Activity responses of two species of Peromyscus (Rodentia, Muridae) to varying light cycles.

Activity of Peromyscus floridanus and P. gossypinus was studied through a sequence of light-dark regimes including 12-hr light, 12-hr dark; 18-hr light, 6-hr dark; continuous light; and 12-hr light, 12-hr dark. Two populations of each species and both wild caught ("field") and first generation laboratory conceived and reared ("lab") mice of each stock were tested. All mice were strongly nocturnal during the initial 12-hr and 18-hr photoperiods, although gossypinus was relatively more active during the light phase than floridanus. Both species tended to become aperiodic in constant light, but floridanus showed a greater tendency to adhere to the nocturnal cycle of the preceding 18-hr photoperiod. Although the two species were relatively more active in light during the second 12-hr photoperiod than during the first 12-hr light, 12-hr dark regime, floridanus reestablished a stronger nocturnal cycle than gossypinus. Lab groups exhibited essentially the same trends between species and populations as the field animals, thus indicating some genetic basis for the observed differences. P. gossypinus had a higher over-all absolute activity level than floridanus. There was also less difference between the performance of field and lab groups of gossypinus than of floridanus. Both species showed a decline in activity with increased photoperiod, the effect being more pronounced in floridanus.The difference in the activity responses of the two species appears to be correlated with the characteristics of their habitats and nest sites. The stricter nocturnality of floridanus is associated with relatively xeric, open habitat in which daytime activity would seem to involve higher risk of predation and greater heat strss and water loss than in the case of the more humid denser forest environments typical of gossypinus. P. floridanus also nests in burrows, whereas nests of gossypinus are typically located in more accessible places where a lower threshold for activity in light might increase a mouse's chances of escaping from a predator when surprised in the nest during the day. The floridanus populations tended to differ more in percentage of activity in light and level of absolute activity than the corresponding gossypinus stocks. The difference in the activity patterns of the two floridanus populations was correlated with the density of vegetation in their respective habitats. Relative eye size was not clearly associated with species and population differences in activity responses to light.