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Bouskila, A.,; Lourie,E.; de Vries, H.; Hermans, Z.M .; van Dierendonck, M. |
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Sex, but not relatedness nor age, affect the social network of horses in a semi-natural reserve |
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2015 |
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Proceedings of the 3. International Equine Science Meeting |
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Proc. 3. Int. Equine. Sci. Mtg |
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For some time now, social network analysis provides tools to describe concisely the social structure of animals. Nevertheless, the factors that shape the social network and determine the frequency of different types of affiliations are often unclear. For example, the effects of relatedness on social networks have been only studied on relatively few species. Both among social or sub-social species the effects ranged from no effect to a substantial effect. Here we test the hypothesis that relatedness has an effect on the social network of horses (Equus caballus) that live freely in semi-natural conditions, and specifically, that individuals with high values of relatedness to their neighbors in the network will have fewer links Wolf et al, 2011). This hypothesis is based on the notion that related individuals have common interests and are likely to assist each other, and may need less links to other, unrelated individuals. In addition, individuals of similar age and sex are likely to have common needs, and thus are more likely to be associated. We thus tested a second hypothesis: homophily to individuals with similar age and sex will have a role in determining the associations within the social network. The field study was conducted on 27 horses in the Blauwe Kamer reserve in the Netherlands (1.1 sq km). We videotaped horses and their groups and used the information from 22 horses, after excluding the five foals from the analysis. Relatedness was calculated from the pedigree, which was based on parentage, determined by DNA analysis. The social network was constructed based on spatial proximity data. We assessed the influence of relatedness, age-homophily and sex-homophily on the network structure with Multiple Regression Quadratic Assignment Procedure (MRQAP) (Krackhardt 1988), with the R package sna. The results show that there was no significant effect of relatedness on the network, nor an effect of age-homophily. Nevertheless, we found a significant effect of sex-homophily, the tendency of individuals to associate with individuals of the same sex. We argue that the lack of a relatedness effect is not likely to have been caused due to the inability to detect who is kin. The structured social system in horses includes strong associations between often unrelated individuals, alongside with young individuals leaving their natal harem as part of the inherent inbreeding avoidance (Linklater & Cameron 2009; Boyd et al. in press). The significant effect of sex-homophily could stem from the protection females get from associations with other females, e.g., in the reduction of harassment from males. Previous studies on feral horses showed that mares that were better connected with other females in their harem benefited from higher survival rates to their foals (Cameron et al. 2009). The associations among bachelor males could also contribute to the strength of the sex-homophily effect.
In order to generalize from our results, one needs to examine additional populations of horses, because the conditions in the Blauwe Kamer reserve may not be representative, mainly due to the limited opportunity for dispersal in a restricted area.
Keywords: Long-term affiliation; spatial proximity, kin detection
Boyd, L., Scorolli, A. L., Nowzari, H., & Bouskila, A. (2016). Chapter 2: Social organization. In J. I. Ransom, & P. Kaczensky (Eds.), Wild equids. Baltimore, Maryland (in press): The Johns Hopkins University Press.
Cameron, E. Z., Setsaas, T. H., & Linklater, W. L. (2009). Social bonds between unrelated females increase reproductive success in feral horses. Proceedings of the National Academy of Sciences, 106(33), 13850-13853
Krackhardt, D. (1988). Predicting with networks: Nonparametric multiple regression analysis of dyadic data. Social Networks, 10(4), 359-381.
Linklater, W. L., & Cameron, E. Z. (2009). Social dispersal but with philopatry reveals incest avoidance in a polygynous ungulate. Animal Behaviour, 77(5), 1085-1093.
