FoA’ response to Jay Kirkpatrick’s Op-ed in the Salt Lake City Tribune:
Wild-horse contraceptives are based on sound science
Into/ Disclaimer: As an initial matter, it is odd that Kirkpatrick places the burden on
Friends of Animals to disclose and study the effects of a pesticide which he manufactures
and promotes. Nonetheless, we welcome the opportunity to respond to his many questions
and attacks.
Kirkpatrick’s Question: 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.) 1
Explain why this hasn't even happened in the population where this theory originated (Cape
Lookout).
Response: Multiple studies confirm that PZP has caused social disruption in Cape Lookout
area, as well as other herd management areas.

Shackleford Banks Island, North Carolina (Cape Lookout).

Mares drugged with PZP change groups or bands approximately 10 times
more than untreated mares. Such animals may therefore be at higher risk of
chronic stress. (Madosky, et al., 2011; Nuñez et al., 2009).

Mares changing groups often experience increased harassment from males,
including chases, and increased reproductive interest; and increased
harassment from resident females, including kicks, chases, and bites. (Nuñez,
et al., 2014).

Additionally, mares transferring bands more frequently are often subject to a
negative feedback loop: their behavior leads to harassment which induces
additional group changes, resulting in further harassment and so on. (Nuñez,
et al., 2014).

This social instability has significant impacts on stress physiology of the
animals. The stress has at least some lasting effect on mare physiology, as

documented by higher cortisol levels for at least two weeks post- group
transfer. (Nuñez, et al., 2014).
I am not sure where Mr. Kirkpatrick gets his definition of disruption, which suggest that
anything short of the total elimination of all herds and social behavior cannot qualify as
social disruption. To the contrary, wild horses live in highly structured social units, and
experts can and have identified disruptions through careful observations and testing of
hormones in their fecal matter.
1
1


Other studies of the Shackleford Banks Island confirm the negative effect of
the PZP on harem stability, and that mares drugged with PZP visited and
changed groups significantly more than mares never treated. Once the
majority of these mares had been treated with PZP, there was a significant
increase in the historic rate of group changes (from approximately 10% up to
66-70%). The change in behavior was observable in breeding and nonbreeding seasons regardless of how many time the horses were drugged with
PZP. (Madosky, et al., 2010; Nuñez, et. al. 2009).
A study of the three herds (Little Book Cliffs Wild Horse Range, located in Mesa
County, Colorado; McCullough Peaks Herd Management Area in Park County,
Wyoming; Pryor Mountain Wild Horse Range, located in Bighorn County, Wyoming
and Carbon County, Montana) confirmed that PZP contributes to increased
reproductive behaviors both from and towards the treated mares at suboptimal
times of the year, causing additional stress on mares and foals. (Ransom, et al.,
2013).
Kirkpatrick’s Question: Identify any wild horse population where PZP has disrupted social
organization or social stability to the point of decreasing reproductive success. Explain why
this hasn't happened where the theory originated (Cape Lookout).
Response:

This question regarding reproductive success is puzzling, as the goal of PZP is to
decrease reproductive success. According to Kirkpatrick’s own research, the
administration of PZP for 5-7 years is associated with some ovulation failure and
depressed urinary oestrogen concentrations (Kirkpatrick, et al. 1996).

Another study of three herds (Little Book Cliffs Wild Horse Range, located in Mesa
County, Colorado; McCullough Peaks Herd Management Area in Park County,
Wyoming; Pryor Mountain Wild Horse Range, located in Bighorn County, Wyoming
and Carbon County, Montana) found that the probability of post-treated females
producing offspring was 38.5% lower than for untreated females, after controlling
for differences due to age, precipitation during conception year, and fertility history
(Ransom, et al., 2013).

Linklater, et al. (1999) proposed that stable relationships between females and a
harem male or group of males serve to reduce intraspecific aggression and the
resultant reproductive costs. Altering harem stability may therefore alter overall

reproductive rate, effective population size, and patterns of genetic diversity critical
to maintaining long term stable populations.
2
Kirpatrick’s Question: Identify any wild horse populations where PZP has increased the
length of the foaling season and resulted in decreased foal survival. Include Cape Lookout in
this answer.
Response:

Studies on mares in Shackleford Banks, North Carolina (Cape Lookout) for 4 years
pre- and 8 years post-contraception management with PZP demonstrated that
foaling occurred over a significantly broader range than it had before the
contraception program. In addition, current recipients of PZP foaled later in the year
than did prior recipient and non-recipient mares. Females receiving more
consecutive PZP applications gave birth later in the season than did females
receiving fewer applications. (Nuñez, et al., 2010).

A study of the three herds (Little Book Cliffs Wild Horse Range, located in Mesa
County, Colorado; McCullough Peaks Herd Management Area in Park County,
Wyoming; Pryor Mountain Wild Horse Range, located in Bighorn County, Wyoming
and Carbon County, Montana) confirmed that PZP contributes to increased
reproductive behaviors both from and towards the treated mares at suboptimal
times of the year (Ransom, et al., 2013).

