Phenotypic plasticity of the heart and skeletal muscles in cold acclimated Red Junglefowls (Gallus gallus)

Abstract: The ability of the heart and skeletal muscles to remodel to environmental demands, their plasticity, is of interest when studying animals’ adaptation to environment changes. Temperature variation due to seasonal change seems to lead to the development of a cold acclimated phenotype in small birds. To endure cold conditions a higher metabolism is required for shivering thermogenesis in aerobic skeletal muscles. This in turn leads to several physiological changes, including a heart and muscle hypertrophy as well as an increased oxygen carrying capacity of the blood. In this study were Red Junglefowls (Gallus gallus) bred indoors and outdoors and physiological aspects such as body size, growth rate, relative size of heart and skeletal muscles (pectoralis major and gastrocnemius maximus) as well as hematocrit and hemoglobin concentrations of the blood were compared between the groups. Observed significant differences included a slower growth rate in fowls bred outdoors, 2.5 (0.7) g/day than indoors, 3.8 (0.4) g/day, as well as a larger relative size of the heart and gastrocnemius muscle. The average relative size of the heart was more than twice as big in fowls bred outdoors, 0.97 (0.08) %, than indoors, 0.40-42 (0.05) %. The average relative size of the gastrocnemius muscle for the fowl bred outdoor was significantly larger than for fowl bred indoors (0.95 (0.11) %, vs. 0.58-0,63 (0.09) %). In addition, fowl bred outdoors showed an increased capacity for oxygen transportation, with blood hematocrit values of 43 (3) % higher than 35-37 (3) % for the indoor animals. Fowls bred outdoors also showed higher hemoglobin concentrations in the blood, 127 (7) g/l, than fowls bred indoors, 113 (7) g/l. Findings indicate a cold acclimated phenotype among the outdoor bred fowls.