Last week, our French Bulldog, Hugo, woke up as if possessed. He acted wild and crazy and was overly assertive with his two much larger stepbrothers (American Bulldog and Coonhound x Rottie).
Our neighborhood morning stroll was like walking a whirlwind. Also Hugo urinated four times as much AND he lifted his leg for the first time ever! Hugo is castrated and 3.5 years old. Watching him lift his leg, I felt a certain amount of masculine pride and began to reminisce about my many years of researching the development of sexually dimorphic behaviors at UC Berkeley. I thought, there are probably a lot of dog owners who are unaware of the effects of testosterone and castration, and so was born this Monthly Woof.
Primary Sexual Characteristics
As with most mammals, male and female dogs differ in many ways. Sexual differentiation occurs during the first third of fetal development. In genetically female (XX) fetuses, the cortex of each gonad develops into an ovary and the puppies grow up as anatomical females that act like females. In genetically male (XY) fetuses, the gonadal medullae develop into testes that soon begin to secrete testosterone-like hormones that cause many permanent changes in anatomy and behavior.
Testosterone causes male fetuses to develop male genitalia. (In the absence of testosterone, female genitalia will develop as the default setting). Testosterone also affects the brain. In the absence of fetal testosterone, the hypothalumus retains its cyclic influence over the release of pituitary hormones, which in turn cause the cyclic release of ovarian hormones, giving rise to the adult female dog’s seasonal “heat”. In males however, fetal testosterone both de-feminizes and masculinizes parts of the brain, permanently suppressing the cyclic nature of the hypothalamus, which results in a permanent “on” switch in adulthood.
Additionally, fetal testosterone permanently decreases the adult likelihood of certain physiological changes and display of female mating behaviors, while permanently increasing the likelihood of male mating behavior in adulthood (the propensity to pursue and mount anything with a pulse).
Secondary Sexual Characteristics
Aside from the obvious primary sex differences of genitalia and mating behavior, fetal testosterone differentiates many other aspects of the brain and behavior. Thus, compared to female dogs, males strongly prefer female odors and do their urine-marking using the leg elevation posture. They tend to be higher-ranking in a pack and much more aggressive. These behaviors are sexually differentiated at birth and the differences become more pronounced as the animal matures.
In male dogs, blood levels of testosterone start to rise around four to five months of age and peak at about ten months. The rise in testosterone correlates well with the behavioral signs of puberty: sexual preferences, mounting behavior, leg lifting, olfactory preferences, marking, and aggression. After ten months of age, testosterone levels begin to fall and reach adult levels by 18 months of age. Thus, adult dogs have five to seven times lower testosterone levels than ten-month-old adolescents. This is unusual in the mammalian world, as are the effects of castrating male dogs.
In most male mammals, castration (neutering) fairly quickly reduces or eliminates primary and secondary sexual characteristics. but it has little, if any, effect on sexual orientation and olfactory preferences. These were permanently differentiated during fetal development. Castration decreases the fervor of sexual interest but not necessarily the vigor of mounting. On the contrary, some neutered males appear to mount more frequently and vigorously than they did prior to castration, or compared with non-castrated counterparts. The developmental and/or long-term effects of castration appear to impair the development of the “doodad,” hence it is more difficult to get it to do its business. But few castrated dogs give up trying... hence, the increased mounting behavior.
Castration does not appear to change the dog’s social rank and has little if any direct effect on reducing male-male fighting or dog-human aggression. Castration causes a reduction in urination frequency but does not resolve house-soiling problems. And castration does not eliminate urine marking or leg-lifting.
For most mammals, the increase in testosterone at puberty and high levels in adulthood are essential for the expression of sexual behavior and the manifestation of secondary sexual characteristics. Certainly, pubertal testosterone facilitates sexual development in male dogs, but it does not appear to be essential, as evidenced by the behavior of castrated male dogs, Hugo included.