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Sexual Behavior - 
Current Topics in Applied Ethology and Clinical Methods
Sue M McDonnell
Equine Behavior Laboratory
University of Pennsylvania School of Veterinary Medicine
New Bolton Center, Kennett Square, PA USA
Voice 610-444-5800      Fax 610-925-8124


            Social organization and sexual behavior of horses, both under natural free-ranging conditions and domestic hand-breeding conditions, have been well described in the literature over the last several decades (feral horses and ponies: Feist, 1971; Klingel, 1975; Miller, 1981; McCort, 1984; Keiper, 1985; Keiper and Houpt, 1984; semi-feral and pasture bred domestic horses and ponies: Tyler, 1972; Bristol, 1975; hand-bred horses and ponies: Asa, 1985; Asa, 1986; McDonnell, 1986; overall review of early work, Waring, 1983).

Modern stud farm practices vary considerably around the world, but in general impose considerable differences in sociosexual environment and breeding behavior from natural conditions for both mares and stallions. Some of the modern horse husbandry practices are for safety and practicality, but many are more or less tradition.  While most domestic horses breed successfully under intense domestic management, there is a considerable amount of sexual behavior dysfunction, and/or sexual behavior related management and performance problems in horses managed under these conditions.  This presentation will briefly review hypotheses and examples of work in applied ethology and veterinary clinical research addressing sexual behavior of domestic stallions and mares.

General Management and Handling of Horses for Breeding

            From a comparison of sociosexual environment and breeding behavior of horses under domestic management with that of free-ranging horses, striking differences can be identified that may account for some of the more common reproductive behavior problems.  For example, standard stud farm practice includes varying degrees of social isolation of stallions from mares other than for the few minutes of breeding contact per service, and not at all outside the breeding season.  In contrast, under natural social systems, a harem stallion interacts continuously year round with his harem mares.  Similarly, direct contact of stallions is rarely permitted on modern breeding farms.  If a stallion is the only stallion on a farm, he may be isolated from direct or fence line contact with any other horses.  Where there are multiple stallions on a farm, they are typically stabled in barns together in individual stalls or paddocks within close proximity with other mature stallions and away from any mares. In contrast, under natural conditions, harem stallions interact regularly in rituals and battles, apparently aimed at establishing and maintaining an order of dominance.  But at the same time it is normal for stallions under natural sociosexual conditions to be able to retreat and remain generally at a distance from other stallions most of the time.  Problems ranging from inadequate libido to overly aggressive behavior and self directed inter-male aggression (self-mutilation) to specific apparent preferences and aversions of a significant percentage of breeding stallions can be logically attributed to inadequate or inappropriate social exposure (Bedford et al, 2000; McDonnell et al, 1987). Simple modification of husbandry to better model the natural social environment of horses, while still considering safety of animals and handlers, is often immediately effective in overcoming as well as in preventing behavior problems.

In the breeding procedure itself, there are some key domestic practices and stallion-handling difficulties that appear to inadvertently lead to sexual behavior dysfunction and infertility. For example, on stud farms, standard breeding shed protocols include varying degrees of mare restraint.  The restraint may interfere with normal interactive behavior, receptive postures of the mare, the ability of the mare to adequately support the stallion and to normally facilitate and accommodate intromission and thrusting. Skilled and informed handling of breeding animals, while critical to preventing and rehabilitating sexual behavior dysfunction in mares and stallions, is a “dying art.” (McDonnell et al, 1994).

Specific Normal Behaviors of Domestic Horses That Are Misunderstood as Abnormal

            Another area of study that should lead to better welfare and reduce the frequency of reproductive behavior problems concern normal equid behaviors that are misunderstood as aberrant or misbehavior in domestic horses.  One example is the behavior known within the horse breeding industry as masturbation.  This involves normal periodic erections and penile movements.  This behavior, both from the descriptive field studies cited above and in extensive study of domestic horses, is now understood as normal, frequent behavior of male equids (McDonnell et al, 1991).  Attempting to inhibit or punish masturbation, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behavior (McDonnell and Hinze, in preparation). 

Play sexual behavior in young horses is another example of a normal, frequent equine behavior that is commonly misunderstood as abnormal.  All major elements of both male and female behavior occur in all fillies and colts from a few days of age (McDonnell et al, 1998). Play sexual behavior directed toward the dam usually peaks during her estrus.  Play sexual behavior with foal cohorts continues for fillies until maturity; with colts it is more frequent and continues into maturity as a bachelor stallion. Simply understanding that these interactions are normal and possibly important to development should reduce managers’ concerns about the behavior and practices aimed at eliminating it.

Social Modulation of Stallion Reproductive Function

            It has long been established that in equid breeding systems some of the mature stallions gain access to a harem or breeding territory, and some remain non-breeding “bachelors” (Keiper, 1985; Klingel, 1975, 1982).  Important behavioral and physiological differences resulting from harem or bachelor social status are just recently being understood and appreciated.  Harem status imparts an upgrading of reproductive endocrinology and function including increased androgen levels, sexual and aggressive behavior, accessory sex gland size and character, testicular size and character, and semen quality; bachelor status imparts changes in the opposite direction (McDonnell and Murray, 1995).

