Why is paternal lactation commoner in birds than in mammals?

Lactation occurs in both birds and mammals, albeit by means of completely different anatomical structures and physiological processes (https://en.wikipedia.org/wiki/Crop_milk).

The analogy is strong, despite the weakness of homology.

However, there is an apparent paradox, as follows.

It is mammals in which lactatation is definitive to the Class. By contrast, lactation occurs only in three orders of birds (Columbidae, Phoenicopteridae, Spheniscidae).

Yet it is in birds that paternal lactation is more common.

In mammals, paternal lactation has been recorded in only three species, all bats (https://en.wikipedia.org/wiki/Male_lactation#:~:text=of%20this%20trait.-,Nonhuman%20animal%20male%20lactation,the%20nursing%20of%20their%20infants.), all belonging to Pteropodidae.

By contrast, in birds paternal lactation is recorded (or assumed) in all of the 344 species of columbids (https://en.wikipedia.org/wiki/Columbidae), all of the six species of phoenicopterids (https://en.wikipedia.org/wiki/Flamingo#Feeding), and one species of spheniscid (https://en.wikipedia.org/wiki/Emperor_penguin).

How can this anomaly be explained?

Let us set aside the case of the emperor penguin, because this species is ecologically and socially extreme, to a degree paralleled by no mammal, or even any other bird.

In the case of columbids and phoenicopterids, the breeding pair is monogamous, and the father lactates along with the mother. This degree of paternal feeding of the offspring does not occur in even the most extremely adapted mammals, such as Lycaon pictus (https://en.wikipedia.org/wiki/African_wild_dog), in which all adult male individuals in the group assist in the care of the litter of the breeding pair, but lactation is absent in males

My explanation is as follows.

In mammals, lactation tends to be associated with a gain in body mass. This is both in the mammae themselves, which tend to store milk to some degree between bouts of suckling (extreme in e.g. Crocuta crocuta), and in the gastrointestinal tract, which tends to hypertrophy in order to meet the lactational needs.

By contrast, in columbids, the parents tend to fast during lactation, and there is no analogue for a full udder.

This is consistent with a fundamental difference between the classes: in birds there is a premium on somatic lightness, in aid of extreme mobility (usually but not necessarily volant).

Because it is essential for birds to remain as light as possible, it is adaptive for the lactational burden to be shared between males and females.

Pteropodid bats are not as evolutionarily committed to mobility as are most birds. However, the fact that the few instances of paternal lactation occur in volant mammals would seem to support my rationale, in the sense of a kind of evolutionary convergence.

Posted on July 09, 2023 11:37 PM by milewski milewski

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