Influence of season and external morphology on thermal comfort and physiological responses in bulls from two breeds adapted to a subtropical climate

ABSTRACT Animals in subtropical regions can be exposed to periods of thermal stress. Locally adapted breeds are especially important in this context, but these have little information available in their natural environments. The aim of this study was to assess whether season affects thermal comfort and physiological responses to increased temperatures of two breeds of bulls in a subtropical climate. Four Angus and four Crioulo Lageano bulls were used in this study. Body and testicles measurements, skin pigmentation and thickness, hair number, length and pigmentation, respiratory rate, panting score, rectal temperature and haematological profile were recorded on three occasions in summer and winter, morning and afternoon. The surface temperature of the flank, eye and scrotum were obtained by infrared thermography. Hair coat colour was determined by CIELAB method. Thermal comfort indexes were calculated from meteorological data. Statistical analysis included analyses of variance, correlations and a partial least squares regression in determining which traits were the most important in thermal adaptability. External morphology of the bulls was important for explaining physiological changes in both seasons, but their contribution was greater in summer. Bulls experienced moderate heat stress in summer, which led to significant physiological responses, which were more pronounced in Angus bulls. The main differences in thermal adaptation found between breeds were the hair coat characteristics and respiratory rate. Despite using different mechanisms to cope with environmental challenges, all bulls were able to maintain optimal testicular thermoregulation as well as systemic normothermia throughout the seasons, showing good adaptation to the climate conditions.

摘要 亚热带地区的动物会经历阶段性的热应激。在此背景下，地方适应性品种尤为重要，但针对其原生环境的相关研究资料却较为匮乏。本研究旨在探究亚热带气候下，季节是否会对两个肉牛品种公牛的热舒适度以及高温诱导的生理响应产生影响。本研究共选用4头安格斯（Angus）公牛与4头克里奥洛·拉盖阿诺（Crioulo Lageano）公牛。分别于夏季和冬季的上午、下午三个时段，记录了公牛的体尺与睾丸尺寸、皮肤色素沉着与厚度、毛发密度、长度与色素沉着情况、呼吸频率、热应激喘气评分、直肠温度以及血液学指标。采用红外热成像（infrared thermography）技术采集公牛肋部、眼部及阴囊的表面温度。毛发颜色通过CIELAB颜色空间法进行测定。基于气象数据计算热舒适度指数。统计分析采用方差分析、相关性分析以及偏最小二乘回归法，以明确热适应性相关的关键性状。公牛的外部形态特征可解释两个季节的生理变化，且该解释能力在夏季更为显著。夏季时公牛会遭受中度热应激，进而引发显著的生理响应，且安格斯公牛的此类响应更为明显。两个品种间热适应性的主要差异体现在毛发特征与呼吸频率上。尽管两个品种应对环境胁迫的生理机制存在差异，但所有公牛在整个观测周期内均能维持良好的睾丸温度调控能力与全身体温稳态，展现出对该气候条件的良好适应性。