No evidence for differential maternal allocation to offspring in the house sparrow (Passer domesticus)

We tested the differential maternal allocation hypothesis in a population of house sparrows. We experimentally altered the attractiveness of males by treating them with implants filled with crystalline testosterone (T) or left empty (C). We subsequently monitored maternal investment as a function of male hormonal treatment and the size of the black patch of feathers on the throat (i.e., the badge), a sexually selected trait. The differential allocation hypothesis predicts that females should adjust their investment with respect to the benefits they receive by mating with an attractive male. Given that both circulating levels of T and badge size are condition-dependent traits, we expected that females mated with T males and/or with large-badged males should invest more into current reproduction. Contrary to this prediction, we found no evidence that suggested differential maternal allocation in this population of house sparrows. Female investment in yolk T, yolk mass, clutch size, chick brooding, and feeding was not affected by male hormonal treatment or by male badge size. As expected, T males invested less into chick brooding and feeding. More surprisingly, females did not compensate the reduced paternal contribution to chick feeding. As a consequence, the breeding success of T pairs was largely reduced compared with that of C pairs. The absence of differential allocation in a system in which it could have an adaptive role raises the question about the possible constraints or overriding factors operating on patterns of reproductive investment in this species.