Anim. Behav., 1996, 52, 743–758 Reproductive behaviour and operational sex ratio in Richardson’s ground squirrels GAIL R. MICHENER & IAN G. McLEAN Department of Biological Sciences, University of Lethbridge (Received 18 July 1994; initial acceptance 17 September 1994; final acceptance 17 January 1996; MS. number: 7034) Abstract. Activity budgets, use of space and social interactions of adult Richardson’s ground squirrels, Spermophilus richardsonii, during the annual mating season were documented in relation to daily operational sex ratio (OSR) for males and in relation to reproductive status for females. For males, mating was a stressful period characterized by weight loss and injury. Male–male conflict was highest when receptive females were most abundant and OSR was least male-biased. Neighbouring males seemed to jockey for position on a daily basis to increase their proximity to those females in oestrus that day. Males spent more time monitoring conspecifics and less time feeding during the mating period than at other times, especially in the late afternoon of the few days on which most females copulated, often with several males. Between emergence from hibernation and emergence of the litter 8 weeks later, the activity budget of females changed little except for the day of oestrus, when females fed less and spent time hiding. Some oestrous females spent more than 20% of their time above-ground hiding, and some precipitated aggression between males by using a hide-then-run behavioural pattern to move outside their usual area of residence. Females in oestrus temporarily expanded their home range, and they had some ability to determine the identity and sequence of their mating partners.  1996 The Association for the Study of Animal Behaviour Most North American species of ground squirrels, Spermophilus, are hibernators characterized by relatively synchronous emergence of individuals within each age and sex class, but with a 1- to 3-week delay between emergence of sexually mature males and the subsequent emergence of sexually mature females (Michener 1983, 1984a). Females mate a few days after emergence from hibernation (Michener 1985), and they are sexu- ally receptive for several hours on only one day (Schwagmeyer & Parker 1987; Sherman 1989; Boellstorff et al. 1994; Murie 1995; Lacey et al., in press). During the mating season, a sexually mature male ground squirrel must seek mates in a social environment that is in daily flux as new females emerge from hibernation, recently emerged females enter oestrus and the proportion of already mated females steadily increases (Michener 1990). The sex ratio among adults in populations of Richardson’s ground squirrels, S. richardsonii, is always strongly biased, with females outnumber- ing males by more than 3:1 (Michener & Michener 1971; Schmutz et al. 1979; Michener 1989a, 1990). Normally all females mate (Michener 1985, 1989b), yet males deplete their fat reserves and sustain severe injuries during the mating season (Michener & Locklear 1990), indicating that males compete for access to mates despite the numerical abundance of females in the popu- lation. Emlen & Oring (1977) proposed the use of operational sex ratio (OSR; i.e. number of oestrous females per sexually active male), rather than population sex ratio, to indicate the relative availability of receptive females to males and hence to predict the tendency for males to monopolize females. In a between-population comparison of the mating system of Richardson’s ground squirrels, Davis & Murie (1985) used a single estimate of OSR for the entire mating season. They reported Correspondence: G. R. Michener, Department of Biological Sciences, University of Lethbridge, Lethbridge, Alberta, Canada T1K 3M4 (email: michener@hg.uleth.ca). I. G. McLean is now at the Department of Zoology, Canterbury University, Christchurch, New Zealand. 0003–3472/96/100743+16 $25.00/0  1996 The Association for the Study of Animal Behaviour 743