The horns of the African aurochs and its evolutionary implications

I did a post on the African aurochs, Bos primigenius mauretanicus, a few years ago. The African aurochs is said to be morphologically very similar if not identical to the European aurochs, so that van Vuure (2005) concludes it might be exclusively geographical [1]. The only difference to the European aurochs mentioned in my old article is the fact that bulls likely had a colour saddle. However, there might be morphological differences in the horns of the African aurochs, which have previously not been recognized in the literature. 

 

In 2015, the oldest aurochs skull found so far was described [2]. It is from Tunisia and is very large-horned. But what was interesting to me about the horns was not their size, but their shape and orientation relative to the skull. They have an angle of 40° relative to the snout, which is narrower than in the European aurochs, where the range of angles of the horns relative to the snout is between 50 and 80° on average, and larger in single exceptions (such as in the skull exhibited at Horsholm). Also, the horns seem to be dorsoventrally compressed at the base or proximal half, while the distal end of the horn seems to be round in cross section. 

Due to the age of the skull (700.000 years), I was unsure whether to classify it as a member of B. p. mauretanicus, or maybe as a basal aurochs that lived before subspecies differentiation. 

However, I recently found a depiction of another African aurochs skull, that, because of its geologically younger age, certainly is a member of B. p. mauretanicus. The anatomical drawing is from a work from 1931 and is labelled as Bos opisthonomus, which is a junior synonym of B. p. mauretanicus. It shows the same anatomy as the 700.000 years old skull from Tunisia: horns facing forwards in a narrow angle (35°) and dorsoventrally compressed at the base or proximal half. The image is from Duerst (1931) [3]. 

An anatomical drawing of an African aurochs skull, from Duerst 1931

Thus, it seems that these two traits (dorsoventrally compressed horns at the base, horns facing forwards in an angle sharper than in the European aurochs), were general anatomical traits of the African aurochs. That the horns of the African aurochs faced forwards in a sharp angle also fits historic descriptions, because Herodotos describes that in North Africa there were bovines with horns so long and forwards-facing that they had to graze backwards [1]. Likely this referred to the African aurochs. Also ancient depictions of African aurochs show horns growing parallel to the snout, what suggests that the horns were facing forwards in a narrow angle. In European aurochs, and sometimes also domestic cattle, the horns were/are oval in cross section at the base as well (in Pleistocene European aurochs more so than in Holocene ones) [1], but this trait is definitely more expressed in the African aurochs. 

 

Apparently, the aurochs varied in horn orientation from subspecies to subspecies. While the African aurochs had the narrowest angle, the Indian aurochs and possibly B. p. suxianensis had the largest angle, and the European aurochs was in between. Whether the difference in horn orientation has a functional purpose or if the variation was merely a result of genetic drift is unclear to me. 

 

It is interesting to speculate about the evolutionary implications of the horn anatomy of B. p. mauretanicus. There are two conflicting hypotheses on the origin of Bos and the aurochs. One of the hypotheses postulates that Bos and the aurochs in particular evolved in Africa from Pelorovis oldowayensis, the other one suggests that Bos originated from Leptobos in Asia, and the aurochs from the very large-horned Bos acutifrons in India. The fact that the horns of the Indian aurochs are more upright than those of the other aurochs subspecies seems to support the latter hypothesis, as the horns of Leptobos are rather upright. However, the fact that the horns of the African aurochs have such a narrow angle between horns and snout and the dorsoventrally compressed horn bases support the origin from Pelorovis oldowayensis, as this species has horns with a very narrow angle relative to the snout and dorsoventrally compressed horns. Other cranial traits seem to support an origin from Leptobos, on the other hand. The origin of Bos probably can only be resolved by finding more fossils of relevant taxa. 

 

Literature

 

[1] van Vuure: Retracing the aurochs – history, morphology and ecology of an extinct wild ox. 2005. 

[2] Martinez-Navarro et al.: The early middle Pleistocene archaeopaleontological site of Wadi Sarrat (Tunisia) and the earliest record of Bos primigenius. 2014. 

[3] Duerst: Grundlagen der Rinderzucht – eine Darstellung der wichtigsten für die Entwicklung der Leistungen und der Körperformen des Rindes ursächlichen, physiologisch-anatomischen, zoologisch-paläontologischen, entwicklungsmechanischen und kulturhistorischen Tatsachen und Lehre. 1931.