Feh, C., de Mazieres, J. 1993. Grooming at a preferred grooming site reduces heart rate in the horses. Animal Behaviour 46, 1191-1194.

Being groomed has a calming effect as reflected in heart rate reduction.It is commonly suggested that the principal function of allogrooming is to reduce social tension between group members, but direct evidence of the physiological consequences of grooming at particular sites is lacking. By filming allogrooming sequences in a herd of Camargue horses, Equus caballus , their preferred grooming site, which lies on the lower neck, was identified. Experimental imitation of grooming at this site reduced the heart rate of the recipient while grooming on a non-preferred area did not, in both adults and foals. This preferred site lies close to a major ganglion of the autonomic nervous system