Feral Horse Populations Research Articles

Multiscale analysis reveals restricted gene flow and a linear gradient in heterozygosity for an island population of feral horses

We studied the genetic (microsatellite) diversity of a feral population of horses ( Equus caballus L., 1758) on Sable Island, Nova Scotia, Canada (1983–2003), at two spatial scales: (1) for the island as a whole and (2) at the level of four equally sized subdivisions along the length of Sable Island, which is a long (42 km) and narrow (1.5 km) vegetated sand bar. At the island scale (n = 264 horses), observed heterozygosity over 10 loci was 0.647 ± 0.035 (mean ± 1 SE), while expected heterozygosity was 0.696 ± 0.029; we observed significant heterozygote deficiency with all loci considered (P < 0.0001). At the subdivision scale, observed heterozygosity ranged from 0.589 to 0.694 in a gradient from west to east. We observed a corresponding gradient in effective number of alleles and allelic richness. Pairwise values of FSTwere significant for most subdivision pairs, ranging as high as 0.067 from west to east. Western areas showed highest levels of inbreeding (FIS = 0.113) with outbreeding indicated in the east (FIS = –0.008). Our results suggest that for a large mammal that lives in polygynous social groups, like the feral horse, gene flow along linear habitats (corridors) may be restricted (relative to the dispersal capabilities of the species), even over short distances.

Immunocontraception decreases group fidelity in a feral horse population during the non-breeding season

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 30 horses on Shackleford Banks, North Carolina, USA 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 = 22); 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 h of observation. Contracepted mares changed groups more often than control mares ( P = 0.04). Contracepts also visited more harem groups than did control mares ( P = 0.02) and exhibited more reproductive interest ( P = 0.05). For both contracepted and control females, the number of group changes ( P = 0.01) and number of groups visited ( P = 0.003) decreased with the proportion of years mares were pregnant. 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.

Maternal investment results in better foal condition through increased play behaviour in horses

Play behaviour is widespread in mammals, but benefits to play have been difficult to demonstrate. Physical training is one of the many proposed hypotheses, suggesting that males and females should play differently, that increased maternal investment should lead to increases in play, and that increases in play should result in physical advantages. In a population of feral horses, Equus caballus, males and females did not differ in their play behaviour except that males initiated more of their play bouts than females. Maternal condition influenced play behaviour only in males, with sons of mothers in good condition playing more. However, when we controlled for maternal effects by comparing a son and a daughter of the same mother, daughters played more when their mother was in poor condition and sons played more when their mother was in good condition. Mothers of foals that played more lost more condition. Therefore, the difference in play behaviour could not be explained by offspring sex or maternal condition alone, but play behaviour mirrored variation in maternal investment. In addition, those individuals that played more survived better and had better body condition as yearlings despite weaning earlier. Since increased activity has been linked to enhanced musculoskeletal development in domestic horses, we suggest that play provides a link between increased maternal investment, increased body condition and future reproductive success in feral horses, and probably in other species.

Management of Feral Horses at the North Carolina National Estuarine Research Reserve

Abstract Feral horse (Equus caballus L.) populations are found on properties managed by governmental agencies in western states, the Missouri Ozarks, and on several Atlantic coast barrier and estuarine islands. These animals are descendants of free-roaming horses introduced decades to centuries earlier. Public sentiment has influenced development of policies that have allowed the herds to remain. The North Carolina National Estuarine Research Reserve is a state program with federal support and oversight that manages four properties; two (Rachel Carson in Beaufort, NC, and Currituck Banks near Corolla, NC) include feral horse populations. Current reserve policies include maintenance of these herds; however, impacts on salt marshes and other ecosystems represent a conflict with federal regulations. Among the Atlantic Coast herds, conditions at the Rachel Carson site are least accommodating for the animals. With a combination of pertinent research results plus 20 years of site-specific management experience ...

