Helpers increase the reproductive potential of offspring in cooperative meerkats

Abstract

In both animal and human societies, individuals may forego personal reproduction and provide care to the offspring of others. Studies aimed at investigating the adaptive nature of such cooperative breeding systems in vertebrates typically calculate helper 'fitness' from relationships of helper numbers and offspring survival to independence. The aim of this study is to use observations and supplemental feeding experiments in cooperatively breeding meerkats, Suricata suricatta, to investigate whether helpers influence the long-term reproductive potential of offspring during adulthood. We show that helpers have a significant and positive influence on the probability that offspring gain direct reproductive success in their lifetimes. This effect arises because helpers both reduce the age at which offspring begin to reproduce as subordinates and increase the probability that they will compete successfully for alpha rank. Supplemental feeding experiments confirm the causality of these results. Our results suggest that one can neither discount the significance of helper effects when none is found nor necessarily estimate accurately the fitness benefit that helpers accrue, unless their effects on offspring are considered in the long term.

Figure 1

Effect of helper-mediated pup weights at independence and supplemental feeding on lifetime reproductive probability of offspring. Offspring that were (a) reared heavy at independence (GLMM: χ₁²=5.87, p=0.015), and (b) supplementally fed during development (GLMM: χ₁²=6.94, p=0.008) were significantly more likely to gain reproductive success in their lifetimes. Figures show predicted means (±1 s.e.) for the sexes separately (overall sex effect, p>0.3). Effects of group size (p<0.05) were controlled in (a), while repeated measures were controlled in (a) and (b) (random terms: litter identity, p<0.05; mother identity, p>0.5; and group identity, p>0.9). Values show number of individuals.

Figure 2

Effects of helper-mediated pup weights at independence and supplemental feeding on underlying offspring life-history traits. Helper-mediated weight at independence was positively associated with the probability that: (a) offspring attempted to breed early as subordinates (GLMM for females, χ₁²=7.10, p=0.008; GLMM for males, χ₁²=4.67, p=0.031); (c) males (χ₁²=5.18, p=0.023), but not females (χ₁²=1.55, p=0.21) will have dispersed by 2 years of age; and (e) offspring competed successfully for alpha rank (Paired t-test, t₂₂=2.40, p=0.025). Supplemental pup feeding was positively associated with: (b) the probability that offspring attempted to breed early as subordinates (GLMM for females, χ₁²=8.38, p=0.004; GLMM for males, χ₁²=5.19, p=0.027); (d) the probability that males (χ₁²=3.11, p=0.078), but not females (χ₁²=0.02, p=0.88) dispersed by 2 years of age; and (f) the proportion of individuals that competed successfully for alpha rank (G-test=9.47, d.f.=1, p=0.002). Values show number of individuals. (a–d) Show predicted means (±1 s.e.), after controlling for any significant effects of group size and random terms (litter identity (p<0.05), mother identity (p>0.5) and group identity (p>0.9).

Figure 3

Weight at independence and its maintenance into adulthood. (a) Weight at independence is positively correlated with weight at 2 years (REML: χ₁²=36.29, p<0.001). (b) Pups that were supplementally fed during development are heavier at independence (REML: χ₁²=24.11, p<0.001), 6 months (χ₁²=6.79, p=0.005), 12 months (χ₁²=5.11, p=0.027), and 18 months (χ₁²=3.61, p=0.057). Figures show predicted means (±1 s.e.) after controlling for average group size between independence and 2 years (a) and random terms (a and b) (litter identity, p<0.05–0.01; mother and group identity, p>0.9). Values show number of individuals.