Milking it

Evolutionary biologists have long pondered the origin of lactation in mammals: converting food into milk is energetically expensive and can consume up to 25 per cent of the food's original value. But it is ubiquitous amongst mammals although absent in birds. A new study Proceedings of the Royal Society series B (published online), has modelled the potential benefits of lactation. They have contrasted the performance of mothers that provide milk for dependent offspring from their reserves compared with otherwise equivalent mothers that feed their offspring directly from the food they find. The researchers find that the selective advantage to lactating mothers, which can feed at a cost without having found food recently, can be substantial when food supplies are risky and there are few opportunities for future attempts at reproduction under a wide range of circumstances. Therefore it is likely that unreliability associated with the lifestyles of small, primitive mammal-like reptiles that evolved extended maternal care, selected for fully-fledged milk production and consumption, prompting the evolution of true mammals. The researchers also believe that coping with unreliable food access during food provisioning may underlie key life-history differences between birds and mammals because the constraints imposed by flight restrict the level of reserves that mothers can carry and utilise. The authors believe they have provided the first formal demonstration that lactation offers substantial selective advantages over providing food found directly when supplies are risky, despite the inefficiencies involved. " This suggests that the evolution of lactation, a definitive mammalian feature, may have been selected for by the insectivorous lifestyle of oviparous mammal-like reptiles that evolved extensive maternal care, " the authors report. Helping hand: A new study suggests that lactation, despite its energetic costs, may have been a crucial feature in the development of mammals and their lifestyles.