Ultradian oscillations appear to persist in some, of the variables monitored in muskrats at Cedar Creek (Tester, 1987) as well as in a number of other small mammals (Gerkema and Daan, 1985). Rate of digestion of food is a potential underlying factor for ultradian rhythms, with smaller organisms being required to alternate feeding activity with periods of digestion more frequently than larger animals. Similarly, an ultradian rhythm would be exhibited by a song bird gathering food for its young, feeding the young, and then gathering food again. However, Gerkema and Daan (1985) believe that the control of the phase of some ultradian rhythms is related to social synchrony.
Ultradian oscillations in the range of 30 to 240 minutes have been singled out as a special group found in many plants and animals (Koukkari, Bingham and Duke, 1987). Observations on muskrats, reported above, and on kestrels (Falco sparverius) (Nunn, 1986) support this conclusion.
All studies reviewed were conducted under the natural LD cycle and the activity patterns are referred to as daily rhythms. Circadian rhythms have been observed in many wild species, under laboratory conditions. It is important to note that masking caused by biological and-environmental factors has been reported to influence rhythms under a wide range of experimental conditions (Minors and Waterhouse, 1989; Page, 1989). The biological and environmental factors discussed in this review can all be considered as potential masking factors.
Variable rhythms that display responses to seasonal changes, as reviewed in this paper, might be classified as infradian. For example, courtship, estrous, and care of young are seasonal in timing for many species. Similarly, reduction in total activity due to cold temperature by organisms living in high latitudes or the development of migratory restlessness in birds are seasonal responses to environmental change. Many of these activity patterns are believed to be endogenous and are referred to as circannual rhythms (Gwinner, 1986).
This review of investigations of rhythms in free- ranging birds and mammals suggests that wild animals have the ability to quickly adapt their activity patterns to rapid changes in biological and environmental conditions. We believe that such adaptations are widespread in all groups of animals, arising in the context of evolutionary fitness and survival in the complex natural world.
While the species considered in this review may have little direct significance to agriculture, the concept of plasticity in activity rhythms in organisms living in the natural environment is highly relevant. Livestock maintained under ranch conditions may be expected to exhibit activity rhythms quite different from rhythms of livestock maintained in individual cages or pens. These differences may be significant in relation to aspects of management such as time of feeding, forage supplements, exercise and breeding.
This paper reviews information on the effects of biological and environmental factors on activity rhythms of wild animals monitored by radio telemetry. Variations in radio signals received from free-ranging animals are used to determine the pattern of activity and rest. Telemetry is especially effective for obtaining activity data from wild animals at night and from those living in dense vegetation or underground.
Biological factors such as breeding behavior, care' of young, time of last eating, and food storage cause changes in daily activity patterns. Similarly, environmental^ factors such as temperature, snow cover, food supply and disturbance caused by humans in an urban setting also cause changes in daily activity patterns. The observed modifications of activity rhythms show that controlling mechanisms allow wild animals to quickly respond to changing biological and environmental factors.