Ethogram of agonistic behaviour for thoroughbred horses

Applied Animal Ethology, 8 (1982) 5-25 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

ETHOGRAM HORSES

OF AGONISTIC

BEHAVIOUR

5

FOR THOROUGHBRED

G.W. ARNOLD Division of Land Resources Management,

CSZRO, Wembley, W.A. 6014 (Australia)

A. GRASSL4 Division of Mathematics and Statktics, CSZRO, Wembley, W.A. 6014 (Australia) (Accepted for publication 23 October 1980)

ABSTRACT Arnold, G.W. and Grassia, A., 1982. Ethogram of agonistic behaviour for thoroughbred horses. Apple Anim. Ethel, 8: 5-25. Social interactions between individual horses were observed in two herds each comprising a stallion and a number of mares. In one herd, the animals were observed whilst grazing and resting; in the other, nearest neighbours were recorded when the animals were grazing, and social interactions were noted when the animals were feeding on hay. In both herds, the horses showed marked preferences for the company of specific individuals when they were grazing. In one herd, the associations were mainly between individuals that had been associated prior to being put in the herd. In the other herd, this was not the case. A new statistic was produced for testing for specific company preference. In both herds, the stallion was dominant over all mares and never received any aggression. The complete social hierarchy could not be determined for the herd which was observed only when grazing because social contact was restricted to that within groups or pairs that associated together. In the herd to which hay was fed, a non-linear hierarchy existed. Statistics were produced to quantify both the general level of dominance of a horse and its specific dominance or subordination to every other horse. It is suggested that these statistics, and one for quantifying the general aggressiveness of a horse, could be widely used. A principal component analysis allowed the horses to be character&d socially according to aggressiveness, their attitude to other horses and their attractiveness to other horses.

INTRODUCTION

Social interactions in the horse hme received scant examination. There are descriptions of the types of aggression - biting, snapping, head and rear threat kick, head swaying, head bump used to assert dominance (Grzimek, 1944; Montgomery, 1957; Feist, 1971; Wells and Goldschmidt-Rothschild, 1979), and of other postures and expressions used in social contact such as

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0 1982

Elsevier Scientific Publishing Company

6

head shaking (Schoen et al., 1976), greeting (Tyler, 1972) and teeth clapping (Feist, 1971). Dominance rankings have been shown to persist for long periods of time (Grzimek, 1944; Tyler, 1972) and to be influenced by age and weight or size, with weight being more important at the top of the hierarchies and age at the bottom (Tyler, 1972). However, no descriptions have been given that enable an ethogram of social dominance to be constructed. Relationships between individual animals have rarely been analysed. There are statements that horses show preference for specific individuals, and that pairs of horses often graze and rest together (Hafez et al., 1962). Tyler (1972) found that New Forest ponies formed stable groups and members were rarely seen apart for any length of time, but no quantitative data exist on the strength of associations between individuals. Wells and GoldSchmidt-Rothschild (1979) worked with Camargue horses run under natural conditions. In this situation, the family unit of mare, foal and yearling offspring formed the basis of the social structure. In a group of seventeen Highland ponies, Clutton-Brock et al. (1976) found that individuals spent most time near to individuals of similar rank and age. Also, the ponies differed significantly in the amount of time they spent alone. In the studies reported here, two herds of thoroughbred horses were studied and associations between individuals analysed. In one of the herds, social interactions during feeding of hay were statistically analysed to provide quantitative information of social dominance and other aspects of social behaviour. METHODS

Study 1

The first study was done in a 12-hectare paddock on the property of Wyndyallia Stud, Baker’s Hill, Western Australia (Fig. 1). The paddock was grazed by cattle from February to September, and by horses from September to December. The herd comprised a four-year-old stallion and thoroughbred mares of various ages which were present for various periods of time. Details of the horses are given in Table I. In all, 16 different mares were present during the study, of which four were there throughout. Procedures The horses were observed during periods in October, November and December 1975. Observations were continuous for about 2 h each morning and afternoon. It was usually possible to observe all animals from a single vantage point and so obtain data on all interactions. Records were obtained of: (1) locations of individual animals when resting (November); (2) all incidents of agonistic behaviour and the individuals involved (October, November, December);