Interactions between Feral Horses and Desert Bighorn Sheep at Water

We studied sympatric populations of native bighorn sheep (Ovis canadensis) and feral horses (Equus caballus )t o quantify their spatial and temporal overlap and to determine whether horses interfered with use of water by bighorn sheep. We observed no evidence of direct competition, but our field experiment, which involved placing desert-acclimated domestic horses near watering sites used by bighorn sheep, demonstrated that bighorn sheep avoided sites with horses nearby. The presence of domestic horses near a watering site preferred by bighorn sheep resulted in a 76% reduction in the number of groups of bighorn sheep coming to water at that location and a concomitant increase in the number of bighorn sheep watering at other sites. An experimental approach to studying competition between large mammals has been problematic and to our knowledge this study constitutes the 1st manipulative field experiment to test for competitive interactions between feral horses and native ungulates.

Live Trapping and Monitoring Mountain Lion Movements within a Feral Horse Population in Storey County, Nevada, 2005 - 2007

Author(s): Gray, Meeghan; Spencer, Jack, Jr.; Thain, David | Abstract: The depredation of feral horses by mountain lions is usually a rare phenomenon and only a few cases have been documented in scientific literature. While such reports indicate that mountain lions are easily capable of killing feral horses, these studies have focused solely on the feral horses and have neglected to consider the mountain lion’s perspective (i.e., movement patterns, prey choice). Today, feral horses have created an artificial prey base for mountain lions, and even if natural ungulate species were not present, mountain lions appear to survive and flourish while consuming feral horses. During a feral horse behavior study conducted in 2005, a resident mountain lion in the Virginia Mountain Range was deemed responsible for several feral horse deaths, with most of the carcasses found being young foals or juvenile horses. A large live trap was developed and was strategically placed in a mountain lion travel corridor where the depredated horse carcasses were found. Bait in the live trap was changed on a weekly basis to prevent spoilage. The trap was set from October through December 2006 and monitored each morning. A 7-yearold, 60-kg female lion was caught in the trap after 3 months of trapping efforts. It was tranquilized, weighed and measured, and fitted with a satellite GPS collar. The satellite collar gave 4 locations per night. Based on the those GPS locations, we determined the lion had depredated on many feral horses, and it continued to range in the same area, even though other native wildlife species, such as mule deer, were in low densities. We monitored her progress until October 2007 to determine overall movement patterns and prey choice.

Simulation model for contraceptive management of the Assateague Island feral horse population using individual-based data

The National Park Service (NPS) manages a culturally significant population of feral horses (Equus caballus) inhabiting the Maryland portion of Assateague Island, a barrier island in the eastern United States. Rapid growth of this population over the past few decades from 28 to 166 horses negatively impacts native species and ecological processes on the island. Since 1994, contraception via porcine zona pellucida vaccine has been used to control horse numbers, although herd reduction has been slower than initially expected, leading NPS to consider other management options. An individual-based stochastic simulation model was developed using the Vortex software program to examine the effects of different management strategies on the population. Data from the managed population were used to populate the model parameters. Model projections over the next 50 years using current management practices show an average rate of population decline of 13% per year, suggesting that the population will reach the management target of 80–100 horses in 5–8 years. The effectiveness of contraception to reduce the herd and maintain it at various target sizes of 20–100 horses was also assessed. The accumulation of inbreeding at each target population size was also modelled.

Genetic Diversity in a Feral Horse Population from Sable Island, Canada

The present-day Sable Island horse population, inhabiting an island off the eastern coast of Canada, is believed to have originated mainly from horses confiscated from the early French settlers in Nova Scotia in the latter half of the 18th century. In 1960, the Sable Island horses were given legal protected status and no human interference has since been allowed. The objective of this study was to characterize the current genetic diversity in Sable Island horses in comparison to 15 other horse breeds commonly found in Canada and 5 Spanish breeds. A total of 145 alleles from 12 microsatellite loci were detected in 1093 horses and 40 donkeys. The average number of alleles per locus ranged from 4.67 in the Sable Island horse population to 8.25 in Appaloosas, whereas the mean observed heterozygosity ranged from 0.626 in the Sable Island population to 0.787 in Asturcons. Various genetic distance estimates and clustering methods did not permit to support that the Sable Island horses originated from shipwrecked Spanish horses, according to a popular anecdote, but closely resemble light draft and multipurpose breeds commonly found in eastern Canada. Based on the Weitzman approach, the loss of the Sable Island horse population to the overall diversity in Canada is comparable or higher than any other horse breed. The Sable Island horse population has diverged enough from other breeds to deserve special attention by conservation interest groups.

