Last revised April 14, 2017.
One of the most common harem social behaviors is allogrooming (also known as mutual grooming). It is expressed by the lateral parallel body position of two horses that allows for nibbling along the back or withers of each horse. While this behavior can be considered grooming, it is also thought to facilitate pair-bonding and dominance structure between band mates.1
Allogrooming (social grooming or mutual grooming) is mutual grooming done by two members of the same species. Many species engage in social grooming, including insects,2 birds,3 bats,4 primates,5 and ungulates (hoofed animals) such as cows and horses.6 Allogrooming in horses is sometimes operationally defined as rhythmic scratching with the incisors, for a period of 3 minutes or more.7
In humans, allogrooming goes far beyond picking lice from another’s head. Psychologists consider allogrooming in humans as “any behavior in which an individual removes or mimics removal of something from the skin or body.8” In addition to social scratching, allogrooming in humans includes activities such as removing loose hair, swatting away insects, and cleaning wounds.9 All of these activities are performed by horses on each other, and should be included in our notions of allogrooming in horses.
Autogrooming means self-grooming, and needs only one for this dance. Autogrooming is done much the same way as allogrooming. Because we there are some places we can’t easily reach, humans may ask others to scratch their backs and otherwise groom the areas that are difficult to reach. Your horse can’t reach his head, neck, withers, or back very well, and will often present these areas to you or another horse for work.
Percentage of time spent allogrooming for different parts of the cow’s body.10
Chimps allogroom for lice and other treasures.11
Humans allogroom for lice and other treasures.
Benefits of Allogrooming
Allogrooming delivers three important benefits to horses.
Hygiene. When a horse simply grooms itself (“autogrooming”), his hygiene improves. Wounds are cleaned, dead skin and dirt are removed, and external parasites may be removed. But when one horse grooms another, much more happens. One horse can reach places on another horse that that horse cannot reach, especially back and withers.12 Allogrooming seems most frequent in April, around the time that a horse sheds his winter coat. Does a shedding coat itch?
Stress Reduction. There is evidence that social relationships between horses are important mediators of social stress, particularly through allogrooming.13 Allogrooming has a calming effect, lowering the heartrate.14 Allogrooming sometimes results in such relaxation that a recipient falls asleep.15 The shade of a tree on a hot summer day will draw horses together; allogrooming under the tree helps reduce the stress that may be generated by this proximity.
Bonding. Allogrooming builds social bonds and stimulates the production of oxytocin. More on this later.
But “benefits” do not explain behavior. Benefits are a consequence, not a cause. What causes animals to groom each other?
All Because of Lice?
We don’t like lice. Lice may produce mild to severe pruritus (itching), patchy alopecia (bald spots), excoriation (compulsive scratching), exudation (oozing wounds), ill temper and loss of condition. We try to avoid people with lice, and kids with lice are often sent home from school. Could lice be the basis of allogrooming?
Lice are certainly common enough in horses: In one study, only 20% of horses showed no evidence of lice.16 When lice were found in this study, they were most often along the neck and below the mane (71%), in the ventral neck (62%), and the dorso-lateral trunk area (52%). These places seem to be favorite grooming spots by my lice-free horse and mule. But lice on a horse can also be found on the abdomen, the head, and all four legs. And so the maps of where lice like to live and where horses like most to be groomed don’t match up well.
Lice began to evolve at least 115 million years ago.17 Their roots are possibly in the feathers of dinosaurs. As modern birds emerged, modern bird lice did too. As mammals later emerged, so did lice that targeted them. Today, most species of birds and mammals support lice, and any one species of louse appears to depend on just one species of host. (Some hosts will support multiple species of lice.). Early humans likely had to deal with lice that bothered them all over their bodies, but when we lost our body hair, lice began to specialize. Today, there are three species of lice that affect humans. One lives only in the hair on our head. Another lives only on our pubic hair (affectionately called “crabs”). And a third lives on our clothing.