I

1 2 3 4 5 6 7 6 9 10 11 12 13 14 16 16 17

Scarlett Edition Plain Joan Alopian Lady Tudor Nytasia Bright Peak Keltan Vid Miss Daniel Countess Go Go Go John Elegant Girl Lady Bronwyn Rosemary Island optic star Linka John Vanity Queen Banana Princess

Age (years)

4 15 14 4 3 4 3 3 16 21 15 24 6 12 17 3 11

Sex

d Q Q Q Q Q Q 0 0 P Q Q 0 Q 0 Q Q 12.7.75 12.7.75 12.7.75 22.10.76 31.10.75 24.10.76 10.9.76 10.9.76 31.10.76 31.10.75 3.11.75 3.11.76 3.11.75 12.7.75 12.7.76 3.12.75

Date of entry

9, 11 had been together before foaling. 1, 2, 3. 9. 10, 14, 15. 17 belong to stud and had been together 12, 13 mother-daughter.

Number

X X X

X X X

previously.

X X

X

X X X X X X X X

24.10

X X X

l-15.10.75

Period

in the first study (X marks animal Present)

Name

Details of horses observed

TABLE

out

X X

X X X

out

X X X X

26.10

out X

X X X X X X X X X. X

31.10 X X X X X X X X X X X X X out out

3.11 X X X X X X X X X X X X X X X X X

3.12

Foaled Foaled Foaled

Stallion

on 1510.75 on 1510.75 on 26.10.75

Other information

CREEK

WATER

TROUGH -

Fig. 1. Typical locations

w

WATER TROUGH -

of individual horses when standing resting in Study 1.

(3) mutual grooming and the individuals involved (October, November); (4) details of movements including animals initiating and following them (October, November). Tables of frequency of each event were made from these records. Study

2

The second study was made in a l&ha paddock at the CSIRO Yalanbee Experiment Station, Baker’s Hill, on the twelve thoroughbred horses detailed in Table II which were in the paddock between January and September 1976. The animals were allowed the first two months to settle in and to form a herd. Competition for hay In April and May, when the pasture

was short, the horses received a chaff and grain supplement which was fed in a trough twice weekly. They also received 45 kg of oaten hay daily. Social dominance was measured on 35 days as follows. The hay was split into seven heaps in a circle of 20 m dia-

9

TABLE

II

Details of horses observed in second study Name

Number

Charity Fair Star Nymph Parteta Autumn Moon Countess Elain Trixie Countess Genevieve Fortunes Orbit Cumbie Messmarie Victory Lad Mare Crepone

1 2 3 4 5 6 7 8 9 10 11 12

Sex

Age (years)

Height (hands)

9

4 14 24 28 22 32 24 17 22 13 19 16

15 15 15 15 16 14 15 15 15 15 15 16

Q

9 Q Q Q Q Q

9 Q 0 d

2 1 2 1 2 3 3 1 3% 2 1%

All had been together since Aug. 76 but 1, 11 came from one owner, 5, 7 came from one owner, 2, 3, 6, 10 came from one owner, 4, 8 had been together in the past and 9 had no previous contact with any of the other horses.

meter. This meant that the horses had to choose where to feed. The feed lasted for about 20 min, after which the normal pattern was for one or two horses to stop feeding and walk towards water or shade. The others invariably followed within.5 min. A continuous record of all movements and agonistic actions was kept whilst all horses were present. This was done from a vantage point from which all animals could be seen. The rate of interactions was such that all interactions could be dictated by one observer into a tape recorder for subsequent transcription. In addition, 8 mm cinefilm was used on several days by a second observer to check the accuracy of the taped record. The behaviour of one animal towards others was categorised as shown below. = A horse feeding by itself or with other horses Leaves leaves a heap of hay. = Horses left behind when one or more horses Is left leave a heap. = Horses that pass by other horses when seeking Avoids a place to feed. = Horses that are passed by. Is avoided = A horse or horses join one or more horses. Joins = The horses that are joined. Is joined Chooses not to be alone = A horse that joins other horses rather than feeding alone.