Ecological and human dimensions of management of feral horses in Australia: a review

In Australia, the management of feral horse populations is a contentious issue, owing to their pluralistic status as an introduced pest and a national icon. In this review, we synthesise current knowledge of the ecological effects of feral horses and the human dimensions of feral horse management, using case studies from around the world to illustrate contentious and successful management practices. We highlight gaps in the literature and suggest that more peer-reviewed research would be beneficial in reducing the current public controversy surrounding management of feral horses.

Saving feral horse populations: does it really matter? A case study of wild horses from Doñana National Park in southern Spain

In the 1980s, a conservation programme involving a feral horse population, the Retuertas horses from the Guadalquivir marshes, was started in the Doñana National Park. The analysis of an extensive genetic survey of this population, which now numbers 100 animals, and 10 additional European and North African breeds using DNA polymorphisms from 22 microsatellites is presented. Highly significant fixation indexes were obtained for all pairwise comparisons between the Retuertas population and other breeds. A population neighbour-joining breed phenogram was built using different distance measures, but the Retuertas population failed to cluster with either of the two major clades of European and North African breeds, highlighting its uniqueness. In fact, the Retuertas population was positioned at the base of the trees, which were rooted using donkey samples. Furthermore, assignment tests and the individual Q-matrices obtained with the structure programme isolated the Retuertas breed from the other breeds with only four K groups. Interestingly, some local semi-feral horses, known as Marismeño, also currently living in the Guadalquivir marshes, have some microsatellite genotypes that fall well within the Retuertas cluster. This raises the possibility of incorporating horses from the Marismeño population in a future conservation programme.

Applying dynamics and comparing life tables for Pleistocene Equidae in anthropic (Bau de l'Aubesier, Combe-Grenal) and carnivore (Fouvent) contexts with modern feral horse populations (Akagera, Pryor Mountain)

In this study, we propose a method based on life tables currently used in population dynamics and ecology to compare the mortality curves of Equidae from anthropic and carnivore fossil assemblages with those from modern populations. The combined analysis of frequency ( f x ) with various ecological parameters such as mortality rate ( q x ), survival rate ( l x ), fecundity rate ( m x ), and killing factor ( k x ), established for each age class, allows us to characterize the stability of fossil populations. These ecological parameters, developed in demography, correspond to many curves and constitute a complementary approach to interpret the two main mortality models, attritional and catastrophic, often evoked in zooarchaeology. The age structures of fossil horses from the anthropic sites of Bau de l'Aubesier (Vaucluse, France) and Combe-Grenal (Dordogne, France) and the hyena den of Fouvent (Haute-Saône, France) are compared to typical catastrophic modern populations of Equidae from the National Park of Akagera (Rwanda). We also used data collected by the Bureau of Land Management for present day feral horses from Pryor Mountain (Montana, USA) which constitutes a reliable referential of a stable living population. The characterization of age structures in those different contexts is discussed as well as the impact of Neanderthals and hyenas on fossil assemblages.

