ABSTRACT

Sexual activity of the male lesser mouse lemur can be induced by a long photoperiod, and plasma testosterone concentrations increase from 20 to 220 nmol/l within 3 weeks of photoperiodic stimulation. When isolated males were exposed to the volatile compounds from the urine of an active dominant male for 4 weeks at the beginning of the long daylight period, they demonstrated a significant decrease in testosterone concentrations (134± 11 nmol/l) compared with controls (210 ± 26 nmol/l) within 2 weeks. Lowering concentrations of prolactin by daily injections of bromocriptine prevented the decrease in testosterone in males simultaneously exposed to the odorant stimulation. Increasing concentrations of prolactin by daily injections of sulpiride mimicked the effect of the odorant stimulation in males receiving only fresh non-odorized air. The decrease in testosterone was strengthened when sulpiride was administered concurrently with exposure to urine. These results support the conclusion that variations in the concentration of prolactin are involved in the neuroendocrinological process mediating the pheromone-like sexual inhibition in the male lesser mouse lemur. However, daily injections of bromocriptine in males which were photoperiodically stimulated but not exposed to dominant male urine odour, also induced a significant decrease in testosterone concentrations. This finding suggests that two different systems involving prolactin and leading to opposite effects might be implicated in the regulation by environmental factors of sexual activity in the male lesser mouse lemur.

J. Endocr. (1987) 114, 279–287

ABSTRACT

The body weight and sexual activity of the male lesser mouse lemur, a prosimian primate, undergo cyclic changes that are photoperiod-dependent. Exposure to long daylength (LD; 14 h light/day) led to sustained sexual activity, fully developed testes and high plasma testosterone concentrations (228 ±25 nmol/l, n = 10). After 14 weeks under LD, a marked decrease in testosterone levels occurred. Gonadal regression was completed within 20 weeks under LD without concomitant changes in body weight. Exposure to short daylength (SD; 8 h light/day) reinforced the sexual quiescence and was associated with a high ponderal gain (from 60 to 110 g). However, independent of the date of gonadal arrest, the sexual activity of males spontaneously resumed at a fixed time after exposure to SD. The testes developed, testosterone concentrations increased to 155 nmol/l and the body weight decreased (from 110 to 80 g) within 20 weeks under SD exposure. The timing for refractoriness appeared very similar under inhibitory and stimulatory photoperiods. This is consistent with the hypothesis that the perception of a critical daylength is used to regulate the timing of the following sexual phase through a mechanism involving photorefractoriness.

Contrary to cricetid rodents, a direct response to photoperiodic signals for both body weight and sexual activity were not prevented by olfactory bulb removal in male mouse lemurs. In bulbectomized males (n = 12), sexual activity was stimulated by LD and inhibited by SD. Their plasma testosterone levels, however, significantly differed from those of controls in both photoperiods. Likewise, ponderal cycles remained intact but the fattening phase was delayed and reduced. Finally, whereas the sexual recrudescence of bulbectomized males occurred under SD with a delay of only 2–4 weeks, spontaneous testicular regression under exposure to LD did not appear in our experimental photoperiodic conditions.

Journal of Endocrinology (1993) 137, 511–518

ABSTRACT

The effect of dominant urine odour on plasma levels of testosterone and cortisol was studied in the prosimian primate Microcebus murinus. This species shows a photoperiod-dependent cycle of sexual activity. In particular, mean testosterone levels vary from 15 nmol/l during the annual rest period to 245 nmol/l during the breeding season. When males of this solitary and territorial species are artificially grouped in captivity, they develop a social hierarchy which in turn results in physiological disorders, especially of reproductive function, in non-dominant individuals. Since olfactory behaviours appear to be important in the establishment and maintenance of the social structure, we have tested the effects of dominant odorant signals upon the sexual inhibition observed in male conspecifics. A preliminary experiment showed that a decrease in plasma testosterone concentrations could be induced by dominant scent marks. Furthermore, dominant urine odour alone was found to be sufficient to induce this modification. Plasma cortisol levels also increased in these animals. Conversely, when sexually inhibited males were olfactorily isolated from dominant urine odour, testosterone and cortisol concentrations returned to a normal seasonal level. These effects were observed even in males which had had no previous contact with the dominant urine donor. It is inferred from these results that a pheromone-like process could lead to sexual inhibition in male Microcebus murinus exposed to an odorant urinary signal produced by a dominant individual. Nevertheless, the endocrine response seems to vary according to the seasonal period of the sexual activity cycle which suggests that the social effect described is modulated by other external (e.g. photoperiodic) or internal (e.g. reproductive physiology) factors.

J. Endocr. (1984) 102, 143–151