Elsevier

Behavioural Processes

Volume 121, December 2015, Pages 54-62
Behavioural Processes

Context-dependent third-party intervention in agonistic encounters of male Przewalski horses

https://doi.org/10.1016/j.beproc.2015.10.009Get rights and content

Highlights

  • Przewalski stallions intervene in encounters of group members.

  • Interveners displayed ‘social control roles’ when new horses were introduced.

  • Interveners tended to have a higher social rank in in standard social situations.

  • Stallions intervened in introduction situations for alliance formation.

  • Interventions served the reduction of social disruption in standard situations.

Abstract

One mechanism to resolve conflict among group members is third party intervention, for which several functions, such as kin protection, alliance formation, and the promotion of group cohesion have been proposed. Still, empirical research on the function of intervention behaviour is rare. We studied 40 cases of intervention behaviour in a field study on 13 semi-wild bachelor horses (Equus ferus przewalskii) in (a) standard social situations, and (b) when new horses joined the group (i.e. introductions). Only interventions in agonistic encounters were analysed. Eight of 13 animals directed intervention behaviour toward threatening animal in agonistic encounters of group members. One stallion was particularly active. The stallions did not intervene to support former group mates or kin and interventions were not reciprocated. In introduction situations and in standard social situations, the interveners supported animals which were lower in rank, but targeted, threatening animals of comparable social rank. After introductions, stallions received more affiliative behaviour from animals they supported and thus appeared to intervene for alliance formation. In standard social situations, interveners did not receive more affiliative behaviour from animals they supported and may primarily have intervened to promote group cohesion and to reduce social disruption within the group.

Introduction

Conflict among individual group members poses a severe threat to the cohesiveness and integrity of social groups and therefore needs to be resolved (Bernstein, 1976, de Waal, 1977, Ehardt and Bernstein, 1992, Flack et al., 2005, Flack et al., 2006). Third-party interventions may provide one of several ways to maintain group stability (Aureli and de Waal, 2000, Smith et al., 2010), when interveners interrupt interactions between initiators and recipients through direct physical contact, interposition, or threats (Widdig et al., 2006, Jennings et al., 2009, Smith et al., 2010; Fig. 1).

Especially when animals intervene in agonistic encounters of group members, third-party intervention is potentially costly for the intervener, who risks physical injury and invests energy and time in scanning group behaviour (Clutton-Brock, 2009, Smith et al., 2010). Nevertheless, the consistent and widespread occurrence of this behaviour in numerous species of animals including humans suggests that interveners benefit from their behaviour. Benefits may be indirect and direct fitness gains.

First, interveners which support distinct animals may support or protect kin, as in primates (Hamilton, 1964, Gouzoules and Gouzoules, 1987) and spotted hyenas (Crocuta crocuta) (Zabel et al., 1992, Smith et al., 2010). Second, animals may attempt to secure the reciprocation of aid for future encounters from the individual they support through intervention (Trivers, 1971), as shown in many primate species (de Waal and Luttrell, 1988, Silk, 1992, Schino et al., 2007), as well as in coaties (Romero and Aureli, 2008). Third, by supporting particular animals interveners may support or win alliance partners, as in male baboons (Papio spp.), female rhesus macaques (Macaca mulatta, Noë et al., 1991, Noë, 1992, Noë and Hammerstein, 1994, Noë and Hammerstein, 1995), spotted hyenas (Crocuta crocuta) (Zabel et al., 1992, Smith et al., 2010), zebras (Equus quagga, Schilder, 1990) and domestic horses (Equus caballus, VanDierendonck et al., 2009, Schneider and Krueger, 2012, Granquist et al., 2012). The formation of alliances is a frequent strategy in group living animals to gain mutual access to resources, such as grooming partners, mating partners, food, shelter, etc. (Noë and Hammerstein, 1994, Connor, 1995, Schülke et al., 2010, Schneider and Krueger, 2012), to reduce predation pressure and to improve reproductive success (Cameron et al., 2009). Interventions of the first three causalities may be displayed by animals of any social rank as described for chimpanzee females, which were of middle rank (de Waal, 1982).

Fourth, middle to high ranking animals may intervene in agonistic encounters of group members without supporting specific animals. This may promote group cohesion (Bernstein, 1976, Ehardt and Bernstein, 1992) through reducing tension (de Waal, 1977) and social disruption within the group (Flack et al., 2005, Flack et al., 2006, Jennings et al., 2009, von Rohr et al., 2012) and stabilize the group’s social rank order (Packer, 1977, Flack et al., 2005, Flack et al., 2006).

We would expect that only a few group members intervene in agonistic encounters of group members, as they run high risks of physical injury (Frank, 1996). Indeed, in primates, including humans, only certain members of a social group take over the social role of an intervener (i.e., specified social context; Hinde, 1978, Ehardt and Bernstein, 1992, Salmivalli et al., 1996, Flack et al., 2005, Flack et al., 2006 on Rohr et al., 2012). Such interveners may engage exceedingly in social activities of the group, as suggested by Granquist et al., 2012.

