Do olfactory cues mediate interactions between rodents on northern shortgrass prairie? Paul Stapp and Beatrice Van Horne Abstract: We counted captures of free-ranging deer mice (Peromyscus municulatus) in odor-baited traps to determine whether deer mice use olfactory cues to detect and evade grasshopper mice (Onychomys leucogaster), a predator and possible competitor on northern shortgrass prairie. Avoidance was measured using the frequency of captures in traps containing grasshopper mouse odors compared with that in traps containing odors of an innocuous rodent (Reithrodontomys megalotis) and in clean traps. We predicted that deer mice would be most deterred by odors in areas where grasshopper mice were common, and during winter, when alternative prey for grasshopper mice are less abundant. We also expected reproductive females to show greater avoidance because of the vulnerability of litters in burrows. Surprisingly, deer mice showed no evidence of avoidance in any experiments. These results are consistent with trapping records from capture-recapture studies on our site. We assert that the presence of grasshopper mice may affect the surface activity of deer mice, but that deer mice apparently do not use olfactory cues to avoid grasshopper mice. Taken with other studies noting the lack of avoidance of predator odors by deer mice and similar species, our results suggest that the response of rodents to predator odors is more variable than was previously appreciated. RCsumC : Nous avons comptC le nombre de Souris sylvestres (Peromyscus municulatus) capturCes dans des pikges h odeurs pour dCterminer si ces souris utilisent les odeurs pour repCrer et Cviter les Souris-h-sauterelles borCales, Onychomys leucogaster, prCdateurs et compCtiteurs probables dans la prairie boreale d'herbes courtes. La rCaction d'aversion a CtC mesurCe par comparaison de la frCquence des captures dans les pikges h odeurs de souris-h-sauterelles h la frCquence des captures dans les pikges h odeurs d'un rongeur inoffensif (Rheithrodontomys megalotis), ou dans les pikges propres. Nous avons posC en hypothkse que les souris fuiraient les odeurs des souris-h-sauterelles, surtout dans les zones oh ces animaux sont communs, et au cours de l'hiver, au moment oh les proies de rechange des souris-h-sauterelles sont moins abondantes. Nous nous attendions aussi h ce que les femelles en phase reproductrice Cvitent encore davantage les pikges h odeurs de souris-h-sauterelles h cause de la vulnCrabilitC de leur progCniture dans les terriers. ~tonnamment, les souris n'ont montrC d'aversion particulikre pour aucune des odeurs. Ces rksultats concordent avec les rCsultats des pidgeages effectuCs au cours d'expkriences de capture-recapture sur le meme territoire. Les souris-h-sauterelles peuvent affecter 1'activitC de surface des Souris sylvestres, mais celles-ci ne semblent pas se guider sur les odeurs pour Cviter les souris-h-sauterelles. Combines aux rCsultats d'autres Ctudes qui dknotent l'absence d'aversion pour les odeurs des prCdateurs chez les Souris sylvestres et chez d'autres espkces semblables, nos rCsultats indiquent que la rCaction des rongeurs aux odeurs des prCdateurs est plus variable qu'on ne l'avait cru h ce jour. [Traduit par la Redaction] Introduction Numerous studies have documented the importance of olfac- tory cues in mediating social and interspecific interactions of small mammals (Drickamer et al. 1992 and references therein). Many rodents use odor cues to detect and avoid both potential competitors (Daly et al. 1980; Drickamer et al. 1992) and mammalian predators (Madzer et al. 1976; Dickrnan 1992; Drickamer et al. 1992; Jedrzejewski et al. 1993; Nolte et al. 1994; Zimmerling and Sullivan 1994; and references therein). Such cues may be particularly useful for prey spe- cies because they allow individuals to recognize and evade predators without visual or direct contact, and because they may remain in an area for an extended period of time and pro- vide information on the temporal status of a predator's activity. I Received May 12, 1995. Accepted September 22, 1995. I P. Stappl and B. Van Horne. Department of Biology, Colorado State University, Fort Collins, CO 80523, U.S.A. The northern grasshopper mouse (Onychomys leucogaster) and deer mouse (Peromyscus maniculatus) are nocturnal murid rodents that inhabit desert and semi-arid grasslands of the western United States and southwestern Canada. Grass- hopper mice are unusual among North American rodents in being mostly arthropodivorous (McCarty 1978), and numerous studies suggest that they prey on other rodents, including deer mice (Bailey and Sperry 1929; Egoscue 1960; Horner et al. 1965; Ruffer 1968; Cole and Wolfe 1970; Flake 1971 ; Rebar and Conley 1983). In many regions, deer mice are omnivorous and consume many insects; this dietary overlap with grasshopper mice may therefore result in interspecific competition. Adult grasshopper mice are approximately 33 % larger than adult deer mice (Armstrong 1972) and, as poten- tial predators, their presence may influence the surface activity of deer mice. Rebar and Conley (1983) found that kangaroo rats (Dipodomys ordii) shifted rnicrohabitat use in the presence of grasshopper mice, and it seems likely that grasshopper mice may have a similar or more significant impact on deer Author to whom all correspondence should be addressed mice because of their greater similarity of size and ecology. I (e-mail: stappalamar . colostateedu) . If grasshopper mice pose a threat to other small mammals, Can. J. Zool. 74: 226-232 (1996). Printed in Canada 1 ImprimC au Canada Can. J. Zool. Downloaded from www.nrcresearchpress.com by COLORADO STATE UNIV LIBRARIES on 08/14/14 For personal use only.