This is the second article of a two-part dvm360 series focusing on neutering and the effect of hormones on the development of both physical and behavioral concerns ((see
the Related Links section below for a link to part 1). We will explore the relevant data demystifying the connection between
hormones, behavior and health.
Much attention has been given to the role of sex hormones and behavioral concerns in cats and dogs, but there is surprisingly
little data on the subject. The most common stereotypes involve testosterone in intact males. It may be appropriate to view
testosterone as a behavioral modulator that could help bring about or escalate the aggressive state.1 An intact dog may react more easily, escalate any response more quickly, plateau at a higher level of reactivity, return to baseline at a slower
rate and possibly alter his baseline to a higher level than would a neutered dog.
That said, both neutered and intact dogs may exhibit behavioral concerns, and only some of these concerns are affected by
sexual dimorphism. Dimorphic behaviors associated with the presence of testosterone include urine marking with lifted leg,
roaming and some types of mounting. Mounting is an unclear issue because it occurs in a number of contexts, most of which
are not sexual, and both sexes do it. Castration results in an androgen drop within six hours, and most hormones that decrease
do so within 72 hours.2
In the 1976 Hopkins study, which looked at male behavioral patterns and the effect of castration, roaming decreased by approximapattely
90 percent, male-to-male aggression by approximately 75 percent, urine marking by approximately 60 percent and mounting by
80 percent in male dogs that were neutered. However, marking, mounting and fighting are complex behaviors not wholly controlled
by hormones. There is a significant learning component involved in practicing these behaviors that won't be redressed by castration.
Context also matters, so space and social interactions need to be considered. Finally, no distinction is made between normal
and pathological behaviors, and one might expect castration to affect them differently.
Researchers have paid less attention to the role of female sex hormones and aggressive behavior, but one study has made a
connection within a fairly restricted population.3 Only female puppies that were already showing signs of "dominance aggression" became worse after spaying. Spaying had no
effect on any other age and behavior group combinations.4 It is possible that for these young, aggressive bitches, sex hormones play a helpful role in modulating their reactivity.
If these dogs are in responsible households where breeding is prohibited, allowing them to have a heat cycle may be beneficial
in terms of ameliorating their aggression, but there is little data on this.