10

Chooses to be alone

= A horse that feeds alone rather than feeding with other horses. Found alone = Horses observed to be feeding alone when all horses were feeding quietly. Aggressive acts = Threats to kick or bite, and kicks and bites. The number of times each of the 17 behaviours was shown by each horse is shown in Table VIII. Nearest

Two July to corded animal

neighbours

when grazing

mares, numbers 6 and 9, were removed at the end of June. From September the nearest neighbour, when all were grazing, was reon 33 occasions. If an animal was more than 30 m from any other it was considered to be alone.

Statistical methodology Associative

behaviour

of grazing horses

Grassia (1978) discussed the extension of concepts of general and specific combining ability (Griffing, 1956) to a study of aggregative behaviour of grazing sheep to determine the association or spatial pattern of animals. The analysis was extended to the case in which the nearest-neighbour approach is used to assess the associative behaviour of grazing animals. In Grassia’s discussion (1978), the data consisted of a triangular table with the elements being the proportions of times the ith animal has been observed in the same quadrat with thejth animal (i=l, 2,. . . , L;j=l+l, 1+2, . . . ) L) and Method 4 of Griffing was used. The data here consisted of a square table (main diagonal missing) whose elements were the proportions (PQ) of times ajth animal was observed as the nearest neighbour to the ith animal out of all possible times they were both present in the survey. The table is not likely to be symmetric. Therefore, Method 3 of Griffing (1956) was used. Random or fixed-effects models could be considered. Two concepts, gregariousness and specific preference, were introduced in Grassia (1978) which correspond to Griffing’s general and specific combining ability, respectively. Some restrictions imposed in our data do not allow the estimation of the effects due to gregariousness as discussed by Griffing (1956) for the general combining ability, because in our case, for a given animal, the proportion of times the other animals were its nearest neighbours should strictly add to one. In fact, the proportions do not add to one in our data as animals standing alone at some observation time were ignored. However, the figures were adjusted for this so that totals close to 1 were obtained. The mathematical model for Method 3 of Griffing (1956) also has a term accounting for reciprocal effects. In our case, this measures whether the ith animal has been nearest neighbour to the jth animal significantly more often than thejth to the ith animal. The reciprocal effects are not likely to be sig-

11

nificant in our study, and calculations showed this to be so. If they had been, it would have been difficult to attach a behavioural meaning to them. The statistical analysis was carried out on angular transformed data to achieve homogeneity of variance. For further details about the analysis, Grassia (1978) could be consulted. Behaviour when competing for hay Some of the 17 variables identified describe a behavioural relationship between pairs of horses, e.g. one horse leaving or joining another, or aggressive acts. Others describe the relationship between a single animal and a group, e.g. one horse “avoids” a group of horses or joins a group; still others imply a relationship between a single animal and all the other animals (e.g. “Found alone”). With the exception of variables in the last category, the same data can express pairs of variables in the sense that the action of one animal is received by another animal, singly or as a part of a group. The data were tabulated in the form of diallel tables (matrices), with the main diagonals blank. The data in one direction, say in rows, show the frequencies of the action done by each individual animal; in the columns these become the frequencies of actions born by each of the other animals. We confined the diallel analysis to the table of aggressive acts (Variables 16 and 17), because these are the only data for which there are large amounts of information and which involve the relationship between pairs of animals. However, all variables were used in a principal component analysis of the means to detect various combinations of behavioural acts that separated the individual horses in broad behavioural terms. The first four principal components were used. This technique is well described in the statistical literature, e.g. Gnanadesikan (1977), Section 2.2.1. In brief, it consists in finding p new variates which are linear combinations of p given correlated variates. The new variates, called principal components, are uncorrelated, account in turn for as much of the variation as possible, and, hopefully, in our case can be given a behavioural interpretation. While the Griffing (1956) model was appropriate for the study of the associative behaviour of horses, a slightly different model, discussed by Gallais (1970), is considered more appropriate for the study of the diallel table of aggressive acts. We used Analysis I of Gallais (1970), which operates on the sums and differences of symmetric observations in the diallel table but with a single error variance. The analysis was carried out on the square root of the number of aggressive acts given (and received), plus a half to achieve homogeneity of variance. We have interpreted the statistics derived from the sums of symmetric observations as an expression of “competitiveness”, and those from the differences between numbers of acts given and received as an expression of “dominance”. Competitiveness and dominance can be general (or average) and specific; the former applies to the relationship of a single animal to the whole group, and the latter to the relationships of individual animals. It is

