Great tit (Parus major) males, when confronted with a source of danger that is either classifiable or inclassifiable and close to their brood, mob more than females. By staying near the source of danger longer, by mobbing it longer and more vigorously, and by approaching it more closely, males incur a greater risk than females do and therefore, appear to behave more altruistically. Since the only two accepted functional explanations, i.e. kin selection theory and self-estimation of reproductive value, fail to account for the extent and nature of this sexual difference, a new, as yet unknown determinant is postulated. It is conceivable that this cost factor operates in conjunction with self-estimation of reproductive value.