Adaptive mechanisms of sheep to climate change

Sheep rearing is the most integral part of animal production particularly in tropi cal regions. Climate change is observed to have devastating effects on sheep farming through constraints such as heat stress, lower grazing lands, water scar city and higher pest and disease incidences. These environmental constraints may lead to compromised productive functions in sheep. The cumulative effects of heat, nutritional and walking stress occurring in the hotter parts of the year compromise the productive and reproductive performances of the sheep through reduced feed intake, modified endocrine profile, lower rumination and nutrient absorption and higher maintenance demands. Indigenous sheep breeds particularly in the tropical region are found to have bet ter adaptability to hot climates than exotic and crossbreds. Sheep try to cope with these adverse environmental stressors through morphological, behavioural, endo crine, blood biochemical and cellular adaptations. Sheep indigenous to tropical and subtropical regions are observed to have carpet-type wool which helps them in elic iting cutaneous evaporative cooling mechanism to dissipate extra heat load during summer. Exposure to higher ambient temperature triggers all behavioural and phys iological mechanisms in sheep to cope with the existing condition. Similarly, their endocrine mechanisms also play a major role in their adaptive processes. Cortisol being the primary stress-relieving hormone initiates various physiological modifica tions in these sheep to adapt to the adverse thermal conditions. Further, heat stressinduced hyperthermia also decreased thyroid gland activity and thyroid hormone levels in the blood to control metabolic heat production. Higher expenditure of energy along with lower feed accessibility altered blood biochemical parameters such as glucose, protein, cholesterol, globulin and albumin in sheep. The activation of cellular and molecular mechanisms is considered to be the most important responses by which the animals survive the heat stress condition. The activation of these pathways may help to identify various thermotolerant genes to quantify heat stress condition in sheep. It has also been found that respiration rate, rectal tempera ture, plasma cortisol, thyroxin, triiodothyronine and heat shock protein 70 are estab lished to be the ideal biological makers for environmental stress in sheep