A whorl of difference: Scientists compare horses and donkeys

Share
Comparison between standardbred, left, and donkey head profiles. Donkeys have shorter and narrower faces than Standardbreds, but they did not differ in the nasal, cranial or, mandibular indices. While the nasal profile index also did not differ, donkeys did exhibit a smaller cranial profile index, as portrayed by the distinct forehead in the donkey.
Comparison between Standardbred, left, and donkey head profiles. Donkeys have shorter and narrower faces than Standardbreds, but they did not differ in the nasal, cranial or, mandibular indices. While the nasal profile index also did not differ, donkeys did exhibit a smaller cranial profile index, as portrayed by the distinct forehead in the donkey.

Researchers have literally gotten inside the heads of horses and donkeys in a study looking for relationships between skull morphology and temperament.

Similar studies have been conducted in different dog breeds, but little is know when it comes to equids. Horses and donkeys belong to the genus Equus, but important differences exist between the species, many of which affect their management and welfare.

Dr Katrina Merkies, associate professor and University of Guelph researcher in Canada, collaborated on a project with Paul McGreevy, professor of animal behavior and animal welfare science at the University of Sydney, and Georgios Paraschou, a veterinary pathologist at The Donkey Sanctuary in Britain. They took many measurements comparing the skulls of Standardbred horses to those of donkeys and found that their olfactory bulbs differed in both size and placement.

Diagram of the various measurements taken from horse and donkey skulls. The orbital notch is marked by the red dot.
Diagram of the various measurements taken from horse and donkey skulls. The orbital notch is marked by the red dot.

Results of the pilot study have been published in the journal Animals in a special issue on advances in donkey and mule research.

The olfactory lobes are tied to the sense of smell, learning and memory. Researchers found that the olfactory bulb in donkeys was smaller than horses and rotated toward the centre of the brain. The differing size and location raise questions as to whether horses may pick up on a wider range of odors than donkeys.

Future studies may also reveal how equids differ in processing external stimuli and in their social responses (two of many functions associated with olfactory functions).

The researchers said that while donkeys and Standardbred racehorses both typically exhibit convex nasal profiles, the physiology and behaviour of these two equids are markedly different. “Temperament in donkeys differs from horses in that they are generally less overtly reactive, leading them to be defined as stoic,” they noted.

“Donkeys, evolving from arid climates, are more affected by intemperate weather than horses, but are less reactive to biting insects. Donkeys can thrive on lower-quality forage than horses, but this also puts them at risk for metabolic diseases when donkey owners feed them like horses. While horses show more obvious behavioural responses to pain than donkeys, both donkeys and mules show more metabolic changes indicative of stress than horses when subjected to transportation and mixing at auction houses.”

Diagram of brain measurements on horse (n = 14) and donkey (n = 16) skulls. The olfactory pitch (OP) was measured as the angle between the hard palate (green line) and the longitudinal axis of the olfactory bulb (purple line). The brain pitch (BP) was measured as the angle between the hard palate and the longitudinal axis of the brain (blue line).
Diagram of brain measurements on horse (n = 14) and donkey (n = 16) skulls. The olfactory pitch (OP) was measured as the angle between the hard palate (green line) and the longitudinal axis of the olfactory bulb (purple line). The brain pitch (BP) was measured as the angle between the hard palate and the longitudinal axis of the brain (blue line).

Merkies said that an unexpected discovery came up pertaining to the placement of whorls.

The study revealed that whorl placement in horses almost always corresponded with the location of the olfactory bulbs. Not so in donkeys, where the hair whorl was located much further down the nose.

There may be something to the folklore on whorls, which implies a whorl above the eyes denotes a difficult horse, one between the eyes suggests a manageable horse, and one below the eyes is the sign of a clever and calm horse.

“The donkey’s whorl below their eyeline matches their known calm temperament,” Merkies says.

Merkies hopes the study results may be linked to other documented differences between horses and donkeys, particularly when it comes to differences in behaviour.

Predictors of behaviour could mean better selection of equids for the jobs they are intended for thus improving communication, safety and welfare for all involved.

Morphometric Characteristics of the Skull in Horses and Donkeys—A Pilot Study. Katrina Merkies, Georgios Paraschou, and Paul McGreevy. Animals 2020, 10(6), 1002; https://doi.org/10.3390/ani10061002

Leave a Reply

Your email address will not be published. Required fields are marked *