J. comp. Physiol. 128, 21-36 (1978) Journal of Comparative Physiology. A 9 by Springer-Verlag 1978 Seasonality of Endogenous Circadian Rhythms in a Diurnal Rodent Ammospermophilus leucurus and a Nocturnal Rodent Dipodomys merriami * G.J. Kenagy ** Max-Planck-Institut f/Jr Verhaltensphysiologie, Seewiesen and Erling-Andechs, Federal Republic of Germany Accepted June 30, 1978 Summary. 1. Circadian locomotor rhythms were mea- sured continuously for 16 months in 27 animals which were exposed during most of this time to the natural daily cycle of illumination (ND) at 47o58 ' N. Lat., and held at 19 ~ However, at two solstices (June and December) and two equinoxes (March and Sep- tember) the animals were transferred to continuous illumination (LL) for one month, where two groups of each species were exposed to two different inten- sities of LL, and then returned to ND. 2. Circadian period (z) was virtually constant throughout the year. Dipodomys merriami showed the positive correlation between r and light intensity predicted by AschofFs rule, but mean z in Ammosper- mophilus leucurus was essentially unchanged over a wide range of light intensities. The theoretically predicted seasonal relationship between z and phase relationship of activity to zeitgeber signals (~b) was not found. 3. The duration of activity (e) in A. leucurus was unusual in showing little seasonal change in ND, de- spite marked seasonal changes in photoperiod. Per- haps due to this relative constancy of c~ in ND (aND), the change of a upon transition into LL was generally not significant. There were also no signifi- cant differences between any of the four seasonal values of a in LL (aLL). In D. merriami ctr<D generally paralleled seasonal changes in length of night, and change of a upon transition into LL was significant at all seasons. Mean aLL fell within a narrow range over the year, during which there were almost no significant season-to-season differences. 4. Neither species showed a marked seasonal cycle of body weight. However, body weight of A. leucurus * Expanded version of a presentation made at a meeting of the Alexander yon Humboldt-Stiftung at Erling-Andechs, Federal Republic of Germany, in September 1975 ** Fellow of the Alexander von Humboldt-Stiftung. Present ad- dress : Department of Zoology, University of Washington, Seattle, Washington 98195, USA consistently increased upon each transfer from ND to LL. 5. Male A. leucurus, investigated by external exam- ination, showed an annual gonadal cycle with a peak in spring, whereas most D. merriami developed and then maintained testicular maturity throughout the study. 6. The role of z in seasonal homeostasis of the circadian system may lie in its availability as a con- stant reference signal. It cannot be directly responsible in these two species for stimulation of adaptive sea- sonal functions. However, complementary changes in either c~ or ~ may interact with z to promote some component of adaptive seasonal cyclic functions. The apparent endogenous circannual gonadal cycle of A. leucurus could be either generated or entrained via such interactions, but the opportunistically responsive gonadal function of D. merriami probably has rela- tively little relation to such circadian functions. Introduction The study of interrelations between circadian rhythms and the seasonal changes in environmental conditions and physiological state of animals is a field of investi- gation which has recently begun to develop more in- tensively. Several decades of research in circadian rhythms (reviews by Bfinning, 1973 ; Saunders, 1977) serve as a basis for this approach to understanding adaptive seasonal physiological changes, i.e., annual cycles of reproduction, hibernation, migration, molt, body weight, metabolism, etc. The involvement of circadian rhythms in the mechanism of photoperiodic time measurement has only recently been established in mammals (review by Elliott, 1976). Seasonal and latitudinal changes in the daily duration of activity (a) and in the phase relationship (~) between activity and the daily environmental light-dark cycle are con- 0340-7594/78/0128/0021/$ 03.20