Huddling in horses

In a previous post, the concept of affiliative behavior was discussed and defined, and the phenomenon of affiliation in horses introduced, as a distinct class of behaviors’ (Read: Affiliative behavior in Equus caballus), much like that afforded to agonistic behavior.

Affiliative behavior was defined as “(…) the activities between two or more (dyadic, triadic or polyadic) individuals within a social group with the function of developing, maintaining or enhancing social bonds.” {Equus Ethogram Project}

In particular we will focus on huddling behavior and describe some of the intricacies of this cooperative behavior in several species with emphasis on horses, Equus caballus (or Equus ferus caballus, for those considering this taxonomic line of thought).

Many organisms’ bunch, crowd, or stand closely together with the function of keeping warm, conserving energy, strengthening social bonds, avoiding insects or as an anti-predatory response amongst others.

This “(…) active and close aggregation of animals” (Gilbert, 2010) is usually referred to as a huddle. Horses huddle too, but what for?

Most often, huddles are linked to thermoregulatory processes, and this social thermoregulation, or “(…) the ability of some species to use sociality or grouping to regulate their body temperature” (Gilbert, 2010), is a common energy saving strategy for many endothermic species (Canals, 1998; Alberts, 1978).

By bunching together, individuals reduce the body surface area exposed to inclement weather, consequently reducing energy spent in regaining a conservative equilibrium (Humphreys, 1933), or homeostasis.

For example:

Pups of Norway rats, Rattus norvegicus, were found to lessen individual body heat loss and oxygen consumption when huddled. (Alberts, 1978)

Mottled gray pulmonate slugs, Limax pseudoflavus, “(…) form closely packed huddles within their day-time resting sites” (Cook, 1981), resulting in water conservation.

Male Emperor penguins, Aptenodytes forsteri, huddle tightly to battle polar winds and freezing temperatures. Emperors’ huddle in groups of up to 6000 individuals achieving an air temperature within the huddle of up to 35°C, while air temperatures in the perimeter are well below 0°C. (Gilbert, 2007)

Recent studies by Zitterbart (2011) highlight the continuous self-regulatory and coordinated movements of huddled male Emperor penguins which allowed those in the periphery temporal access to warmer positions within the huddle, while the whole time balancing their mates egg on their feet.

See video:

In terms of horses huddling as a means of social thermoregulation, there really is very little work done. During cold weather horses have been observed to huddle or crowd together, on windy or rainy days; horses typically stand close to one another with “backs to weather” or “backs to natural windbreak”, as described by McDonnell (2003):

Backs to weather – Typically observed during windy or rainy days, Two or more horses stand closely together with their “(…) hindquarters into the wind.” (from the Equid Ethogram p. 79)

Backs to natural windbreak – Two or more horses stand closely together with their “(…) hindquarters protected from the wind by vegetation or other feature of the environment.” (from the Equid Ethogram p. 78)

McDonnell (2003), suggests that the backs to weather behavior reduces the body surface area exposed to inclement weather, thus minimizing heat loss; in short it serves a thermoregulatory function.

With thermoregulation in mind and considering that standing close together exposes each individual’s surface area to the others’, these confronting surfaces are likely to maintain a constant temperature, thus non-evaporative heat loss decreases (Morgan, 1996).

Social thermoregulation aside, huddling is suggested to perform a social function as well (Alberts & Brunjes, 1978; Dunbar, 1991; Spruit, 1992; Takahashi, 1997). Many of the species studied, present a significant correlation between preferred partners, based on the frequency, distance, and duration individuals are proximate to one another.

In a study on the Japanese macaque (Macaca fuscata) troop on Kinkazan Island, Hiroyuki Takahashi of Kyoto University found that huddling was more frequent among kin and affiliative dyads, than it was among non affiliative dyads (Takahashi, 1997). Furthermore, Takahashi (1997) suggested that huddling strengthens social affiliations between non-related individuals.

In horses, the social bonds between unrelated mares, friendships, contribute to reproductive success as suggested by Cameron et al (2009) in a study on the Kaimanawa feral horses of New Zealand.

Horses have preferred partners within their band or herd with whom they associate more often with than other members of the group. Claudia Feh (1987), found that in the Camargue horses, horses had up to two, rarely three, preferred partners. These affiliative interactions are characterized by individuals sharing “personal space” (Dierendonck & Goodwin, 1992), and synchronizing activities.