12

not likely that large effects of “general competitiveness” occur, because the averages of the sums of symmetric observations tend to balance out and therefore this is a concept of limited importance here. “Specific competitiveness” is also unlikely to be very meaningful unless considered jointly with “specific dominance”, when it could acquire the concept of a “challenge” between pairs of animals of approximately equal rank, and “dislike” of an animal for another of different rank. However, in the latter case, average values are likely to arise because the large number of aggressive acts of the dominant animal is usually associated with a low number by the dominated animal. RESULTS

Study 1

During the first period, when eight horses were present, there were few conflicts and most information was obtained on social events occurring when the animals were grazing (Table III). The major role of the stallion is clear. He initiated more moves than any mare, he vocalised more often and he moved around (joined) horn mare to mare more than any mare. The other feature was that Plain Joan was frequently alone, that is more than 30 m from any other horse. Little mutual grooming was seen in October, with only five pairings being involved out of a possible twenty-eight. The stallion was involved in 18 of the 35 mutual groomings in October and November, involving six mares. Two mares, Keltan Vid and Miss Daniel, were involved in 12 mutual groomings, six in October and six in November. The dominance hierarchy was clearer in December when 17 horses were present. Table IV gives the numbers of aggressive acts given and received by

TABLE

III

Characteristics Horse

Scarlett Edition Banana Princess AIopian Plain Jane Optic Star Linka John K&an Vid Miss Daniel

of social Sex

d

9 Q Q Q Q

9 9

hehaviour Number

(October

-

ten days)

of times

Led

Followed other horses

Joined other horses

13 0 4 1 2 2 2 3

5 17 12 11 11 13 12 12

14 2 2 0 1 3 0 0

Involved in grooming

Grazing or standing alone 4 0 0 12 1 0 2 2

Vocalised

IV

9 11 12 13 16

0

1

2

2

17

2

33 6

113

3

2

7

7

14

5

5

15

8

4

26

2

38

42

3

2

6 6 3

7 6 4

35

5 4

73

6

8

5 4

5

4 4

5 7 7 25444111 6 4 6 3 5 7 3 3 4

19

7

4

10

3

2

9

6

5

11

13

26

2

38

2

4 3

9 11 243 5 5 4 5 3 4

12

28

4

6

5

10

16

293

293

95 38 44 38 30 0 24 17 0 0 3 2 0 0 2 0 0

Total

16 12 8 8 8 0 6 5 0 0 1 1 0 0 1 0 0

No. of horses dominated

Zero values occurred in blank cells. Note that the horses in the left hand block were present in the first period of the study and those in the second block were introduced later. Horses 9, 10, 11 were mares with foals.

Total

g

0

ru

.z 5 M 3 P al .z 3 Iz

1 17 3 2 14 15 7 8 4 5 106

Horse No.

Matrix of interactions between individual horses in Study 1, December (seventeen days)

