Testicle size and social structure may be directly related. (CC image: ben pollard)

Evolution Happens

March 30, 2010 | 1 Comment

Category: Biology, Social, Social Structure, Society

Some passages from Endocrinology of Social Relationships, edited by Ellison and Gray.

“Not surprisingly, males of pair-bonding bird species have been shown to undergo an endocrinological shift to lower testosterone levels in parallel with the behavioral shift from territorial defense and mate attraction to parental behavior. Manipulations that evoke territorial responses in nesting males, such as playing the song of an invading male, both undermine parental behavior and lead to an increase in testosterone….Recently evidence has even begun to accumulate suggesting that lower testosterone levels may be typical of human males in stable mating relationships and perhaps even lower levels in men who are fathers of infant children.” (p. 70)

“…This led to the ‘challenge hypothesis,’ which states: high plasma levels of testosterone occur during periods of social instability in the breeding season (resulting from male-male competition for territories and mates) but are at a lower breeding baseline in stable social conditions thus allowing paternal care to be expressed.” (p. 83)

“Furthermore, there is growing evidence that patterns of testosterone in tropical species that may have long breeding seasons are very different from northern species (Goymann et al., 2004). Tropical species with long breeding seasons tend to have extremely low levels of testosterone that generally do not change markedly with social challenges.” (p. 85)

In 1998, I hypothesized that when there are great apes with larger testicles, suggesting males competing with other males in an environment characterized by female choice, with sperm production becoming more important than muscle mass, the testosterone levels would decrease even though the testicles were larger. Testes produce both sperm and testosterone. I calculated that an emphasis on one would diminish the other. Gorillas have small testicles and patrifocal male control of procreation. Bonobo have large testicles with a matrifocal, horizontal social structure.

The passages above suggest a relationship between testosterone production and social structure. Even testosterone fluctuations within an individual over time suggest that different procreation strategies are accompanied by different testosterone levels. If male testosterone levels are instrumental in the choices made at any time regarding degrees of cooperation or family orientation, and testosterone levels inform maturation rates, then there is a direct connection between social structure, maturational delay and acceleration.

For reasons I do not really understand, there seems to be little academic attention directed toward the possibility that testosterone manages rates of maturation. Testosterone is associated with handedness, and left-handedness is associated with low testosterone. Left-handedness is associated with maturational delay. Yet, testosterone is rarely visited as related to maturational acceleration and delay. Even further from the minds of theoreticians is the possible influence of estrogen on the timing of the rates of maturation. Grasping that seems to require an understanding of how maturation rates change under the influence of testosterone.

Human and nonhuman endocrine systems are moved by countless different variables in turn influenced by myriad environmental effects. Nevertheless, it seems central that social structure, which deeply influences evolution, is guided by a balance between testosterone and estrogen levels. These levels change according to the season, the environment and the circumstances of life. As these changes occur, maturation rates and timing transform and evolution happens.


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