Lice have been clever in their selection of hideouts: they always choose something other than bare skin. So the distribution of lice on most birds is everywhere under their feathers, and the distribution of lice on most mammals is everywhere under their fur. When humans lost their fur, they lost their lice, too, and modern lice are programmed to stay in the hair, and not come down. Finally, there is an advantage to being an old man.
Birds seem to have tried two basic strategies for dealing with their lice. Some, such as gulls, evolved to stay a small distance from others of their species, preventing lice from jumping ship from one bird to the next. But other birds, such as cockatiels, parakeets, conures, crows, parrots, and finches all will make physical contact in the course of the day, allowing the spread of lice from one to the next. Social/colonial birds that are ripe for lice all seem to love allogrooming. Perhaps it is because they had to become more sociable to enable allogrooming, it is the social birds that we choose as pets.
In territorial birds the maintenance of personal space helps slow the spread of lice, but does not prevent it18, and territorial birds that manage to get a louse can be overwhelmed — because their need for personal space prevents allogrooming. But on average, maintaining personal space does seem to help defend against lice.
An itch might motivate allogrooming in birds in the same way that I will suggest that itches motivate rolling in horses: add a vague itch to those areas where lice hang out, let grooming address that itch, and voilà! With lice, adding the itch is easy: the saliva of a louse apparently triggers an allergic reaction, which feels itchy. At least, this is what humans report.19 We should assume that lice in all species cause allergic reactions and itches. In social animals, such an itch could motivate autogrooming and efforts to take part in allogrooming, as can be seen in the previous three photos. But note that all of this allogrooming is of head and neck.
The itch from a louse does not explain why horses might allogroom in the absence of lice. And a louse bite that produces an itchy allergic reaction is not going to be dramatically reinforced by scratching, because the itch will persist for some time after the louse is gone. And why would allogrooming always seem to focus on limited real estate — head and neck in birds, back and withers in horses?
The answer might be mother nature. Social birds might groom each other to try to cure an itch, but negative reinforcement — stopping the itch by scratching — can’t account for the severe pleasure they show when they groom each other. There must be special nerves arranged in social birds and horses that trigger the sensation of pleasure. The presence of such nerves could make grooming feel great.
The distribution of such nerves could account for why some parts are preferred as grooming sites. Rub a horse’s chin, and perhaps you stimulate such nerves. Rub their hoof, and you probably don’t. A hoof rub doesn’t feel bad — it just isn’t likely to feel good.
So we should look for such magic pleasure nerves in those areas where allogrooming in the horse is most popular: back and withers. But do such nerves even exist? Without much effort, we find that “there exists in the skin of the body a population of unmyelinated mechanosensory nerves that respond optimally to precisely the kinds of touch that typify many grooming behaviors — gentle moving touch.20” For more on this topic and these “CT afferents”, see the chapter on Pleasure.
Since we find that birds and mammals both prefer grooming of the head and neck, it is possible that the preference began with the first ectoparasites, who would have been unreachable by the victim’s mouth. (Of course, any victim with hands could have scratched them off — perhaps this is what T. rex did with his tiny hands and arms.) Because cattle, sheep, and chickens all allogroom head and neck, even though their lice may be evenly distributed around their bodies,21 it is likely that parasites, erogenous zones, and allogrooming date back a very, very long time. Early animals might have evolved with concentrations of CT afferents positioned in the hard-to-reach places favored by parasites. With this, autogrooming and allogrooming of those areas would have developed, causing problems for those parasites.
Allogroomers Choose a Friend, Build a Friendship
Everyone, including your horse, is ticklish. But ticklishness seems to only work when the tickler is well-known and well-liked by the ticklee. In return, there is the Midas effect, in which touch doesn’t produce gold, but produces recipients with more positive emotions and goodwill toward others.22 Allogrooming is like tickling: it is done between close friends, and it strengthens their bond.23 In humans, allogrooming is important in pair bonding, helping develop trust and playing a role in courtship and flirtation.24 Your horse has the same uses for it.