Despite huddles not being extensively studied in horses, it is frequently mentioned in equid related literature, especially regards to group rest (Tyler, 1972; McDonnell, 2003; Ransom & Cade, 2009), or social grooming such as in mutual insect control. (McDonnell, 2003; Ransom & Cade, 2009)

Group rest in horses is a synchronized activity wherein most, if not all, members of the band or herd rest at the same time, typically in close proximity of one another. (Tyler, 1972) Furthermore, Tyler (1972) suggested that group rest is socially facilitated: the behavior of one or more individuals’ trigger or initiate similar behavior in other members of the group, in other words; the behavior of one individual is socially contagious. (Thorpe, 1963)

Before going any further, let’s differentiate two types of huddles, namely; tight huddles and loose ones (Behnke, 2012). Tight huddles are those in which the majority of group members are in physical contact with one another, or separated by < 50 cms, leaving no gaps between individuals (Behnke, 2012). In contrast, loose huddles are those in which the majority of group members are in close proximity; from > 50 cms to < 150 cms, but not in physical contact with one another. Both between individual distances are currently in use in the ongoing Equus Ethogram Project.

Feist & McCullough (1976) observed horses typically resting in tight groups, or alternately, in groups of one or two pairs. (Feist & McCullough, 1976) Whether horses are grouped tightly or loosely may seem trivial, but for the sake of alienating the functional characteristics of different huddles, and their forms, the distance between individuals are of prime importance.

In tightly huddled horses, insect control is facilitated between group members. (Ransom & Cade, 2009) Several studies suggest that animals tend to group together when biting fly density or harassment is high (Bergerund, 1974; Schmidtmann and Valla, 1982; Rutberg, 1987; Rubenstein and Hohmann, 1989).

In the warmer months, which tend to correlate with an increase in insect harassment, two or more horses stand close together, typically tail to shoulder to the nearest neighbor. This is usually referred to as antiparallel standing. In this position, individuals take advantage of one’s proximity to another to keep pesky biting insects at bay. Typically, horses have flies swished from their faces by the tail of a neighbor, but this can also be achieved by rubbing or bumping with those close by.

Social grooming, or allogrooming (McFarland, 1987), is characterized by one individual grooming another. In this sense, it is plausible to consider differentiating uni-directional grooming from mutual grooming.  As the name implies, uni-directional grooming involves one individual grooming another without being groomed in return (at that moment), whilst mutual grooming involves two or more individuals grooming each other simultaneously. (McDonnell, 2003).

The function of allogrooming is twofold, as it not only serves to relieve horses of discomfort, such like that caused by itchy skin or biting insects, but it lowers the heart rate and cortisol levels of the individuals when directed at certain locations. (Feh & Mazieres, 1993; Haverbeke et al., 2002; Dierendonck & Goodwin, 1992).

“In mammals, grooming behavior appears not only to remove ectoparasites, but to play a role in cementing social relationships” (Dugatkin, 1997)

To initiate a mutual grooming bout, one or both horses approached the other head on (Feist & McCullough, 1976), “(…) with its ears forward and its mouth occasionally open with the lower teeth slightly exposed”. (Tyler, 1972)

A typical bout consists of two standing horses facing each other; nibbling at the neck, mane, forelegs and withers of their partners (Tyler, 1972). Alternately they stand antiparallel, with head to shoulder for the mutual grooming session.

Feist & McCullough (1976) noted that mutual grooming only occurred between members of the same group and in all possible combinations between group members.

Unlike mutual grooming described in the competition-reconciliation model, where the number of times groomed by another is relative to status, in horses this seems not to be the case as suggested by the random and multivariate grooming sessions afforded by feral horses.

Mutual grooming will be discussed further in a following section.


Reference list

Alberts, J.R. (1978). Huddling by rat pups: multisensory control of contact behaviour. Journal of Comparative and Physiological Psychology 92, 220–230.

Alberts, J. and Brunjes, P.C.  (1978) Ontogeny of Thermal and Olfactory Determinants of Huddling in the Rat. Journal of Comparative and Physiological Psychology; Vol. 92, No.5, pp. 897-906

Alberts, J.R. (2007). Huddling by rat pups: ontogeny of individual and group behaviour. Developmental Psychobiology 49, 22–32.

Behnke, T. (2012) To Huddle or Not to Huddle: That is the Question . A Brief Study of the Basis for Huddling Behavior in Eulemur rubriventer. In dependent Study Project (ISP) Collection. Paper 1470

Berger, J. (1986). Wild horses of the great basin: social competition and population size. University of Chicago Press, Chicago.

