Social Behaviors of Wild and Domestic Horses
A Multi-Faceted Social System
Horses, like most species of ungulates, are highly social animals. Under feral conditions or even at pasture, horses live in groups called harems or bands. In the wild, a harem will typically consist of one to six stallions, several mares, and the mares' offspring that are up to five years old. Harems are not limited to any certain geographic area, as they usually travel continuously in search of food and water. Harem size may range anywhere from 2 to 21 horses, with multiple stallion harems typically being larger than single stallion harems. At the center of the harem is the mares themselves, who will remain together even if the stallion dies or leaves the herd. One stallion, the highest-ranking male of the harem, does most (if not all) of the breeding, and serves to protect the herd from threats. However, this is not to say the stallion is always the highest ranking horse in the herd, as older mares may just as easily assume the most dominant position. Unsurprisingly, the offspring of dominant mares tend to also become higher-ranking individuals in their herds later in life. This is indicative of both genetic and experience components in the herd hierarchy system.
Relationships among harem members are multifaceted, and are dependent on multiple factors. Herd hierarchy seems to be linear, and associated with age or the ability to survive in challenging situations; it is not necessarily based on height, weight, gender, or time within the harem, as many people would assume. Status within a herd also depends on the ages and sexes of the other members; the more members, and the more members within each age and sex group, the less likely a dominance hierarchy will occur. This is extremely important to consider when stabling horses, as attention to management is required when housing horses together or introducing new horses into an already established group.
Herd hierarchy seems to be linear, and associated with age or the ability to survive in challenging situations; it is not necessarily based on height, weight, gender, or time within the harem, as many people would assume.
The dominant stallion has first priority to mares in heat, is often the one to remove a filly or colt from the harem, and is usually the one to steal mares from other harems. Unless they become pregnant, mares have 21-day cycles during the spring and summer. Most fillies and colts born into the harem will remain with the group until they become sexually mature (usually by about two years), by then which the highest stallion will chase them from the herd. Even colts and fillies that are not removed from the herd will usually leave on their own by age five (when they are socially mature) to join or establish other harems. Fillies that fail to leave their original harem tend to have fewer offspring. These are all effective methods by which nature combats inbreeding.
Young stallions that are removed from their original herds may remain on their own for a few months before joining other solo males, forming “bachelor” herds. The most dominant individual of these bachelor stallions is usually the first to acquire a mare and start a harem, after which this cycle continues with the other stallions. Young fillies that are newly chased from their herds may choose to temporarily join a bachelor herd for protection, but are also often merged into other, more established harems by their dominant stallion. Aside from the “bachelor phase,” stallions are rarely alone; if this occurs, the stallion is usually too old or otherwise unfit to join or maintain a harem.
While herd ranking in stallions is based mostly on their access to mares and fillies, ranking among mares is usually determined by which mares can lead the herd to resources or offer the herd protection. When a harem moves as a unit, the dominant female often leads at the front, while the dominant stallion follows close behind the herd to ensure that all of his mares and foals are keeping up. Since harems consist mostly of females, it is the females that make the decisions about whether they leave or stay with the harem; this is typically based on factors such as the number and quality of stallions, and the amount of resources available. Dominant females may effectively interfere with the nursing of the foals of less dominant females; this may be a variant of “survival of the fittest,” as the foals of more dominant mares are more likely to survive if they are not competing for resources with the foals of less dominant mares. Like many social herd animals, the mares may form “friendships” and preferentially groom each other. As unforgiving as some of these behaviors may seem, this pattern is typical of many herd species; hierarchy is determined primarily by way of lower-ranking animals deferring to those that are higher-ranking, not by the results of fighting or killing.
The effects of ranking not only exists between individuals, but also exists between entire herds themselves. Herds with multiple stallions are dominant over harems with only one stallion. This is most likely because lower-ranking stallions within a herd conduct most of the fighting that occurs between herds in an attempt to steal mares for themselves. Herds that are occupying an area or using a resource (such as a watering hole, grazing area, etc.) tend to retain it for long periods of time, keeping other harems away. Harems, as well as the individual horses within them, follow specific patterns of fecal marking as a form of communication.
As unforgiving as some of these behaviors may seem, this pattern is typical of many herd species; hierarchy is determined primarily by way of lower-ranking animals deferring to those that are higher-ranking, not by the results of fighting or killing.