Traditional group housing of stallions may inadvertently impose bachelor status on breeding stallions.  For example, stabling of stallions in barns with other stallions led to suppressed reproductive function, including lower sexual interest and arousal, compared to stabling as the only stallion with mares.  Both in normal research animals and in clinical cases of sexual behavior dysfunction, we have found that simply manipulating the amount of ongoing exposure that a stallion has to mares and other stallions can markedly affect libido, testosterone levels, testicular volume, and sperm production efficiency. On most farms, sociosexual conditions can usually be easily manipulated to effect physiological and behavioral conditions corresponding to those of bachelor and harem status (McDonnell, 1995). 

Pharmacologic Aids for Modifying Sexual Behavior

            Another growing area of study regarding sexual behavior of horses concerns the development of pharmacologic aids to modifying sexual behavior.  To date there are reasonably effective protocols for enhancing and suppressing sexual behavior both for mares and stallions.  Approaches include psychotropic and endocrine agents (reviews, McDonnell, 1992, 1999a, b).  Most recent promising work involves development of methods of reversible chemical castration, using GnRH super agonists to down-regulate the pituitary-gonadal axis (PF Daels and B Colenbrander, personal communication).


Asa CS (1986) Sexual behavior of mares. Veterinary Clinics of North America Equine Practice Vol 2 (3): 519-534.

Asa CS, Goldfoot DA, Ginther OJ (1979) Sociosexual behavior and the ovulatory cycle of ponies (Equus caballus) in harem groups. Horm Behav 13: 49-65.

Bedford SJ, McDonnell SM, Tulleners E, King D, Habecker P (2000) Squamous cell carcinoma of the urethral process in a horse with hemospermia and self-mutilation behavior. J A m Vet Med Assoc 216(4): 551-553.

Bristol F (1982) Breeding behavior of a stallion at pasture with 20 mares in synchronized oestrus. J Reprod Fert Suppl 32: 71-77.

Feist JD  (1971)  Behavior of feral horses in the Pryor Mountain Wild Horse Range.  Master’s thesis, University of Michigan, Ann Arbor.

Keiper R (1985) The Assateague Ponies.  Tidewater Publishers, Centreville, Maryland.

Keiper R, Houpt K (1984) Reproduction in feral horses: An eight-year study. Am J Vet Res 45: 991-995.

Klingel H (1975) Social organization of reproduction in equids. JReprod Fert Suppl 23: 7-11.

McCort WD (1984) Behavior of feral horses and ponies. J Anim Sci 58 (2): 493-499.

McDonnell SM (1986) In Houpt KA, Crowell-Davis S (Eds), Veterinary Clinics of North America Equine Practice 2 (3)(Behavior): 535-555.

McDonnell SM (1995) Stallion behavior and endocrinology.  What do we really know?  Proceedings Annual Meeting American Association of Equine Practitioners.  Lexington, KY.

McDonnell SM (1992) Normal and abnormal sexual behavior.  In Blanchard TL, Varner DD (Eds), Veterinary Clinics of North America Equine Practice, Stallion Management 8 (1): 71-89.

McDonnell SM (1999a) Stallion sexual behavior.  In Samper J (Ed), Equine Breeding Management and Artificial Insemination.  WB Saunders, Philadelphia, pp 53-66.

McDonnell S M (1999b) Libido, erection, and ejaculatory dysfunction in stallions.  Compendium 21 (3): 263-266.

McDonnell SM (2000) Reproductive behavior of stallions and mares: comparison of free-running and domestic in-hand breeding.  Anim Reproduc Sci 60-61: 211-219.

McDonnell SM, Diehl NK, Oristaglio Turner RM (1994) Modification of unruly breeding behavior in stallions.  Comp on Cont Ed17 (3): 411-417.

McDonnell SM, Lutz, MM, Ewaskiewicz EH, Ruducha A (1998) Ontogeny of sexual behavior of foals in an established semi-feral population. Proceedings VII IERS, Pretoria, South Africa.

McDonnell SM, Henry M, Bristol F (1991) Spontaneous erection and masturbation in equids. Proceedings Vth International Equine Reproduction Symposium. JReprod Fert Suppl 44: 664-665.

McDonnell SM, Kenney RM, Meirs RS (1987) Self-mutilation in horses.  American Veterinary Society of Animal Behavior Annual Meeting, Chicago, Illinois, July 20.

McDonnell SM, Murray SC (1995) Bachelor and harem stallion behavior and endocrinology.  Biol of Reprod Mono 1: 577-590.

Miller R (1981) Male aggression, dominance, and breeding behavior in Red Desert feral horses.  Z Tierpsych 57: 340-351.

Tyler SJ (1972) Behavior and social organization of the semi-feral ponies.  Anim Behav Mono 5. 

Waring GH (1983) Horse Behavior: The Behavioral Traits and Applications of Domestic and Wild Horses, Including Ponies. Noyes Publications, Park Ridge, New Jersey.