Maternal lactational investment is higher for sons in chimpanzees

Iulia Bădescu,M Anne Katzenberg,David P Watts,Daniel W Sellen

doi:10.1007/s00265-022-03153-1

Iulia Bădescu, M Anne Katzenberg + Show 2 more

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https://doi.org/10.1007/s00265-022-03153-1

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Abstract

Maternal lactational investment can affect female reproductive rates and offspring survival in mammals and can be biased towards infants of one sex. We compared estimates of lactation effort among mothers, assessed as their potential milk contribution to age-specific infant diets (mother-infant differences in fecal stable nitrogen isotopes, δ15N), to the timing of weaning (infant age at last nursing bout) and to maternal inter-birth interval lengths for male and female infant chimpanzees (Pan troglodytes) at Ngogo, Uganda. Infant males had greater proportions of milk in their age-specific diets, indicated by higher mother-infant differences in δ15N (Generalized Estimating Equation, GEE: p < 0.01). This may mean that mothers of sons showed greater lactation effort than mothers of daughters. Infant males stopped nursing at older ages than infant females (Kaplan–Meier product limit estimate, Breslow estimator: p < 0.05). Mothers of sons showed longer interbirth intervals than mothers of daughters (GEE: p < 0.01). All three measures indicated maternal lactational investment was higher for sons. Male infants may cost mothers more to ensure infant survival than female infants because males are more vulnerable and/or because maternal genetic returns on investment are greater for sons than daughters, as male philopatry means that chimpanzee mothers can have more influence on the reproductive success of sons. Chimpanzee females may trade off growth-related benefits of high lactational investment in male offspring against reduced reproductive rates.Significance statementMaternal investment via lactation affects the reproductive success of female mammals and their offspring and can be biased towards infants of one sex. We investigated lactational variation among wild chimpanzees in relation to infant sex using three proxies for maternal lactational investment: fecal stable nitrogen isotopes, a physiological biomarker that may provide an estimate of lactation effort; observations of nursing, which we used to establish weaning ages; and the lengths of intervals between births of surviving infants. Chimpanzee mothers biased lactational investment toward sons on all three indicators and showed reduced fecundity due to longer inter-birth intervals for mothers of sons than for mothers of daughters. These results would be expected if greater maternal investment toward sons leads to better condition and higher reproductive success for sons later in life, thus to greater inclusive fitness for mothers.

Highlights

Age at weaning, or the length of maternal investment via lactation, is an important life history characteristic in mammals that has fitness implications for both infants and mothers (Lee et al 1991; Lee 1997; Kennedy 2005)
We investigated lactational variation among wild chimpanzees in relation to infant sex using three proxies for maternal lactational investment: fecal stable nitrogen isotopes, a physiological biomarker that may provide an estimate of lactation effort; observations of nursing, which we used to establish weaning ages; and the lengths of intervals between births of surviving infants
Both these analyses indicated that milk constituted a greater proportion of the male infant diet than the female infant diet for all age categories except ages 1 to ≤ 1.5, 2 to ≤ 2.5 and 3 to ≤ 3.5 (Figs. 1 and 3) and were consistent with the hypothesis that overall lactation effort was higher for mothers of sons than for mothers of daughters

Summary

Introduction

The length of maternal investment via lactation, is an important life history characteristic in mammals that has fitness implications for both infants and mothers (Lee et al 1991; Lee 1997; Kennedy 2005). Behavioral Ecology and Sociobiology (2022) 76: 44 behavioral and psycho-social aspects of nursing interactions (Sellen 2006, 2009; Hinde and Milligan 2011). These benefits likely apply to all mammals, since lactation is a derived adaptation that has not been lost in any extant lineage (Bronson 1989). Female mammals balance the extra metabolic requirements of maintaining milk synthesis throughout the lactation period allocated to current offspring (i.e., “lactation effort”: Lee et al 1991; Deutsch et al 1994; van Jaarsveld et al 1995) against their own somatic maintenance, remaining development (if they are not yet fully grown), and future reproductive investments (Trivers 1974; Clutton-Brock 1991; Hayssen 1993; Sellen 2006; Langer 2008). Continuing lactational investment in current infants often means that females delay their ability to conceive again (Bronson 1989); this can lengthen interbirth intervals and lower female fecundity (Lee 1997)

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