Allonursing, the nursing of another female’s offspring, is assumed to impose a substantial energetic cost given the high cost of lactation to mothers. However, these costs have not been quantified. In cooperatively breeding mammals where helpers contribute to lactation, they might be expected to modify their behavior to mitigate these potential costs. Here, we show that overnight weight loss during lactation did not differ between allonurses and controls. However, meerkat helpers that allonursed do not gain weight over a reproductive bout as non-allonursing subordinate females did, suggesting that allonurses may incur some cost. Allonurses may mitigate the costs by increasing foraging effort during lactation. Allonurses do not, as expected, reduce investment in other cooperative behaviors during lactation. We suggest that the increase in cooperative behavior, including allonursing, may serve a social function, but further work is needed to confirm this hypothesis.