Hyenas: the why of female masculinization (Part 2)

This healthy cub apparently survived the arduous birth passge & possible attacks of a sibling

This healthy cub apparently survived the arduous birth passage & possible attacks of a sibling. Click to enlarge

The disadvantages for the female hyena of having a masculinized urogenital tract when giving birth are clear (see part 1) but are there advantages for the hyena’s society of a having a masculinized and very dominant female?

The “why” question implies that there was a reason for this adaptation. Looking for a reason in adaptation (teleology) incites fierce debates which I am not equipped to get involved in. A well-known example of this “why” reasoning is the one that the giraffe developed a long neck to be able to reach the leaves of tall trees. But Darwinian teaching stresses there is no purpose behind  mutations. They are chance events & the changes it brings may be advantageous for the animal in its present living conditions.  Thus it is more able to cope & survive, and this mutation is passed on.  Thus it is “survival of the fittest” by gradual small changes allowing optimal adaptation to changing conditions.

Thus to return to the masculinized female genitalia it is hard to argue that this adaptation developed to allow the fittest to survive. The frequent obstruction in the very narrow genital tract produces a very high maternal & newborn mortality rate!

Steven Gould & Richard Lewontin in a famous paper in 1979 used an architectural analogy to explain this strange adaptation. In cathedrals certain structures called spandrels are spaces in the ceiling that exist as a byproduct of need to support essential building elements. They are often highly decorated but they were produced by architectural constraints when building vaults and not because the spaces were needed to place ornaments in. Thus the two scientists they argued that in evolutionary adaptation one cannot just look at individual changes in the animal. Some of the changes may just be spandrels. They are the frills that accompany the essential evolutionary changes. In fact sometimes these individual changes can be a serious disadvantage i.e. masculinized genitalia. It is the total sum of the advantages & disadvantages of all the changes e.g. anatomical, biochemical, social, produced by a mutation that will decide whether it is a successful adaptation and the animal more suited for his environment.  But the mutation may also require the animal to carry extra baggage filled with disadvantageous features e.g. masculinized genitalia.

So what are the possible advantages for a species of having heavily masculinized females? Are there other reasons that would increase their chances of survival despite the difficulties at birth? Various theories have been proposed.

  1. Female dominance & aggression: – In lion society the large male are dominant. They eat first at kills, with females getting the left overs. Thus when game is scarce the females are not well nourished & their milk supply may dwindle. Furthermore if a new lion coalition takes over a pride the existing cubs are often killed & the females come into heat to breed again. Thus the new leaders’ blood line is propagated. On the contrary in hyena society the masculinized female is larger than the male and is dominant. Thus at a kill she and her cubs eat first.  She is also fully capable of  protecting her cubs from paternal infanticide
  2. Siblicide: – This theory suggests it is best to get rid of your potential rival, particularly if female as soon as possible. Hyena pups are typically born in pairs & their aggressive behavior is apparent as soon as they are born. They have advanced motor development & fully erupted teeth at birth, & fights break out immediately with bites to the back & shoulders followed by shaking. The weaker sibling often dies from the wounds or ensuing starvation. The victor will of course now have the optimal conditions to grow up in.

    Hyena war & peace

    Note the large wound on the cub’s neck & the pseudo-penis & pseudo-scrotum of the mother. The cub is approaching the visible right teat (one of two in the spotted hyena). (click to enlarge)

In part 3 we will discuss the “how” of female masculinization and look at the ins & outs of testosterone metabolism in general.

Reference: –

1. Francis RC. 2004. Why men won’t ask for directions. The seductions of sociobiology. Princeton University Press.

2. Gould SJ, Lewontin R. The spandrels of San Marco & the Panglossian paradigm: a critique of the adaptionist programme. Proc R Soc Lon 205:581:1979

 

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