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Behavioral Ecology of Feral Horses Introduction Feral horses live in a wide variety of habitats including deserts, mountains, prairies, swampy estuaries and barrier islands. Because of different environmental pressures, feral horse populations show great variation in their behavior and ecology. Social Organization Most feral horses live in small, permanent groups that inhabit large, overlapping home ranges. These equids display a polygynous mating system, where one male forms a bond with and breeds several females. This harem system of social structure has been considered to be an adaptation to seasonally changing ecological conditions (Klingel, 1975). Because the stallion defends his mares rather than a territory, the band is not restricted in its movements so it can make use of the best available food from season to season. Types of Bands Although the social organization of different feral horse populations is similar, band structure within unmanaged populations varies widely. In all studies of unmanaged populations with natural age and sex ratios, the most common social unit is the harem or family band. Bands consist of 1-26 mares and their offspring, accompanied by 1 or more stallions. Up to half of the bands may contain more than 1 and as many as 5 stallions (Linklater et al, 1999). Band size variation results from differences in reproduction and foal mortality, the adult sex ratio and the age of the stallion. Family bands are stable social units. Stallion tenure averaged 2.11 years for twenty-four stallions in the Granite Range (Berger, 1983) but lasted as long as ten years on Sable Island (Welsh, 1975) and on Assateague Island (Keiper, 1985). The composition of adult mares in the band is also stable, with some mares remaining in the same band for life. In the Pryor Mountains of Montana, for example, only 7.6% of adult mares changed bands in a year (Feist and McCullough, 1975). Dispersal of Young Most changes in band membership result from the dispersal of young. Unlike most mammals, where males disperse and females remain in their natal group throughout their lives, most young horses leave their natal band. Berger (1986) noted that 97% of young animals dispersed between the ages of 1 and 4 years. On Assateague Island 97% of males and 81 % of females had dispersed by 5 years of age. Average age of dispersal was 20.8 months for males and 24.6 months for females (Rutberg and Keiper, 1993). The proximate causes of dispersal seem to vary with the horse’s sex and from population to population. Young males may disperse alone or with peers and form all-male bachelor bands. Young males and females from the same or different bands may unite to form mixed-sex peer bands. Because these animals are usually not sexually mature, these bands are non-reproductive. The bands also tend to be unstable in composition. Home Range and Territoriality In horses the different kinds of bands occupy specific, undefended, nonexclusive geographic portions of their environment known as the home range. This is the area used during the day-to-day activities and incorporates not only grazing sites and waterholes but also shade, wind breaks and refuges from insect pests. Feral horse home ranges vary in size from 0.9 to 48 square kilometers (Keiper, 1986) and utilization varies seasonally as water and forage availability and insect pest densities change. In essentially all the feral horse populations studied, resources were not abundant or evenly distributed, so water, forage and shelter sites were shared among several bands. When bands do compete for access to a limited resource such as a waterhole, one band may dominate other bands. Stud piles are prominent features in the home ranges of feral horse bands. They consist of accumulated fecal matter deposited by stallions on their own previous feces and on feces of other stallions. Since the piles occur randomly within the home range rather than at a boundary, they may function to orient horses to mark a familiar trail or waterhole location, to serve as a warning to other stallions and thus help an area being used by another band, or to provide socially relevant information, such as identity and dominance status, about the marker. The presence of territoriality, where bands defend areas of exclusive use against other bands, has been reported on Shackleford Island off the coast of North Carolina (Rubenstein, 1981). Territories, whose boundaries ran the width of the island, only occurred where the island was narrow, the visibility unrestricted and the essential vegetation zones ran along the island’s long axis. Dominance Relationships In most feral horse bands there is a clear, stable, linear rank order between group members. This hierarchy helps reduce aggression between band members, contributing to band stability. The rank order is strongly correlated with age, with older animals dominant over younger. A second important determinant of rank is temperament; aggressive horses tend to achieve higher ranks than predicted by their size or age. The importance of other factors in determining rank is less clear. Length of residency in a band may increase