In laboratory studies of terrestrial locomotion, researchers usually attempt to control the influence of behavior by testing animals under fixed locomotor regimens. One example is the testing of animals at constant treadmill speeds to determine steady-state rates of oxygen consumption and, hence, energetic costs (e.g. Autumn et al., 1999; Full et al., 1990; Taylor et al., 1982). As another example, studies of locomotor performance abilities attempt to motivate animals maximally and thus eliminate variation in ‘willingness to perform’ (for a review, see Garland and Losos, 1994). Free-ranging animals, however, can choose to move at a range of speeds and for varying durations (D. J. Irschick and T. Garland, in preparation) (Blumstein, 1992; Christian et al., 1997; Irschick and Jayne, 1999; Kenagy and Hoyt, 1989; Weinstein, 1995). Moreover, contrary to the conditions imposed in most laboratory studies, many species from a wide range of taxa exhibit intermittent locomotion in the field (Kramer and McLaughlin, 2001). For instance, ghost crabs observed in the field spent 64 % of overall activity time in motionless pauses (Weinstein, 1995). Similarly, anuran hopping locomotion is typically intermittent (for references, see Anderson et al., 1991). Terrestrial rodents, such as gray squirrels and chipmunks, spend up to 41 % of foraging time motionless (McAdam and Kramer, 1998), and heteromyid rodents typically spent more than 90 % of surface activity in slow, intermittent movement and stops (Thompson, 1985). Some species of bird use flap-bounding, an intermittent pattern of flying (Tobalske et al., 1999). Intermittent swimming and gliding is characteristic of many deep-diving mammals, such as dolphins, Weddell seals and blue whales (Pennisi, 2000; Williams et al., 2000). Many human exercise activities also include periodic rest periods. For example, many occupational tasks are performed intermittently (Mathiassen, 1993; Mathiassen and Winkel, 1992; Mathiassen and Winkel, 1996), and several sports are characterized by intermittent, high-intensity efforts (e.g. soccer and American football). Intermittent exercise has been studied most thoroughly in humans (e.g. Donnelly et al., 2000; Essen, 1978; Rossouw et al., 2000; Spriet et al., 1988). Several studies have shown that endurance can be increased and/or performance can be improved when heavy work is performed in short periods of exercise interrupted by pause periods of low-intensity or no work. In early studies of intermittent exercise in humans, researchers demonstrated that, under a heavy work load, exercise duration was increased when the work was performed intermittently (Astrand et al., 1960). Endurance-trained athletes could remain at maximal oxygen uptake 2.9 times longer and traveled 1.9 times farther before exhaustion during intermittent running at speeds associated with maximal oxygen 4311 The Journal of Experimental Biology 204, 4311–4320 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3612 In nature, many animals use intermittent rather than continuous locomotion. In laboratory studies, intermittent exercise regimens have been shown to increase endurance compared with continuous exercise. We hypothesized that increased intermittency has evolved in lines of house mice (Mus domesticus) that have been selectively bred for high voluntary wheel-running (wheel diameter 1.12 m) activity. After 23 generations, female mice from four replicate selection lines ran 2.7 times more revolutions per day than individuals from four random-bred control lines. To measure instantaneous running speeds and to quantify intermittency, we videotaped mice (N=41) during a 5-min period of peak activity on night 6 of a 6-day exposure to wheels. Compared with controls (20 revs min –1 while actually running), selection-line females (41 revs min –1 ) ran significantly faster. These instantaneous speeds closely matched the computer-recorded speeds over the same 5-min period. Selection-line females also ran more intermittently, with shorter (10.0 s bout –1 ) and more frequent (7.8 bouts min –1 ) bouts than controls (16.8 s bout –1 , 3.4 bouts min –1 ). Inter-bout pauses were also significantly shorter in selection-line (2.7 s) than in control-line (7.4 s) females. We hypothesize that intermittency of locomotion is a key feature allowing the increased wheel-running performance at high running speeds in selection-line mice. Key words: activity, evolution, exercise, intermittent locomotion, selection, wheel-running, house mouse, Mus domesticus. Summary Introduction Selection for high voluntary wheel-running increases speed and intermittency in house mice (Mus domesticus) I. Girard*, M. W. McAleer, J. S. Rhodes and T. Garland, Jr † Department of Zoology, 430 Lincoln Drive, University of Wisconsin-Madison, Madison, WI 53706, USA *e-mail: igirard@facstaff.wisc.edu † Present address: Department of Biology, University of California Riverside, Riverside, CA 92521-0427, USA Accepted 2 October 2001