Effects of anabolic steroids on behaviour in mares

Abstract

The effects of the anabolic steroids, methandriol, testosterone and boldenone on behaviour was studied in two mature mares. Behaviour was assessed using an all-instances scan sampling technique and any interactions either active or passive occurring between the treated mares and the other members of the herd were recorded. A control period was used to determine the mares' normal behaviour and their social rank within a herd of twenty two horses. In the first experiment, methandriol was administered intramuscularly, once only, at the maximum recommended dose (300 mg) to each mare. At this dose rate, both mares showed a significant (P<0.001) increase in abnormal (slightly aggressive) behaviour towards the other horses in the herd. However, this behaviour was not aggressive enough to cause an increase in the mares' social order within the herd. In fact, one mare decreased her social rank to become the lowest ranked horse. Therefore the androgenic effects of methandriol in these two mares were minimal. In the second experiment, testosterone was administered intramuscularly to each mare at a moderate dose (20 mg) every second day. This dose rate caused a highly significant (P<0.0001) decrease in the aggressive activity which other members of the herd displayed to the two treated mares. This showed an indirect form of aggression by the two treated mares. However, again contrary to what was expected, no direct form of abnormal aggressive or masculine behaviour was displayed. None of the effects were great enough to cause any change in social position (ie, it remained the same as it was at the conclusion of the methandriol trial. In the third experiment, boldenone undecylenate was administered to each mare intramuscularly as a single dose rate of 1 mg per 100 kg of body weight. As with testosterone, this caused a highly significant (P<0.0001) decrease in the aggressive activity other members of the herd displayed to the treated mares. With boldenone undecylenate, mares also exhibited 'male like' behaviour including teasing, knickering, herding and trying to mount oestrus mares. Boldenone undecylenate caused enough extreme abnormal behaviour to cause both treated mares to increase their social rank. Therefore the behavioural effects of various anabolic steroids vary and before their use, knowledge of their effects must be considered so that appropriate management and handling can be performed.