Seasonal dietary stress in a forest monkey (Cercopithecus mitis).

Blue monkeys (Cercopithecus mitis nyasae Schwarz) were studied for a period of 15 months on the Zomba Plateau in Malawi, which experiences pronounced seasonality of climate. 2) As might be expected, the monkey's diet contained most protein per item, and least potential digestion inhibitors, during the warm rainy season of the Malawian summer. 3) During the period, however, from October to November, which constitutes the hottest time of the year when the greatest number of trees are flowering and fruiting, protein intake was relatively lower than at other times of year and digestion-inhibitor intake tended to be relatively higher. 4) In the same period, the monkeys ranged more widely than during the winter months, when suitable food was available on the fewest number of tree species. 5) During the winter months, which occur in the first part of the dry season, the monkeys were able to obtain a reasonably nutritious diet from foliar material and some flowers, without recourse to any significant amount of fruit. 6) It is suggested that poor quality of diet during October and November is related to increased chemical defences in edible plant parts which may be partly the result of climatic pressures, and that the monkey's flexible response reflects a high degree of sensitivity to the chemical makeup of such plant parts.