Copulatory behaviour of the ram, Ovis aries. I. A normative study

Comportamiento reproductivo del carnero en un rebaño de ovejas "Obispo" de la montaña del estado de Guerrero, México

Este es el primer reporte del comportamiento reproductivo de los carneros en un rebaño de ovinos “Obispo”. El estudio se llevó a cabo ex situ (18°15'52" LN) en condiciones de mayor temperatura de lo que se presenta en la montaña de Guerrero, México. En el análisis de Componentes principales (CP), el primer componente (CP1) incluyó el peso corporal (PC), circunferencia escrotal (CE) y volumen de eyaculado (VE) y, en el CP2, se agrupó la concentración espermática (CONC) y motilidad progresiva (MP). La CE mostró un incremento a través del año llegando al final del estudio a 32,0 ± 0,3 centímetros a una edad aproximada de 30 meses y mostró correlaciones mayores a 0,5 con VE, CONC y MP. Los valores más bajos registrados en las características fueron en la primavera, con 0,86 ± 0,3 milílitros (mL) para el VE, 4.120,2 ± 162 millones de espermatozoides·mL-1 × 106 para CONC y de 86 ± 1,6 % para MP; mientras que los valores más altos se presentaron en otoño con 1,12 ± 0,02 mL para el VE, 4.519,2 ± 187,0 millones de espermatozoides·mL-1 × 106 para CONC y 92,0 ± 1,1 % para MP. Los carneros conservan una alto libido que les permite mantener un número elevado de servicios (más de tres en un periodo de cinco minutos), con reducidos tiempos de reacción 49 segundos y de recuperación entre cópulas (1,84 minutos).

Influence of sexual behavior of Dorper rams treated with glutamate and/or testosterone on reproductive performance of anovulatory ewes

The aim of this study was to determine if exogenous administration of glutamate and (or) testosterone to male rams during the season of reproductive arrest is able to re-activate male sexual behavior and, later on, to promote through the male effect, both sexual and reproductive competence of anovulatory nulliparous ewes. Therefore, an experiment was performed under long-day photoperiods (spring; photo-reproductive arrest, 26°N). Dorper rams were randomly divided into four homogeneous experimental groups (n = 5 males each) regarding live weight (LW), body condition score (BCS), scrotal circumference (SC) and odor intensity (OI). Then, groups were treated with: i) GG (7 mg kg^-1 LW of glutamate, every 4d × 30d, im.), ii) GGT (7 mg kg^-1 LW of glutamate every 4d × 30d im + 25 mg of testosterone propionate, every 3d × 15d, im.), iii) GT (25 mg of testosterone propionate every 3d × 15d, im, and iv) GC (1 mL of saline, every 4d × 30d, im.). Thereafter, Dorper rams, (n = 4 per group) were selected and exposed to Dorper anovulatory-nulliparous ewes divided in four groups (n = 14 ewes each), and all the appetitive (ASB) and consummatory (CSB) sexual behaviors and indicators of sexual rest (ISR) were registered during the first 48 h of this male-to-females contact. Thereafter, males continued the male-to-female contact for another 8 d, in order to quantify the ewe's sexual and reproductive response through the male effect. During the sexual behavior tests, the GGT rams showed the highest ASB + CSB frequencies (P < 0.05) followed by the GG-rams with the lowest frequencies showed by the GC and GT groups. While the highest ISR behaviors were shown by the GG and GGT groups (P < 0.05) followed by the GC and GT-rams, no differences occurred regarding LW, BCS, and SC along the experimental breeding, with the largest (P < 0.05) OI shown by the GGT-rams and the lowest value observed in the control rams. Regarding the reproductive response of the nulliparous-anovulatory ewes exposed to treated rams, the glutamate + testosterone treated males induced not only an increased (P < 0.05) ovulatory activity and faster (P < 0.05) estrus response but an augmented (P < 0.05) pregnancy rate of the previously anovulatory ewes. To conclude, whereas the GG and GGT treatments generated encouraging sexual and reproductive outcomes, our results are also thought-provoking from a comparative sexual behavior perspective while may embrace potential applications to other animal industries.

Acute and chronic stress-like levels of cortisol inhibit the oestradiol stimulus to induce sexual receptivity but have no effect on sexual attractivity or proceptivity in female sheep

Stress-like levels of cortisol inhibit sexual receptivity in ewes but the mechanism of this action is not understood. One possibility is that cortisol interferes with the actions of oestradiol to induce sexual receptivity. We tested this hypothesis in 2 experiments with ovariectomised ewes that were artificially induced into oestrus by 12 days of i.m. injections of progesterone followed by an i.m. injection of oestradiol benzoate (ODB) 48 h later. In Experiment 1, ewes were randomly allocated to the following groups: saline infusion+25 μg ODB, saline infusion+50 μg ODB, cortisol infusion+25 μg ODB or cortisol infusion+50 μg ODB (n=5 per group). Saline or cortisol was infused i.v. for 40 h beginning at the ODB injection. In Experiment 2, ewes were infused with saline or cortisol (n=5 per group) for 5h beginning 1h before ODB injection. In both experiments, ewe sexual behaviour (attractivity, proceptivity and receptivity) was quantified every 6h. Blood samples were also collected. The cortisol infusion yielded plasma concentrations of cortisol similar to those seen during psychosocial stress. In both experiments, cortisol suppressed receptivity index (number of immobilisations by ewe/courtship displays by ram) and the number of times ewes were mounted but had no effect on attractivity or proceptivity, irrespective of the dose of ODB (Experiment 1). Cortisol also suppressed LH pulse amplitude. These results suggest that both an acute (5h) and chronic (40 h) infusion of cortisol inhibit oestradiol-induced sexual receptivity in ewes and that increasing the dose of ODB does not overcome the inhibitory effects of cortisol.

Mating behaviour in a small flock of lowland sheep

A flock of Polled Dorset Horn ewes joined with three Polled Dorset Horn rams were observed during the hours of daylight, for 17 days, when all the ewes were known to be showing oestrus regularly. The number of approaches, mounts and serves made by the rams and the identities of the ewes involved were recorded.

The mean and SE for the number of services per ewe in oestrus was 4·2 ± 0·3. There were no significant differences in sexual performance between the rams apart from the number of mounts and the mount to service ratio. The number of ewes served per ram and the number of services per ewe in oestrus declined significantly over the 17-day period. Ram performance was significantly related to the number of ewes in oestrus and to the time that the rams had been joined with the same group of stimulative animals (e.g. multiple regression of number of services x with time y and number of ewes in oestrus z is: x = −0·72y + 1·3z + 8·8).

Psychosocial stress suppresses attractivity, proceptivity and pulsatile LH secretion in the ewe

Various stressors suppress pulsatile secretion of luteinizing hormone (LH) in ewes and cortisol has been shown to be a mediator of this effect under various conditions. In contrast, little is known about the impact of stress and cortisol on sexual behavior in the ewe. Therefore, we tested the hypothesis that both psychosocial stress and stress-like levels of cortisol will reduce the level of attractivity, proceptivity and receptivity in addition to suppressing LH secretion in the ewe. In Experiment 1, a layered stress paradigm of psychosocial stress was used, consisting of isolation for 4 h with the addition of restraint, blindfold and noise of a barking dog (predator stress) at hourly intervals. This stress paradigm reduced LH pulse amplitude in ovariectomized ewes. In Experiment 2, ovariectomized ewes were artificially induced into estrus with progesterone and estradiol benzoate treatment and the layered stress paradigm was applied. LH was measured and sexual behavior was assessed using T-mazes and mating tests. Stress reduced pulsatile LH secretion, and also reduced attractivity and proceptivity of ewes but had no effect on receptivity. In Experiment 3, ewes artificially induced into estrus were infused with cortisol for 30 h. Cortisol elevated circulating plasma concentrations of cortisol, delayed the onset of estrus and resulted in increased circling behavior of ewes (i.e. moderate avoidance) during estrus and increased investigation and courtship from rams. There was no effect of cortisol on attractivity, proceptivity or receptivity during estrus. We conclude that psychosocial stress inhibits LH secretion, the ability of ewes to attract rams (attractivity) and the motivation of ewes to seek rams and initiate mating (proceptivity), but cortisol is unlikely to be the principal mediator of these effects.

The ram as a model for behavioral neuroendocrinology

The sheep offers a unique model to study male sexual behavior and sexual partner preference. Rams are seasonal breeders and show the greatest libido during short days coincident with the resumption of ovarian cyclicity in the ewe. Threshold concentrations of testosterone are required for the acquisition and display of adult sexual behavior. In addition, estrogens produced from circulating testosterone by cytochrome P450 aromatase in the preoptic area are critical for the maintenance of sexual behaviors in rams. Sex differences in adult reproductive behaviors and hormone responsiveness are the result of permanent organizational effects exerted by testosterone and its metabolites on brain development. Early exposure to ewes enhances ram sexual performance, but cannot prevent some rams from exhibiting male-oriented sexual partner preferences. Neurochemical and neuroanatomical studies suggest that male-oriented ram behavior may be a consequence of individual variations in brain sexual differentiation.

Monitoring of estrous cycle of ewes by ram-seeking behavior

For 6 years adult, non-synchronized Hampshire ewes (n=2,5,8,8,9 and 12) and rams (n=6,10,12,11,11 and 9) were maintained separately and released to contiguous pastures having in common a woven-wire fence about 80m long and 1.1m high. Natural estrous cycles were monitored daily by observations of ram-seeking behavior as defined as a ewe being persistent in staying at the common fence with attention focused on the ram to the virtual exclusion of grazing. Day 0 of the estrous cycle is the first day that ram-seeking is observed. One hundred and thirty four fence stays and 115 estrous cycles have been observed. For cycles falling within the normal range, the mean cycle duration (+/-SE) was 16.53+/-0.03 days. During the first four study years, a copulation test with an intact ram, carried out on a spot-check basis, indicated that all tested ewes showing ram-seeking, except one, were in estrus. Ram-seeking never occurred outside of the estrous period of the cycle. During the fifth year, daily copulation tests with an intact ram and daily observations for ram-seeking has indicated that estrus generally lasts about two days while ram-seeking lasts for one of these days, so that the two behaviors while overlapping are not coextensive. During the sixth year, daily copulation tests with a vasectomized ram showed again a partial coincidence of the two behaviors, but also that occasionally they appeared to have the same duration. It is concluded that the utilization of ram-seeking provides an alternative, practical means to monitor the estrous cycle.