Breeding and Gestation
The three phases of sexual behavior in horses are courtship, mating, and post-mating behavior. During courtship, the stallion will approach a mare in estrus (or heat), prancing, sniffing, nuzzling, and grooming her while often exhibiting the flehmen response (holding up the head, curling the upper lip, and inhaling through the nostrils) to further determine her hormonal status. If the mare is not yet in her receptive phase, she may squeal, kick, or run away to show the stallion that she is not yet ready to be bred. Ovulation typically occurs 36 hours before the end of estrous, by then which estrous behavior begins to decline. When the mare is ready, she will stand still with her hindquarters towards the stallion, deviate her tail, urinate, “wink” with her vulva, and allow the stallion to mount her. Under natural conditions at pasture, breeding may achieve 100% success in impregnating mares, whereas controlled or “hand breeding” may only achieve 50-60% success rates. This is probably due to increased familiarity between the horses, higher fertility due to longer courtship, and lessened aggression.
Gestation in horses typically lasts 315 to 365 days, with 340 days being the average. Elements that control gestation length include nutritional status, time of year (shorter if bred in late summer), and gender (slightly longer if the foal is male). Mares almost always deliver at night, even if provided with constant artificial light. After delivery, bonding between the mare and the foal beings immediately. The horse is a prey animal, so the foal learns to stand and walk within a few hours of being born. Nursing is initiated instinctively by the foal and stopped by the mare.
Stages and Symptoms of a Mare's Estrous Cycle
Stage of Cycle
Early (Days 1-3)
Mixed signals; may squeal, squat, lift tail, and spray urine, but will not allow a stallion to mount.
Mare wants to interest and excite stallion, but will not yet allow him to breed due to lack of ovulation.
Full (Days 4 & 5)
Will give all signals (squealing, squatting, lifting tail, spraying) and will allow a stallion to mount.
Egg is at or nearing ovulation. Mare will breed, as having sperm in uterine horn(s) at this stage increases odds of conception.
Late (Days 6 & 7)
Mixed signals just as early heat. Some may still allow a stallion to mount, others may not.
Behavior may or may not allow for fertilization to take place, as it is possible within several hours of ovulation, but less likely at this stage.
Anestrus (not in heat)
Seeks no interaction with stallion. If stallion approaches, some mares may act aggressively.
Period of sexual inactivity. If conception occurred, the uterine tract alters its chemical and physical environment to support a fetus.
The horse is a prey animal, so the foal learns to stand and walk within a few hours of being born.
In the first couple of months of life, foals are fully dependent on their mothers and have minimal interaction with the other horses in the harem. At about two months, snapping (tooth chomping) begins. Snapping is a behavioral facial expression in which the lips are retracted and the teeth are clamped together. It is displayed by foals to adult horses, especially stallions. Its function may be to reduce aggression from adults, a way of stating, “I’m just a baby, don’t harm me.” It is also possibly explained as displaced nursing behavior (air nursing). Snapping peaks at two months of age, then steadily declines. This behavior is not the same as smacking; smacking is an aggressive threat in which the ears are laid back, the mouth is open, and the lips are smacking, but the lips are not pulled back.
At about three months of age, foals enter the socialization period. Up until this time, play is usually solitary. At this point, foals start exploring and playing with other foals. There are sex differences in play; colts play more often than fillies, and games between colts are different than games between fillies. Colts focus more on fighting and mounting when playing, while fillies focus more on racing and grooming each other. Fillies will groom both fillies and colts, while colts tend only to groom fillies. This has been interpreted as probable practice for future courtship behavior. Play is an important social experience for normal social development and interaction in adult life. After about four months of age, foals start to develop more independent personalities and spend more time exhibiting adult behaviors, such as grazing and standing while at rest.
Young Horses Displaying Snapping ("Tooth Chomping") Behavior in the Presence of an Older Horse.
The Importance of Movement to the Horse
Free-ranging movement is a major factor in the development of young horses. Many behavior problems in domestic horses are often associated with confinement; they have not evolved to stand in stalls or small paddocks all day. Common confinement-related behaviors include breeding aggression, wood chewing, cribbing, pica, stall walking, weaving, pawing, and self-mutilation. These behaviors can often be prevented by getting plenty of turnout time and physical activity; however, these behaviors are often a headache to reverse or manage once they are established. In feral conditions, horses will spend at least 60% of their day foraging and exploring, and will eat many small meals a day. In general, the rest of a horse's time is spent resting, engaging in social activities with other herd members, and captivating humans with their beauty and free spirit. For veterinary professionals seeking more information about the social behaviors of horses, and how to apply that knowledge to their practice, Equine Behavior: A Guide for Veterinarians and Equine Scientists is an excellent source of more detailed information.
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