Here, we study interventions in agonistic encounters among 13 stallions in a semi-wild bachelor group of Przewalski horses (Equus ferus przewalskii) kept in semi-natural conditions in Tennenloher Forst, Germany. In horses, male offspring disperse from their natal groups, either singly or together with related and unrelated group mates (Tilson et al., 1988, Houpt and Boyd, 1994, Zharkikh and Andersen, 2009). They remain in bachelor groups until maturity. When mature stallions take over harems, they guard the harem mares and their offspring (Berger 1986). Przewalski horses have been observed to support individuals that are attacked by other group members (Tilson et al., 1988, Houpt and Boyd, 1994, Zharkikh and Andersen, 2009). It was suggested that interveners benefit through kin protection (Boyd, 1988, Houpt and Boyd, 1994, Zharkikh and Andersen, 2009) or alliance formation (Tilson et al., 1988).

For the present study, interactions were recorded both in standard social situations and during the introduction of new animals. We predicted that:

  • (a)

    Only a few group members display interventions (de Waal, 1982, Clutton-Brock and Parker, 1995, Flack et al., 2005, Flack et al., 2006, von Rohr et al., 2012),

  • (b)

    Interveners protect former field-mates with whom they might have formed short term alliances for protecting themselves or their resources in the past, but they do not support kin. This is because horses recognize and memorize individual group mates (Proops et al., 2009, Krueger and Flauger, 2011), but have never been shown to recognize unfamiliar kin,

  • (c)

    Intervention is not reciprocal, as reciprocal aid has not been shown in horses or other ungulates (Jennings et al., 2009),

  • (d)

    Interveners are of high social rank and rise faster in the social hierarchy than the animals they target (de Waal, 1992, Clutton-Brock and Parker, 1995, Flack et al., 2005, Flack et al., 2006),

  • (e)

    Interveners are highly social (Granquist et al., 2012) and exchange more affliative than agonistic behaviour with their group members,

  • (f)

    Interveners preferentially support alliance partners or intervene to form long lasting alliances for future, mutual resource protection (de Waal, 1992, VanDierendonck et al., 2009, Schneider and Krueger, 2012, Granquist et al., 2012).

Section snippets

Animals and enclosure

We observed 13 male wild horses (Equus ferus przewalskii) kept by Landschaftspflegeverband Mittelfranken in semi-wild management in a 50 ha enclosure of forest and grassland in Tennenlohe near Erlangen, Germany. They had free access to water, vegetation, and received additional hay and horse feed in winter. For controlling the horses' well-being, they were observed by park rangers daily.

The horses had been raised in the zoos of Nuremberg and Munich, Germany, and were transferred to the enclosure

Occurrence of third-party intervention

In total we observed 40 intervention triads in agonistic encounters of group mates, with individual horses displaying between 1 and 19 interventions each (Table 1, Table 2; Table A.3). Eight of the 13 studied horses displayed third-party interventions. One horse, Dimitri, displayed most of the intervention behaviour (N = 19). We observed 28 cases of intervention triads in introduction situations and 12 cases of intervention triads in standard social situations.

The whole group’s agonistic

Discussion

In the present study on a bachelor group of semi-wild Przewalski horses, eight of 13 stallions engaged in third-party intervention with one stallion, Dimitri, intervening most often.

Contrary to earlier suggestions (Boyd, 1988), the horses in the study did not intervene to protect kin. This might simply reflect a lack of kin recognition, as horses are capable of individual recognition (Proops et al., 2009, Krueger and Flauger, 2011), but discrimination between unfamiliar kin and non-kin has not

Conclusions

In conclusion, we suggest that intervention behaviour in agonistic encounters of male Przewalski horses mostly serves the promotion of group cohesion and the reduction of social disruption within the group in standard social situations and mostly alliance formation in introduction situations.

Acknowledgements

We thank Thomas Bugnyar, and Hans Klingel for helpful discussions, Kate Farmer for language corrections, Knut Krüger for help with the “R” statistical environment, the Landschaftspflegeverband Mittelfranken and especially Verena Fröhlich for providing us with help, their facilities and a picture, and the Tierpark Hellabrunn and Tiergarten Nürnberg for providing us with test horses. Furthermore, we thank fife anonymous referees for their helpful suggestions for the improvement of the manuscript.

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  • Cited by (0)

    1

    University of Hohenheim, Behavioural Physiology of Farm Animals, Garbenstr. 17, 70599 Stuttgart, Germany.

    2

    University of Würzburg, Department of Animal Ecology and Tropical Biology, Biocentre, Am Hubland, 97074 Würzburg, Germany.

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