COPYRIGHTED MATERIAL Social Ecology of Horses Konstanze Krueger University of Regensburg Department of Biology I corresponding author: Konstanze Krueger: [email protected] ‘The original publication is available at www.springerlink.com’ 2008 Ecology of Social Evolution, 195-206, DOI: 10.1007/978-3-540-75957-7_9 Abstract counter when escaping from their human care takers. Since horses show a strong tendency to gather in Horses (Equidae) are believed to formidably demons- groups, it seems to be reasonable to apply Hamilton`s trate the links between ecology and social organizati- selfish herd theory (1971) to herd aggregation and on. Their social cognitive abilities enable them to suc- group cohesion in equids. Hamilton (1971) introdu- ceed in many different environments, including those ced the case of a novel mutant which increases its provided for them by humans, or the ones domestic reproductive strategy by taking cover between its horses encounter when escaping from their human herd companions. Finally, the novel behaviour will care takers. Living in groups takes different shapes spread throughout the whole population and initiate in equids. Their aggregation and group cohesion can herd aggregation, which results in a better protection be explained by Hamilton`s selfish herd theory. How- from predation for the herd members. This theory led ever, when and which group to join appears to be to the assumption that simple selfish movement ru- a conscious individual decision depending on preda- les would decrease predation risk when the predator tory pressure, intra group harassment and resource attacks from outside the flock perimeter (Viscido et availability. The latest research concerning the social al. 2001). And in fact Viscido et al. (2001) found that knowledge horses display in eavesdropping experi- regardless of the predator‘s size and speed predation ments affirm the need for an extension of pure ge- risk always decreases as long as the individuals take netic herd concepts in horses for a cognitive compo- their mates into account. They concluded that the nent. Horses obviously realize the social composition selfish avoidance of danger can lead to aggregation. of their group and determine their own position in it. However, Hamiltons (1971) movement rules are neg- The horses` exceedingly flexible social behavior ea- lected to produce true aggregations, because the re- gerly demands for explanations about the cognitive quired complex individual behavior would be impos- mechanisms which allow horses to determine their sible for most animals to follow. Viscido et al. (2002) individual decisions. called this phenomenon the ‘‘dilemma of the selfish herd”. In their opinion the animal’s ability to detect “Ecology conditions like those that favor the evolution its neighbors is an important factor in the dynamics of open behavioural programs sometimes also favor of group formation. Not only the nearest neighbors the evolution of the beginnings of consciousness, by but also the behaviour of distant neighbors mediates favoring conscious choice. Or in other words, con- information in case of predation. Reluga and Viscido sciousness originates with the choice that are left (2005) suggested that predation-based selection can open by open behavioural programs” (Popper, 1977). even increase the influence of distant neighbors rela- tive to near neighbors. Another critical approach to the nearest neighbor 1 Introduction strategy, proposed by Hamilton’s selfish herd mo- Horses (Equidae) are believed to formidably demons- del, was written by James (2004). He introduced the trate the links between ecology and social organiza- concept of a ‘‘limited domain of danger’’, which re- tion (Berger 1977, Moehlmann 2002). Their social presents either a limited detection range or a limited cognitive abilities enable them to succeed in many attack range of predators. An analysis of individual different environments, including those provided for movement rules showed that animals escape from them by humans, or the ones domestic horses en- danger best, when they use a time minimization stra- tegy rather than a nearest neighbor strategy. 1999). Foals born into a group stay with it for one to Because of the horses` strong tendency to gather five years before they disperse (Moehlmann 2002). in groups, I agree that their basic aggregation and Young females usually leave during their first estrus group cohesion can be explained by Hamilton`s sel- and join other families (Berger 1986). Young males fish herd theory (1971). However, the critical voices, tend to stay for several more years before they de- which express criticism concerning the consideration part to find bachelor groups (Moehlmann 2002) or of conscious neighbor detection should be conside- found a harem of their own. However, Klingel (1972) red, as well. When and which group to join appears hypothesized that in the absence of playmates male to be a conscious individual decision depending on offspring disperses earlier from their natal group. predatory pressure, intra