January 25, 2010 – PZP’s Effects, part 2: Behavior

Wild horses have very well-defined social characteristics. It is thus important that management not cause significant changes in their behavior. PZP’s effect on the behavior of wild horses has been the subject of much speculation, debate, and controversy. However, discussions of such effects should be in the context of findings from well-designed scientific studies. The results of such studies have been performed, and these will be the topic of today’s discussion.

We are asking what seems to be a straightforward question: Do wild horses treated with PZP behave different than untreated wild horses? In reality, this is a pretty complex question to try to answer. To start with, a large sample of treated and untreated individuals must be defined so that thorough observations are made. Also, there must be a set of defined behaviors that can be observed and measured. Numerous observations must then be made over a significant length of time so that a wide range of data is collected. This data must then be analyzed so that any statistically significant differences in the behavior of treated and untreated mares can be detected. This is all made more complex by the fact that wild horses are so socially complex. Despite these challenges, there have been studies done to try and answer the above question.

In the late 1990’s, a study (Powell, D.M., 1999) was done to try to determine how PZP might be effecting the behavior of mares on Assateague Island. During this study, 43 mature females were observed through 1997. It was concluded that PZP did not cause significant changes to the behavior of the studied horses. Another study done on a different barrier island was recently published (Nunez, C.M.V., Adelman, J.S., Mason, C., and Rubenstein, D.I., 2009). This one was done on the Shackleford Banks horses of North Carolina. During this study, 30 females were observed 102.2 hours from December 2005 to February 2006. The study found indications that during the non-breeding season, there was a higher rate of interchange among treated mares than untreated mares. Thus, there are possible differences in the findings of these two studies.

These aforementioned studies regarded behavioral effects among two barrier island herds. There have been some questions asked about the possible behavioral differences that exist between herds in different parts of the country. A recent study (Ransom, J.I., 2009) sought to detect differences between treated and untreated wild horses in the West. This study will be the primary focus of today’s discussion as it is unique in focusing on Western herds. The Western herds studied were in the McCullough Peaks Herd Management Area, the Little Book Cliffs Wild Horse Range, and the Pryor Mountain Wild Horse Range. Observations were made on eight to ten harems in each of these herds. The harems represented the typical range of possible harems in each herd, but they all had at least one stallion along with at least one mare that had been treated with PZP and one mare that had not. Also, an ethogram (a guide defining specific behaviors) was developed to ensure that observations on behaviors remained consistent throughout the study (Ransom, J.I. and B.S. Cade. 2009). Observations were also made on possibly influential factors such as harem size and stability, ages of observed individuals, and body conditions. The study took place from 2003 to 2006. Observations were made in the morning, afternoon, and evening. For each harem, at least nine observations were made each of these time periods each month (April to October). This resulted in a large collection of observations; there were 527 hours of observations made in the McCullough Peaks, 704 hours in the Little Book Cliffs, and 623 hours in the Pryors. These observations were then analyzed so that possible relationships between the different observed variables could be determined.

The study concluded that when a number of factors are considered, PZP minimally influences changes in the behavior of wild horses. Among 6 to 14 year old mares, treated mares showed more reproductive behaviors than untreated mares. In this same age group, it was found that stallions herded untreated mares more than treated mares, though the presence of a foal decreased the amount of time that these mares were herded. Among females over the age of 6, it was found that there were differences in the amount of time treated and untreated mares spent performing certain activities, such as feeding and resting. Differences in spatial relationships between stallions and treated and untreated mares were minimally present; stallions stayed closer to 2 to 5 year old treated mares than untreated mares of the same age group. No other differences in spatial relationships were found. There was evidence that the possible secondary effects of PZP can cause some changes. For example, stallions were found to be more concerned with females with higher body condition scores than those with lower scores, though there were no significant differences in the way stallions otherwise treated and untreated mares. It was found that behaviors varied between the different populations studied; this is thought to be due to differences in the areas the herds live in. For example, during this study, the Pryor horses tended to concentrate in small areas while the other two herds were more distributed across their areas. This could thus lead to more possible harem interaction in the Pryors, which could lead to different behaviors than those found among horses in the other two herds. Given the rarity of significant behavioral changes that could possibly be attributed to PZP, the author of the study believes that “PZP is a promising alternative to traditional hormone-based contraceptives.” The study also recommends that further work be done to determine how different treatment strategies and herds in different ecosystems influence behavioral effects.