This is significant because offspring born at this time face nutritional and
thermoregulatory challenges not experienced by their counterparts born during the
normal foaling season (during the spring and summer), potentially making
developmental benchmarks difficult to achieve. (Ransom, et al., 2014; Nuñez, et al.,
2010).

Without the trophic support of abundant forage, the likelihood of these foals’ deaths
is quantifiably increased as they are born later after the peak forage of the summer
solstice. Ransom, et al. (2013) found that the foals’ risk of mortality increased 1.4%
for every ten days after peak forage that birth occurred. This may seem insignificant,
but for foals born 180 days after the summer solstice (roughly around the winter
solstice), the risk of mortality increases over 25%. Even if the foals survive their
disadvantage, the reduction in forage availability can prevent them from reaching
critical developmental milestones (Nuñez, et al., 2010).
Kirpatrick’s Question: Identify any wild horse population where PZP treatment has resulted
in a decrease in body condition scores, or an increase in adult or foal mortality, or a decrease
in longevity. Conversely, identify any wild horse populations where PZP treatment has
increased body condition scores, decreased mortality and increased longevity
Response:

As explained above, mares treated with PZP are more likely to give birth later in the
season, increasing the rate of foal mortality.
3

Studies on the Shackleford Banks Island herd demonstrated that mares which
change groups more often (such as those treated with PZP) exhibit increased stress
levels and that this increased stress is maintained for at least two weeks after the
group changes occur (Nuñez, et al., 2014). For mares that change herds frequently,
these stress levels can be elevated constantly.

Stress to wild horses causes sustained elevated cortisol levels, which can be
extremely physiologically damaging (Nuñez, et al., 2014). This stress can cause a
multitude of adverse physical effects, including negative impacts to cardiovascular
function, inhibition of reproduction, compromised immune response, and
neurological issues (Nuñez, et al., 2014). Elevated stress response to transferring
herds can also cause increased offspring mortality and increased parasite loads
(Nuñez, et al., 2014).
Kirpatrick’s Claim: The groups have stated, untruthfully, that the Humane Society of the
United States holds a patent on PZP. If this is true, where is the patent number?
Response:

Friends of Animals stated that the Humane Society is the registrant of PZP. When
the Humane Society obtained ESA registration for PZP in 2012 from the EPA, the
organization never provided evidence that PZP doesn’t have negative side effects…it
just provided information about the efficacy of PZP and actually requested waivers
for most of the studies ordinarily required from an applicant seeking pesticide
registration—including a toxicity study, ecological effects and environmental fate
guideline study.
Kirpatrick’s Claim: The groups state the makers of PZP and the Humane Society profit from
the vaccine, but it's actually produced on a non-profit basis. If there were a profit to be made,
why aren't for-profit companies making the vaccine (which is not patented)?
Response:

HSUS benefits from its message that it has an answer to wild horse “problem.” It has
received millions of dollars in grant money for PZP research.
Kirpatrick’s Question: Why, after 15 years management with PZP in the highly social species
of African Elephants, have there been no changes in social organization or social behavior?
Why, after 25 years of PZP management of 85 different species of animals in more than 200
zoos worldwide, have there been no health or behavioral problems among the treated
animals, seen daily by veterinarians and keepers?
Response:

Managing animals in zoos does cause health and behavioral changes to these
animals. Friends of Animals believes these animals have a right to live in the wild
4
free of manipulation and exploitation. Zoos and industries that seek to keep animals
in captivity may claim that these animals are better off in human managed
environment. However science indicates otherwise and Friends of Animals believes
wild horses, and other wild animals, have a right to breed, raise families, and foster
a new generation.
As Aldo Leopold said, “There are some who can live without wild things and some
who cannot.” Friends of Animals cannot.
Kirpatrick’s Question: PZP has helped end or reduce the need for roundups in all herd
management areas where it is routinely used. Roundups and removals continue in areas
where it's not used. What do groups expect to change to make these facts untrue?

FoA does not believe there is a “need” for roundups. We seek to keep wild horses
wild and fight policies that effectively manage them to extinction. Moreover, PZP
does not replace roundups. An example is the BLM is currently seeking comments
on a proposed Roundup in the Pryor Mountains, where PZP has been administered
since 2001. Even the BLM admits in most cases they will still have to conduct
roundups, administer the PZP by hand injection and then re-release the mares back
out on the range. In the BLM’s own words: Darting is generally not practical for BLM
because it is difficult to approach most wild horses closely enough on Western
rangelands. For this reason, the BLM mostly uses a longer-lasting, 22-month, pelleted
PZP agent (PZP-22). The pelleted vaccine has been successfully administered by
darting into a few mares, but typically it is hand-injected after the mares have been
captured. This method of treatment means that during gathers, more mares need to be
captured (for treatment and release) than would actually be removed from the range
if removal was the only goal. While this is usually possible, it can be difficult to capture
a large enough fraction of the population so that significant numbers can be treated
and released.
So despite what PZP proponents say, PZP will never replace roundups unless wild
horse are exterminated from our public lands, and FoA does not view that as an
acceptable solution.
Kirpatrick’s Question: Provide injury and mortality figures for horse populations treated
with PZP and for the same populations during the course of round ups and removal.

