Acclimation, Acclimatization, and Adaptation of Heat Stress in Dairy cattle

Animals have developed coping mechanisms to minimize the impact of these environmental stressors on their biological systems. These responses are termed acclimation, acclimatization, and adaptation. Acclimation is defined as the coordinated phenotypic response developed by the animal to a specific stressor in the environment .while acclimatization refers to a coordinated response to several simultaneous stressors (e.g., temperature, humidity, and photoperiod.

Adaptation involves genetic changes as adverse environments persist over several generations of a species. Generally, there is hardly ever an example under normal environmental conditions where only one variable is changing. Therefore, typically an animal is undergoing acclimatization to a changing environment. Acclimation and acclimatization are induced by the environment and are considered phenotypic and not genotypic change and the responses decay if the stress is removed. Acclimation and acclimatization act to improve animal fitness to the environment. In many cases, the response is induced by sudden environmental change, such as heat or cold stress. In other examples, the acclimation response is driven by slower seasonal changes in photoperiod or other environmental cues such as the lunar cycle which permit the animal to “anticipate” the coming change in the environment leading to seasonal acclimation adjustments in insulation (coat thickness, fat deposition), feed intake, or reproductive activity in advance of the actual environmental change. However, in every case, the process is driven by the endocrine system and is “homeorhetic”; meaning metabolism is coordinated to support a specific physiologic state . In this case, the specific physiologic state is the “acclimatized animal.” If the environmental stressors are present for prolonged periods of time (e.g., years) these metabolic and physiologic adjustments can become “fixed genetically” and the animal is considered “adapted” to the environment.

Acclimation and acclimatization are therefore not processes which involve evolutionary adaptations or natural selection, which are defined as changes allowing for preferential selection of an animal’s phenotype and are based on a genetic component passed to the next generation. The altered phenotype of acclimatized animals will return to the prior state if environmental stressors are removed, which is not true for animals which are genetically adapted to their environment. Acclimatization is a process that takes several days to weeks to occur, and close examination of this process reveals that it occurs via homeorhetic and not homeostatic mechanisms. There are three functional differences between acclimatization responses and homeostatic or “reflex responses.”

1. First, the acclimatization response takes much longer to occur (days or weeks vs. seconds or minutes).
2. Second, the acclimatization responses generally have a hormonal link in the pathway from the central nervous system to the effector cell.
3. Third, the acclimatization effect usually alters the ability of an effector cell or organ to respond to environmental change.

These acclimatization responses are characteristic of homeorhetic mechanisms and the net effect is to coordinate metabolism to achieve a new physiological state. Thus, the seasonally acclimatized animal is different metabolically in winter than in summer. These characteristics of acclimatization into the concept of homeorhesis, which is defined as “orchestrated changes for priorities of a physiological state” . The concept originated from considering how physiological processes are regulated during pregnancy and lactation (Bauman and Currie, 1980), but application of the general concept has been extended to include different physiological states, nutritional and environmental situations, and even pathological conditions. Key features of homeorhetic controls are its chronic nature, hours and days vs. seconds and minutes required for most examples of homeostatic regulation; its simultaneous influence on multiple tissues and systems that results in an overall coordinated response, which is mediated through altered responses to homeostatic signals.

Acclimatization is generally considered to occur in two stages; acute or short term and chronic or long term. The acute phase involves the heat shock response at the cellular level and homeostatic endocrine, physiological, and metabolic responses at the systemic level while the chronic or long-term phase results in acclimatization to the stressors sometimes called “conditioning” and involves reprogramming of gene expression and metabolism . In domestic animals, there is generally a loss in production as animals enter the acute phase and some or even all this productivity is restored as animals undergo acclimatization to the stressors.

The chronic response or stage 2 of acclimatization to stress is driven by continued exposure of the animal to the stressor. It is mediated by the endocrine system and is associated with altered receptor populations which change tissue sensitivity to homeostatic signals resulting in a new physiologic state . Thus, acute heat stress is a homeostatic response driven by the autonomic nervous system and chronic stress responses, acclimatization and seasonal changes are driven by the endocrine system and homeorhetic mechanisms.