A mechanism for this bonding may be oxytocin. Stimulation of some of the spots that horses like to be groomed has been shown to stimulate production of oxytocin. Oxytocin, in turn promotes friendship, bonding, love.
Birds only permit grooming by those they trust and love, and most only permit it on head and neck. If you’ve ever loved a cockatiel, you know that they love to be groomed on the head and neck by someone they love. New feathers coming in have a sheath that must itch. They can break this sheath — on those feathers they can reach — by crunching them systematically with their bills. This leaves the top of their necks and their head with feathers they cannot work on. Those remaining spots must be especially uncomfortable, for cockatiels have a moan or special cluck that they’ll give when someone they trust is rubbing those feathers. The moan means they are enjoying the grooming, and I find it rewarding — so I enjoy it. Animals that allogroom are careful to thank their groomers, as well as to give feedback if grooming becomes too rough and hurts a bit.
If we are to accuse ancient lice of causing our horse to be so affectionate, what of a horse that has no lice? Does he lose interest in us? Nope. The horse biology that makes certain parts feel good when rubbed is older than his last shampoo. It is as old as Equus itself (4 million years ago) or as old as lice (at least 115 million years ago). So if we take away the lice, scratching an itch will still feel good, and rubbing the skin and stimulating a population of CT afferents will still feel good. And besides, it appears that many horses do have lice.
Horses are like social birds. We can probably study either to learn about the other.
Claudia Feh and Jeanne De Mazieres25 are among the few scientists who have studied allogrooming in horses. They have learned that both males and females will engage in allogrooming, but their 1993 study of Camargue horses found that in the spring allogrooming was most likely between male-female pairs of horses, a bit less likely with female-female pairs. In the winter, there was no difference between male-female pairs and female-female pairs. And in neither spring nor winter were two stallions observed allogrooming. This free-living herd contained no geldings, so we can’t learn what we suspect: that mares pull the horse world together through allogrooming, and we can’t know if castration improves a male’s willingness to allogroom. Certainly, we see pairs of geldings engaged in allogrooming every day in pastures with geldings only.
And who gets groomed? In one study, most mutual grooming bouts were between related horses26 or, especially in mares and young horses, with their peers.27
Allogrooming delivers three important benefits to horses: hygiene, stress reduction, and bonding. It is likely reinforced by the pleasure it produces for the receiver (zones groomed are erogenous zones) and for the giver (mouth and tongue used are also erogenous zones) from the release of oxytocin and serotonin. These zones are normally the head and neck area, areas that are difficult for animals to reach in their self-grooming, and which might develop a parasite burden in the absence of this grooming.
1 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs
2 Moore, D.; Angel, J.E.; Cheeseman, I.M.; Robinson, G.E.; Fahrbach, S.E. (1995), “A highly specialized social grooming honey bee(Hymenoptera: Apidae)”, Journal of Insect Behavior, 8 (6): 855–861.
3 Spruijt, B.M.; Van Hooff, J.A.; Gispen, W.H. (1992), “Ethology and neurobiology of grooming behavior”, Physiological Reviews, 72 (3): 825–852
4 Wilkinson, G.S. (1986), “Social grooming in the common vampire bat, Desmodus rotundus” (PDF), Animal Behaviour, 34 (6): 1880–1889
5 Sparks, John. “Allogrooming in primates: A review.”. In Morris, Desmond (Ed). (1967). Primate ethology , (pp. 148-175). New Brunswick, NJ, US: AldineTransaction, xxii, 374 pp.
6 Kimura, R. (1998), “Mutual grooming and preferred associate relationships in a band of free-ranging horses”, Applied Animal Behaviour Science, 59 (4): 265–276
7 Feh, Claudia. “Relationships and communication in socially natural horse herds.” The domestic horse (2005): 83-93.
8 Nelson, Holly, and Glenn Geher. “Mutual grooming in human dyadic relationships: an ethological perspective.” Current Psychology 26, no. 2 (2007): 121-140.
9 Nelson, Holly, and Glenn Geher. “Mutual grooming in human dyadic relationships: an ethological perspective.” Current Psychology 26, no. 2 (2007): 121-140.
