Scientists at the Morris Animal Foundation have published their research on the effects of culling and the use of the contraceptive GonaCon-B.
Wild horses from Theodore Roosevelt National Park in North Dakota were studied for a year before and after no-kill culling and treatment of specific mares with contraceptives.
Culling was found to have a greater effect on the herds behavior than the use of contraceptive.
Excerpted from the study below:
There were no differences in any behaviors between treatment groups before GnRH (GonaCon-B) vaccination and culling. Females vaccinated against GnRH managed their time differently than controls. They fed less, rested more, travelled less, and performed more maintenance behaviors than their control counterparts, but engaged in the same amount of social behavior. The estimates across years, however, indicated that pronounced behavioral changes occurred in how stallions engaged with females in social behaviors. Stallions herded all females less and harem-tended more following the changes in population density, demography, and social perturbation that resulted from culling. The date of observation in relation to forage availability explained some differences between years, while the increased mean age of stallions as well as the decreased band size in 2010 also explained some of the differences. None of the models indicated that herding or harem-tending varied due to the treatment status of females, but rather implied that the paradigm shift in competition influenced how band stallions managed their family groups. Because the same stallions were observed in both years, the effect of age potentially reflected the additional year of experience each male possessed, in the context of competition pressure.
Behavior of feral horses in response to culling and GnRH immunocontraception
Jason Ransom, Jenny Powers, Heidi Garbe, Michael Oehler Sr, Terry Nett, and Dan Baker
•We examine behaviors of feral horses contracepted with GnRH
•We examine social behaviors of feral horses before and after management culling
•Differences in behaviors between treated and control female horses were minimal
•Social behavior of harem stallions changed markedly following population culling
Wildlife management actions can alter fundamental behaviors of individuals and groups, which may directly impact their life history parameters in unforeseen ways. This is especially true for highly social animals because changes in one individual’s behavior can cascade throughout its social network. When resources to support populations of social animals are limited and populations become locally overabundant, managers are faced with the daunting challenge of decreasing population size without disrupting core behavioral processes. Increasingly, managers are turning to fertility control technologies to supplement culling in efforts to suppress population growth, but little is quantitatively known about how either of these management tools affects behavior. Gonadotropin releasing hormone (GnRH) is a small neuropeptide that performs an obligatory role in mammalian reproduction and has been formulated into the immunocontraceptive GonaCon-BTM. We investigated the influences of this vaccine on behavior of feral horses (Equus caballus) at Theodore Roosevelt National Park, North Dakota, USA, for a year preceding and a year following nonlethal culling and GnRH-vaccine treatment. We observed horses during the breeding season and found only minimal differences in time budget behaviors of free-ranging female feral horses treated with GnRH and those treated with saline. The differences observed were consistent with the metabolic demands of pregnancy and lactation. We observed similar social behaviors between treatment groups, reflecting limited reproductive behavior among control females due to high rates of pregnancy and suppressed reproductive behavior among treated females due to GnRH-inhibited ovarian activity. In the treatment year, band stallion age was the only supported factor influencing herding behavior (P < 0.001), harem-tending behavior (P < 0.001), and agonistic behavior (P = 0.02). There was no difference between the mean body condition of control females (4.9 (95% CI = 4.7–5.1)) and treated females (4.8 (95% CI = 4.7–4.9)). Band fidelity among all females increased 25.7% in the year following vaccination and culling, despite the social perturbation associated with removal of conspecifics. Herding behavior by stallions decreased 50.7% following treatment and culling (P < 0.001), while harem-tending behavior increased 195.0% (P < 0.001). The amount of available forage influenced harem-tending, reproductive, and agonistic behavior in the year following culling and treatment (P < 0.04). These changes reflected the expected nexus between a species with polygynous social structure and strong group fidelity and the large instantaneous change in population density and demography coincident with culling. Behavioral responses to such perturbation may be synergistic in reducing grazing pressure by decreasing energetically expensive competitive behaviors, but further investigation is needed to explicitly test this hypothesis.
Gregarious relationships can impart individual benefits such as decreased depredation, increased foraging efficiency, and increased fecundity (Pusey and Packer, 1997). When resources to support populations of social animals are limited and populations become locally overabundant, managers are faced with the daunting challenge of decreasing population size without disrupting core behavioral processes. For example, the limited contiguous habitat and finite resources found on public wildlands cannot sustain overabundant populations of large ungulates, and managers continuously seek innovative and publically-acceptable tools for managing these species (Powers et al., 2011). Historically, such populations have been reduced by culling, using either lethal methods such as hunting, or non-lethal methods such as capture and translocation. Limiting fecundity through the use of immunocontraceptives is becoming a more commonly considered tool for controlling wildlife abundance. However, these tools can be accompanied by physiological changes that may alter behavior and ultimately influence population dynamics in unforeseen ways (Ransom et al., 2014). The paucity of quantitative data on natural behavior of wild free-roaming fauna, especially pertaining to those treated with fertility control agents, impairs our ability to understand species’ influences and roles in ecosystems and thus our ability to effectively manage populations.