Question

It seems to me that an evolutionary path where not only females, but also males could feed the offsprings is entirely plausible: the males anyway have niples, so it is easy for the offsprings to confuse them for female nipples, and depending on hormonal background, some males are even capable to produce fluids with milk glands (even some humans). So, such capacity in males theoretically could improve survival chance. Also, this would somewhat relieve the females who contribute a lot of resources to the offsprings during pregnancy while the males in some species do essentially nothing except insemenation and/or fighting between themselves, thus wasting resources.

Given these speculations, if there is no such species with this trait, what is explanation for this?

Answer 1

The absence of lactating males in species is a result of sexual selection, where males evolved traits to signal genetic fitness rather than directly nurturing offspring. Monogamy presents exceptions, with significant male parental care. Overall, reproductive strategies and physiological roles typically delegate lactation and direct nurturing to females.

Step-by-step explanation:

The absence of lactating males in most species, despite the theoretical plausibility of such a trait, is primarily due to sexual selection and reproductive strategies that differ between males and females. In many species, males develop elaborate ornaments or engage in competitive behaviors as part of sexual selection to signal genetic fitness to potential mates, such as the good genes hypothesis and Fisherian runaway models suggest. This kind of sexual dimorphism is often associated with a variance in reproductive success among males, thus shaping the evolution of traits that may not benefit individual survival but maximize reproductive success, as explained by the handicap principle.

Monogamous systems offer an example of a different reproductive strategy, where males may provide significant parental care, such as in many bird species. This can be explained by theories like the mate-guarding hypothesis, the male-assistance hypothesis, and the female-enforcement hypothesis. But in most cases, male investment tends to be less direct in offspring nurturing and more focused on mating opportunities and competition with other males. Furthermore, physiological differences between males and females often mean that investment in offspring via lactation is typically the female's role due to her biological investment during pregnancy and the nutrient-rich composition of the eggs in oviparous species.