First of all Sorraia is evidently a rare (endangered) breed of horse that is indigenous to Portugal. They were re-discovered in the 20th centurary and there are re-establishment projects in the works. Check out how lovely they are! evidently they are most often grullo (wikipedia)!
The research project examined affiliations among the mares comparing them to the reproductive status, relatedness, age, and dominance of the individuals. An affiliation is defined as a "long lasting relationship between two mares which are reflected in spatial proximity and participation in affiliative interactions, such as mutual grooming" We all have seen this--- domesticated horses (mares and geldings)sometimes preferentially hang around one horse in particular. We catch them grooming each other etc... We make comments like "Beau and Magic are such buddies".
This study found that kinship (relatedness) did not significantly affect these affiliative relationships and that individuals tended to spend more time with others in the same reproductive state. They found these friendships were relatively stable as well but their strength decreases after foaling. I don't know about you but I am seeing many parallels between horses and humans in this respect. :)
I think this research highlights the importance for equine friendships. So what could we learn from this research? Maybe we can all try and be more cognisant of these lasting friendships in our horses when we are moving them around between pastures and between barns etc. I know I have moved Bodhi several times now forcing him to loose several close friends. This of course can't always be helped but if we pay attention to the natural bonds in our horses then we can reduce their stress by not unnecessarily splitting them from their good friends.
Also for managers of breeding facilities it seems this paper highlights the importance of keeping brood mares together especially when they are pregnant. It seems they form the strongest bonds.
Here is another article summarizing the paper
Filipa Heitor and Luis Vicente 2010. Affiliative relationships among Sorraia mares; influence of age, dominance, kinshop and reproductive state. Ethology, 28, 133-140.