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Science - Education | Environment - Nature

"Hermaphrodite" hyenas give cubs hormonal kick-start

Hyena mother with cubs
«Jumpstart with hormonal jolt»

© afrol News / Kay Holekamp
afrol News, 28 April
- Some mothers have more resources than others, enabling them to give kids better chances to compete later in life. This is also true among the African spotted hyenas, researchers have found. High-ranking, dominant hyena mothers pass to their offspring high levels of certain hormones that make cubs more aggressive and sexually vigorous - in other words more likely to survive, thrive and reproduce, a new study shows.

Michigan State University zoology professor Kay Holekamp and her former graduate students have published an article in the latest edition of the science journal 'Nature', demonstrating that spotted hyenas alpha mothers - the dominant females in a hyena group - are packing a hormonal punch that gives their cubs a powerful head start. The study shows that alpha females have higher levels of androgen during the final stages of pregnancy than lower-ranking group members.

"What this means is that there are gifts a mom can give to her baby," said Ms Holekamp. "She can manipulate her offspring's behaviour and help her kids to survive and reproduce successfully by transferring status-related traits via prenatal hormone exposure."

"This research sheds light on mammalian reproductive biology and helps us imagine how evolution might have produced such a bizarre product," Ms Holekamp said. The findings result from nearly two decades of research spent studying wild spotted hyena populations in Kenya. The paper outlines the first instance where researchers have shown that a female mammal's hormones can influence her offspring's behaviour and appearance.

Androgen is just one of the many hormones travelling across the placenta to the developing foetus. This hormone mediates masculine characteristics like aggression, muscle development and male-typical sexual behaviour. But it is not just the male cubs that stand to benefit from the maculinising effects of androgens. Females gain just as much, the study shows.

Normally, when it comes to muscle mass, aggressive behaviour and dominance, males have a leg up, but hyenas take this norm and turn it on its head. "You don't find many mammals where the female is the boss," Ms Holekamp said.

The sex roles in spotted hyenas are completely reversed from those in most mammals: females are larger and more aggressive than males when competing for limited resources and dominate the 40 to 60 members of their social group. In fact, females look so much like males the average person may have trouble differentiating between the sexes. A female hyena's genitals have evolved into something that looks more like a penis than a vagina.

Ms Holekamp and her colleagues speculate that the behaviours attributed to exposure to high levels of prenatal androgens may be evolution's way of offsetting the negative consequences associated with mating and giving birth through a penis-like structure.

A spotted hyena's vaginal canal makes a hairpin turn and exits the body like a penis. The opening of the vaginal canal is at the end of an elongated clitoris, nearly six to seven inches in an adult, which looks remarkably like a penis, Ms Holekamp said.

Not surprisingly, ancient people, like the Greek philosopher Aristotle, thought hyenas were hermaphrodites. The characteristics of the genitalia are responsible for some obvious anatomical challenges that arise when it is time for a hyena to mate and give birth. Mating is tricky to say the least. A male must position himself at just the right angle to enter the female's clitoris.

If the match is successful, a mother hyena will give birth to her cubs through the elongated clitoris, which doubles its diameter for the occasion. "It is really weird genitalia, but it seems to work. Although giving birth through a 'penis' isn't a trivial problem," Ms Holekamp said. "All her female-typical behaviours are there – she has been masculinised without being defeminised."

The physical characteristics and behaviours handed down from alpha mother to their babies may be helpful in understanding how this unique genitalia evolved and why it works.

Mating that culminates in pregnancy and successful rearing of young is contingent on a female's ability to secure food resources. More aggressive females are better able to compete for food when hyenas squabble over carcasses of gazelles, wildebeest and zebras, which are their main prey, Ms Holekamp holds.

Young males exposed to higher levels of prenatal androgen exhibit mounting behaviours more often than the males born to lower-ranking females. In other words, they get more practice at the difficult art of hyena mating, Ms Holekamp said.

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