My research explores changes in the social and sexual behaviour of feral horses that may be correlated with the application of the immunocontraceptive vaccine Porcine Zona Pellucida (PZP).

It has been suggested that stallions (mature males) in populations where mares (mature females) are treated with PZP experience more intermale aggression.

In this photo, Jasper (left) is heading off a threat from the older stallion, Doc. In recent years, Doc has become more territorial, and commonly runs off horses in his vicinity. Doc had charged Jasper's harem, and while the mares moved away to safety, Jasper turned to demonstrate his strength to Doc and prove he was not backing down.

Many conflicts between feral stallions are limited to posturing-or displays of strength. While this photo looks quite violent, neither stallion came into physical contact with the other during this encounter.