group harassment and re- This “playmate hypothesis” is still under discussion. source availability. For greater foraging efficiency, Berger (1986) could not find any correlation between horses have to decide when to spread out, at a cost dispersal age and number of peers in the feral horses of greater predation risk (Janson 1990). of the “Great Basin”, Nevada, Utah, Oregon, where- The social lives of equid herds can be compared to as Rutberg and Keiper (1993) could prove a strong the fission-fusion model (Dyer 2000) in apes` soci- correlation for male offspring dispersal age and the al systems to some extend. Apes (Dyer 2000), ele- presence of playmates in Assateague Island ponies, phants (Moss and Poole 1983) and dolphins (Con- Maryland. On average male offspring dispersed ear- nor et al. 2000) frequently depart and reunite again. lier than female offspring on Assateague Island. However, most equids` social groups are much more When food biomass levels drop below 40 g/m² during stable, even thought stallions change their reproduc- periods of drought, normally stable groups of plains tive strategy and therefore their social affiliation seve- zebras may become unstable (Ginsberg 1988). This ral times throughout their lives. Nevertheless mares suggests that the stability of group and group size in tend to stay with the group they once joined after they equids is bound to the distribution and availability of departed from their natal group in their first estrus. resources. Especially in dry environments, such as Depending on predatory pressure and resource avai- the Danakil Desert of Ethiopia and Eritrea, the scat- lability they sometimes change their affiliation to a tered supply of food and limited water usually does certain social group even later in live. Though these not permit females to forage close to one another and capabilities are probably best known from feral hor- to form consistent groups. Therefore, males estab- ses (Equus ferus caballus) they have to be discussed lish separate territories near a critical source of water in the context of the whole equid family. or food. They then control mating with all females, Living in groups takes different shapes in equids. For which, sometimes accompanied by their offspring, species, which live in wide grasslands, such as the come onto the territory to drink or feed (Moehlmann Serengeti Plain of Tanzania or the valleys of Hus- 2002). African wild asses (E. africanus), feral asses tai National Park in Mongolia (King 2005), food and (E. asinus), Grevy’s zebra (E. grevyi) and Asiatic wild water resources are sufficient enough to allow fema- asses (E. hemionus) organize themselves without les to feed together and to thus form stable groups, permanent bonds between adults although someti- which consist of one or more mares, their offspring mes they form temporary groups. Stallions can do- and usually one, but occasionally up to five males minate their territories for years. They may tolerate (Tyler 1972, Berger 1977, Moehlmann 2002). Surplus other males in this area but monopolize mating. stallions gather in separate bachelor groups that dif- Controlling access to water is critical. Lactating fe- fer in size from 2 to approximately 17 horses (Berger males need to drink at least once a day, and so they 1977). Such a system is referred to as “harem”, “fa- will stay as close to a pond or stream as possible. mily,” or “band.” Many bands form a structured social A female comes into estrus a week or two after gi- unit, called “herd,” which shows the same migration ving birth and, if she is not fertilized then, again about patterns within a common home range (Miller 1979). a month later. Thus, the territorial male has several Berger (1977) observed a herd of more then 210 feral chances to father a new foal. The females, in turn, horses grazing, clustered in groups. Harem groups do not only gain access to water but may also benefit are common in Plains and Mountain zebra (Equus from reduced harassment from bachelor males and burchelli and E. zebra), Przewalski’s horses (E. fe- better protection from predators (Klingel 1977, Mo- rus przewalskii), and feral horses (E. ferus caballus) ehlmann 2002). and provide a relatively safe environment, as stable groups and the presence of a stallion help to fend off 2 Ecology and social organization of feral horses predators, such as wolves, lions, and hyenas. Mature females of a band often remain together According to genetic data, horses, E. caballus, were throughout their whole lives, while stallions may domesticated repeatedly from several distinct popu- change their reproductive strategy several times, de- lations of wild horses (Jansen et al. 2002). Today, all pending on their age and fighting ability and the num- domestic and feral horses are organized in social ber of competitors they have to contend with (Feh groups. For a full understanding of the evolutionary roots of their social behavior, a comparison with their mating, several times throughout their lives (Miller wild ancestors would be of immense value.