Physiology & Behavior 104 (2011) 464–468

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Physiology & Behavior

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Asymmetry of behavioral responses to a human approach in young naive vs.trained horses

Carol Sankey a,⁎, Séverine Henry a, Caroline Clouard a, Marie-Annick Richard-Yris b, Martine Hausberger a,b

a Laboratoire d'éthologie animale et humaine-UMR 6552 CNRS/ Université de Rennes 1, Station Biologique, 35380 Paimpont, Franceb Laboratoire d'éthologie animale et humaine - UMR 6552 CNRS/Université de Rennes 1, Avenue du Général Leclerc, Campus de Beaulieu, F-35042 Rennes Cedex, France

⁎ Corresponding author. Tel.: +33 2 99618155.E-mail address: carol.sankey@univ-rennes1.fr (C. Sa

0031-9384/$ – see front matter © 2011 Elsevier Inc. Aldoi:10.1016/j.physbeh.2011.05.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 September 2010Received in revised form 12 April 2011Accepted 6 May 2011

Keywords:LateralityEmotionalityHuman approachHorse

The aim of this study was to investigate the impact of training experience on young horses (Equus caballus)’lateralized responses to an approaching human. The results show that the one year old untrained horsesdisplay asymmetrical responses to an approaching human, with more negative reactions (escapes, threats)when approached from the left side, while approaches towards the right shoulder elicited more positivebehaviors. On the contrary, two years old trained horses reacted equally positively to approaches and contacton both sides. Our findings support those of previous studies investigating a link between emotionality andlaterality and confirm the role of the left hemisphere in the processing of novel or negative stimuli. Moreover,the data underline the impact work and training can have on this laterality in horses.

nkey).

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© 2011 Elsevier Inc. All rights reserved.

1. Introduction

Long believed to be a human specificity, brain lateralization (i.e.asymmetry of brain function and behavior) is now accepted as awidespread characteristic of vertebrates [1,2] and recent evidencesuggests it is also present in some invertebrate species (e.g. [3]).Throughout vertebrates, the general pattern of lateralization observedis that the right hemisphere is involved in the expression of intenseemotions and controlling rapid responses, whereas the left hemi-sphere is concerned with responses that require inhibition ofresponding until a decision is made [4]. Behavioral laterality hasbeen investigated in both intra- and interspecific encounter situa-tions. For instance, a lateralized approach pattern has recently beendemonstrated in mangabeys showing that when individuals approachanother group member, they position themselves as so to have theapproached individual in their right visual field [5]. In humans, a headturning bias towards the right side has been shown in human adultkissing behavior [6]. Most studies investigating interspecific in-teractions looked at the specific case of predator–prey encounters(e.g. in toads: [7], dunnarts [8]) or agonistic interactions (e.g. in geladababoons [9]). Thus, these studies have focused on eye preference orlateralized escape responses as the outcome of a negative encounter.

In domestic animals, the case of human animal interactions andmore specifically the way to approach an unknown animal has notreceived particular attention. A single study, conducted by Austin andRogers [10] investigated the asymmetry of flight turning response to a

human approaching with a frightening object (umbrella) in domestichorses. However, it was more the novel and frightening aspect of theobject, rather than the human, that was being tested. Other studieshave involved novel objects, including in marmosets [11], chicken[12] or even fish [13]. Here, our study aimed to investigate thebehavioral responses of young domestic horses to a human approach-ing them from the left or right side, as well as to use visual laterality toevaluate the possible impact of horses' training experience on the waythey perceive this approaching human (positive, neutral or negative).Horses are traditionally approached and handled on the left side. Yet,understanding lateralization in horses has many practical implica-tions in order to understand current training methods, develop newtraining paradigms but also help for the selection of horses for thevarious disciplines and work practices [10]. So far, horses have beenshown to display both motor [14] and sensory (e.g. auditory [15])lateralization patterns. When it comes to visual laterality, horsesprove to be a particularly interesting model, for they have laterallyplaced eyes and an almost complete decussation of the optic fibres[16,17], meaning that what is seen in the right monocular visual fieldis processed by the left hemisphere and what is seen in the leftmonocular visual field is processed by the right hemisphere.Interestingly, recent research has shown a link between emotionalityand visual laterality in horse: adult horses show greater reactivitywhen approached by a human holding a novel object in their leftmonocular visual field [10] and horses that have the highestemotionality index tend to fixate a novel object with their left eye[18]. Up until today, the way this lateralization pattern developsremains unclear, but factors such as age and training experience(Larose et al. [18], Farmer et al. [26]) have been suggested to impacton horses’ emotional laterality.

