Variability of weaning age in mountain gorillas (Gorilla beringei beringei)

Abstract

Objectives: Weaning is a key life history milestone for mammals that represents both the end of nutritional investment from the perspective of mothers and the start of complete nutritional independence for the infants. The age at weaning may vary depending on ecological, social, and demographic factors experienced by the mother and infant. Bwindi mountain gorillas live in different environmental conditions and have longer interbirth intervals than their counterparts in the Virunga Volcanoes, yet other life history characteristics of this population remain less well known. We use long-term data from Bwindi Impenetrable National Park, Uganda to examine factors related to weaning age. Materials and methods: We analyzed data on infants born in four mountain gorilla groups in Bwindi to quantify their age of weaning (defined as last nipple contact) and to test if the sex of offspring, parity, and dominance rank of mother influences age of weaning. We also compared the age at weaning and time to conception after resumption of mating in Bwindi and Virunga gorillas. Results: Bwindi gorillas were weaned at an average age of 57.5 months. No difference was found between age of weaning for primiparous and multiparous mothers, nor did maternal dominance rank influence age of weaning, but sons were weaned at a later age than daughters. The majority of Bwindi mothers were still suckling when they resumed mating and mothers generally conceived before they weaned their previous offspring. The age of weaning was significantly later in Bwindi than in Virunga gorillas. After mothers resumed mating, the time to conceiving the next offspring was not significantly longer for Bwindi females than Virungas females (6 vs. 4 months). Discussion: Later weaning age for sons than daughters is similar to findings of other studies of great apes. Bwindi mountain gorillas are weaned at approximately the same age as western gorillas and chimpanzees, which is more than a year later than Virunga mountain gorillas. The results of this study suggest that variation in ecological conditions of populations living in close geographic proximity can result in variation in life history patterns, which has implications for understanding the evolution of the unique life history patterns of humans.

Ontogenetic changes to metacarpal trabecular bone structure in mountain and western lowland gorillas

“…As the locomotor transition from highly arboreal to mainly terrestrial is more pronounced in mountain gorillas than western lowland gorillas, the former is used to define the age categories. Importantly, we also note that age categories above based on changes in locomotor behaviours and epiphyseal fusion are not necessarily consistent with other developmental changes documented in Virunga mountain gorillas specifically, which reach independence (e.g., weaning at ~3.5 years; Eckardt et al, 2016 ) and age at first birth (~8+ years; Williamson & Gerald‐Steklis, 2001 ) earlier than Bwindi mountain gorillas and western lowland gorillas (Robbins & Robbins, 2021 ).…”

Maternal lactational investment is higher for sons in chimpanzees

“…Such seems to be the case when mothers of species with marked sexual dimorphism invest more resources in sons than daughters because male infants are more energetically costly (Clutton-Brock et al 1985;Hewison and Gaillard 1999;Lindstrom 1999;e.g., gorillas, G. beringei : Meder 1990; Leigh and Shea 1996;Eckardt et al 2016;Robbins and Robbins 2021; sea lions, Zalophus spp. : Oftedal et al 1987;Piedrahita et al 2014).…”

Comparative life history patterns of female gorillas

“… 1 Furuichi, 1989; 2 Eckardt et al, 2016; 3 Robbins & Robbins, 2021; 4 Nowell & Fletcher, 2007; 5 Lonsdorf et al, 2020; 6 Matsumoto, 2017; 7 Bray et al, 2018; 8 Bădescu et al, 2017; 9 van Noordwijk et al, 2013; 10 Nishida et al, 2003; 11 Wittig & Boesch, 2019; 12 Wallis, 1997; 13 van Noordwijk et al, 2018; 14 Reynolds, 2005; 15 Sugiyama, 2004; 16 Colchero et al, 2021; 17 Wich et al, 2004. …”

“…Despite the paucity of evidence for clear energetic differences, studies have reported marked differences in life history. Virunga mountain gorillas have the earliest age of weaning and shortest interbirth interval, with Bwindi mountain gorillas having comparable values to western gorillas (Eckardt et al, 2016; Nowell & Fletcher, 2007; Robbins, Gray, et al, 2009; Robbins & Robbins, 2021; Robbins, Stoinski, et al, 2009; Stoinski et al, 2013). Western gorillas have slower physical maturation and are considered to be adults at later ages than Virunga mountain gorillas (Breuer et al, 2009; Galbany et al, 2017).…”