Op-ed: Wild horse contraception not without unintended consequences

In a recent op-ed, Jay Kirkpatrick suggested that our research on the side effects of contraception in feral horses was conducted at an "unusual location," implying that it is uninformative for managing feral horses. Although we agree with Kirkpatrick that contraception is the best option for managing feral horses in the U.S., we disagree with several of his statements about our work and encourage readers to evaluate this science for themselves.

Our research focuses on Cape Lookout National Seashore, N.C. This region is ecologically similar to Assateague Island, where Kirkpatrick has studied contraception with porcine zona pellucida (PZP) for decades. Our writings argue that PZP is highly valuable and effective, but, as with any good tool, can be misused. We maintain that any "science-based and workable strategy for helping horses," as Kirkpatrick puts it, should include analyses of PZP's unintended consequences. Below, we address three of Kirkpatrick's specific points, referencing relevant studies.

First, Kirkpatrick states that horse advocates have relied on "data regarding the small and unusual horse population at Cape Lookout..."

In actuality, this population may not be so "unusual." PZP-treated females at Cape Lookout change social groups more often, display more reproductive behaviors and experience more male harassment. Similarly, treated females from three populations in the western U.S. (Colorado, Wyoming and Montana) received more reproductive behaviors from males. Such results have not been seen on Assateague. The fact that behavioral changes were documented in four out of five populations raises the question, is Cape Lookout "unusual"?

Second, Kirkpatrick challenges horse advocates to "identify wild horse populations where PZP has disrupted the social structure or social behaviors of the horses. (By definition, this means the disappearance of harem groups, bachelor groups, social hierarchy or other fundamental social behaviors.) Explain why this hasn't even happened in … Cape Lookout ... "

Although Kirkpatrick is a highly accomplished scientist, he is not a behavioral ecologist and his definition of disruption does not reflect a consensus among animal behaviorists. Behavioral ecologists have long considered mare fidelity, group stability and reproductive behavior crucial to the well-being of natural feral horse populations. As referenced above, if we include these behaviors, several populations have shown important behavioral changes with PZP treatment. Suggesting that important behavioral changes must include massive reorganizations of a species' social system, as Kirkpatrick does, sets an unrealistic standard.

Finally, Kirkpatrick asks horse advocates to "identify any wild horse populations where PZP has increased the length of the foaling season and resulted in decreased foal survival. Include Cape Lookout ... "

PZP has altered the foaling season in several populations. At Cape Lookout, mares treated repeatedly with PZP gave birth over a wider range of months and later in the year than did untreated mares. In western populations, previously treated mares also gave birth later in the season, even after stopping PZP treatment.

Kirkpatrick is correct that data on these foals' survival is not available, reflecting an important and open question. It is reasonable, however, to hypothesize that animals born later in the season, when fewer nutritional resources are available, would not fare as well — an established principle across diverse species.

Again, we agree with Kirkpatrick that PZP is the best means currently available for managing feral horses in the U.S. His recent op-ed accurately highlighted several important benefits of PZP, including increased body condition, increased longevity and, critically, the need for fewer roundups. However, dismissing research that identifies PZP's unintended consequences also dismisses opportunities to optimize wild horse management. Our response merely serves to clarify our research and reiterate our position that as with any valuable tool, PZP's use should be carefully and continually evaluated when possible.

Cassandra Nuñez is adjunct assistant professor of Natural Resource Ecology and Management at Iowa State University. Jim Adelman is an assistant professor in the Department of Natural Resource Ecology and Management at Iowa State University. Dan Rubenstein is a professor at Princeton University who studies the behavior and ecology of horses, zebras and wild asses.