Horses may indeed be “thick skinned”, but this does not protect them from any discomfort arising from whip use, according to researchers.
Scientists who compared horse skin to human skin found that both species have the equivalent basic anatomic structures to detect pain in the skin.
They found that the dermis, the skin’s collagen layer, which is not involved in pain detection, was significantly thicker in horses. However, the outer layer, the pain-detecting epidermis, was little different to that of humans.
“Although horse skin is thicker overall than human skin, the part of the skin that is thicker does not insulate them from pain that is generated during a whip strike,” Lydia Tong and her fellow researchers reported in the open-access journal Animals.
The Australian study was carried out to increase understanding of the capacity of horse skin to detect pain when directly compared to human skin.
The work focused on the gluteal skin where horses are most often struck with whips during racing.
The researchers used microscopic studies of skin from 20 Thoroughbred-type horses, euthanized for reasons unrelated to the study, and 10 deceased humans to explore any differences between the species in their skin structure and nerve supply.
They found no significant difference between humans and horses in either the concentration of nerve endings in the outer pain-detecting layer (the epidermis) or in the thickness of this layer.
“The superficial pain-sensitive epidermal layer of horse skin is as richly innervated and is of equivalent thickness as human skin, demonstrating that humans and horses have the equivalent key anatomical structures to detect cutaneous pain.”
It was found that this layer was deeper on the right than the left — something they described as a highly unexpected finding which cannot yet be explained.
Racing’s whip use under scrutiny
The authors, laying out the basis for their study, noted that the use of whips in horse racing is increasingly being questioned on ethical, welfare, social sustainability, and legal grounds.
“Racing industry proponents argue that ‘the whip is used for safety (of both rider and horse) or to encourage the horse to perform to its best when in contention’, and that horses in many jurisdictions are protected from pain by whip padding and rules that govern whip use.
“Some industry commentators have asserted that racing horses are immune to any pain from whip strikes chiefly because they are adrenalinised.
“Others point out that horses may be physically brutal with one another in social interactions or conflict and have therefore evolved to be robust and stoic.
“At the core of the debate is a fundamental question — do horses experience pain when whipped?”
The study team said it is broadly accepted in science and society that most animals experience pain when struck.
“This is reflected by current societal norms, ethics, and laws that prohibit striking or hitting animals. However, this protection is not extended to certain animal species — these being almost exclusively large domestic species — including the horse.”
It is, they said, reasonable to question why this assumption exists, and crucially, what scientific evidence exists to demonstrate that horses do or do not feel pain when being whipped.
They said assumptions that horses are insensitive to pain may be influenced by historic factors and colloquialisms. “The term ‘thick-skinned’, to denote insensitivity, has been in the English lexicon for over 400 years,” they noted.
Are horses really thick-skinned?
In recent decades, limited scientific information has become available that documents the full skin thickness of horses, with it reportedly ranging between 1.2mm to 7mm.
“This is indeed considerably thicker than full-thickness human skin, reported at 1.2mm thick on average in multiple studies, with a range of 0.52 to 2.0mm.
“However, interpreting the skin thickness as a whole as an indicator of skin sensitivity fails to account for the complexity of the skin organ and, crucially, that detection of pain by mammalian skin overwhelmingly occurs in the very superficial layer of the skin (the epidermis), where the source of the painful stimulus comes into contact with these cells.”
The capacity of skin to detect pain must also depend on the presence and density of the microscopic neural structures of the epidermis.
“Pain is a complex and primordial physiological function key to survival because it provides a powerful incentive to avoid tissue damage.”
Discussing their findings, the authors said their study does not attempt to assume, based on the findings, that the end-experience of pain in the horse is the same as in humans.
“Such a comparison is currently outside the capability of science, even when comparing the experience of pain between two humans.
“However, this does mean that broad similarity between species permits valid comparisons about pain perception, even in the absence of absolute equivalence of skin structures.”
Whip use common in racing
They noted that an observational study of horses racing in Australia found 83% of whip strikes caused indentations of the skin of the horses whipped.
They said repeated whipping is common in racing even though the International Federation of Horseracing Authorities forbids “using the whip with excessive frequency”.
“The accumulated damage from successive strikes is likely to create more pain and inflammation,” the researchers said.
In Australia, racing Thoroughbreds may be struck no more than five times prior to the final 100 metres, but after this point they can be struck at the jockey’s discretion.
“This can equate to a strike per stride and effectively this means that 18 strikes per race are within the rules.”
They continued: “The current findings suggest the ethical future for racing is one in which whips are carried but not used for so-called encouragement. Fortunately, this model is already well established in the UK where apprentice jockeys compete in hands-and-heels series.”
Tong and her colleagues said the role of pain in horse sports is part of the growing debate about what constitutes ethical equitation.
“In a flight animal, such as the horse, being unable to resolve aversive cutaneous stimulation causes distress. Horses have evolved to run away from such stimuli since the most likely natural cause of such stimulation is contact from a predator.”
The concept of ethical equitation advocates a process for stakeholders who seek to retain the social license to operate, the authors said.
“It demands that we identify the causes of distress in the horses we ride, that we mitigate these stressors as much as possible and we justify the retention of those that cannot be mitigated.”
The study team comprised Tong, with the Taronga Conservation Society Australia; Melinda Stewart, with Starling Scientific; Ian Johnson and Richard Appleyard, with Macquarie University; Bethany Wilson and Paul McGreevy, with the University of Sydney; Olivia James, with Australian Veterinary Equine Dentistry; and Craig Johnson, with Massey University in New Zealand.
Tong, L.; Stewart, M.; Johnson, I.; Appleyard, R.; Wilson, B.; James, O.; Johnson, C.; McGreevy, P. A Comparative Neuro-Histological Assessment of Gluteal Skin Thickness and Cutaneous Nociceptor Distribution in Horses and Humans. Animals 2020, 10, 2094.