Maternal condition and maternal investment during lactation in mantled howler monkeys

Lactation is a costly reproductive state for females, as it may entail decreased body condition, increased morbidity, and increased mortality risk. To offset these costs, mothers should allocate investment in infant care as a function of their physical condition. Here, we assessed the relationship between maternal condition and maternal investment in wild mantled howler monkeys (Alouatta palliata) to determine: (1) whether maternal physical condition varied as a function of food availability, activity levels and feeding effort throughout lactation; (2) whether maternal investment was affected by maternal condition and offspring age.

We studied six adult females while rearing their immature offspring. We determined weekly food availability, C-peptide levels (as a measure of energy balance) in urine samples (306 samples), mothers' time budgets, feeding rates, time spent vigilant, and time spent carrying their offspring (for two years; 600 observation hours).

C-peptide levels were positively related to food availability. Maternal care in terms of vigilance and offspring carrying peaked at early lactation and decreased progressively with offspring age. Carrying was positively related to C-peptide levels and was also influenced by variation in maternal condition throughout lactation.

These results converge with previous theoretical and empirical studies on this topic and contribute to the growing evidence that nonhuman primate mothers allocate maternal care based on their current condition to maximize lifetime reproductive success.