Difference in behavioral effects of hormones on thyroid functioning in males and females. A comparison between humans, chimpanzees, and bonobos.

Bonobos stay young longer

Following my somewhat vague speculations in my last post, I found several articles discussing emerging research in behavioral genetics about the significance of sex hormones in affecting thyroid development and contributing to notable behavioral differences between sexes.  There is also speculation that such neurochemical differences may also exist in the brains of other gendered individuals, though experimental evidence of this is currently lacking.

In this particular article, a key difference in the role of the thyroid hormones Triiodthyronin (T3) and Thyroxin (T4) was found to play a role in several behavioral differences between male and females in the closest relatives to humans; chimpanzees and bonobos.   While chimpanzees and bonobos experience a similar setting and behavioral patterns early in life, in adulthood differences in thyroid hormones indicate the role the thyroid plays in regulating behavior.   The major difference between bonobos, chimpanzees, and humans is that bonobos continue to have high levels of thyroid hormones after puberty, where instead chimpanzees and humans both experience a reduction in hormone levels. 

As a result of elevated levels of hormones in adulthood:  “Male bonobos are less aggressive, engage in lasting friendships with females and receive life-long support from their mothers. In contrast, the social network of male chimpanzees consists of a mixture of male-male cooperation and aggressive behavioural [sic] strategies in males that aim on gaining and maintaining high social status.  The consequence is that the two sister species live in different social systems.”

Further, male bonobos were found to have even higher thyroid levels than females, and that female bonobos preferred “thyroid hormone ‘doped’ mates.” Speculating from my own knowledge, It is perhaps possible to speculate then that trends in natural selection differ between bonobos, chimpanzees, and humans (since it plays a role in mate selection).  Therefore, trends of male dominance are genetically selected by the process of evolution in humans and chimpanzees.     

I would further speculate that even though non-humans arguably do not possess metacognition, as humans do, the role of metacognition is likely offset significantly by neurochemistry and the role of the unconscious.  Take for example, the thyroid’s known influence on attention deficit hyperactivity disorder (ADHD), and the vital role psychotropic medicine plays in moderating ADHD.

Sources:
Bonobos Stay Young Longer
Behavioral Issues with Thyroiditis