Helper at the Nest Among Nonhuman Primates
A helper is an individual that contributes to breeding by conspecifics through providing alloparental care. The helper may be nonreproductive, or its own reproduction may have failed.
In the majority of primate species, infant care is provided solely by mothers, although other group members may interact with infants and carry them for short periods. In some pair-living primates, social/genetic fathers may provide paternal care. Particularly in night monkeys and titi monkeys, males are the principal infant carriers, while the contribution by helpers is negligible (Huck and Fernandez-Duque 2012). Regular helping behavior through transport of and food sharing with infants occurs in marmosets and tamarins, of the New World primate family Callitrichidae. Callitrichids (except Goeldi’s monkey) are characterized by flexible mating systems, including polyandry (particularly in tamarins), monopolization of reproduction by a single female per group, and twin births (Goldizen 1988). The energetic costs of carrying twins which at birth may correspond to 15–20 % of maternal body mass have been postulated as a driver for the evolution of the helper system in callitrichids (Goldizen 1990).
How Do Helpers Help?
In callitrichids, helpers basically provide two types of infant care: transport and food sharing. Transport is by carrying infants on the back. During the first weeks of life, twins are usually carried together, but when they become heavier are transported separately. Infant carrying ceases after 3–4 months of age; thereafter helpers carry infants only in case of danger (e.g., predatory threats).
Food sharing extends for almost the entire first year of life, although it diminishes in frequency when offspring become older. Shared food mainly includes prey (arthropods, small vertebrates) that infants are not yet capable to capture and fruits that require force for opening the husk. Food sharing may be passive through not resisting “stealing” by infants, but in some callitrichids, food may actively be offered, including the emission of “food calls.”
Is Helping Costly?
As infant body mass is high in callitrichids, it can be reasonably assumed that their transport incurs energetic costs. In fact, captive studies have demonstrated that helpers loose body mass during periods of infant carrying (Sánchez et al. 1999). Changes in activity budgets, particularly the reduction of feeding and foraging, in wild callitrichids suggest similar energetic costs (Huck et al. 2004). It is conceivable that infant carrying might also reduce the speed of escape, thus increasing the risk of predation for helpers.
Why Do Helpers Help?
The costs incurred by helping must be compensated by benefits to maintain this behavior evolutionary stable. By helping with the rearing of younger siblings, helpers may gain inclusive fitness benefits. High levels of intragroup genetic relatedness suggest that such benefits might indeed be realized (Huck et al. 2005). However, intensive helping has also been observed in individuals unrelated to infants, suggesting that other benefits must be realized as well. Such a benefit could be an increased probability of survival by remaining in a group until opportunities for own reproduction emerge; helping could then be a form of “pay to stay.” Inheritance of breeding positions by female helpers suggests that such a benefit can also apply (Goldizen et al. 1996).
Helpers may receive more allogrooming from reproductive females (“pay for help”; Lazaro-Perea et al. 2004) which renders social, psycho-physiological, and/or hygienic benefits as an additional possibility. Whether or not helpers can benefit from gaining rearing experience for own later reproduction is contended.
Does Helping Make a Difference?
Several studies, both in captivity and in the wild, have shown a relationship between the number of helpers present in a group and the probability of infant survival (Culot et al. 2011; Rothe et al. 1993). Group productivity can thus be increased by helping, although the ideal measure for such an effect would be the amount of help received per infant rather than the number of helpers.
Helpers in Other Primates
Primate infants are generally attractive to other group members, including older siblings, but also related and unrelated adults who may handle, play, or carry infants. This is particularly noteworthy in capuchins and squirrel monkeys (Huck and Fernandez-Duque 2012), the closest phylogenetic relatives of the callitrichids. However, the amount of maternal care always exceeds the care provided by others, unlike callitrichids, and so far there is no evidence for an influence of the number of caretakers on infant survival.
A special case is grandmothering: post-reproductive females caring for the offspring of her daughters. Grandmothering has been observed in some nonhuman primate species.
Among nonhuman primates, helpers at the nest in the strict sense are uniquely found among the New World marmoset and tamarin monkeys. Helpers provide transport and food for heavy infants to whom they may or may not be related. Benefits of helping may mainly rest in inclusive fitness gains and increased survival by remaining in a group. Helping may increase the probability of infant survival.
- Huck, M., & Fernandez-Duque, E. (2012). When dads help: Male behavioral care during primate infant development. In K. B. H. Clancy (Ed.), Building babies: Primate development in proximate and ultimate perspective (pp. 361–385). New York: Springer.Google Scholar