In 1945, as Allied troops tightened the noose around Hitler’s Germany, they also began the long overdue process of liberating POW camps. What they found shocked them. A long-term diet of black bread and thin soup had taken its toll on the bodies of the prisoners. The skin taut on their shorn skulls was yellowed with jaundice. They looked like walking skeletons, swimming in a fetid mass of grimy overcoats and whatever other scraps of clothing they had managed to cobble together over the months and years of imprisonment. GIs wept at the sight.
Internally, prison had done major damage to the organs of the prisoners. Their testicles, liver, and pituitary glands had shrunk and atrophied. Returned to the West, and nursed back to health by the dedicated staff of Allied hospitals, their bodies began to regain normal function. First the testicles (of course), then the pituitary. Hair regrew, and wasted muscle and depleted fat stores began a slow return to normal. But in some of the former prisoners, a strange phenomenon began to occur. Some of the men began to lactate.
They were, at first, understandably confused. But doctors quickly sussed out the cause. The loss of testicular and pituitary function during the long period of malnutrition had resulted in vastly decreased hormone levels. Just out of the prison camps, the men had very low levels of testosterone, estrogen, and other sex hormones. The rapid replenishing of their food stores caused their body to become flooded with newly produced hormones, trying to make up for the deficiency. However, the liver was still on vacation, and so excess hormones were not being metabolized. The higher-than-normal levels of estrogen in the former prisoners triggered lactation in some of the men. As liver function returned, their hormone levels normalized and the lactation eventually ceased.
Male lactation can occur, it’s just rare. Male mammals generally have the hardware for it: particularly in primates, sexual differentiation of the mammary glands doesn’t occur until puberty – before that, we’re all the same. And lactation can be induced in some male mammals (including humans) by stimulating the nipples, or by close contact with infants and lactating women. If the hormone balance of a male is altered slightly towards a more “female” complex, males will start to lactate. So if it’s so easy, why don’t they do it more often?
Lactation is a hallmark of mammals. Few parents are as dedicated as mammal mothers, which produce milk for their infants at a tremendous energetic cost to themselves. Most studies estimate that milk production is the most energetically demanding part of infant-rearing in mammals: up to a third of a mother’s daily calorie intake goes towards milk production. Researchers hypothesize that it evolved as an adaptation to deal with variable environments. If you don’t know whether your child is going to be born into a world where food is bountiful, or a drought-plagued wasteland, then its best to have an independent food source. By lactating, female mammals ensure that their offspring have a supply of rich, nutritious food early in life – and the quality of milk produced by a mother correlates with the survival and growth of that infant far later on in life. If males have the hardware for lactation, why don’t they help out? If infants could have twice as many food sources from a young age, it would decrease the burden on each parent, and increase the infant’s success. It would also allow female mammals to have larger litters, which would increase the reproductive success of both mother and father.
Unfortunately, most male mammals are bad dads. In over 90% of mammal species, the male makes only one contribution to their infant – the sperm that fertilizes the egg. Then they’re out the window and heading for the hills. Evolutionary biologists hypothesize two reasons why males don’t lactate. The first is the ease of desertion. Because fertilization in mammals is internal, one of the sexes must carry the gestating fetus to term. In mammals, this has evolved to be females – which makes it very tricky for a pregnant mammal to leave her baby behind, and just “duck out for a pack of cigarettes.” Male mammals, on the other hand, can desert a female easily after fertilization – usually so he can go and chase other females.
The second hypothesis is paternity uncertainty. Again, because fertilization is internal, the males are never 100% sure whether that new bundle of joy is their baby, or if it belongs to that hunky primate sun-bathing lasciviously in the next tree over. This is especially true of promiscuous animals, where males and females have both mated with multiple partners during a mating season. In this case, males are unlikely to invest in the energetically demanding act of lactation when there is a chance that the effort is being wasted on another male’s child.
But these hypotheses allow us to predict a situation where male lactation could evolve as a good strategy. In a paper published in Trends in Evolution and Ecology, Thomas Kunz and David Hosekn did just that. They hypothesized that in an animal species that mates monogamously (genetic monogamy, not just social monogamy), and in which the male has limited opportunity for pursuing…extra-curricular activities, it would make biological sense for the father to lactate and handle a share of the feeding duties. Because the hormone profile needed for lactation has other effects – slight decreases in muscle mass and aggression – this would also need to be a species with limited male-male competition.
Meet the Dayak fruit bat. The Dayak fruit bat is a rare species of bat, native to Peninsular Malaysia and Borneo. It’s a small, fig-loving, and rarely studied bat. But it has one curious trait that sets it apart from its relatives. The males lactate. The family life of the Dayak fruit bat is little studied, but they appear to be monogamous, and researchers hypothesize that male lactation has evolved in them to relieve some of the female’s burden, just as we hypothesized our imaginary animal should behave.
There are still a lot of questions left to answer before we know for certain whether the Dayak fruit bat meets the requirements of our hypothetical lactating dad. How often do they lactate? Do all males do it? There needs to be paternity testing to determine just how monogamous they are, and there is a plausible alternative hypothesis that needs to be investigated: that they lactate because of the high phytoestrogen levels found in their preferred foods. But it’s entirely possible that Dayak fruit bats are the super-dads of the animal kingdom (although admittedly, not a difficult title to gain).
Daly, M. 1979. Why don’t male mammals lactate? J. Theor. Biol. 78:325-345.
Kunz TH and DJ Hosken. 2009. Male lactation: why, why not and is it care? TREE 24: 80-85.