Injuries and blemishes in a semi-feral herd of ponies

With the goal of characterizing the type and degree of injuries and blemishes incurred by horses living under natural social conditions, all members of a semi-feral herd of ponies were inspected on 4 occasions over a period of 28 months. Two occasions were during breeding/foaling season and 2 during non–breeding/foaling season months. On each occasion, each animal was examined (N = 47-65) and all injuries or blemish marks were recorded. Based on the types and number of injuries or blemishes, an injury/blemish grade was assigned for each inspection, ranging from 1 for no blemishes to 6 for more than 1 open wound. Almost all injuries and blemishes recorded were extremely mild. Only 12 of the 213 inspections and 14 of the 858 total injuries and blemishes involved wounds affecting tissues other than hair and skin. Males had significantly greater mean injury/blemish grade than females ( P < .0001). For all social categories and ages, the mean injury/blemish grade was significantly greater in the breeding/foaling season months than during the non–breeding/foaling season ( P < .0001). All foals were blemish-free at the non–breeding/foaling season inspections (n = 24) and relatively blemish-free (9 of 14) at the breeding/foaling season inspections. The rump and the barrel areas were the most common site of injuries or blemishes, both as a percentage of the total injury count and as a percentage of inspections involving those areas. These findings are consistent with the seasonal and gender patterns of aggressive behavior seen in this herd. Compared with reports of truly feral horse populations, it appears that injuries are fewer and less severe in this semi-feral herd. In addition, there appear to be fewer leg injuries in this semi-feral herd than has been described for truly feral horse populations.

Use of Remote Sensing Techniques to Determine the Effects of Grazing on Vegetation Cover and Dune Elevation at Assateague Island National Seashore: Impact of Horses

The effects of grazing by feral horses on vegetation and dune topography at Assateague Island National Seashore were investigated using color-infrared imagery, lidar surveys, and field measurements. Five pairs of fenced and unfenced plots (300 m2) established in 1993 on sand flats and small dunes with similar elevation, topography, and vegetation cover were used for this study. Color-infrared imagery from 1998 and field measurements from 2001 indicated that there was a significant difference in vegetation cover between the fenced and unfenced plot-pairs over the study period. Fenced plots contained a higher percentage of vegetation cover that was dominated by American beachgrass (Ammophila breviligulata). Lidar surveys from 1997, 1999, and 2000 showed that there were significant differences in elevation and topography between fenced and unfenced plot-pairs. Fenced plots were, on average, 0.63 m higher than unfenced plots, whereas unfenced plots had generally decreased in elevation after establishment in 1993. Results demonstrate that feral horse grazing has had a significant impact on dune formation and has contributed to the erosion of dunes at Assateague Island. The findings suggest that unless the size of the feral horse population is reduced, grazing will continue to foster unnaturally high rates of dune erosion into the future. In order to maintain the natural processes that historically occurred on barrier islands, much larger fenced exclosures would be required to prevent horse grazing.

Feral horse demography and population growth in the Kaimanawa Ranges, New Zealand

Although feral horses are a common management problem in numerous countries, detailed and long-term demographic studies are rare. We measured the age and sex structure, and pregnancy, birth and death rates in a population of 413 feral horses in New Zealand during 1994–98 and used them to construct a model simulating population growth. Survivorship increased with age (0–1 years old = 86.8%, 1–2 = 92.3%, 2–4 = 92.4%, ≥�4 years old = females 94%, males 97% per annum). Birth sex ratio parity, a slight female bias in the adult sex ratio (92 males per 100 females) and higher adult male survivorship indicated lower average survivorship for young males than females that was not detectable in mortality statistics. Pregnancy and foaling rates for mares ≥�2 years old averaged 79 and 49%, respectively. Foaling rates increased as mares matured (2–3-year-old mares = 1.9%, 3–4 = 20.0%, 4–5 = 42.1%, ≥�5 = 61.5% per annum). Young mares had higher rates of foetal and neonatal mortality (95% of pregnancies failed and/or were lost as neonatal foals in 2–3-year-old mares, 70.6% in 3–4, 43.2% in 4–5, and 31% in mares ≥�5 years old). Population growth was 9.6% per annum (9.5–9.8, 95% CI) without human-induced mortalities (i.e. r = 0.092). Our model, standardised aerial counts, and historical estimates of annual reproduction suggest that the historical sequence of counts since 1979 has overestimated growth by ~50% probably because of improvements in count effort and technique.

