When did the horse population reach its peak?

Reproduction of the horse in nature

Current topics, top articles by Nicole Basieux and Dominik Burger * //

It is every breeder's dream: to have a mare population that can reproduce with a fertility rate of 95 percent. This may be the case with herds that still live wild in nature. But due to the influence of humans, the pregnancy rates are lower. The "Horse Week" shows in a multi-part series how horses reproduce, how the sexual organs and sexual behavior of stallions and mare work, what reproductive techniques there are and what optimal management looks like.

Horses live in small groups in the wild. They stay together all year round and usually consist of a stallion and several mares and their foals. The mares are so-called "seasonally polyostrical". This means that during the warmer season (February to July) they go through several cycles during which they are receptive.

During the colder months of the year in our latitudes, mares are therefore anesthetic and their ovarian activity is suspended. As soon as the days get longer again, the horse cycles begin and the first ovulations set in - so-called ovulation. The entire sexual cycle, with an oestrus (horses, estrus, receptive time) of five to seven days and a diostrus (intermediate horses) of about two weeks, lasts about three weeks. When a mare lives in a natural social structure, the horse is characterized by repeated approaches to the stallion, frequent urination, lifting the tail to the side and spreading the hind legs. During the intermediate steed, the mare avoids the stallion, can show aggression towards him and even give him hoofbeats if he continues to woo her.

The stallion covers the mare while her foal horses. The foal is there.

However, the stallion and his mares have a permanent social relationship; they graze together, groom each other and this without sexual interaction. The mares - in contrast to other hoofed and hoofed animals - can sporadically show horse behavior during the non-ovulatory phase, which is probably due to the excretion of certain hormones (estrogenic steroids) by the adrenal cortex. These year-round "attractive" steed signs of the mare are probably there to maintain the social structure in which the stallion stays with the herd throughout the year.

The stallion and his harem

Throughout the year, the stallion makes considerable efforts to keep the mares and young horses within his harem. He keeps them away from other stallions and protects them from danger. The harem stallion seems to continuously monitor the status of his mares' cycle by periodically checking the smell of their body, urine and excrement. The frequency of this behavior increases considerably when a mare is in horses. In this case, the stallion spends a lot of time near her, pays more attention to her, reacts to her movements and shows a typical stallion posture opposite her with the arched neck, the raised tail and vocalizations. Even erections without mating are often observed in the stallion in the wild and do not have to be a sign of frustration.

Ladies choice

Under natural conditions, the role that the mare plays in behavior before and during mating is very important. The mare draws the stallion's attention to her side and seems to determine the time of mating - either by actively simplifying the mounting and insertion of the penis or by refusing to mate. Almost 90 percent of the interactions are initiated by the mare before mating.

Most copulations take less than a minute and it is a rather quiet event after which the horses often stay close to each other for a few minutes. During the peak of the horses, which lasts one to two days, the mare is usually covered several times. This in time intervals of a few minutes up to one to two hours and more often at sunrise and sunset.

Even if there are several mares in the horse, some stallions mate with an excellent fertility rate. In contrast to the horses, which are kept by humans, almost all copulations follow one or more ascents without erection. The purpose of these is to confirm or bring about the acceptance of the mare.

The birth

Although mares rarely give birth to a foal before the age of three, they reach sexual maturity between the ages of one and two years; the males between two and three years of age. The maturity of the latter's sexual behavior is not reached until five to six years. It often happens that the mares only foal every two years, or every two to three years.

A few days before the birth, the stallion feels sexually attracted to the pregnant mare, which normally rejects him during pregnancy. Even during childbirth, the stallion is interested in the smell of liquids and excretions and he also shows erections in this context. If the food supply is good, the mare can be fertilized again at the first ovulation, which usually takes place between eight and 15 days after birth (foal horses).

Protection against inbreeding

From around four months onwards, the foals are weaned more and more independently of their mother and at nine to ten months they are weaned completely. Results from French studies show that the social development of young horses is influenced by the presence and interaction with adult horses. Sex games are often observed in both sexes. A form of youthful marking behavior can already be observed during the first week of life. But this behavior, as a sign of hormonal change, primarily characterizes the beginning of the age of puberty. The juvenile detachment from the herd takes place around the age of two and aims to prevent inbreeding. When the young females are in horses and they still belong to the group in which they were born, the stallion of the harem will loosen his supervision a little, allow them to be mated outside the group and then come back again. Likewise, the young males over two years of age in most harems seem free to leave the herd and come back during a transitional phase.

The bachelors

The male animals' detachment from the harem is followed by their inclusion in a bachelor group (Bachelors) up to the age of five to six years, in which social interactions with the other males are numerous. In a population of Przewalski horses, a predictable connection between the hierarchical ranking of the bachelors and the number of their later offspring has recently been shown.

After separating from the herd of birth, each stallion will try to form its own group by acquiring mares in the following ways:

• Bringing together isolated mares (young mares who have just left their birth group or adult mares who are separated from their group).
• Challenge a stallion in order to conquer all mares in his group.
• Targeted attack to take possession of part of a harem.
• Team up with another young stallion to get a mare.
• Remain in his birth group in order to inherit the harem later.

Around 70 percent of the stallions formed a harem up to the age of five. The new groups are mostly created during the mating season and are often unstable at the beginning, which means that the young stallions can often lose their mares after a few weeks. Groups are also described in which two stallions live (both sexually mature and often unrelated) - one is responsible for reproduction and the other has a role as an assistant (satellite stallion).

Only 70 to 85 percent of the foals in a harem are therefore the offspring of the actual harem stallion. The others come either from mares who changed groups during pregnancy, from covering outside the group or from the satellite stallion, which was generally accepted by the harem stallion for individual mares.

How does nature do it?

The Institut suisse de médecine équine ISME conducts intensive research - in collaboration with the Universities of Lausanne, Neuchâtel and Hanover (GER) - on the interactions between stallions and mares. Various interdisciplinary projects aim to improve the reproductive success of horses. In nature, this is around 95 percent. Under the aegis of the people, it is often up to 30 percent lower, despite sometimes intensive management.


* Scripts and articles by the Institut suisse de médecine équine ISME, especially by Mireille Baumgartner (†).

(Published in Horse Week No. 01/2013)