A Taurus steer in Hortobagy, Hungary

Steers can be very interesting to look at as they differ in morphology from functional bulls. They grow taller, have longer legs, longer horns and a longer snout. Therefore, they are more aurochs-like in these respects. One possible explanation might be that castration reduces the effect of the developmental delay caused by domestication that results in the morphology we see. So the horns continue to grow when they would stop growing in a functional bull, and so also other parts of the skeleton. You see that very clearly in Chianina steers, which have considerably longer horns than functional bulls of the breed. Thus, the horns of domestic bulls which are almost always shorter and less curved than in the aurochs, even in aurochs-like breeds, might be another symptom of so-called paedomorphy. 

The morphology of steers is a very strong hint that development contributes a lot to the typical domestic cattle morphology, and not mutated novel alleles for the actual traits alone. Altering the developmental processes and timing will result in the domestic phenotype, reversing these changes might let the aurochs phenotype surface again (for the trait looked at). It is therefore always interesting to look at steers and compare them with functional bulls of the same breed. 

In Hortobagy, where they have the largest Taurus cattle breeding site with about 400 individuals, they also have bull herds. Some of these bulls are castrated. I have seen couple of photos of Taurus steers before, and recently I discovered a photo online. Go here for the photo. 

The hump is well-expressed, the colour and the head look like those of a cow. As the degree of melanisation in the fur is linked to testosterone level in E+ cattle, the steer does not have enough testosterone to develop the black colour of bulls and therefore it retained the reddish brown colour scheme of a cow. Castrated banteng bulls show the same phenomenon (van Vuure, 2005). The horns grew much longer than in functional Taurus bulls and thus are more strongly curved. Not as curved and inwards-facing as in aurochs, but more so than in functional Taurus bulls. 

It is particularly interesting to look at the horns of steers, as they reveal the genetic potential for horn growth and curvature that functional bulls of the breed have. While in functional individuals the horns might only be shaped like bananas, they actually might have the genetic potential for an aurochs-like horn curve if they were not stopped to grow by development. 

This, in turn, has implications for “breeding-back”. It means that selective breeding on the actual horn shape might not be that effective, because the “error” does not lie in the horn genetics of the animal, but rather the developmental calibration of the whole organism. The insufficient horn curvature is thus a symptom of the basic domestic nature of the animal, which is even harder to reverse by selective breeding. As long the animals are domestic, their horns will stop growing earlier than in an aurochs and thus are shorter and less curved. 

However, some Lidia and Maronesa bulls happen to have an aurochs-like horn shape despite being domestic and displaying other domestic traits.