Long-lasting effects of the presence of male siblings in utero on subsequent reproductive performance.

Laboratory studies with rodents indicate that in utero proximity of a female to male foetus can affect female's subsequent reproduction due to elevated testosterone exposure during early development. It remains unknown whether these findings can be generalised to non-laboratory species because the need for caesarean section makes it difficult to determine the intrauterine position outside laboratory conditions. As an alternative, some studies have compared the reproductive performance of individuals born in male-biased litters to those born in female-biased litters. We identified 44 of those studies in 28 viviparous species for a total of 176 relationships between litter sex composition around the time of birth and subsequent reproductive performance (fertility, fecundity, age at first reproduction, interbirth intervals or post-natal survival of offspring). Those relationships are discordant and complex both within and across species. Some factors can mask an actual association between litter sex composition and reproductive performance. Conversely, a part of significant relationships between litter sex composition and reproductive performance likely arises via pathways other than androgen- and oestrogen-transfer between foetuses of different sexes.