Brunjes, P.C. and Alberts, J. (1979) Olfactory Stimulation Induces Filial Preferences for Huddling in Rat Pups. Journal of Comparative and Physiological Psychology, Vol. 93: 3: pp. 548-555

Canals, M., et al. (1997) Geometrical aspects of the energetic effectiveness of huddling in small mammals. Acta Theriologica 42 (3): 321-328

Contreras L. C. (1984) Bioenergetics of huddling: test of a psycho-physiological hypothesis. Journal of Mammalogy 65: 256-262

Crowell-Davis SL  (1994)  Daytime rest behavior of the Welsh pony (Equus caballus) mare and foal.  Appl Anim Behav Sci 40: 197-210.

Dierendonck, M. and Goodwin, D. (????) Social contact in horses: implications for human-horse interactions

Dugatkin, L.A. (1997) Cooperation Among Animals: An Evolutionary Perspective. Oxford University Press. Oxford, U.K.

Dunbar, R.I.M. (1991) Functional significance of social grooming in primates. Folia Primatol 57: 121-131.

Feh C (1987) Etude du développement des relations sociales chez des étalons de race      Camargue et de leur contribution à l’organisation sociale du groupe. Thèse d’université, University of Aix-Marseille, France.

Feh C, de Mazières J (1993) Grooming at a preferred site reduces heart rate in horses. Anim Behav 46: 1191-1194.

Feist, J.D. & McCullough, D.R. (1976) Behavior Patterns and Communication in Feral Horses. Z. Tierpsychol, 41, 337-371

Gilbert, C.G., Robertson, Y., Le Maho, Y., Naito, A. and Ancel, A. (2006) Huddling behavior in emperor penguins: dynamics of huddling. Physiology & Behavior , 88(4-5), 479-488

Gilbert, C., McCafferty, D., Le Maho, I., Martrette, J. M., Giroud, S., Blanc, S. and Ancel, A. (2010) One for all and all for one: the energetic benefits of huddling in endotherms. Biol. Rev. 85: 545–569

Haverbeke, N.S. et al. (2002). Heart rate reduction by grooming in horses (Equus caballus). Dorothy Russell Havemeyer Foundation Workshop. Horse Behaviour and Welfare. Holar. Iceland

Haig, D. (2008) Huddling: brown fat, genomic imprinting and the warm inner glow.  Current Biology 18(4): R172-R174.

“Huddle.” Accessed January 18, 2014.

Jefimow, M.; Glabska, M. and Wojciechowski, M. (2011) Social thermoregulation and torpor in the Siberian hamster. The Journal of Experimental Biology 214, 1100-1108

Linklater, W.L. (2000). Adaptive explanation in socio-ecology: lessons from the equidae.Biological Reviews of the Cambridge Philosophical Society, 75(1), 1-20.

Linklater, W.L. et al. (2000). Social and spatial structure and range use by Kaimanawa wild horses (Equus Caballus, Equidae). New Zealand Journal of Ecology, 24(2), 139-152.

McDonnell, S.M. (2003) The Equid Ethogram: A Practical Field Guide to Horse Behavior . Eclipse Press, Lexington, Kentucky

Ransom, J.I. and Cade, B.S. (2009) Quantifying equid behavior— A research ethogram for free-roaming feral horses:U.S. Geological Survey Techniques and Methods 2-A9

Rees, L. (1984). The Horse’s Mind. London: Stanley Paul.

Rubenstein, D.I. & Hohmann, M.E. (1989) Parasites and Social Behavior of Island Feral Horses. Oikos 55: 312-320.

Sealander J. A. (1952) The relationship of nest protection and huddling to survival in Peromyscus at low temperature. Ecology 33: 63-71

Spruijt, B.M., Van Hoof, J.A.R.A.M., Gispen, W.H. (1992) Ethology and neurobiology of grooming behavior. Physiol Rev 72(3), 825-852.

Takahashi, H. (1991) Huddling Relationships in Night Sleeping Groups Among Wild Japanese Macaques in Kinkazan Island During Winter. Primates, 38(1): 57-68

Tyler, S.J. (1972) The behaviour and social organization of the new Forest ponies, Animal Behaviour Monographs, 5: 85-196

Zitterbart DP, Wienecke B, Butler JP, Fabry B (2011) Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle. PLoS ONE 6(6): e20260

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