Evidence for a priming effect on maternal resource allocation: implications for interbrood competition.

It is commonly assumed that there exists interbrood competition mediated by in utero growth. This could be manifested by a female reallocating saved resources to future broods. Here, we report results of a manipulation experiment designed to detect such reallocation. Two groups of female mice were allowed each to produce two broods. In the first brood, the test females were mated with phenotypically normal males heterozygous for an insulin-like growth factor 2 (Igf2) null allele, while the control females were mated to a wild-type male. The test sample females invested 20% less into their first brood than did the control sample. In both test and control groups the females were mated with a wild-type male in the second round of mating. Surprisingly, we found that females that invested little into their first brood also invested little (compared with other second broods) into their second brood. This 'priming' effect suggests that the assumptions of classical models of parent-offspring conflict are overly simplistic but cannot disprove the existence of interbrood competition.