The post-natal environment in which young develop can substantially impact development, adult phenotype and fitness. In wild mice, competition among litter-mates affects development rate and adult behaviour. We manipulated post-natal litter size in a cross-fostering design to investigate the effects of enlarged and reduced litter sizes on sexual signalling, oxidative stress and the links between them. Oxidative stress causes somatic damage that can limit reproductive success and lifespan, and is predicted to mediate investment in life-history traits, including sexual signals. We predicted that litter enlargement would cause an increase in potential oxidative stress, inhibit growth and reduce sexual signalling in male mice. Males reared in enlarged litters were smaller at weaning and, despite rapid growth immediately after weaning, remained smaller at 10 weeks of age than those reared in smaller litters. Females from enlarged litters were consistently smaller throughout post-weaning development and showed no increase in growth rate compared with females from reduced litters. In enlarged litters, protein thiol concentration was lower at weaning in the liver and kidneys, with this trend continuing at 10 weeks of age in the kidneys only. Aconitase enzyme activity was also lower in mice from enlarged litters at weaning and 10 weeks of age in the kidneys. Male mice from enlarged litters scent marked more frequently and had larger preputial glands than those from reduced litters, indicating greater sexual signalling investment irrespective of this increased oxidative challenge. The results of this study are the first to reveal oxidative costs of developmental stress in small mammals.

In mammals, allocation to reproduction can either be primed or suppressed in relation to cues from other individuals. Some conspecifics (e.g. potential mates) may enhance an individual's ability to reproduce but others may have a detrimental effect on reproductive success. One widely studied response to conspecific cues, the 'Bruce effect', occurs when pregnant females abort their pregnancies after exposure to a novel male. It has been suggested that this response has evolved as a counter-tactic to the threat of infanticide posed by novel males. In some species, like mice, pregnancy termination will only occur if females are exposed to the unfamiliar male during a brief critical period early in pregnancy, which is surprising considering that an unfamiliar male threatens infanticide whenever present, and in particular near to birth. We demonstrate that female mice experiencing novel males during late pregnancy also alter their investment in progeny, but in a more subtle manner than previously observed. Females exposed to an unfamiliar male during late pregnancy give birth to offspring of a comparable weight to those produced by females exposed to the paternal male, but these offspring grow more slowly over lactation. As a consequence, offspring from these females weigh less at weaning. Modification of their growth trajectory, however, allows these offspring to catch up to normal weights by adulthood. Thus, cues of unfamiliar males, and possibly their associated threat of infanticide, can produce more wide-ranging effects on maternal investment than previously recognized.