Thermal Adaptation - Sweating

Mammals are endothermic organisms, meaning that they have to physically or behaviourally change in order to maintain optimal body temperature. This is crucial in maximising the operation of bodily functions such as metabolism, osmoregulation and heart function. In excessive heat, some mammals have evolved with sweating mechanisms allowing thermal regulation. Humans possess three types of sweat glands (apocrine, eccrine, and apoeccrine) which are located on various parts of the body and serve different functions (Asahina et al 2014).

Fig. 1 Diagram of sweat glands present in some mammals

(Asahina et al 2014)

Apes, horses, monkeys and goats have evolved with similiar types of sweat glands that humans possess (apocrine, eccrine and apoeccrine) (Baker 1989). Sweat glands are merocrine glands, a class of exocrine glands. All types of sweat glands are tubular epithelial structures, apocrine being more prevalent in hair follicles, and apoeccrine located on soles/palms and eccrine in normal skin (Asahina et al 2014). Apocrine and eccrine sweating works by coating the outer skin with moisture which is then evaporated, expending heat energy from the body to the atmosphere. It is theorised that apocrine and eccrine sweating in early hominids allowed easier hunting of prey without thermoregulatory systems. The prey would tire faster than the hominids making them easier to ambush and kill (Noakes 2006). This proves the value of sweating as a thermoregulatory structure throughout evolution.

Fig. 2 Hominids ambush exhausted deer
(The Natural History Museum)

Sweating that occurs on the palm/sole of many mammals is referred to as ‘emotional sweating’. This process increases friction while grasping or performing delicate tasks (Asahina et al. 2014). In cats and dogs it is beneficial in increasing ground traction when running. Humans possess apoeccrine sweat glands likely due to our relation to primates, where it was advantageous in gripping tree branches. The sweat is produced as a result of anxiety or stress, so when primates were attacked or predated, their palms sweat, increasing friction and allowing a swift escape (Asahina et al. 2014).

Sweating is an important thermal adaptation to combat heat exhaustion caused by physical exertion or the environment. It has aided the evolution of humans in hunting prey, and allowed for primates and other mammals to escape quickly in times of stress. Sweating continues to be a relevant and important thermal adaptation for humans and other mammals.

References:

Asahina, Masato et al 2014, ‘Sweating on the palm and sole: physiological and clinical relevance’, Clin Auton Res, Chapter 25 pp 153-158

Noakes, Timothy 2006, ‘Exercise in the heat: Old ideas, new dogmas’, International SportMed Journal; Vol. 7 Issue 1, p58

Baker, M.A 1989, ‘EFFECTS OF DEHYDRATION AND REHYDRATION ON THERMOREGULATORY SWEATING IN GOATS’, Journal of Physiology 417, pp. 421-435. 

Fig, 1:
Asahina, Masato et al 2014, ‘Sweating on the palm and sole: physiological and clinical relevance’, Clin Auton Res, Chapter 25 page 154

Fig. 2: 

The Natural History Museum, date unknown, 'Paleolithic'.