Girard, I.*; McAleer, M. W.; Rhodes, J. S.; Garland, T., Jr.: Increased intermittency of locomotion in house mice selectively bred for high voluntary wheel running.

After 21 generations of artificial selection for increased wheel running, female house mice (Mus domesticus) from replicate selection lines demonstrated greatly increased voluntary locomotion on running wheels (circumference 1.12 m; Behav. Genet., 1998, 28:227-237), compared with individuals from random-bred control lines (L.S. Mean � SE of wheel running on day 6 of a 6-day test: selection = 10,846 � 2,023 rev/d; control = 2,444 � 551 rev/d). In nature, many animals use intermittent locomotion, which, in laboratory studies, has been shown to increase distance endurance as compared with continuous exercise. We hypothesized that increased intermittency may have evolved in the lines of mice selected for high wheel running. To describe detailed patterns of running, we videotaped wheel running on day 6 of a 6-day test. For each mouse (n = 41), we analyzed 5 minutes of videotape in which the mean running speed (revs per 1-minute interval) was 75-100% of the maximum rpm observed over the entire day 6. Proportion of time spent running did not differ between animals from selected and control lines. Selection-line females ran significantly more frequently (7.8 �1.2 bouts/min), faster (41 � 3.0 rpm while actually running), and in shorter bouts (9.9 � 2.5 sec/bout), as compared with controls (3.4 � 1.2 bouts/min; 20 � 1.1 rpm; 16.8 � 2.5 sec/bout). Running bouts were interrupted by the mouse coasting or exiting the wheel: inter-bout pauses were shorter in selection-line (2.7 � 0.6 sec) than in control-line (7.4 � 0.6 sec) females. Because selection-line females may be running at speeds near their aerobic threshold, intermittency in locomotion may be a key feature allowing the increased wheel-running performance of these mice. NSF IBN-9728434 and NIH P30 HD03352.