Behavioral effects are possibly one of the most difficult of the possible effects of PZP to understand. It makes sense that this is the case as other effects can be more easily observed and measured. It is one thing to count injection site reactions among treated horses, but it is an entirely different issue to make substantial observations on behavior among treated and untreated horses. We have seen the results from three different studies today, and the results from these studies have differences. Two of the studies indicate that PZP has minimal effects on the behavior of wild horses while the other study indicates that it can possibly lead to higher rates of interchange during the non-breeding season. There are even differences between the two studies that indicate that PZP has minimal effects on behavior. I think a major point to realize with behavioral studies is that they are very complex. They are complex to design, they are complex to perform, they produce complex data that requires complex analysis, and their results are often complex to understand. They are not something that can be casually performed. Nonetheless, they are important; and so it is likely that more of these types of studies will be done in the future.

Though I have tried to summarize these studies, I think that it is worthwhile to take a look at them. I have links to full text versions of all these studies but Powell, D.M., 1999. Next time, we will discuss another complex and controversial topic, which is whether or not PZP has an impact on the number of out of season foals born.

References

Nunez, C.M.V., Adelman, J.S., Mason, C., and Rubenstein, D.I., 2009, Immunocontraception decreases group fidelity in a feral horse population during the non-breeding season: Applied Animal Behaviour Science, v. 117, p. 74–83

Powell, D.M., 1999, Preliminary Evaluation of Porcine Zona Pellucida (PZP) Immunocontraception for Behavioral Effects in Feral Horses (Equus caballus): Journal of Applied Animal Welfare Science, v. 2, p. 321-335

Ransom, J.I., 2009, Understanding the influences of immunocontraception of equids through competing models of behavior: Master’s Thesis, Colorado State University

Ransom, J.I. and B.S. Cade. 2009, Quantifying equid behavior–A research ethogram for free-roaming feral horses: U.S. Geological Survey Techniques and Methods Report 2-A9

Advertisements
Published in: on January 25, 2010 at 7:53 pm  Comments (5)  

The URI to TrackBack this entry is: https://pryorwild.wordpress.com/2010/01/25/january-25-2010-pzps-effects-part-2-behavior/trackback/

RSS feed for comments on this post.

5 CommentsLeave a comment

  1. SInce you ask commenters to identify themselves, shouldn’t the author of this article do the same? Please let me know who wrote this.

    Ms. Nunez’s study (Princton Univ.), which you site in your references, reaches a different conclusion than the study you chose to concentrate on. She states that “Our study shows that the application of PZP has significant consequences for the social behavior of Shackleford Banks horses. In gregarious species such as the horse, PZP application may disrupt social ties among individuals and inhibit normal social functioning at the population level.” It would be nice for the sake of balance to get some of the info from the other investigators.

    they further found: “The behavioral effects of the immunocontraceptive agent porcine zona pellucida (PZP) have not been adequately studied. Important managerial decisions for several species, including the wild horse Equus caballus, have been based on this limited research. We studied 25 horses on Shackleford Banks, North Carolina to determine the effects of PZP contraception on female fidelity to the harem male. We examined two classes of females: contracepts, recipients of the PZP vaccine (n=17); and controls, females that have never received PZP (n=8). We conducted the study during the non-breeding season from December 2005 to February 2006, totaling 102.2 hours of observation. Contracepted mares changed groups more often than control mares (P = 0.03), the number of group changes increasing with the number of PZP applications (P = 0.01). In addition, contracepts visited more harem groups than did control mares (P = 0.02), and exhibited more reproductive interest (P = 0.01).”

    I look forward to seeing my comment posted. thank you.

    • Hello,
      Thanks for your comment. I really appreciate that you’ve taken the time to understand this issue enough to write a comment like this. My name is Matt Dillon, and I write everything posted on this blog.