Influence of Predation by Mountain Lions on Numbers and Survivorship of a Feral Horse Population

Influence of Predation by Mountain Lions on Numbers and Survivorship of a Feral Horse Population

Effects of Roundups on Behavior and Reproduction of Feral Horses

Roundups are used to maintain feral horse populations in balance with rangeland grazing capacity, but little is known about short-term and long-term effects of roundups on horses. We evaluated the effects of roundups on behavior and reproduction of feral horses. The study was conducted near Challis, Ida. during 1994-1995, and repeated near Lander, Wyo. during 1995-1996. Horses were randomly assigned to 3 different treatment groups. One group (ADOPTED) was gathered by a Bureau of Land Management roundup crew using a helicopter. These horses were removed and placed in the Adopt-A-Horse Program. The second group (SIMULATED) consisted of horses that were gathered by helicopter, but these horses evaded capture and remained in the study area after the roundup. Horses in the third group (CONTROL) were not herded by helicopter. Horse behavior was monitored in the SIMULATED and CONTROL groups before and after roundups. Behavioral variables analyzed were the percentage of time spent resting, feeding, vigilant, traveling, and engaged in agonistic encounters. Neither foraging or social behavior of feral horses was affected by roundups in either study area (P > 0.10). Reproduction was monitored within the SIMULATED, CONTROL, and ADOPTED groups during the year following roundups. The percentages of mares with live foals did not differ (P > 0.10) among the 3 treatment groups in Idaho or Wyoming. Foaling success rates in Idaho were 29%, 31%, and 43% for CONTROL, ADOPTED, and SIMULATED mares, respectively. In Wyoming, foaling success rates were 29%, 42%, and 48% for CONTROL, ADOPTED, and SIMULATED groups, respectively. We found no evidence that roundups had deleterious effects on behavior or reproduction of feral horses. DOI:10.2458/azu_jrm_v53i5_hansen

Adaptive explanation in socio-ecology: lessons from the Equidae.

Socio-ecological explanations for intra- and interspecific variation in the social and spatial organization of animals predominate in the scientific literature. The socio-ecological model, developed first for the Bovidae and Cervidae, is commonly applied more widely to other groups including the Equidae. Intraspecific comparisons are particularly valuable because they allow the role of environment and demography on social and spatial organization to be understood while controlling for phylogeny or morphology which confound interspecific comparisons. Feral horse (Equus caballus Linnaeus 1758) populations with different demography inhabit a range of environments throughout the world. I use 56 reports to obtain 23 measures or characteristics of the behaviour and the social and spatial organization of 19 feral horse populations in which the environment, demography, management, research effort and sample size are also described. Comparison shows that different populations had remarkably similar social and spatial organization and that group sizes and composition, and home range sizes varied as much within as between populations. I assess the few exceptions to uniformity and conclude that they are due to the attributes of the studies themselves, particularly to poor definition of terms and inadequate empiricism, rather than to the environment or demography per se. Interspecific comparisons show that equid species adhere to their different social and spatial organizations despite similarities in their environments and even when species are sympatric. Furthermore, equid male territoriality has been ill-defined in previous studies, observations presented as evidence of territoriality are also found in non-territorial equids, and populations of supposedly territorial species demonstrate female defence polygyny. Thus, territoriality may not be a useful categorization in the Equidae. Moreover, although equid socio-ecologists have relied on the socio-ecological model derived from the extremely diverse Bovidae and Cervidae for explanations of variation in equine society, the homomorphic, but large and polygynous, and monogeneric Equidae do not support previous socio-ecological explanations for relationships between body size, mating system and sexual dimorphism in ungulates. Consequently, in spite of the efforts of numerous authors during the past two decades, functional explanations of apparent differences in feral horse and equid social and spatial organization and behaviour based on assumptions of their current utility in the environmental or demographic context remain unconvincing. Nevertheless, differences in social cohesion between species that are insensitive to intra- and interspecific variation in habitat and predation pressure warrant explanation. Thus, I propose alternative avenues of inquiry including testing for species-specific differences in inter-individual aggression and investigating the role of phylogenetic constraints in equine society. The Equidae are evidence of the relative importance of phylogeny and biological structure, and unimportance of the present-day environment, in animal behaviour and social and spatial organization.