Fig. 1. Diagram of the paths taken by the experimenter to approach the pre-definedzones, commencing 8 m away.

465C. Sankey et al. / Physiology & Behavior 104 (2011) 464–468

In this study, we investigated 1) whether one year old, naivehorses show a lateralization in their response to a human approachingthem from the left or from the right and 2) whether the potentiallateralized response pattern differed in two years old horses that hadundergone initial training at an earlier age (traditionally performedfrom the left side).

2. Material and methods

2.1. Study subjects

This study was conducted at the “station expérimentale deChamberet” in France with 39 Angloarabian (N=33) and Frenchsaddlebred (N=6) young horses, living in groups at pasture.

A first group was composed of 16 one year old horses (10 femalesand 6 castrated males). The particularity of the group was that theirexperience with humans was limited tominimum veterinary care andfeed distribution in the winter period. This group was considered asthe “naïve” horses group.

A second group was composed of 23 two years old horses (15females and 8 castrated males). In addition to the veterinary care andwinter feeding, these horses had received a two months initialtraining at the age of one year (cf. [19]), during which they had beentrained to remain immobile on a vocal command given from their leftside and then to accept usual handling procedures (such as beingfitted with a halter and a surcingle, giving their feet, etc.). This groupwas here considered as the “trained” group.

Otherwise, both groups were raised in similar conditions duringtheir first year: from birth until weaning at the age of 6 months, theywere brought up as a group with their dams in a pasture. Weanlingsthen spent their first winter in groups of 5 to 6 individuals in a stallwhere they stayed until the next Spring. Group 2 horses spent theirsecond summer all together in a pasture, and were pushed in a largeindoor stall for a few hours every day in order to carry out their initialtraining (~4 months). They spent their second winter in individualindoor stalls, where they could see and hear each other and alsophysically interact with their neighbors through the stall bars. In theSpring 2008, both groups were released in 2 different (but equivalentin size) pastures, sufficiently distant from one another so that theycould not see or hear each other.

2.2. Experimental procedure

In both groups, approach tests were carried out in 1 to 2 dailysessions, lasting 1 to 2 h and took place at different times every day,between 8.30 am and 7.30 pm. In the two groups, all approaches werecarried out by the same experimenter (CC, woman who always worethe same dark green coat). She was blind to the horses' earlierexperience with humans. During a single session, the experimenternever approached the same horse more than twice. In order to assessthe young horses' responses to an approaching human, we defined 7zones: shoulder, flank and croup on both left and right sides and front(Fig. 1). Each approach followed the same pattern: after choosing animmobile but awake (i.e. eyes open) animal, the experimenter placedherself at a distance of 8 m from the horse and started approaching itfollowing an imaginary line, walking at a constant average speed ofone step per second. She walked in a neutral position with her armsbeside her body. During the whole approach, she looked at the horse,in the direction of the target zone. When she was close enough(distance b0.2 m), she extended her left arm to gently touch thetarget zone, while her other arm stayed beside her body. An approachended when she established contact and touched the target zone or atwhatever distance if the animal attempted to avoid, escape orthreaten her by any means. Lastly, if another horse interrupted theapproach by placing itself in the experimenter's trajectory, theapproach was canceled and repeated later. The order in which the

animals were approached was determined by the animals' availabilityin the pasture but the order in which the zones were approached foreach horse was randomized. Overall, the experimenter approachedeach horse 7 times, once towards every predefined zone.

