An interesting article on our sister site has me thinking about herd dynamics with my own horses.
Mares' Social Bonds Might Enhance Reproductive Success on TheHorse.com summarizes a larger study that seems to indicate the social structure of herds helps to increase mares' ability to conceive, carry, and deliver foals. The research was conducted on feral mares rather than domesticated horses, but author Christa Leste-Lasserre believes the behvior's benefits translate to horses in captivity.
I have always been an advocate of herd management -- keeping mares in groups rather than alone. (The rule I heard growing up was that horses should be kept in groups of "two, four, six, or more" due to pair-bonding.) Now, I'll be even more cognizant of the specific need for mares to bond with other mares.
I'm down to one broodmare now -- pastured with my yearlings and 2-year-old -- and have been on the lookout for another mare or two to add to my breeding program. Now I have just the excuse I needed....