[Jacob-list] Occypital condylar dysplasia

[Pbs123 at aol.com]

In a message dated 4/19/02 9:58:46 AM Eastern Daylight Time, 
NLGrose at Yadtel.net writes:


> How does this compare to other sheep with horns? ...With two horned Jacob 
> rams?
> We have noticed massive skulls on the back of the heads of these 
> characters, I had assumed it was a primitive trait to support the horns 
> when fighting. Males of primitive sheep seem to not be geared toward long 
> life, only toward getting a chance to pass on their genes.
>  
> 

       If there is one characteristic that distinguishes jacob four-horn rams 
from other horned sheep it's the inordinate thickening that takes place in 
the skull with age, especially in the rear as you mentioned. Two horned rams, 
both jacob and those of other breeds, never attain anywhere near the kind of 
mass you see in those with four horns. This is explained by the fact that 
multiple horns almost never exhibit a symmetrical growth pattern.
        Bone thickening takes place in response to stress. In this case the 
unorthodox positioning of polycerate sheep horns results in stresses on the 
skull during fights that are extreme and unnatural. 
       The vertical surface at the rear of the skull is the point of 
attachment for a large number of muscles that cause various head movements. 
When head butting, unusual  impact forces cause the head to torque and 
greatly stresses the connection between the muscles and the skull. For this 
reason the rear of the skull often accrues great amounts of roughened, 
irregular bone.  
       This thickening is called pachyostosis and is seen, for example, in 
the bones around the ears of whales that dive to great depths in the ocean. 
There is however another related phenomenon that I see in jacobs, both male 
and female, called hyperostosis. In hyperostosis, cartilage in certain areas 
of the skull are unnaturally replaced by bone with time. This often results 
in the skull appearing as one massive bone, as opposed to various bones 
separately by "mortise" joints.

Peter 
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