TABLE

14

each horse, and the matrix of conflicts between individuals. There were no conflicts between many of the horses, so that the hierarchy could be established for only the top seven horses. These were all horses present during October. Horses 4,5, 6,13 and 16 received a lot of aggression. They were all horses introduced to the paddock after mid-October. Nearly all resting was done under three trees (Fig. 1) for which the horses formed very close associations. Thus, when 13 horses were present in Novem. ber for 17 observations, the following groupings occurred: were always together; 192, 394 were always together standing a short way from 1, 2, 3, 4 as 5, 6 illustrated in Fig. 1; were always together and adjacent to 1-6; 778 9, 10 both with foals, were always together and on ten occasions, 11, also with a foal, was with them; 11 was found by itself on seven occasions, the only horse to keep to itself; were always together well away from other horses. 12,13 The shade was not used territorially because no aggression over shade was observed. However, the horses showed a regularity of use of particular trees. Horses 1,2, 3 and 4 were found more often at Tree B (6 times) than 5, 6 (4 times), 7, 8, 9 and 10 (3 times). Horses 11,12 and 13 (2 times) also used the tree. Conversely, Tree C was used by 9 and 10 (5 times) but only once by 7 and 8. Study 2 Competition for hay The pattern of behaviour was for the horses to gallop the 100 m from their area of shade to the hay. For l-2 min there was considerable movement and running from heap to heap. This was followed by a period of movement of horses, often in a chain reaction. An aggressive act by one horse on another in a group would force that horse to leave the other horse or group of horses. The horse then circled the groups of horses and either joined another horse or group of horses or took up a vacant heap of hay if one was available. An aggressive horse would approach a group with ears flat to the head and might force one or two horses to leave. This forced ejection was classed as part of aggression and was therefore ignored in compiling the number of times a horse “left” others. Leaving was defined to be a voluntary action taken without obvious (to the observer) threat or aggression. The major variables and the numbers of times each indiv%.~al was involved in them are given in Table V, and the conventional dominance hierarchy in Fig. 2. Certain characteristics of the individuals are readily apparent. The stallion was dominant over all the mares but achieved this with fewer aggressive acts than dominant mares. Horse number 9 was the most aggressive mare and was “avoided” by other horses when alone. However, given the choice of

5 6 0 2 2 1 9 5 10 I 6 11

1 2 3 4 5 6 7 8 9 10 11 12

51 36 42 23 39 10 41 25 27 21 31 30

Leaves’

34 52 28 34 36 24 39 24 6 18 33 20

Is left

19 30 44 46 37 33 34 36 28 42 30 6

Joins

104 113 45 79 98 48 111 72 92 120 56 42

25 35 24 19 36 15 43 16 62 34 36 48 129 101 107 102 71 64 69 67 38 49 71 94

37 32 34 30 34 19 36 33 30 32 19 21

Is joined when alone

activities

Is joined

in different

Is avoided

2 was recorded

Avoids

each horse in Study

16 10 36 18 8 13 2 16 2 7 13 4

Chooses to be alone

10 1 2 4 1 11 3 19 16 5 7

0

Chooses not to be alone 10 16 33 19 7 26 45 35 70 61 21 30

Found alone

’ The horses may leave, join or avoid one or several other horses. In most cases there was a correlation between, gxoup and the number of times it left one other horse. Thus, the figures given above are for number of events.

No. dominant over

of occasions

Numbers

Horse

V

TABLE

e.g.. the number

2 5 3 1 2 4 13 4 40 13 9 13

Isavoided when alone

113 32 127 101 53 17 32 71 25 47 72 0

Aggressive acts received

of times a horse left a

66 99 1 34 61 24 127 41 139 132 46 62

Aggressive acts given

16 12

c 10 2 5

8 11 1 6 \4 3 Fig. 2. Dominance

hierarchy

of horses

in Study

2.

joining a group or being alone it rarely “chose” a vacant heap of hay (see Table V). By contrast, Horse number 3, the lowest ranking mare, “chose” to be alone twice as often as any other horse. Grassia’s (1978) technique was used to obtain a ranking of horses in terms of general dominance and social competitiveness, and to give estimates of specific dominance (Table VI) and specific social competitiveness. Analyses of variance for this are given in Table VII. TABLE

VI

Average and specific values of dominance (lower triangle) and social competitiveness (upper triangle) in Study 2 derived from a diallel table of numbers of aggressive acts (X) transformed to square root (fi). (Highly positive values in the table imply high dominance or aggressiveness for the horse whose number appears in the row) ____Horse

Specific competitiveness 1

0 B z ..M E 2 .z % : P

Average dominance

1 2 3 4 5 6 7 8 9 10 11 12

2

3 0.043

-0.099** -0.005 -0.097** -0,006 -0.084 0.076 -0.100** 0.206*** -o.oa9* -0.050 0.047 -0.036

