Fig. 4: Pups gain less investment from a partner mother than expected from female food intake, leading to poorer growth among less competitive younger pups in communal nests.

a Relative investment gained from own mother according to the mother’s food intake relative to her partner (a measure of relative milk production) by pups in sister (red) and unrelated (blue) litters (dashed line shows expected investment based on own mother intake). b Relative investment gained per pup from a randomly assigned focal vs other mother, corrected for the bias expected from relative female food intake (see Table 1). The randomly assigned focal mother was either the pup’s mother (green) or partner mother (yellow). Effect of communal litter size (competition) on the proportion of milk gained from the partner mother minus the proportion expected from female food intake for pups in first-born (c) or second-born (d) litters in sister (red) or unrelated (blue) pairs. Increasing competition reduced the proportion of milk gained from unrelated partner mothers in second-born litters (d, blue regression line). e In first-born litters, pups fed by sisters (red) gained more weight per g female food intake due to greater food intake among unrelated females (blue). f In second-born litters, pups that gained less partner milk than expected from female food intake gained less weight across both sister (red) and unrelated (blue) nests. Data are means per litter (large open circles) or median ± IQR boxes with whiskers showing full range, and individual data points (small filled circles; n = 44 sister and n = 41 unrelated pups in first-born litters; n = 33 sister and n = 40 unrelated pups in second-born litters). Corresponding statistical analyses are shown in Table 2 (a, b), Table 3 (c, d) and Table 4 (e, f). Regression lines shown where there was a significant relationship (solid lines).