Adaptation to thermal stress in animals

Pashu Sandesh, 21 Feb 2022

Sriti Pandey, Abhishek Rajput

INTRODUCTION

Adaptation is the process by which an animal adapts itself to new changed environmental conditions. Adaptability can be evaluated by the animal’s capability to adjust to environmental conditions and climatic extremes. Many factors can affect livestock production in tropical and subtropical areas as availability and price of imported raw materials, quality of local feed resources, sanitary problems, etc. The climatic environment is one of the main limiting factors of production efficiency in these regions. The increasing concerns on production losses because of high ambient temperature is justifiable not only for the tropical area but also for countries occupying the temperate zone in which heat stress is an occasional problem during the 2 to 3 summer months and/or during hot spells.

The increase in global average surface temperature by 2100 may be between 1.88C and 4.08C. These predictions suggest that negative effects of heat stress on livestock production will become more apparent in the future as the world population and food supply continue to increase rapidly especially in tropical and subtropical regions, while the free land gets limited (i.e. desertification, intensive agriculture, ethanol (biodiesel) production, etc.).

The animal’s climatic environment is complex, especially in outdoor conditions. Practically, in such conditions, air Ta alone cannot be a representative measure of thermal environment and relative humidity (RH), solar radiation and wind speed must also be considered. However, scientists attempt to define it in an index value representing the influence of sensible and latent heat exchanges between the animal and its environment. Heat exchange could be accessed directly from physiological measurements like rectal, cloacal and skin temperatures, respiratory rate, panting and heat production or indirectly from animal performance as growth rate, egg and milk production that is related to the animal’s ability or inability to efficiently cope with acute or chronic hot conditions.

Under tropical and subtropical conditions, livestock animals are heat challenged most of the time. The long-term thermoregulatory responses underlying heat acclimation (or acclimatization) increase the physiological strains, which in most cases are accompanied by a reduced performance. These responses occur within the lifetime of the animal and include a reduction of metabolic rate, changes in the cardiovascular system, efficient alteration in heat loss (vasomotor response: vasodilatation response), changes in behaviour response and in the general morphology of the animal. In cattle, most acclimation occurs within the 3 to 4 days after the onset of a heat challenge. Similar data were obtained in pigs (Renaudeau et al., 2012). However, in the latter study, we showed that the time course for the total acquisition of thermal acclimation takes several weeks and varies with the magnitude of the heat challenge. In poultry, a similar pattern of acclimation was demonstrated; that is, several days for a vasomotor response, but several weeks for crucial changes in the cardiovascular system or endocrine activity.

WAYS OF ADAPTATION

i) Biological Adaptation-

Morphological, anatomical, biochemical and behavioural characteristics of the animal promoting its welfare and favouring its survival in a specific environment refer to the “biological adaptation”.

(ii) Genetic Adaptation-

The changes taking place over many generations of a population in a particular” environment refers to “genetic adaptation”. This includes heritable characteristic alterations favouring the survival of animal populations in a specific environment and evolutionary changes taking place over many generations.

(iii) Physiological Adaptation-

It involves the capacity and process of adjustment of the animal to the external physical environment.

ADAPTATION TO HEAT STRESS

The hot climate of a desert is a testing environment for living creatures. The hot days and cold nights mean they need to be well equipped to deal with the extremes. These factors, along with the hot climate’s lack of water and shelter, have resulted in animal’s adapting their bodies to suit the climate.

a) Behavioural Patterns

Animals in hot climates have adapted behavioural patterns to avoid the hottest part of the day or season. For example, Costa's hummingbird breeds in late spring and leaves the area for the hot summer. Meanwhile, reptiles and mammals are only active at dusk or night. Burrowing is also a useful mechanism. Lizards bury themselves in the sand during the day, while rodents create burrows and plug the entrance to keep out hot air.

b) Dissipating Heat

To keep cool, animals have created mechanisms to encourage air circulation around their bodies and to dissipate heat. Camels have a thin layer of fur underneath their bellies to help lose heat, while a thicker layer across their humps shade them. Owls, nighthawks and poorwills fly around with their mouths open so water is evaporated from the mouth. Vultures urinate on their legs so it cools them down as it is evaporated. They can also fly high in the air to experience cooler air flows. Physiological reactions of high-yielding dairy cows to hot ambient temperatures and temperature humidity indexes above 68 to 72 include panting, elevated evaporation and increased rectal temperatures (Bernabucci et al., 2010).