      I agree that the Nunez et al study is interesting. I think it would be very interesting for a follow-up study to occur on the Shackleford horses that has a higher sample set and takes into account other factors, such as the removal of young horses from untreated mares. I’ve had the privilege to talk with Ms. Nunez, and I’ve been able to learn more from her on this study. I do mention the study earlier in this post: Another study done on a different barrier island was recently published (Nunez, C.M.V., Adelman, J.S., Mason, C., and Rubenstein, D.I., 2009). This one was done on the Shackleford Banks horses of North Carolina. During this study, 30 females were observed 102.2 hours from December 2005 to February 2006. The study found indications that during the non-breeding season, there was a higher rate of interchange among treated mares than untreated mares. Thus, there are possible differences in the findings of these two studies. The change in behavior discussed in the Nunez et al study related to non-breeding season interchange rates; I feel that I touched on this in my writeup. The study did not really go into any other effects beyond this one.

      I only briefly mentioned the two barrier island herd studies for a few reasons. For one, people (from all sides) often are critical of PZP studies done on barrier island herds. Thus, I thought that the study done on Western horses would be an interesting one to discuss. Second, the Nunez et al study seems to be brought up a lot in these conversations, and so I thought it may be useful to focus on a different study. Also, I feel that the study by Jason Ransom was very comprehensive. His results reflected hundreds of hours of observations on many horses in three unrelated herds over many years. These two barrier island studies did not have such a sample set. Numerous behaviors were studied in the Ransom study. He also took into account many different factors that I feel are relevant to these studies; there are obvious risks with correlation not implying causation in discussions of the effects of PZP on behavior. This isn’t to say the other studies were faulty; they are what they are.

      The Ransom study does allude to the fact that there is a good probability that behavioral effects will vary from herd to herd just as the baseline of behavior of horses in different herds varies. Perhaps this is one of the reasons why there are basically three different conclusion sets in the three different papers (The Ransom study describes results that are different than those of the Powell study). I think that it would be generally agreed on from most parties that this is a topic worthy of further study in more unrelated herds across the country.

      For me personally, whenever I read reports on any of the effects PZP, or other fertility control agents, may cause, I think about whether the horses that would possibly be affected would be better off getting removed. It’s not the most positive way of thinking, but it is fairly realistic: If fertility control is not used, then roundups probably will be. Each strategy has its pros and cons, and so such comparisons between the two are very important when formulating opinions about wild horse management.

      I hope this has helped answer your questions. Don’t hesitate to further discuss these issues if you would like.
      Matt

  2. The behavioral effects of the immunocontraceptive agent porcine zona pellucida (PZP) have not been adequately studied. Important managerial decisions for several species, including the wild horse Equus caballus, have been based on this limited research. We studied 25 horses on Shackleford Banks, North Carolina to determine the effects of PZP contraception on female fidelity to the harem male. We examined two classes of females: contracepts, recipients of the PZP vaccine (n=17); and controls, females that have never received PZP (n=8). We conducted the study during the non-breeding season from December 2005 to February 2006, totaling 102.2 hours of observation. Contracepted mares changed groups more often than control mares (P = 0.03), the number of group changes increasing with the number of PZP applications (P = 0.01). In addition, contracepts visited more harem groups than did control mares (P = 0.02), and exhibited more reproductive interest (P = 0.01).

  3. Hi Matt,
    Just want you to know these PZP posts are fantastic. Gets people thinking and discussing the merits, based on what science has been done. But again, as you say, if not PZP, then what? More roundups. More horses sitting in government holding pens? I hope not.
    As an aside, I know you have been busy compiling these blogs, but whenever you get out to the Dryhead, I’d love to know how Blizzard, and his mare and foal are doing. I think Blizzard is one of the most handsome stallions. I will never forget hearing his nicker from the ridge, but not being able to see him. And then, out of the juniper he came, the mares and foal in front of him. His copper coat reflected the late afternoon sun, and it was sure one of those sights that made you feel glad to be alive, as they headed out across Mustang Flats towards the river.
    Regards, Wendy

  4. This blog is a wonderful research for anyone interested in learning more about the practical implementations of pZP. Thank you for sharing your knowledge and enthusiasm!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: