Sperm head ellipticity as a heat stress indicator in Australian Merino rams (Ovis aries) in Northern Patagonia, Argentina

Abstract

In Northern Patagonia, Argentina, the ovine mating season starts on March 15, which is the time when rams are submitted to summer temperatures. This study assessed the adaptability of 12 Australian Merino rams, six unshorn and six shorn, half of which were treated in a heat chamber for fivedays (09.00hours to 17.00hours) that gradually reached 40°C. In an attempt to quantify the effects of heat stress on sperm head morphology, ellipticity was analyzed to establish the relationship between the distributions of subpopulations, light hours, temperature and humidity. Ellipticity was measured on 9224 sperm heads that were obtained over 12 weeks starting in the summer time. Four sperm head subpopulations (S) were identified by comparison with a sperm head population of ejaculates obtained in the late breeding season without the effect of heat stress (S1=heads with ellipticity≥2.00; S2=sperm head with range of ellipticity between 1.80 and 1.99; S3=sperm head with range of ellipticity from 1.60 to 1.79; and S4=sperm head with range of ellipticity from 1.30 to 1.59). The variable sperm head ellipticity for each ejaculate was expressed as the means and frequencies of subpopulation. The results demonstrate changes in ram sperm head ellipticity in different conditions (control/treated, unshorn/shorn) throughout the experiment (P<0.05). Treated shorn rams had a higher mean ellipticity and frequency of elliptical heads (mean ellipticity value=2.06 and S1 frequency=76.35%), peaking in the seventh week posttreatment (on the basis of the action of heat stress on seminiferous tubules). According to this study, unshorn rams were better adapted to heat stress than the shorn ones.