Bins, bouts and wheel-running speed

In a recent paper Koteja et al. (1999a) discussed trait differences in mice bred for high and low running. In one of their analyses they concluded that animals bred for high levels of running ran faster. For their estimate of average running speed, Koteja et al. divided the number of wheel turns by the number of 1-min bins with some running. This method of determining average running speed is not uncommon, and others have based estimates of running speed on the division of the number of wheel turns by the number of bins that include at least some running (Jakubczak 1973; Mondon et al. 1985; Eikelboom & Mills 1988; Swallow et al. 1998; Koteja et al. 1999b). There are, however, two problems with this procedure. First, it is not clear that this procedure results in an accurate measure of instantaneous running speed. Second, differences in running speed determined by this procedure may be due to differences in other running characteristics rather than speed differences. The procedure of counting the number of bins with at least one wheel turn does not consider the natural pattern of the animal’s running. If the natural bout of running (a period of more or less continuous running) is shorter than the bin size, then counting the bins in which the animal runs will overestimate the running time. There will be periods of no running included in the total bin time, resulting in an underestimation of the instantaneous running speed. With rats there is evidence that the bout duration is short (usually only a few wheel turns) and that the running speed (in a standard wheel) approaches one wheel turn/s (Premack & Schaeffer 1962, 1963). In this case, using a bin size of 1 min would greatly underestimate the running speed. Not only might this procedure result in an underestimation of running speed, but it may also make suspect speed comparisons between groups. Consider a situation where the running bout length is different in two groups, but animals in both groups run at the same speed: if both short and long bouts are shorter than the bin size, then the bout length difference will be evident as a speed difference. Both the long and the short bout will usually occur within a single bin and result in high and low measures of running speed, respectively. Conversely, if the running speed is different in two groups of rats, but the number of turns per bout remains the same, the speed difference will not be observed at all. Because the bouts occur within a single bin for both groups of animals, the bouts will result in the same running speed. Working with rats, Rodnick et al. (1989) connected a tachometer to the wheel, which permitted a direct measure of running speed for each running bout. Using this system, they compared high, medium and low runners and concluded that the average running speed was similar in each group (ca. 41–45 m/min) but that the bout length increased as the amount of running increased. This suggested that high and low runners, at least for rats, did not differ in running speed. Lapveteläinen et al. (1997), using a system that counted quarter wheel turns every second, found that the average running speed of their adult male mice was 23 m/min, a figure that is as high as Koteja et al.’s (1999a) selectively bred high runners. These empirical findings and theoretical concerns suggest that it might be worthwhile to look further at running speed determined from data collected into different sized time bins. In our laboratory, we collect wheel-running information in 5-s bins. This makes it possible to determine running speed with short and longer bins by recompiling the data into bins of different size. I reanalysed the data for 22 adult male Sprague Dawley rats that were controls in several published (Mueller et al. 1997, 1999) and unpublished experiments (initial comparisons revealed no differences among the studies). All rats lived in standard wire cages with an attached running wheel (34 cm diameter, 11 cm wide mounted on a rack of 12 cages) for 28 days (12:12 h light:dark cycle) till running had reached a plateau level. Wheel turns were counted using a reed switch aligned with a single magnet on the wheel. Each Correspondence: R. Eikelboom, Department of Psychology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada (email: reikelbo@wlu.ca).