Friday, March 15

Deadbeat Moms

Why some fathers get left holding the baby.

Scientists have cracked a 140 year old mystery as to why, for some animals, it’s the father rather than the mother that takes care of their young. Researchers from the Universities of Bath, Sheffield and Veszprém (Hungary) found that role reversal was caused by an imbalance in the numbers of males relative to females.

Another paper published in Nature. This is exciting stuff, as the press articles say, this has been a mystery "140 years old", which is to say, as old as Darwin's work, since it has always been an irrefutable fact that the males of some species do sometimes help rear children. Many researchers have grappled with this problem over the past century and a half. Earlier work tended to appeal to ecological or life history explanations. 

Ecological theories suggest that the physical habitat which the animals inhabit drives males to be involved in the rearing of young. A theoretical example (solely for the sake of discussion): an environment turns dry due to drought, resources are scarce, and if a male and mate succeed in producing live young, the male would be drawn to invest in the infant because the chances of successfully producing more progeny is severely diminished. His energy is better spent helping out. 

Similarly, a life history explanation would suggest that as males age, their ability to compete with other males for mating rights with multiple females decreases. The alternative is to monopolize a female's time, and invest energy is making sure a small number of progeny reach adulthood. While these explanations may seem entirely plausible, the evidence simply does not support these theories.

Until now, thanks to the Liker et al's new theory. In the article's original title, the authors only claim to have answered the question in birds. It may be that no primate species are affected, that is, that role reversals in primates are not caused by an imbalance in sex ratio. Then again, there aren't many primate species in which the male ever takes over rearing for the female.

In many species, the males stick around, but do not get involved. In baboons, the males (and usually fathers) are ever present, but they don't engage in child rearing. They will interact with babies on occasion, but that is a far cry from rearing. Species in which the males play an active role in raising infants: owl monkeys and, of course, humans.

Humans are an interesting case, and not just because all my readers are human. When modern human males take over the rearing of children, is it caused by an imbalance in sex ratio? Probably not.
But then again, humans are not a good study subject for this sort of question; the environments we inhabit are not "ecological". However, our close relatives, chimpanzees and bonobos, do not engage in role reversals (nor do other apes), so the particular type of sexual role reversal we've evolved may be unique.

Liker A, Freckleton RP, & Székely T (2013). The evolution of sex roles in birds is related to adult sex ratio. Nature communications, 4 PMID: 23481395


  1. I don't agree with your comment that, "While these explanations may seem entirely plausible, the evidence simply does not support these theories." Certainly with humans, there is substantial evidence that paternal involvement increases the reproductive success of progeny.

    Moreover, while I haven't read the paper yet, discussions like this always make me a bit uncomfortable. Finding "the reason" for a phenomenon like this isn't always possible. The evolutionary fitness function has a horrendously complicated, contingent structure. This complexity leads people into all sorts of problems; looking for a single fundamental reason for an observed trait is one of them; misunderstanding of genetic drift is another big one

  2. Well, like the authors say, the paper only supports the new model in bird species. Evidence beyond birds likely proved too difficult to obtain and analyze.

    And yes, determining the ultimate cause of an adaptation is always a tricky business, particularly in field studies where confounding variables are extraordinarily difficult to control. Nonetheless, at least in birds, there is evidence to back up the author's theories (for now).