c) Water Adaptations

It is a common misconception that a camel stores water in its hump. In fact, a camel has adapted to the heat by being able to go for a long time without drinking water at all. Mammals have adapted to extract water from cacti. Small insects obtain nectar from the stems of plants, while larger animals derive water from leaves. Interestingly, kangaroo rats burrow into holes and recycle the moisture from their own breath to get retain water. As the rat exhales, water condenses on its nasal membrane. This process means the rat can conserve a lot of water so it doesn't need to drink for the day.

d) Thermoregulatory responses 

Farm animals are homeotherms as they can keep relatively constant body core temperature within narrow limits despite wide variations in the climatic environment. Thermoregulation is the balance between heat production and heat loss mechanisms that occur to maintain a relatively constant body temperature. Under high thermal conditions, animals reduce heat storage by reducing metabolic heat production and improving heat losses by latent and sensible pathways. An animal can lose heat by evaporation, conduction, convection and radiation. Although for evaporation the main driving force is the level of humidity in the surrounding air, for convection, radiation and conduction it mainly depends on the thermal gradient between the animal surface area and the surrounding air (radiation convection) and objects (conductance). In both cases, the body surface area that is in contact with the surrounding environment also plays a crucial role in the efficacy of the heat loss process. 

The early response of an animal is to increase its respiratory ventilation rate and thus the respiratory evaporative heat loss. First, rapid shallow breathing called thermal polypnea leads to an increase in the amount of air passage through the upper region of the respiratory tract. When temperature continues to rise, this thermal polypnea shifts to a slower deeper panting phase (thermal hyperpnea) characterized by an increase of alveolar ventilation rate. 

e) Other Adaptations

Some animals have adapted in unique ways to survive in hot climates. Some rodents have additional tubules in their kidneys to remove extra water from their urine so that it can be returned to the bloodstream for hydration. Reptiles and birds have adapted by excreting uric acid as a white compound that lacks moisture. This means they can retain vital water for their bodily functions. Other animals, like camels, also have a large surface-area-to-volume ratio to get rid of heat effectively.

ADAPTATION TO COLD

Animals survive in these harsh conditions by reducing the percentage of body heat that is lost to the environment. This can be by physical means (generally evolved over many generations) or patterns of behaviour. Generating heat that is sufficient to maintain a steady body temperature requires that the animal is able to generate enough heat in the first place and then hang onto it, this is done by:

  • Taking enough energy as food to generate the heat
  • Anatomical, physiological and behavioural adaptations to retain the heat generated

These two are bound tightly together, unless you can raise and maintain your temperature, you cannot be active enough to gather food, so there aren't any large cold-blooded terrestrial animals in polar regions, once cold they would never get warm again.

Thick, windproof or waterproof coats

Many Antarctic animals have either a windproof or waterproof coat. Emperor penguins are a very good example of this. These birds have four layers of scale-like feathers. These layers overlap each other, forming good protection from the wind, even in blizzard conditions.

Thick fat (or blubber) layers

Whales, seals and some penguins have thick fat layers. These fat layers act as insulation, trapping body heat in. This is a little like wrapping yourself in a blanket but on the inside. In some animals, this is even further refined, with the animals selectively able to reduce blood flow to the blubber layers. The further the blood is from the skin surface, the less heat is lost. Blubber layers can also be used as an energy reserve, for example, male elephant seals can live off their fat reserves during summer.

Small 'extremities'

The term extremities are used to mean any body part that is removed from the main body. In humans, our hands and feet count as 'extremities'. These are often the first places to feel cold in winter. The same applies to animals. Emperor penguins have a very small bill and flippers, which means less blood is required to these areas, thus less heat is lost.

CONCLUSION 

Animals achieve thermal balance through a combination of physiological, behavioural and physical processes. As their environmental temperature changes, they may elect to redistribute internal body heat or alter their exposure to different microhabitats in order to achieve their optimum temperature. 

REFERENCES

D. Renaudeau , A. Collin, S. Yahav, V. de Basilio, J. L. Gourdine, R. J. Collier. Adaptation to hot climate and strategies to alleviate heat stress in livestock production. Animal (2012), 6:5, pp 707–728.

Bernabucci U, Lacetera N, Baumgard LH, Rhoads RP, Ronchi B, Nardone A. Metabolic and hormonal acclimation to heat stress in domesticated ruminants. Animal. 2010; 4(7):1167–83.

Bartholomew, G. A. Role of behaviour in temperature regulation of masked booby. Condor, 1966, 68, 523–535.