Sex differences in cell-mediated immunity in the Magellanic penguin Spheniscus magellanicus

Males in domestic and laboratory populations usually display a lower immuno competence than females due to differential action of steroid hormones, but information for natural populations is scant. Here we show that male Magel lanic penguins Spheniscus magellanicus had a significantly lower cell-media ted immunity at hatching of their chicks than females. This was assayed thr ough the phytohemagglutinin injection assay, a standard method of measuring T-cell-dependent immunocompetence in birds. Breeding phenology affected co ndition at laying and cell-mediated immunity at hatching for males, with hi gher values for late-breeding males. In females, condition at laying declin ed with breeding date, with no link with immunocompetence at hatching. Earl y breeding males may incur date-dependent costs in terms of condition and s ubsequent immunocompetence.