rank. In most studies gender was not appear to affect rank. Dominance does not seem to be related to physical factors such as height, but in some studies there was a significant correlation between weight and rank. Where resources are scarce, a dominance hierarchy occurs between bands. In the Red Desert (Miller and Denniston, 1979) and the Grand Canyon (Berger, 1977) larger bands were dominant over smaller ones and had first access to water sources. Low ranking bands often waited for hours for the opportunity to drink. In contrast Stevens (1988) found that resident bands (those that were at the waterhole first) were dominant over newly arrived bands even if they were smaller in size. Predation Historically wolves were successful predators of feral horses but they have little impact today because of their low numbers and restricted range. While predation has been reported on several feral horse populations, it is considered to be incidental or uncommon. In the Montgomery Pass Wild Horse Territory in central California, however, Turner, Wolfe and Kirkpatrick (1992) found that while there was no evidence of mountain lions preying on adult horses, their predation limited the growth of the population by reducing foal survival rate. Biting Flies Tabanid horse flies are ectoparasites that might influence the social organization of feral horses. On hot, humid summer days these biting insects can consume 300-500 cubic centimeters of blood a day, create open sores that can become infected, transmit serious diseases such an Eastern Equine Encephalitis, which has over a 90% mortality rate, and Equine Infectious Anemia. Finally the biting flies disrupt normal activities and induce long-distance movements and habitat shifts. In response to insect harassment the feral horses of Assateague Island move from grazing areas in the marsh and on the dunes to other habitats that serve as anti-insect refuge sites. These include mudflats, the beach and the waters of Chincoteague Bay. These habitats were used significantly more in summer, when pest densities are higher, than in winter (Keiper and Berger, 1982). Despite the environmental differences horses in the Granite Range of Nevada displayed daily and seasonal patterns of habitat use similar to the Assateague horses. In summer they grazed in low meadows early in the day when it was cooler and pest densities lower but moved to the higher, cooler, windy slopes and ridges where they rested during the afternoon. Often they stood in snow patches (Keiper and Berger, 1982). Management of Feral Horse Populations It may seem paradoxical to talk about feral horses and how to manage them in the same breath, but all but the most radical wild horse advocates realize some form of population control is necessary. Several studies have reported annual population growth rates of 15 – 20% and such uncontrolled growth results in excessive grazing leading to habitat degradation. In the past feral horse populations were primarily controlled through periodic gatherings and removals but these have proven to be exceedingly expensive and incapable of placing all removed horses. One alternative is a vaccine derived from the zona pellucida of pigs stimulates the mare’s immune system to produce antibodies that inhibit fertilization and thus prevent pregnancy. References Berger J (1977) Organizational systems and dominance in feral horses in the Grand Canyon. Behav Ecol Sociobiol 2: 131-146. Berger J (1983) Induced abortions and social factors in wild horses. Nature 303: 59-61. Berger J (1986) Wild Horses of the Great Basin: Social Competition and Population Size. University of Chicago Press. Berger J, Rudman R (1985) Predation and interactions between coyotes and feral horse foals. J Mammal 66: 401-402. Feist J, McCullough D (1975) Reproduction in feral horses. J Reprod Fert Suppl 23: 13-18. Keiper R (1985) The Assateague Ponies. Tidewater Press, Cambridge, MD. Keiper R (1986) Social structure. In Crowell-Davis S, Houpt K (Eds), Veterinary Clinics of North America: Equine Practice. WB Saunders Co, Philadelphia, pp 465-484. Keiper R, Berger J (1982) Refuge-seeking and pest avoidance by feral horses in desert and island environments. Appl Anim Ethol 9: 111-120. Klingel H (1975) Social organization and reproduction in equids. J Reprod Fertil Suppl 32: 89-95. Linklater W, Cameron E, Minot E, Stafford K (1999) Stallion harassment and the mating system of horses. Anim Behav 58: 295-306. Miller R, Denniston R (1979) Interband dominance in feral horses. Z Tierpsychol 51: 41-47. Rubenstein D (1981) Behavioral ecology of island feral horses. Eq Vet J 13: 27-34. Rutberg A, Keiper R (1993) Proximate causes of natal dispersal in feral ponies: some sex differences. Anim Behav 46: 969-975. Stevens E (1988) Contests between bands of feral horses for access to fresh water: the resident wins. Anim Behav 36: 1851-1853. Turner J, Wolfe M, Kirkpatrick J (1992) Seasonal mountain lion predation on a feral horse population. Can J Zool 70: 929-934. Welsh D (1975) Population behavioural and grazing ecology of the horses of Sable Island, Nova Scotia. PhD dissertation. Dalhousie University, Halifax. |
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