Abstract
BackgroundIn animals with altricial offspring, most growth occurs after birth and may be optimized by post-natal maternal care. Maternal effects on growth may be influenced by individual characteristics of the mothers, such as social status, individual investment strategies and the length of association with offspring. The prolonged juvenile dependence seen in humans is a distinctive life history adaptation, which may have evolved to facilitate sustained somatic and brain growth.In chimpanzees, offspring are typically weaned at approximately 4 years old, yet immature individuals continue to associate with their mothers for up to 10 years beyond weaning. Whether this lengthy association or the individual characteristics of mothers influences growth patterns in this species is not clear.The relationship between urinary creatinine and specific gravity is an established non-invasive measure of muscle mass in humans and chimpanzees. We analysed the urinary creatinine and specific gravity of 1318 urine samples from 70 wild chimpanzees from the Taï Forest, Ivory Coast aged 4 to 15 years.ResultsWe showed a clear increase in urinary creatinine levels with age in both males and females, replicating established growth curves in this species and reaffirming this measure as a reliable proxy for lean body mass. Comparing those who experience maternal loss (orphans) with non-orphan chimpanzees, maternal presence beyond weaning age and into late juvenility positively influenced offspring muscle mass throughout ontogeny such that orphans had significantly less muscle mass than age-matched non-orphans. In age-matched offspring with mothers, those with high-ranking mothers had greater muscle mass. Accounting for variation in muscle mass attributable to maternal presence, we found no effect of maternal investment (length of inter birth interval, from own birth to birth of following sibling) on offspring muscle mass.ConclusionChimpanzee mothers have an extended and multi-faceted influence on offspring phenotypes. Our results suggest that maternal investment extends beyond lactation and into early adulthood and has clear benefits to offspring physical development. Therefore, prolonged juvenile dependence, although unique in its form in human societies, may be a trait with deeper evolutionary origins.
Highlights
In animals with altricial offspring, most growth occurs after birth and may be optimized by post-natal maternal care
We found that maternal loss in weaned individuals had a significant negative effect on offspring’s muscle mass, such that orphans had lower muscle mass compared with non-orphans (P = 0.012; Fig. 1; Table 1)
Age had a positive association with creatinine concentrations in female subjects (Estimate ± SE: 0.134 ± 0.044, 95% confidence intervals: 0.037–0.227) and we observed a more pronounced effect of age on muscle mass in males compared with females (Estimate ± SE: 0.162 ± 0.047, 95% confidence intervals: 0.092– 0.272)
Summary
In animals with altricial offspring, most growth occurs after birth and may be optimized by post-natal maternal care. There is great variation among mammal species in terms of the duration and extent of post-natal maternal effects, human life history is considered distinctive due to the length of juvenile dependence on mothers [14, 15]. This extended mother-offspring association and sustained provisioning in humans is considered adaptive; it facilitates sustained somatic and brain growth, as well as providing extensive social learning opportunities for offspring [14], to the extent that early-life conditions and maternal or care-giver relationships can be key determinants of adult physical and psychological phenotypes in humans [16, 17]. Whether post-weaning maternal presence enhances offspring physical development remains unclear
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