4 0.042 0.026

0.050 0.009 0.012

0.034 0.053 0.075

0.057 -0.002

0.017

-0.024 --0.072 0.035 -0.083* -0.090* -0.050 0.014

0.049 0.016 0.005 -0.003 0.047 0.002 -0.025

-0.070 -0.019 -0.018 0.043 -0.057 -0.071 0.074

-0.072

-0.038

0.019

5

6

-0.041 0.044 0.003 -0.051

-0.013 -0.052 -0.038 0.012 0.006

-0.066 0.044 -0.014 -0.052 0.093* 0.074 0.006

0.030 -0.067 -0.002 -0.015 0.026 0.012

-0.005

-0.033

_

S.E. of specific competitiveness and specific dominance within the body of the table “0.038. S.E. of the difference in average competitiveness and average dominance “0.017. *P < 0.05.

-

17

Only between Horses 3 and 12 was the specific competitiveness significant when tested against zero. However, there were major differences in general competitiveness between the twelve horses. For example, the stallion had the lowest value because it made few threats and never received them. Mare 6 was also in this category. At the other extreme were Mares 1,2,7 and 9, which were equally competitive. Many of the specific dominance values between horses were significant. The highest values were mostly for horses next in order in the social rank, e.g. 9 and 7, 10 and 7, 8 and 11, etc. (see Table VI). Horse number 1 had a high number of significant specific dominant associations with other horses, i.e. with five out of the eleven animals. Most horses had only one or two significant dominance associations, and Horses 3 and 6 had none. Average dominance produced a ranking as follows: 9, 12, 7,10, 2, 3,11,8,6,1, 4, 3. The principal component analyses provided some objective classification of the individuals. The first four principal components accounted for 38,20, 17 and 9%, respectively, of the variance. The latent vectors of these PCPs are given in Table VIII. It is not always possible to give meaningful descriptive names to each vector in such analyses. However, the following descriptions seem best to describe the ones in Table VIII. PCPl Dominance PCP2 Activity (restlessness) PCP3 Social attitude (specific preferences) PCP4 Social attractiveness

AVerage

competitiveness 7

8

9

10

11

12

0.057 -0.019 -0.029 0.018 -0.036 -0.016

-0.020 -0.009 -0.018 -0.017 0.008 0.049 -0.003

0.021 0.006 0.039 -0.041 0.032 0.015 0.066 -0.019

-0.049 -0.012 -0.038 0.029 -0.020 0.003 0.032 0.012 -0.022

-0.024 -0.010 -0.013 0.024 0.042 -0.005 -0.029 0.043 -0.043 -0.007

-0.031 -0.024 0.090** -0.038 0.013 0.040 -0.030 -0.024 0.005 -0.027 0.024

-

0.028 -0.166*** 0.126** -0.000 -0.040 0,049

0.001 -0.055 0.079* 0.045 -0.014

0.089 -0.027 -0.090*

0.034 -0.015

-0.000

0.063

0.029

-0.014

0.052

0.024 0.017 -0.003 -0.001 -0.016 -0.021 0.019 -0.024 0.020 0.011 -0.012 -0.030

18 TABLE

VII

Analyses of variance of social dominance and competitiveness in Study 2. Analysis variance on number of aggressive acts (x) transformed to square root (Jx) Source of variation

Degrees of freedom

Competitiveness Average Specific

11 54

Total

65

(a)

Dominance Average Specific

11 55

Total

(b)

66

Total

(a + b)

Mean squares

Variance ratio

0.00654 0.00253

3.94*** 1.52*

0.03860 0.01020

23.25*** 6.14***

of

135

Error

0.00166

*P < 0.05. ***p < 0.001.

