fawns furtherthan white-tailed deer females (hereafter whitetails, white-tail female or fawn). Mule deer are more likely to defendfawns and will defend fawns that are not their own off-spring, including whitetail fawns (Lingle et al. 2005).Mule deer females confront coyotes, Canis latrans, and de-fend fawns and even other adults throughout the year(Lingle & Pellis 2002). In contrast, whitetail females gener-ally defend only their own offspring and only during thefirst few months of a fawn’s life. Subsequently, whitetailfawns as well as adults rely on flight to avoid predationwith no direct assistance from other deer.Active defence by mule deer females is clearly advanta-geous for individuals being helped, because it usuallybrings attacks to an end (Lingle & Pellis 2002; Lingle et al.2005). Yet, there is no indication that it is advantageousfor females to defend fawns that are not their own. Onthe contrary, there appear to be real costs. Battles betweencoyotes and mule deer are vigorous and prolonged, lastingseveral minutes and sometimes hours, so at a minimum,these consume time and energy (Hamlin & Schweitzer1979; Truett 1979; Wenger 1981; Wilkinson & Douglass2002; Lingle et al. 2005). Females that defend fawns areCorrespond ence: S . Lingle, Department of Psychology, University ofLethbridge, Lethbridge, AB T1K 3M4, Canada (email: [email protected]).9070003e 3472/ 07/$30.00/0 ! 2007 The Association for the Study of Animal Behaviour. Published by Elsevie r Ltd. Al l rights reserved.ANIMAL BEHAVIOUR, 2007, 73, 907e916doi:10.1016/j.anbehav.2006.11.004sometimes attacked themselves. Furthermore, mule deercommonly leave their offspring behind to defend anotherdeer, potentially exposing their own fawn to greater risk.The simplest explanation for why females help un-related fawns is that such defence is a by-product ofparental care (Brown 1983), in which females try to de-fend their own fawn or the area surrounding their fawn(Curio 1978) and incidentally help other fawns that arenearby. A second possibility is kin selection, in whichcase females obtain indirect as well as direct genetic bene-fits by preferentially assisting close relatives (Hamilton1964a, b). A third possibility is reciprocal altruism, inwhich case females help the offspring of their closest com-panions, which would be expected to reciprocate suchhelp in the future (Trivers 1971).For any of these selective processes to lead to altruisticbehaviour, individuals need a mechanism by which theycan recognize suitable recipients of their help. However,recognition mechanisms are not perfect and may notfunction equally well in all contexts (Reeve 1989). Bothempirical and theoretical work have shown that it is essen-tial to consider recognition constraints to