The role of maternally transferred antibodies in maternal performance in red deer

Maternal effects are ubiquitous. Yet, the pathways through which maternal effects occur in wild mammals remain largely unknown. We hypothesise that maternal immune transfer is a key mechanism by which mothers can affect their offspring fitness, and that individual variation in maternally derived antibodies mainly depends on a mother's characteristics and the environmental conditions she experiences. To test this, we assayed six colostrum‐derived antibodies in the plasma of 1447 neonates in a wild red deer population. Neonatal antibody levels were mainly affected by maternal genes, environmental variation and costs of prior reproductive investment. We found consistent heterogeneity in maternal performance across traits, with mothers producing the heaviest calves also having calves with more antibodies. Unexpectedly, antibody levels were not associated with calf survival. We provide a unique example of how evolutionary theory on maternal effects can be used to gain insight into the causes of maternal effects in wild populations. Maternal immune transfer is a key mechanism by which mothers can affect their offspring. We used a long‐term individual‐based study to investigate the causes and consequences of variation in maternally transferred antibodies in a red deer population. For the first time in a wild mammal, we report that past reproductive effort negatively affected maternal immune transfer. However, neonatal antibody levels were not associated with calf survival.