Weaned age variation in the Virunga mountain gorillas (Gorilla beringei beringei): influential factors

Abstract

Weaning marks an important milestone during life history in mammals indicating nutritional independence from the mother. Age at weaning is a key measure of maternal investment and care, affecting female reproductive rates, offspring survival and ultimately the viability of a population. Factors explaining weaned age variation in the endangered mountain gorilla are not yet well understood. This study investigated the impact of group size, group type (one-male versus multi-male), offspring sex, as well as maternal age, rank, and parity on weaned age variation in the Virunga mountain gorilla population. The status of nutritional independence was established in 69 offspring using long-term suckling observations. A Cox-regression with mixed effects was applied to model weaned age and its relationship with covariates. Findings indicate that offspring in one-male groups are more likely to be weaned earlier than offspring in multi-male groups, which may reflect a female reproductive strategy to reduce higher risk of infanticide in one-male groups. Inferior milk production capacity and conflicting resource allocation between their own and offspring growth may explain later weaning in primiparous mothers compared to multiparous mothers. Sex-biased weaned age related to maternal condition defined by parity, rank, and maternal age will be discussed in the light of the Trivers-Willard hypothesis. Long-term demographic records revealed no disadvantage of early weaning for mother or offspring. Population growth and two peaks in weaned age within the Virunga population encourage future studies on the potential impact of bamboo shoots as a weaning food and other environmental factors on weaning.Female reproductive rates are key in determining the viability of endangered populations. Using suckling observations from the Virunga mountain gorilla population spanning nearly forty years, we investigated factors affecting variation in age at weaning, a principal determinant of reproductive rate. Our findings provide evidence that females in one-male groups that have higher risk of infanticide wean offspring earlier than those in multi-male groups, that there is differential investment in males depending on age and reproductive experience of the mother, and that we should focus more on the availability of easily digestible foods (here bamboo) in relation to age at weaning. Improved understanding of such influencing factors enables us to more readily predict future population dynamics of these great apes as they grow towards their carrying capacity with continued change in their environment. 1996; Quinn 2013) as it can determine both female future reproductive rates and offspring fitness in mammals (

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

“…and chimpanzee ( Pan troglodytes spp.) weaning ages range between 3.5 and 7.2 years of age (gorillas: Eckardt, Fawcett, & Fletcher, ; chimpanzees: see Table ). Further, earlier weaning in humans is integrated with other derived life history traits and behavioral changes, such as cooperative breeding and offspring provisioning, increased fertility, slow childhood growth, extended period of brain maturation, and postreproductive lifespans (works in Hawkes & Paine, ).…”

“…This uncertainty is exacerbated by the fact that our understanding of weaning variation in wild great apes, an important comparative reference for human evolution, is still limited in key respects. Only one study to date has thoroughly examined the correlates of weaning variation (mountain gorillas: Eckardt et al, ). Here we provide the first such report in wild chimpanzees ( P. troglodytes ) by examining weaned age variation, defined as the cessation of suckling, using long‐term data from Gombe National Park, Tanzania.…”

“…For example, within species, later weaning and/or longer interbirth intervals (hereafter, IBIs) have been reported in natural foraging or food limited environments compared to captive or otherwise food‐enhanced environments (e.g., table 1 in Lee, ; savanna baboons [ Papio cynocephalus ]: Altmann & Alberts, ). Maternal dominance rank and parity have also been shown to significantly influence maternal investment strategies, with low ranking and/or first time mothers weaning offspring at later ages and exhibiting longer surviving IBIs (e.g., olive baboons [ Papio anubis ]: Smuts & Nicolson, ; chimpanzees: Pusey, Williams, & Goodall, ; mountain gorillas [ Gorilla beringei ]: A. M. Robbins, Robbins, Gerald‐Steklis, & Steklis, , Eckardt et al, ; rhesus macaques [ Macaca mulatta ]: Maestripieri, ; but see Nuñez, Grote, Wechsler, Allen‐Blevins, & Hinde, ). In particular, primiparous mothers may face trade‐offs between investing in current offspring or completing their own somatic growth to maximize future reproduction (Charnov & Berrigan, ; Stearns, ).…”

Variation in the social organization of gorillas: Life history and socioecological perspectives

“…34,137,138 Political conflict broke out in Rwanda in 1990, leading to a decade of civil unrest that restricted the ability for foreign researchers to work at Karisoke, but routine monitoring of the Karisoke study groups continued by Rwandan field staff. 139 Since 2000, studies have been conducted on a variety of topics including, but not limited to, male relationships, 40,120,140,141 social ontogeny and personality, 142,143 feeding ecology and ranging patterns, 42,144 feeding competition, 145 behavioral endocrinology, [146][147][148] body size and growth patterns, 149 and reproductive success. 55,71,90,150 Fossey's legacy lives on with 2017 marking 50 years of research at the Karisoke Research Center.…”

Alloparenting is associated with reduced maternal lactation effort and faster weaning in wild chimpanzees

“…Inter-individual variation in weaning age occurs at Ngogo [46], as in other mammals (e.g. primates: mountain gorillas, Gorilla beringei beringei  [63]; humans, H. sapiens  [64,65]; rhesus macaques, M. mulatta  [51]; eastern chimpanzees, P. t. schweinfurthii  [52]; non-primates: northern fur seals, Ca. ursinus ; California sea lions, Z. californianus  [48]; beluga whales, D. leucas  [49]; Steller sea lions, E. jubatus  [50]).…”