Activity rhythms of vertebrates, and the biological clocks which control these rhythms, have been of great interest to both laboratory and field biologists. These rhythms have proven to be persistent under a wide variety of experimental conditions. The presence of endogenous oscillators controlling such rhythms is the subject of active research. In spite of the presence of such oscillators and the exceptional control which they exert on activity under controlled conditions, activity of wild animals in the natural environment does not exhibit high precision nor does it maintain the same pattern throughout the year. Pohl (1982) reported that this variability in activity rhythms is probably related to special ecological requirements of the species. The variability is manifested in terms of the time of onset and end of activity and as a result, changes in such circadian characteristics as period and phase relationships.
Temporal information is provided to the animal through the length and direction of change of the photoperiod.
Thus, animals determine time of year and anticipate changes in the environment which enable them to initiate such activities as food storage, migration, and reproduction to enhance survival and the production of offspring. Additional information may be provided by such biological and physical parameters as nutritional state, social status, stress, food availability, temperature or humidity. Recent studies suggest that multiple sources of biotic and environmental information interact with hormonal signals to produce a particular behavior.
Development of radio telemetry techniques for monitoring activity of unrestrained animals living in their natural environment has made it possible to obtain longterm data on rhythms of many species. These data reveal striking changes in seasonal levels of activity and great plasticity in circadian aspects, suggesting that constancy of rhythms is rare in free-ranging animals. This paper reviews information on variability in activity rhythms as a result of such biological factors as courtship, care of young, time since last food intake, and aspects of food storage and fat reserves, and environmental factors such as photoperiod, weather, food supply, and disturbance.
All of the species mentioned in this review may occur in and be a part of agricultural ecosystems. However, none should be considered as domesticated.
BIOLOGICAL FACTORS INFLUENCING ACTIVITY RHYTHMS
Ruffed grouse (Bonasa umbellus) studied year around at the Cedar Creek Natural History Area in Minnesota are normally diurnal and both sexes show a bimodal wave form with activity peaks near sunrise and sunset (Tester, 1987), similar to many other birds (Aschoff, 1967). Beginning in early April and extending into July, male grouse carry on their courtship display of drumming. Drumming may occur during either night or day, with the pattern changing ,as the season progresses. Archibald (1976) showed that during the peak of the courtship season, drumming occurred throughout the 24 hour day except for brief spans near sunrise and sunset when the male grouse were feeding. Combining the minutes spent drumming, feeding and preening indicate that male grouse are active nearly 100 percent of the 24 hours during the peak of the breeding season (Archibald, 1976).
In contrast, activity during other seasons of the year indicates a diurnal pattern with distinct crepuscular peaks.
Care of Young
Prior to egg-laying, female ruffed grouse at Cedar Creek Natural History Area exhibited a typical bimodal activity pattern. During incubation hens were on the nest for nearly the entire 24-hour period. Immediately after hatching, the hens altered their activity rhythm, beginning activity as much as several hours later in the morning and ending activity earlier in the evening. Maxson (1977) believed that this delay of onset and early cessation was related to brooding behavior by the hen. Brooding during the cool morning and evening hours reduced the exposure of chicks, helping them to maintain their body temperature.
In studies of barred owls (Strix varia) at Cedar Creek, Fuller (1979) observed that non-breeding owls exhibited nocturnal activity patterns with peaks of activity occurring just after sunset and before sunrise. However, when the female was incubating, the male increased his length of activity time, presumably to obtain food for the female. Fuller (1979) indicated that the marked increase in activity of both the male and female following hatching was the result of increased hunting for food for the growing young.
The demands of caring for young were considered to be responsible for seasonal differences in activity among different sex and age groups of black bears (Ursus americanus) studied in southeastern United States by Garshelis and Pelton (1980). Females with cubs were more active than the other sex and age categories in spring and fall.
Activity patterns of goshawks (Accipiter eentilis) studied in Sweden revealed that activity increased as the level of hunger increased. Widen (1982) found that the ratio of activity to rest increased significantly on days following the time of last food intake. Further, the percent of total time active per day increased markedly as hunger level increased.
Food Storage and Fat Reserves
Gray squirrels (Sciurus carolinensis) and muskrats (Ondatra zibethicus) studied at Cedar Creek both showed marked seasonal changes in activity patterns (Tester, 1987). A striking increase in activity of gray squirrels in September and October was related to the collection and storage of food for winter. Similarly, high levels of activity of muskrats in late summer and early fall were believed to be related to the time required to build lodges and store food for winter. After ice formed, preventing further lodge construction and food storage, the minutes of activity per 24 hours were reduced by about half.