EXTEROCEPTIVE FACTORS IN SEXUAL PERIODICITY

Abstract

Summary 1. In all mammals and probably in all vertebrates there is an internal endocrine sexual rhythm with alternating periods of rest and activity. This cycle is usually adjusted to external seasonal changes and with some marked exceptions spring is the period of greatest sexual activity, but there is much specific variation and among ruminating mammals breeding is generally in autumn. 2. Among tropical and subtropical animals breeding is not nearly so restricted to definite seasons, and there is evidence that ecological factors play a part in its recurrence. 3. In individual animals which are made to cross the equator there may be two seasons of sexual activity in the year and the times of breeding are eventually reversed though the original endocrine rhythm may be partly maintained for some time. 4. Experiments with light and ultra‐violet irradiations indicate that with the majority of vertebrates exteroceptive stimuli dependent upon these processes are of primary importance in adjusting the cycle to changing periodic environmental conditions. 5. The evidence shows that the factors in question act through the intermediation of the central nervous system and the anterior pituitary body. 6. The cycle is liable to considerable modification by exteroceptive stimuli arising from the relations between the sexes and between the mother and her offspring. Thus ovulation in various species of vertebrates may depend upon stimuli normally arising from the male but which may be imitated by experimental methods. 7. Sexual display and courtship phenomena among various classes of animals, while undoubtedly in some species serving to keep the pairs together and thus securing a ‘marriage bond’, have the further and much more general function of promoting an effective synchronization of the male and female sexual processes, thus favouring successful procreation. 8. The synchronization is mainly effected by pituitary stimulation which is often mutual, the stimuli acting through the intermediation of the hypothalamus, as in the case of other stimuli. 9. Whatever the exteroceptive stimulus the anterior pituitary is the regulator of the gonadal function.