Wolf, J. B., Traulsen, A., & James, R. (2011). Exploring the link between genetic relatedness r and social contact structure k in animal social networks. The American Naturalist, 177(1), 135-142. |
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Bouskila, A. |
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Equine Behaviour @ team @ |
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5881 |
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VanDierendonck, M.C.; de Vries, H.; Schilder, M.B.H. |
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Title |
An Analysis of Dominance, Its Behavioural Parameters and Possible Determinants in a Herd of Icelandic orses in Captivity |
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Journal Article |
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1995 |
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Netherlands Journal of Zoology |
Abbreviated Journal |
Netherl. J. Zool. |
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45 |
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3-4 |
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362-385 |
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Dominance; rank order; horses; Icelandic horses. |
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Th e applicability of the concept of dominance was investigated in a captive herd of  Icelandic
horses and  ponies of diff erent breeds. Eight out of  behaviours possibly related
to dominance occurred frequently enough to be investigated in detail. For these eight agonistic
behaviours the coverage, the unidirectionality in the exchange, and the degree of
transitivity (Landau`s linearity index) were calculated. Four off ensive behaviours, together
with avoidance, were suitable for further analysis with regard to dominance. Th e patterns
of asymmetries with which these behaviours were exchanged were suffi ciently similar as to
justify the application of the dominance concept and to construct a (nearly) linear dominance
hierarchy. Th e rank order of the castrated stallions was completely linear, the hierarchy
of the mares was almost completely linear. Th e results suggest that off ensive and defensive
aggressive behaviours should be treated separately and that the concept of dominance
is applicable. However, ritualized formal dominance signals between adult horses appear to
be (almost) absent. Th e rank positions of the individuals were correlated with age and residency
in the herd but not with height. Middle ranking horses tended to be more frequently
in the close vicinity of another horse than high ranking or low ranking horses. Over and
above this correlation at the individual level, it was found that pairs of horses close in rank
to each other were more often also spatially close to each other. Being in oestrus did not infl
uence the dominance relationships between mares. For castrated stallions the rank positions
were correlated with the age at which they were castrated. Th is suggests that in male
horses experience prior to neutering infl uences the behaviour afterwards. |
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English |
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Gammell, M.P.; de Vries, H.; Jennings, D.J.; Carlin, C.M.; Hayden, T.J. |
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Title |
David's score: a more appropriate dominance ranking method than Clutton-Brock et al.'s index |
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Journal Article |
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2003 |
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Animal Behaviour. |
Abbreviated Journal |
Anim. Behav. |
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66 |
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3 |
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601-605 |
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453 |
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de Vries, H.; Stevens, J.M.G.; Vervaecke, H. |
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Title |
Measuring and testing the steepness of dominance hierarchies |
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Journal Article |
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Year |
2006 |
Publication |
Animal Behaviour. |
Abbreviated Journal |
Anim. Behav. |
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Volume |
71 |
Issue |
3 |
Pages |
585-592 |
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In the analysis of social dominance in groups of animals, linearity has been used by many researchers as the main structural characteristic of a dominance hierarchy. In this paper we propose, alongside linearity, a quantitative measure for another property of a dominance hierarchy, namely its steepness. Steepness of a hierarchy is defined here as the absolute slope of the straight line fitted to the normalized David's scores (calculated on the basis of a dyadic dominance index corrected for chance) plotted against the subjects' ranks. This correction for chance is an improvement of an earlier proposal by de Vries (appendix 2 in de Vries, Animal Behaviour, 1998, 55, 827-843). In addition, we present a randomization procedure for determining the statistical significance of a hierarchy's steepness, which can be used to test the observed steepness against the steepness expected under the null hypothesis of random win chances for all pairs of individuals. Whereas linearity depends on the number of established binary dominance relationships and the degree of transitivity in these relationships, steepness measures the degree to which individuals differ from each other in winning dominance encounters. Linearity and steepness are complementary measures to characterize a dominance hierarchy. |
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454 |
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de VRIES, H.A.N. |
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Title |
Finding a dominance order most consistent with a linear hierarchy: a new procedure and review |
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1998 |
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Animal Behaviour. |
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Anim. Behav. |
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55 |
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4 |
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827-843 |
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A procedure for ordering a set of individuals into a linear or near-linear dominance hierarchy is presented. Two criteria are used in a prioritized way in reorganizing the dominance matrix to find an order that is most consistent with a linear hierarchy: first, minimization of the numbers of inconsistencies and, second, minimization of the total strength of the inconsistencies. The linear ordering procedure, which involves an iterative algorithm based on a generalized swapping rule, is feasible for matrices of up to 80 individuals. The procedure can be applied to any dominance matrix, since it does not make any assumptions about the form of the probabilities of winning and losing. The only assumption is the existence of a linear or near-linear hierarchy which can be verified by means of a linearity test. A review of existing ranking methods is presented and these are compared with the proposed method. |
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Albers, P.C.H.; de Vries, H. |
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Title |
Elo-rating as a tool in the sequential estimation of dominance strengths |
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2001 |
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Animal Behaviour. |
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Anim. Behav. |
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61 |
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2 |
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489-495 |
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De Vries, H.; Appleby, M.C. |
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Finding an appropriate order for a hierarchy: a comparison of the I&SI and the BBS methods |
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2000 |
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Animal Behaviour. |
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Anim. Behav. |
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59 |
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1 |
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239-245 |
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