Foraging energetics in patas monkeys (Erythrocebus patas) and tantalus monkeys (Cercopithecus aethiops tantalus): implications for reproductive seasonality.

The patas monkeys (Erythrocebus patas) in Kala Maloue, Cameroon, have their birth season in the mid-dry season, whereas closely related, sympatric tantalus monkeys (Cercopithecus aethiops tantalus) have their birth season in the wet season. To evaluate the optimality of a species-specific birth season, I estimated the daily intake of available energy and gross protein, and energy expenditure for one individual of each sex of each species between respective birth and mating seasons. The monkeys obtained a larger amount of available energy and gross protein in the birth season than in the mating season. No significant seasonal differences in energy expenditure between the birth and mating season were found. Thus, the birth season appears to be timed to the season when the monkeys can obtain more surplus energy and protein. Interspecific differences in the optimality of birth season were attributed to widely exploitative foraging, supported by the patas' high locomotive ability, which may enable them to obtain more energy from seeds of Acacia seyal and gums of A. sieberiana, and more protein from grasshoppers and seeds of A. seyal in the mid-dry season than the tantalus monkeys. A review of preceding studies suggests that the availability of seeds of Acacia fruiting during the dry season may exert the dominant influence on timing of birth not only in patas but also in savanna monkeys (Cercopithecus aethiops), which include the tantalus monkeys.