Seasonal changes in nutritional requirements

Changes in diet often follow the changes in seasons. When researchers at Sea World, Durban, traced the annual food consumption of their female dusky dolphin (Lagenorhynchus obscurus) over a 13-year period, they found that her annual food intake jumped from 4,784 lb (2,170 kg) when she was five years old, to nearly 6,393 lb (2,900 kg) the year after. This increase coincided with the installation of a cooling system that was used in the summer in the years thereafter, and after her sixth year, her food consumption fluctuated between 5,290 lb and 6,173 lb (2,400-2,800 kg) per year. In general, her food intake was above average during autumn and winter, and below average during spring and summer, and the average pool water temperature fluctuated seasonally.

A similar study of California sea lions (Zalophus californi-anus californianus) found that the voluntary decrease in food intake during summer was associated with increased aggres sive behavior in males, while the seasonal fluctuation in non-reproductive females was negligible. Seasonal fluctuations in male food intake was especially pronounced between the ages of four and eight, when sexual maturity was reached.

Territorial male California sea lions defended their territories in the breeding season, during which they do not feed, and remain in their territory for an average of 27 days. In captivity, they have been shown to lose as much as 198 lb (90 kg) during the breeding season—independent of food availability, suggesting the possibility of an endogenous rhythm. The simultaneous increase in aggression suggests testosterone involvement as well. The females, on the other hand, showed less profound fluctuations in monthly food intake than their male counterparts, possibly because females are non-territorial and do feed during the breeding season.

Seasonal variation in temperatures may also be important as male sea lions in particular ate less when air and water temperatures were high and a thick fat layer was less important for maintenance of constant body temperature.

Sheep have also been observed to alter their diets according to the shifting seasons. They are known to consume a more fibrous type of forage such as tussock grass during the winter season or seasons with a scarcity of resources. Two independent studies, in the semiarid rangelands of Argentina and Australia, respectively, reported that sheep preferred tussocks only in winter, and avoided them during the growing season. Although sheep behave generally as bulk grazers, they will also consume, when offered, a considerable amount of shrubs in the fall and winter seasons. This preference for evergreen shrubs corresponds with times of the year when grasses are less available or nutritious. Scottish sheep, in a comparable high-latitude oceanic climate, also displayed a similar feed-

Scottish Mammals Images
A brown bear (Ursus arctos) feeds on salmon in the summer months. (Photo by © John Conrad/Corbis. Reproduced by permission.)
Groundwater Seasonal Changes
Barren-ground caribou (Rangifer tarandus) favor lichens, especially caribou moss, but they are not essential for survival. (Photo by Michael Gian-nechini/Photo Researchers, Inc. Reproduced by permission.)

ing pattern, i.e., they consumed a high proportion of shrubs only in winter. Shrubs are also a type of forage that maintain a relatively high-protein content during the colder seasons.

Variation in the diet of a species over seasons may also be the result of habitation in different landscapes of the same region, thus linking prey use with availability. The swift fox (Vulpes velox), for example, occupies two distinct landscapes in western Kansas that are dominated by either cropland or rangeland. In spring and the fall, plants such as sunflower seeds, and birds were consumed more frequently in cropland than in rangeland. However, birds were more common in the swift fox diet in cropland during the fall.

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