It is known that the sexual attributes of men are important for their ego.  The bigger it is, the better!  Still, big is not always a good sign.  An American study for the least provocative suggests that men with large testicles become less good fathers than those with small ones.  Evolutionary biologists have been observing for years that male primates who often mate to promote the next generation do not spend significant time with their offspring.  For example, a gap exists between the behavior of gorillas and those of chimpanzees towards their progeny.  Chimpanzees have testicles twice as large as those of humans, and therefore, since they produce a large amount of sperm, they can procreate significantly.  However, they do not make any effort to spend time with their descendants, and even ignore them.  On the contrary, gorillas, having smaller testicles, and not producing much seed, spend significant time caring for their young pups and are very careful.  It is precisely this behavior that American scientists wanted to test in humans.  To conduct their study, the researchers selected a group of 70 men, fathers of babies aged one to two years, and measured the volume of their testicles and brain activity at the sight of their children, as well as their level of the male hormone, testosterone.  In a parallel survey, the spouses evaluated the subjects’ paternal behavior and their willingness to care for their babies.  The results are similar to those in primates: they show that the most caring fathers who look after their children have a lower testicular volume and also have lower levels of testosterone.  Men whose paternal quality was assessed as weak have larger testicles, but lower brain activity at the sight of a picture of their offspring.

We should note that testes are often used as an object of observation for several scientific studies dealing with evolution, development or reproduction.  For example, a few years ago, a study suggested that the outer position of the testes was determined by evolution.  Scientists have been able to understand that the outside temperature influences sperm, and thus the gender of the offspring.  In reptiles, it is known that temperature is decisive for the gender.  Small alligators that grow at or below 30 degrees become females, while eggs heated to over 30 degrees become males.  In mammals, a gene specifically located on the male Y chromosome can act as a switch and block the development of a male in favor of a female.  However, the temperature at which the testicles, and thus the spermatozoa, are kept, also seems to be a considerable factor in the determination of sex and this is also perhaps the reason why the testicles are located outside the bodyIndeed, in humans, statistics show that male births predominate in the tropics.  In order to slow the development of the males, the evolution would have placed the testicles on the surface of the body, to keep the sperm cold.


  1.  Mascaro JS, Hackett PD, Rilling JK (2013).  Testicular volume is inversely correlated with nurturing-related brain activity in human fathers.  Proc Natl Acad Sci 110 (39) : 15746-51.
  2.  MacLachlan JC, Storey H (2003).  Hot male: can sex in humans be modified by temperature?  J Theor Biol 222 (1) : 71-2


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