Why don't male mammals lactate?

Why has male lactation never evolved? This question is discussed with attention to (a) the physiological and ontogenetic modifications that such an evolutionary development would require, and (b) the reproductive strategies of monogamous mammals and the costs and benefits of extant forms of parental investment. Functional male lactation would require changes in sexually differentiated ontogenetic processes at both prepubertal and circumpubertal stages, as well as some male analogue of lactogenic events in late pregnancy. None of these modifications seems impossible, but together they constitute a formidable barrier to the evolution of male lactation. Moreover, it is by no means clear that such an evolutionary development could enhance its bearers fitness. Other factors than maternal lactational capacity evidently limit the reproductive potential of monogamous mammals. Much more information is needed on the economics of mammalian reproduction.