Identifying domestic horses, donkeys and hybrids from archaeological deposits: A 3D morphological investigation on skeletons

The first evidence for the domestication of donkeys (Equus asinus) dates back to at least 6000-5000 BP in Northeast Africa, and their dispersion is attributed to the ancient Romans. Latin authors described donkeys as being particularly suitable for the transport of goods and farm work. In addition, they were also bred to produce prized hybrids, particularly mules, which were perfectly adapted to the long-distance transport of people and goods. However, although the historical sources extensively describe their economic importance, both donkey and hybrid remains are surprisingly scarce in the archaeological record. This apparent contradiction is probably due to the difficulties involved in correctly identifying their bones: relatively few bones displaying morphological and metrical criteria can be used for identification, so it is often based purely on bone size. The aim of this study, therefore, is to propose solutions to identify domestic equid bones using 3D geometric morphometrics on isolated and combinations of anatomical elements. A set of 3D coordinates were registered on the 18 main skull and limb bones of 111 modern reference specimens (i.e. 42 horses, 44 donkeys and 25 hybrids). In this paper, we present the classification rate obtained on this reference sample using the k-Nearest Neighbors algorithm. The application of this method on archaeological skeletons from Roman to modern sites is also presented. The percentage of correctly classified specimens was between 77% and 95% for all 18 bones, and higher than 80% for 10 of the fragmentation patterns we defined. Using a combination of several bones enabled us to increase the rate of correct reclassification to a maximum of 97%. The application to archaeological skeletons proved the ability of this method to identify domestic horses and donkeys from archaeological samples. Correspondingly, some bones, and especially combinations of bones, provided good rates to identify hybrids. This method has proved reliable in detecting the presence of donkeys and hybrids from the archaeological samples of equid bones, and should enrich our knowledge regarding their spread across Europe.