TABLE Vectors

VIII of principle

components

Variable

Leaves one or more Leaves one other horse Is left by other horse(s) Is left alone Avoids other horse(s) Avoids a lone horse Is avoided when alone or in group Is avoided when alone Joins horse(s) Joins a lone horse Is joined when alone or in group Is joined when alone Chooses to be alone Chooses not to be alone Found alone Number of aggressive acts given Number of aggressive acts received

obtained

from

data in Study

Dominance

Activity

PCP 1 ~~10 1 19 29 19 28 26 ~ 36 ~ 1 1 27 14 29 32 25 - 28 36

2 (x 10’)

PCP 2

Social attitude PCP 3

Social attractiveness PCP 4

42 20 38 13 0 m-15 27 1 42 0 23 38 -13 14 -13 28 0

-11 30 0 10 -47 -25 15 5 -27 41 26 -21 -14 ~~25 -29 -23 9

29 -30 ~ 29 -- 30 ~ 6 -26 -13 8 13 46 12 27 29 - 1 29 9 27

19

Reasons for these terms will be given as the relationships between individual horses are discussed. These are plotted in Figs. 3, 4,5 and 6. The top four horses in the dominance hierarchy appear on the left-hand side of Fig. 3, and the bottom horse in the hierarchy on the extreme right-hand side. Besides being dominant, Horses 7, 9 and 10 were aggressive and “chose” more often not to be alone but were “avoided” more than subordinate horses. .2 .7

0.4 -

81 .5 0.2 -

I

09 I

1.0

I

I

-0.5

06

I

I

1

0.0

I

0.5

DOMINANCE

1.0

PC,,,

Fig. 3. Plot of horses in terms of first and second principal components. Note: On this and Figs. 4-6, the numbers alongside the points are the horse identities.

910

.8

.9 .7

?? 4

L --I.”

-0.5

0.0 DOMINANCE

0.5 PC /II

Fig. 4. Plots of horses in terms of first and fourth principal components.

1.0

20

1-

.I0

z_ 03

.8

.9

?? 7

?? 11 .I2

06 .4

I

L

I

-0.26

-0.6

in terms

of second

0.6

0.25 PC,21

and fourth

principal

components.

.10

.3

.4

5 .2

SPEClFlC

Fig. 6. Plot of horses

0.5

0.0

-0.6

in terms

PREFERENCES

of third

.2

I

00 RESTLESSNESS

Fig. 5. Plot of horses

.6

I

I

PC

and fourth

principal

components.

21

22

PCP 2 indicates activity or restlessness in the sense that Horses 6 and 9 (Fig. 3) had low frequencies of leaving one heap to find another, and of being left. (They were not often joined when alone but this variable does not relate to activity and indicates a mixed vector.) Horse 2, at the top, had high frequencies of leaving, of being left and of being joined when alone, i.e. it was very active or provoked activity in other horses. The most attractive horse socially was Number 1, which had the highest frequency of being joined by other horses and low frequencies of being avoided and being found alone. Horses 2 and 5 were least attractive socially (Fig. 4). They were frequently avoided, were not joined very often, and were left as lone horses more than other horses. The differences in specific preferences, i.e. social attitude to others (PCP 3), can be seen in Fig. 6. The stallion had no preferences, rarely avoiding another horse, and Mare 1 was similar. However, another of the top ranked horses, Number 10, had the highest incidence of avoiding other horses. Associations

between

individuals

when grazing

Each horse was commonly close to at least one other horse and it kept well away from at least one horse (Table IX). The largest number of significant associations (positive values) and dissociations (negative values) for individual horses was six for Horse number 3, and the lowest was two for Horse number 4. DISCUSSION

The two studies showed that horses show marked preferences for associating with particular individuals when grazing and resting. In the first study, associations when resting was between animals that had been in the same grouping previously or between animals with foals and those without foals. Schoen et al. (1976) noted a separation of mares with foals from mares without foals. Hafez et al. (1962) stated that pairs of horses often graze and rest together. The associations between individuals when grazing (Study 2) were not as clear cut. Six pairings of horses could have been expected to show specific preference for each other’s company because they had been together previously, but only two pairs did so. Even in a competitive situation, some horses showed specific preferences. For example, Horse 10 was particular to avoid certain other horses when choosing which heap of hay to feed at. Mutual grooming was not common, as is also the case with Camargue mares (Wells and Goldschmidt-Rothschild 1979). Mutual grooming appears, from Study 1, to be associated with social bonding and perhaps with sexual behaviour; on two occasions the stallion was involved in allogrooming with a mare in oestrus. Only one pair of individuals that associated closely when grazing and resting were involved in frequent grooming. These two mares had not been together previously, and the mutual grooming, in their case,