Population Characteristics of Feral Horses on Cumberland Island, Georgia and Their Management Implications

Control of feral borse populations (Equus caballus) on public lands is restricted to nonlethal methods. These methods can limit population growth, however, efficacy may very among populations with differing demography and social structures Characteristics of many western feral horse populations are well documented, but fewer data are available for Atlantic barrier island populations. Therefore, we monitored a population of feral horses on Cumberland Island, Georgia from 1986 to 1990, Population growth averaged 4.3% annually with a stable age structure. Mean size of bands with ≥ 1 stallion was 4.6 horses. The adult sex ratio for the population was 0.6 females: 1.0 males. About 66% of mares foaled during a given year and no juvenile females foaled. Annual survival rates averaged 61.1% for female foals. 58.8% for male foals, and 92.4% for mares. Most bands contained a dominant or codominant stallion, mares and-or juvenile females, and foals Most mares (62.5%) changed bands ≥ 1 time during our study. Bachelors represented 50.4% of stallions. These population characteristics differed from other feral horse populations. The small band size, large number of bachelors, high degree of band instability, and codominance of band stallions on Cumberland Island likely will linut the efficacy of some forms of nonlethal population control.

Effects of roundups on behavior and reproduction of feral horses

Roundups are used to maintain feral horse populations in balance with rangeland grazing capacity, but little is known about short-term and long-term effects of roundups on horses. We evaluated the effects of roundups on behavior and reproduction of feral horses. The study was conducted near Challis, Ida. during 1994-1995, and repeated near Lander, Wyo. during 1995-1996. Horses were randomly assigned to 3 different treatment groups. One group (ADOPTED) was gathered by a Bureau of Land Management roundup crew using a helicopter. These horses were removed and placed in the Adopt-A-Horse Program. The second group (SIMULATED) consisted of horses that were gathered by helicopter, but these horses evaded capture and remained in the study area after the roundup. Horses in the third group (CONTROL) were not herded by helicopter. Horse behavior was monitored in the SIMULATED and CONTROL groups before and after roundups. Behavioral variables analyzed were the percentage of time spent resting, feeding, vigilant, traveling, and engaged in agonistic encounters. Neither foraging or social behavior of feral horses was affected by roundups in either study area (P &gt; 0.10). Reproduction was monitored within the SIMULATED, CONTROL, and ADOPTED groups during the year following roundups. The percentages of mares with live foals did not differ (P &gt; 0.10) among the 3 treatment groups in Idaho or Wyoming. Foaling success rates in Idaho were 29%, 31%, and 43% for CONTROL, ADOPTED, and SIMULATED mares, respectively. In Wyoming, foaling success rates were 29%, 42%, and 48% for CONTROL, ADOPTED, and SIMULATED groups, respectively. We found no evidence that roundups had deleterious effects on behavior or reproduction of feral horses.

Preliminary Evaluation of Porcine Zona Pellucida (PZP) Immunocontraception for Behavioral Effects in Feral Horses (Equus caballus)

Successful management of captive populations of wild animals requires effective control of reproduction. Contraception is one tool for controlling reproduction of animals in zoos; however, the options available to the animal manager are limited. Contraceptives vary in efficacy, reversibility, and side effects, and thus may not be suitable for widespread use. One consideration when selecting a contraceptive is its potential for side effects on behavior, especially given the fact that reproduction plays such a prominent role in the biology of any species. To date, there have been few evaluations of contraceptives for behavioral effects, and those that have been conducted have focused on hormone-based contraceptives. This study sought to evaluate a novel method of population control, imrnunocontraception, for behavioral effects in a population of feral horses. Porcine zona pellucida (PZP) immunocontraception prevents fertilization of ova and does not alter normal hormone secretion patterns. It therefore should leave the animal behaviorally intact in terms of reproductive behavior. The study examined the behavior of 43 sexually mature mares on Assateague Island during the 1997 breeding season and, with help from Earthwatch volunteers, collected observations over a 3-month period. The study found no significant differences between treated and untreated mares in general activity budget, aggression given or received, and spatial relationships relative to the stallion. These preliminary findings indicate that PZP contraception seems to have no acute behavioral effects on the behavior of individuals. The study findings also suggest that PZP could be a desirable and effective management tool for captive species in which social behavior plays an integral role in group dynamics. Analyses of group level effects and population level effects are continuing.