All horses and donkeys belong to the genus Equus but anatomical and behavioural differences exist among species. Equus caballus displays distinctive conformational attributes among breeds provisionally related to ganglion cell distribution and skull and brain morphology. Equus asinus shows less variation in skull shape, and little is known about brain organisation. The current research compared skull and brain morphology between horses and donkeys. Skulls of Equus caballus, primarily of Standardbred type (N=14) and Equus asinus (N=16), were obtained postmortem. All animals had been humanely euthanised for reasons unrelated to this study. Heads were sectioned sagitally along the midline and photographed for measurement of various skull structures using Image J software. Measurements included: skull index (SI)=zygomatic width*100/skull length; cranial index (CI)=cranial width*100/cranial length; nasal index (NI)=zygomatic width*100/nasal length; cranial profile index (CPI)=rectangular area bordered by an 80mm line from orbital notch and occiput; nasal profile index (NPI)= rectangular area bordered by 80mm line from orbital notch and tip of nasal bone; olfactory lobe area (OLA); OL pitch [angle between hard palate and the OL axis]; brain pitch [angle between longitudinal axis of the cerebral hemispheres and the hard palate]; and whorl location (WL) [distance of OL from the level of the forehead whorl]. A General Linear Model determined the main effect of species with Sidak’s multiple comparisons of species’ differences among the various measurements. Donkeys had shorter heads (cranial lengths) than horses (19.7±2.5 vs 23.6±1.4cm respectively; F1,23=51.49, P<0.0002). Donkeys also had smaller cranial widths (13±3.4cm; F1,17=15.91, P<0.001) and mandibular depths (24±2.6cm; F1,21=13.05, P<0.002) than horses (19±0.8 and 27.2±1.1cm, respectively). There was no species difference in SI, ZI, or NI (P>0.40), but donkeys tended to have a smaller CI than horses (F1,17=3.59, P<0.08). Similarly, donkeys had a smaller CPI than horses (F1,21=7.54, P<0.034), but there was no difference in NPI (F1,21=0.05, P>0.83). Donkeys also had a smaller OLA than horses (1.4±0.3 vs 2.3±1.3cm2 respectively; F1,13=4.96, P<0.05) although there was no difference in brain pitch (F1,23=0.69, P>0.43). The greatest difference was seen in WL, which corresponded to the level of the OL in horses, but was extremely rostral in donkeys (F1,21=24.29, P<0.0001). These results show clear differentiation in skull morphology between horses and donkeys which may be linked to behaviour. This may be useful in validating different approaches in the training and management of horses versus donkeys.
Horses demonstrate specific behaviours which may be associated with skull shape, although nothing is known about this relationship in donkeys. This pilot study has shown that donkeys have smaller brain cases and olfactory lobes than Standardbred horses. Donkeys’ facial whorls are located lower down the face while horses’ are in close proximity to the brain’s olfactory lobe. Clarifying differences between horses and donkeys is crucial to understanding species-specific behavioural responses and providing appropriate management and training practices.