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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?

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  • $\begingroup$ It's not easy to give a definitive answer ("...what is explanation for this?") for a reason something doesn't exist. Why don't we have 31% O2 in the atmosphere instead of 21? What's the reason twins don't occur more often than they do? Since flight is a good escape tactic, why did only one mammal develop true flight? The number of such questions are endless. The reason is, it is what is (evolution.) $\endgroup$ Dec 20 '21 at 0:25
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    $\begingroup$ Does this answer your question? Why do some bad traits evolve, and good ones don't? $\endgroup$
    – tyersome
    Dec 20 '21 at 0:30
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In the large fruit-eating bats Dyacopterus spadiceus and Pteropus capistratus the males are known to lactate. Wikipedia has an article on male lactation which mentions these species. However, as discussed in an article by Kunz and Hosken (1), it is unclear why the males of these species lactate. The biology and normal life history of these species is not well known, and apparently there are no reports of males actually suckling young bats. Kunz and Hosken discuss ways in which the male lactation in these bats might be adaptive even if no or little nutrition is provided to the young by the male.

(1): Kunz, T.H. and Hosken, D.J., 2009. Male lactation: why, why not and is it care?. Trends in Ecology & Evolution, 24(2), pp.80-85; [PDF]

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As @mgkrebbs mentioned in their answer, Dyacopterus spadiceus and Pteropus capistratus are the males speciees known to lactate. The Wikipedia link also mentions one more species - goats (Gómez et.al., 1999). In fact, male lactation has been observed in a few domesticated animals, including cats and guinea pigs but on rare occasion though it's not yet clear what is the mechanism to produce milk..

Moreover, Jennifer A Brunton and Thomas H Kunz of Boston University and their colleagues who studied the fruit bats in the Krau Game Reserve in Malaysia, suspect that the mammary glands of these bats contain an enzyme that converts testosterone into oestrogen -- a hormone that excites milk secretion. (source)

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