Testosterone is the major hormone associated with the behavioural patterns exhibited by stallions. However, other hormones, such as dihydrotestosterone, do have a contributory effect. Both these hormones drive the normal libido of the stallion, possibly via their conversion to oestradiol and, through this, exert an effect on the higher centres of the brain (Johnson, 1991a). In addition to these two hormones, GnRH apparently directly affects behaviour, especially at the time of mating. At the sight of a mare, or at sexual stimulation, the frequency and amplitude of GnRH pulse release increase. The result is twofold: an increase in LH and FSH pulse frequency and amplitude, leading to a rise in testosterone secretion and a corresponding enhancement of libido (Irvine and Alexander, 1991); and a direct effect on the higher centres of the brain, stimulating the CNS and so acting as an additional drive to mating behaviour (Pozor et al., 1991) (Fig. 3.26).
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