The meaning of weaning in wild Phayre's leaf monkeys: Last nipple contact, survival, and independence

Abstract

In primates and other mammals, weaning is an equivocal concept, as is reflected in the numerous ways it is measured: a) first intake of solid food, b) conflict over access to the nipple, c) ability to survive without mother, d) maternal resumption of cycling, or e) the cessation of nipple contact. The lack of a consistent definition means that weaning age, although it falls between gestation (fetal growth) and age at first reproduction (most energy diverted from growth), is currently not a reliable life history variable capturing offspring independence. Using data for wild Phayre's leaf monkeys (Trachypithecus phayrei crepusculus) at Phu Khieo Wildlife Sanctuary, Thailand (51 offspring, four groups), we asked whether the end of nipple contact indicates offspring independence as measured by survival to 3 years. To establish a baseline for the onset of independence, we assessed the youngest age at which individuals were orphaned (15-17 months) but then survived to 3 years. Next we determined that offspring age at last nipple contact (19.0 months) was comparable to two other independently calculated measures: offspring age at mother's first postpartum ovulation (11.5 months), and age at mother's re-conception (15.6 months). Using these separate "starting points," we arrived at similar ages for nipple contact cessation (18.4 and 19.2 months, respectively). Overall, in wild (but not in provisioned) Asian colobines, age at last nipple contact was allometrically related to adult female body mass, supporting its designation as a life history variable. Future comparisons need to show if this holds for other taxa.

Cyclical nursing patterns in wild orangutans

“…Unfortunately, there are no reliable observational data on the cessation of suckling in known-aged wild Sumatran orangutans for comparison. One of the main difficulties in documenting this in wild arboreal primates, such as orangutans, is the fact that suckling is often inconspicuous, and it is particularly difficult to assess when individuals are nesting with their mothers at night ( 7 ). …”

“…Experienced field biologists have concluded that observational approaches do not permit comprehensive quantification of suckling duration or milk intake in wild orangutans and other arboreal primates ( 6 ). Infants and older immature individuals may suckle inconspicuously, as well as while nesting with mothers at night, both difficult to detect in an arboreal environment ( 7 ). Moreover, continuous long-term field studies encounter obstacles while documenting a process that may last for seven or more years, and they typically rely on age estimations because exact birth dates are rarely known ( 4 , 8 , 9 ).…”

The evolution of pair‐living, sexual monogamy, and cooperative infant care: Insights from research on wild owl monkeys, titis, sakis, and tamarins

“…For example, van Schaik and Kappeler () write, “unfortunately, data on the relative duration of lactation are not available for a critical set of primate species, so it is not certain whether all of the infant‐cachers are below the infanticide threshold value of relative lactation length and all infant carriers above it” (p. 60). The limited availability of adequate data from wild primates on the duration of lactation and age at weaning is so serious that one should question whether or not to use the index in the first place (Borries et al, ; Borries, Lu, Ossi‐Lupo, Larney, & Koenig, ). However, since the index has been used, we illustrate here just how problematic its use can be, based on data for Aotus .…”

Sources of variation in weaned age among wild chimpanzees in Gombe National Park, Tanzania

“…As such, measures of suckling cessation are currently the only way to examine sources of variation in a comparative manner. In addition, Borries, Lu, Ossi‐Lupo, Larney, and Koenig () conducted a detailed comparison of three metrics used in wild studies to characterize declining maternal investment and increasing offspring independence in Phayre's leaf monkeys ( Trachypithecus phayrei crepusculus ): offspring age at last nipple contact, maternal age at first postpartum ovulation, and maternal age at reconception. They reported that offspring age of last nipple contact represented a better measure of both offspring nutritional independence and maternal investment than the other two metrics (Borries et al, ).…”