Investigator John Splettstoesser discusses a research question posed (in “The Friendship Letter,” No. 50, 2006) by his friend Michael Cooney:
Observations of groups of horses standing freely in a field have led to an intriguing question about their behavior in fly season, when flies attack them with vigor.
Quite often horses stand side by side but head to tail, so each horse can brush flies away from the other horse?s face with its tail. Whether they stand this way deliberately and for that reason is only speculation, but on long-term scrutiny (normally done by graduate students), the question to be looked at is whether they switch their tails at random, or whether the horse is reacting only when the adjacent horse is emitting some kind of signal to inform the other horse that ?flies are about my face, please switch your tail.?
It is also possible that a horse switches its tail when a fly is bothering its own face, although the tail is on the wrong end of the horse to be effective in that case. A horse might switch its tail as a result of flies bothering its posterior, which has some merit, although irrelevant in a case of mutual switching for its neighbor.
It would seem that the position of side by side and head to tail has merit in the fly situation, but is it also valid when it is colder and flies are not a problem? Observation of horses in winter conditions also shows the same configuration, however, so the matter of mutual assistance between horses does not seem to apply for the sake of flies (or other insects). Because an observer must be concentrating on only one end of the horse, the field study requires two graduate students, one for each end of the horse, although it has been noted since historical times that there often seem to be more rear ends of horses than there are horses.
Horses that have their tails wrapped up in a ?bun?, such as draft horses groomed for display, are at an extreme disadvantage in the case of insect pests, as one imagine, and have little alternative but to hope for windy days.