A milk-sharing economy allows placental mammals to overcome their metabolic limits

Maternal resource availability and metabolism have a strong limiting effect on reproductive output. Allomaternal care and domestication increase the energy available to the mother and should correlate with an increase in reproductive output. Here, we take a comparative approach to understand how this increase is accomplished (e.g., litter mass, reproductive frequency, etc.) and the strength of the effect among different forms of external energetic supplementation. We find that domestication and all forms of allocare correlate with increased fertility. All forms of provisioning correlate with larger litters without compromising offspring size. The greatest increase we observe in reproductive power is in species that practice allonursing. Our results suggest that the ultimate factor limiting reproductive output in placental mammals is maternal metabolic power rather than resource availability.