2.3. Data collection

The data were recorded by the experimenter herself using a digitalvoice recorder for later transcription. From the beginning of herapproach, she recorded the horse's behavior continuously. She alsonoted whether the approach led to a contact on the target zone andthe animal's positive or negative response to the contact. Standingstill, looking at the experimenter and sniffing her were consideredpositive reactions, while avoidance, escape and threats, threats ofbiting and of kicking were considered negative reactions [20,21].Thus, no neutral reactions were recorded.

2.4. Statistical analysis

Binomial tests allowed us to compare the frequency of successfulapproaches (i.e. leading to contact) as well as the frequency ofapproaches eliciting positive or negative reactions to frequenciesexpected by chance. Fisher's exact test was used to compare rates ofpositive/negative reactions between groups. The non-parametricMann–Whitney U-test was also used to compare the two groups.Alpha was set at 5% for each analysis.

3. Results

3.1. “Naïve” 1-year-old horses

Most yearlings accepted to be touched at least once by theexperimenter (N=15/16). The mean number of zones where theyaccepted contact was 2.57±0.39 out of the 7 predefined zones.Overall, 35.4% of the approaches from the right led to contact vs. 29.2%of the approaches from the left.

Clear differences appeared regarding horses' preference or rathertheir dislike for certain zones. Left and right shoulders, as well as leftflank weremore difficult to touch than expected by chance: only 4 outof the 16 horses (20%, but not always the same 4 subjects) accepted tobe touched on each of these zones (binomial test, P=0.04; cf.Table 1), while the others withdrew during the approaches. For the

Table 1Number and percentage of one and two-year-old horses accepting to be touched on the different zones and their positive or negative response to contact and statistics (binomialtests).

1 year-old («naïve») 2 year-old («trained»)

Approached zone Success rate P Positive reactions Negative reactions P Success rate P Positive reactions Negative reactions P

Left shoulder 4/16 (25%) 0.04 0 4 NA 18/23 (78%) 0.005 18 0 b0.001Left flank 4/16 (25%) 0.04 2 2 NA 12/23 (52%) 0.5 10 2 0.02Left croup 6/16 (38%) 0.2 1 5 0.1 10/23 (43%) 0.3 8 2 0.05Front 10/16 (63%) 0.2 6 4 0.4 19/23(83%) 0.001 17 2 b0.001Right shoulder 4/16 (25%) 0.04 3 1 NA 18/23 (78%) 0.005 18 0 b0.001Right flank 8/16 (50%) 0.6 3 5 0.4 14/23 (61%) 0.2 11 3 0.03Right croup 5/16 (31%) 0.1 3 2 0.5 18/23 (78%) 0.005 12 6 0.1

Fig. 2. Percentage of horses reacting either positively or negatively to the humantouching their left/right shoulder.

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other predefined zones, the proportion of approaches leading tocontact did not significantly differ from chance level (PN0.05).

Concerning a potential lateralization bias in the horses' positive ornegative responses to contact by zone (see Table 1 for details), it isinteresting to note that amongst the yearlings that let the experi-menter approach and touch their shoulders (most commonlyapproached zone in order to fit horses with their equipment), all ofthem (4/4) displayed negative reactions (i.e. threats or escape) atcontact on the left side, contrary to the right side where most of them(3/4) displayed positive reactions (i.e. immobility or sniffing theexperimenter). Though striking, this difference was not statisticallytested due to the low number of animals that were touched. No suchtendencies were found for the other contact zone (flank, croup).

3.2. Trained horses

Here again, most horses accepted to be touched at least once by theexperimenter in the pasture (N=22/23), and the mean number ofzones where they accepted contact was 4.74±0.42 out of the 7predefined zones. Overall, 72.5% of the approaches from the right ledto contact vs. 57.9% of the approaches from the left.

Clear differences appeared, this time regarding horses' preferencefor certain zones: left and right shoulders, as well as the front,appeared as privileged contact zones, for the experimenter was ableto touch them without the horses fleeing or threatening during theapproach (i.e. before contact) (for both shoulders: N=18/23,binomial tests, P=0.005; for the front: N=19/23, binomial test,P=0.001; cf. Table 1). Most horses also let the experimenter approachtheir right croup (N=18/23, binomial test, P=0.005). For the otherpredefined zones, the proportion of approaches leading to contact didnot significantly differ from chance level (PN0.05).