10 Image source: Val-Laillet, D., Guesdon, V., von Keyserlingk, M. A., de Passillé, A. M., & Rushen, J. (2009). Allogrooming in cattle: relationships between social preferences, feeding displacements and social dominance. Applied Animal Behaviour Science, 116(2), 141-149.
11 Image source: Pilot. “Neuroscience and the Internet #2: Can I Love all 1,000 of My Facebook Friends?”
12 A horse can rub his head and upper neck against a foreleg, and entire neck against a tree. But back and withers remain beyond his reach, except during a dust bath.
13 WIkipedia: “Erogenous zone”. https://en.wikipedia.org/wiki/Erogenous_zone
14 Feh, C., de Mazieres, J., 1993. Grooming at a preferred site reduces heart rate in horses. Anim. Behav. 46 (6), 1191–1194.
15 Smuts, Barbara (1985), Sex and Friendship in Baboons, New York: Aldine Publications, ISBN 978-0-202-02027-3, retrieved 28 April 2010
16 Larsen, K. S., M. Eydal, N. Mencke, and H. Sigurðsson. “Infestation of Werneckiella equi on Icelandic horses, characteristics of predilection sites and lice dermatitis.” Parasitology research 96, no. 6 (2005): 398-401.
17 Dunn, Rob. “Of lice and men: a very intimate history.” New Scientist 216.2889 (2012): 36-39.
18 A study by Rekasi and others found that territorial birds had lower frequencies of lice than colonial birds. Rekasi, J., Rozsa, L. and Kiss, B. J. 1997. Patterns in the distribution of avian lice (Phthiraptera: Amblycera, Ischnocera). – J. Avian Biol. 28: 150-156.
19 WebMD “Lice – Symptoms” http://www.webmd.com/children/tc/lice-symptoms#1
20 McGlone, Francis, Susannah Walker, and Rochelle Ackerley. “Affective Touch and Human Grooming Behaviours: Feeling Good and Looking Good.” Affective Touch and the Neurophysiology of CT Afferents. Springer New York, 2016. 265-282.
21 Milnes AS, O’Callaghan CJO, Green LE (2003) A longitudinal study of a natural infestation in growing cattle over two winter periods. Vet Parasitol 116:67–83; James PJ, Moon RD (1998) Pruritus and dermal response to insect antigens in sheep infested with Bovicola bovis. Int J Parasitol 28:419–427
22 Schirmer, Annett, Maria Teresa Wijaya, and Siwei Liu. “The Midas Effect: How Somatosensory Impressions Shape Affect and Other-Concern.” Affective Touch and the Neurophysiology of CT Afferents. Springer New York, 2016. 283-299.
23 Val-Laillet, D., Guesdon, V., von Keyserlingk, M. A., de Passillé, A. M., & Rushen, J. (2009). Allogrooming in cattle: relationships between social preferences, feeding displacements and social dominance. Applied Animal Behaviour Science, 116(2), 141-149.
24 Nelson, Holly, and Glenn Geher. “Mutual grooming in human dyadic relationships: an ethological perspective.” Current Psychology 26, no. 2 (2007): 121-140.
25 Feh, C., de Mazieres, J., 1993. Grooming at a preferred site reduces heart rate in horses. Anim. Behav. 46 (6), 1191–1194.
26 Keiper, R. R. “Social interactions of the Przewalski horse (Equus przewalskii Poliakov, 1881) herd at the Munich Zoo.” Applied Animal Behaviour Science 21, no. 1-2 (1988): 89-97.
27 Sigurjónsdóttir, Hrefna, Machteld C. Van Dierendonck, Sigurdur Snorrason, and Anna G. Thórhallsdóttir. “Social relationships in a group of horses without a mature stallion.” Behaviour 140, no. 6 (2003): 783-804.; Wells, Susan M., and Bettina Goldschmidt-Rothschild. “Social behaviour and relationships in a herd of Camargue horses.” Ethology 49, no. 4 (1979): 363-380.