Again, FoA rejects the false dilemma of roundups or PZP. The truth is that the two
go together. As indicated above, BLM has stated that the administration of PZP
generally requires rounding up the horses. Moreover, both are based on the premise
that wild horses are a pest that needs to be controlled or suppressed. On the other
hand, FoA believes wild horses should be free and allowed to fill their niche in the
ecosystem.
5

Studies of mares treated with PZP, that changed groups more frequently showed
increased stress levels comparable to the highest levels exhibited by Grevy’s zebra
during captivity after capture and relocation. (Nuñez, et al., 2014).
Kirpatrick’s Question: Do they believe use of public lands for agriculture, energy
development, mining or recreation will be reduced or eliminated to accommodate
unmanaged populations of wild horses?

Yes, we believe that public lands should include habitat to accommodate wild
horses. FoA recognizes that many might see our position as a “pipe dream.” Today,
public lands in the United States are a bleak place—largely ecologically unsound
because of extensive human involvement. We have killed off or limited the number
of nearly every native animal; we have over utilized resources; and we have filled
the landscape with non-native species. But still, it is a dream we cannot give up on; if
we do, then any chance of a return to ecological balance is lost forever. But if we
continue to push, maybe we can someday soon see a real push for true ecological
zones on public lands; zones where the landscape and animals are free from
exploitation and management.
Kirpatrick’s False Dilemma: The truth is that, while these groups await an unachievable
ideal for wild horses, populations continue to increase, as do roundups and removals. This
management scheme costs U.S. taxpayers more than $75 million annually and results in the
death and injury of horses. Thousands of horses are removed from their habitats and kept in
long-term holding. And the problem only continues.
Response:

True if we eliminate wild horses, we would also eliminate roundups, death and
injury. However, we value wild horses and believe that we should focus on
eliminating the roundups and management practices rather than eliminating wild
horses. Again, FoA rejects the false dichotomy of roundups or PZP. Instead, there’s a
third option. We believe there should be place on our public lands for wild animals
to live freely, including wild horses. The BLM should give back to wild horses the
land in the original Herd Areas that has been stolen from them since the Wild Horse
and Burro Act was passed in 1971. Cattle and sheep grazing should not be allowed
in Herd Management Areas.
Kirkpatrick’s claim: True friends of these animals support solutions that will help reduce
and end the problem, not prolong it.
Response:

If one views wild horses as a “problem” then Friends of Animals’ position may seem
odd. However, wild horses are not a “problem,” they are highly social, majestic
creatures that deserve the right to live free of human manipulation and exploitation.
6
*Please see Friends of Animals Petition to Cancel the Registration of PZP for a more in
depth discussion of these issues as well as additional negative impacts of PZP, including
decreased genetic diversity and increased risk of extinction.
7
Works Cited
Kirkpatrick JF, Turner JW, Liu IKM, FayrerHosken R. (1996) Applications of pig zona
pellucida immunocontraception to wildlife fertility control. Journal of Reproduction and
Fertility 50: 183–189.
Linklater, W. L., Cameron, E. Z., Minot, E. O. & Stafford, K. J. (1999). Stallion harassment and
the mating system of horses. Animal Behaviour, 58, 295-306.
Madosky, J. M., Rubenstein, D. I., Howard, J. J., & Stuska, S. (2010). The effects of
immunocontraception on harem fidelity in a feral horse (Equus caballus) population.
Applied Animal Behaviour Science, 128(1), 50-56.
Nuñez, C. M., Adelman, J. S., Mason, C., & Rubenstein, D. I. (2009). Immunocontraception
decreases group fidelity in a feral horse population during the non-breeding season.
Applied Animal Behaviour Science, 117(1), 74-83.
Nuñez, C. M., Adelman, J. S., & Rubenstein, D. I. (2010). Immunocontraception in wild horses
(Equus caballus) extends reproductive cycling beyond the normal breeding season. PloS
one, 5(10), e13635.
Nuñez, C. M., Adelman, J. S., Smith, J., Gesquiere, L. R., & Rubenstein, D. I. (2014). Linking
social environment and stress physiology in feral mares (Equus caballus): Group transfers
elevate fecal cortisol levels. General and comparative endocrinology, 196, 26-33.
Ransom, J. I., Cade, B. S., & Hobbs, N. T. (2010). Influences of immunocontraception on time
budgets, social behavior, and body condition in feral horses. Applied Animal Behaviour
Science, 124(1), 51-60.
Ransom, J. I., Hobbs, N. T., & Bruemmer, J. (2013). Contraception can lead to trophic
asynchrony between birth pulse and resources. PloS one, 8(1), e54972.
8