Moreover, most horses displayed positive rather than negativereactions at contact, whatever the zone touched (binomial tests,P≤0.05), exception made for the right croup for which this differencewas not statistically significant (cf. Table 1). For instance, horses thatlet the experimenter touch their shoulder all displayed positivebehaviors (i.e. remaining immobile or turning their head to sniff theexperimenter) to the contact, whatever the side (N=18/18).

3.3. Effect of training or age: comparison of the 2 groups

As previously mentioned, most yearlings and 2-year-olds acceptedto be touched at least once by the experimenter. However, thetolerance of contact was higher in older horses that accepted to betouched on a larger number of predefined zones than yearlings (meannumber of zones touched: X1 year=2.57±0.39; X2 years=4.74±0.42out of 7, Mann–Whitney U-test, U=74.5, P=0.001). A greaterproportion of 2-year-olds accepted contact on both shoulders (Fish-er's exact test, N1 year=4/16, N2 years=18/23, P=0.001), as well as onthe right croup (Fisher's exact test, N1 year=5/16, N2 years=18/23,P=0.004). There were no significant differences in the tolerance ofcontact between age groups for the other zones.

Besides, it is noteworthy that whereas the 2 groups' horsesbehavior did not differ significantly in response to a contact on theright shoulder (number of horses displaying positive behavior:N1 year=3/4, N2 years=18/18, Fisher's exact test, P=0.2; Fig. 2) andcroup (N1 year=3/5, N2 years=12/18, Fisher's exact test, P=0.4), theyclearly differed in their behavioral response when these zones weretouched on the left side (Number of horses displaying positivebehavior: shoulder: N1 year=0/4, N2 years=18/18, Fisher's exact test,Pb0.001; Fig. 2; croup:N1 year=1/6, N2 years=8/10, Fisher's exact test,P=0.002).

4. Discussion

Overall, the one year old “naïve” horses showed a clear asymmetryin their response to an approaching human. They displayed morenegative reactions when approached from the left side: two out of thethree zones on the left side were amongst the most difficult ones toapproach. Moreover, when they did let the experimenter touch theirleft shoulder, most horses escaped or threatened her. On the contrary,most horses displayed positive behaviors when touched on the rightshoulder. However, it seems that the positive training received by thetwo years old horses alleviated this asymmetry, as these horsesreacted equally positively to approaches and contact on both sides.

In our study, one year old naïve horses appeared more reluctant toaccept contact when approached by the human in their left monocularvisual field and conversely better accepted the human approach intheir right monocular visual field. This result support previous studiesconducted in adult horses, that showed that the left monocular visualfield (information processed by the right cerebral hemisphere) wasassociated with a higher reactivity [10,18] and fits in the theory ofvalence (right hemisphere mainly processing negative emotions andleft hemisphere mostly processing neutral or positive emotions;[22,23]).

However, the negative reactions observed in the naïve horseswhen they were approached on their left side were not displayed bythe two years old trained horses. In fact, they did not displayasymmetrical behavioral responses and accepted approach andcontact equally well on both sides. Agemay be a factor as Søndergaard

