Houpt, K. A., Houpt, T. R. 1988. Social and illumination preferences of mares*. Journal of Animal Science 66, 2159-2164.

When allowed to control their own environment, horses spent the majority of their time (88 to 95%) outside a stall and half of their time in contact with other horses. ... When alone, the horses ate less and walked more. An increase in activity in stressful situations, such as weaning, has been noted in other studies (Houpt et al., 1983) and has been correlated with an increase in corticosteroids. ...Horses worked to increase illumination levels. An environment with some light and an environment with horses in the vicinity appeared to be preferred.*Also published in: Equine Practice 14, 11-16, 1992; Houpt KA, Houpt TR; Social and illumination preferences of mares.