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and Halekoh [24] suggest that the chance of a horse approaching ahuman and the chance of approaching a horse increased with age.Thus, horses' increasing acceptance of an approaching human withage could be attributed to a simple maturation effect, resulting fromthe normal physiological development horses undergo with age.Nevertheless, this same study revealed that handled horsesapproached humans more rapidly than unhandled ones [24], showingthat older horses are also more familiar, by an effect of habituation tohumans due to the daily feeding by humans [24]. In fact, the onlydirect evidence of age dependent shifts in bias during development isfound in humans, chicks and rats [1]. In horses, McGreevy and Rogersreported a shift in motor laterality (foreleg preference) in horses overtwo years of age, compared to horses aged two years or less [14]. Inaddition, Larose et al. [18] found that 3 years old Saddlebred horseshad a stronger visual lateralization than 2 years old ones. However,studies in other species have reported an effect of age over a muchgreater span of years than the one year difference observed in Laroseet al.'study [18] or ours, in which all horses tested can be classified asyoung horses. As previously suggested, the training paradigmsapplied by humans may greatly influence the expression of lateral-ization in domestic horses [18]. The fact that this study's two years oldhorses had undergone initial training at the age of one year is probablya very relevant event. The absence of lateralized responses to thehuman approach could be inferred to the repeated interactions withthe trainer, for handling was mostly performed on the left side of thehorse (cf. [19]). Other studies previously suggested an effect of workon horses' motor [14] and visual [18] laterality. The question thatarises here is one about the type of work that is performed and theemotional memory horses keep about it, for evidence suggests thatdifferent trainingmethods can greatly impact the way horses perceiveworking sessions and more generally the human-horse relationship[25]. In the present study, the initial training of the two years oldhorses included repeated handling sessions using, or not, positivereinforcement. Such positive reinforcement based training has beenshown to have long lasting positive effects on the human–horserelationship [19,25], which could explain the decrease of negativeresponses on the left side. In most studies, horses’ left eye isrepeatedly associated with fear inducing situations; while the rightone is associated with more positive events (Larose et al. [18], DeBoyer Des Roches et al. [29]). In addition, horses spontaneouslydisplay less reactivity to a frightening object on the right side than onthe left one (Austin and Rogers [10]). In our study, we observed lessemotional reactions on themore “reactive” side (i.e. left) in the horsesthat had undergone training. In a recent study, Farmer et al. [26]found, on the contrary, that training in an interactive situationinduced in horses a strong preference for looking at the trainer withthe left eye. The authors argue that the trainers involved were wellknown to the horses and cared for them and fed them, as well asworking with them, so there is no reason to suppose that there wouldbe any negative emotion associated with them. However, in the lightof the literature investigating a link between emotionality andlaterality, one can question the way horses perceived that particulartraining situation, for it involved chasing the horse away beforeturning your back andwaiting for it to return (“join up” technique and“hook on” method). Such techniques could indeed be a source ofstress and negative emotions associated with the trainer, for aprevious study involving the agitation of a stick in front of ponies'heads to make them back up showed that this type of negativereinforcement-based training (i.e. using an aversive stimulus)induced a rise in the ponies' heart rates and a negative perceptionof training [25]. The fact that in Farmer's study the trainer also fed andcared for the horses may not have been sufficient to compensate thepossible negative memory acquired at work, as negative events seemto be stronger in memory than positive ones [27].

Evidence suggests that eye preference to fixate an object may giveinsight into how an object is perceived [28]. A Study by De Boyer Des

Roches [29] revealed that horses use more their left eye to look at anobject with a negative emotional value (veterinary's white shirt) thanto look at a neutral (plastic cone) or positive (feed bucket) one. Thus,we hypothesize that the type of work and how it is perceived byhorses, rather than work itself, impacts differently on horses'perceptual laterality.

Knowing that horses perceive the world differently on each sidehas practical applications. Young or highly reactive horses wouldcertainly learn faster and be more relaxed if first approached on theright side. Beginning their training by handling them on their mostreactive side (i.e. left) may in some cases exacerbate their emotionalreactivity. Besides, knowing that interocular transfer occurs in horses[30], at least from the right eye to the left one (i.e. from left to righthemisphere; [10]), they could progressively learn to accept approachand handling on the left side. Once a positive relationship isestablished, this study suggests that the laterality bias may fade toallow horses to accept human contact equally on both sides.

Acknowledgements

The authors are grateful to the Director and staff of the “Stationexpérimentale des Haras”, Chamberet. We also wish to thank theCOST of the “Haras Nationaux” and the “Region Bretagne” for theirfinancial support. Thanks to Dr Nadav Shashar, Dr Catherine Blois-Heulin, Audrey Maille and three anonymous referees for their usefulcomments on the manuscript. This study complies with the Frenchlaws concerning the use of animals in research.

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