23

was probably involved in bond development. Clutton-Brock et al. (1976) found that, in a group of Highland ponies, all participated in mutual grooming but the frequency varied greatly between individuals. The frequency was unrelated to age or social rank. Most grooming was with a pony’s nearest neighbour, which was usually a regular associate, so it may welI be involved in the bonding of individuals. Clearly, more specific studies are needed to find out how social bonds develop and are maintained in horses. The social context for leadership was differerent in the two studies. In the first study all activities normal to maintenance behaviour were being monitored, i.e. grazing, resting, sheltering and drinking. The stallion was the major initiator of changes in activities, which is in agreement witi Collery’s (1974) observations. In the second study, only two activities followed hay feeding - drinking or sheltering. The stallion was not the major initiator of these two activites; Mares 1 and 8 led as frequently. The difference may simply indicate a low general correlation between dominance rank and leadership in initiating maintenance activities, as has been found in other species, e.g. cattle (Syme and Syme, 1979). Dominance was asserted in Study 1 when there was no specific competition, i.e. when animals are grazing or resting. Animals 4, 5 and 6 received nearly all their aggressive acts from Horses 1,2, 3,7 and 8, with whom they associated when resting. These three, and Horses 12,13 and 16, were all later additions to the group, and had had no previous social contact with the other horses present in the paddock. Both factors may have contributed to their receiving so many aggressive acts, but it would indicate a “group” dominance by the horses which already knew each other. “Group” dominance has previously been demonstrated in rhesus monkeys (Marsden, 1973). Where all individuals became aggregated to feed on hay, then a dominance hierarchy became obvious (Study 2). Dominance and aggressiveness were quite closely correlated (r = 0.78, P < O.Ol), which is in agreement with the finding of Houpt et al. (1978) with herds of both sexes. The correlation was lower than might be expected, because the stallion was never challenged and seldom had to be aggressive to assert its dominance. A point of interest was that the most aggressive mare (Number 9) was dominant over all except one of the lower ranking mares. This latter mare, Number 1, on certain days set out to demonstrate its dominance over Mare 9 by joining the group in which 9 was present and forcing it to leave. The majority of aggressive acts by Mare 1 on Mare 9 occurred on three days. It appeared that Mare 1 was deliberately asserting its dominance. As in studies with other herds (Tyler, 1972), the incidence of aggressive acts by an individual was highest towards animals closest to them lower in the hierarchy. The specific dominance values showed this clearly Montgomery (1957) found that aggressive acts were unidirectional. This was not entirely so in this study. It tended to be more so between the top horses. Montgomery (1957) also found that older animals were in the top of the hierarchy but that weight was more important than age in determining rank. Age and gen-

24

era1 dominance value of mares were not correlated in this present study. Since all but Mare 6 were of similar height, size differences were not involved either. As found by Houpt et al. (1978) for herds of more than six animals, the dominance hierarchy was triangular. The activities recorded in the process of competition for hay did not, in themselves, prove particularly useful in describing the social behaviour of individuals. This was because most of the activities could have occurred for two reasons. For example, one animal might join another either because it was dominant to the one it joined, or because it was gregarious. The principal component analysis was of use in grouping these actions to give several broad descriptions of behaviour which sorted the animals in various ways. For example, a characteristic of dominant mares was that they asserted their dominance by moving around and joining other horses when vacant heaps of hay were available. As might be expected, dominant horses were avoided by subordinate horses. The analysis also showed that the individual horses varied in “popularity” or social attractiveness, and that this is a separate characteristic from social dominance. For example, Mare number 3 was lowest in social rank but was “unpopular”. These various characteristics of members of a herd represent parts of an ethogram of social behaviour which can be described by the principal components derived in this study. The principal component analyses and Grassia’s technique gave very similar rankings for dominance. The ranking with PCPl, which was termed “dominance”, was 9, 12, 10, 7, 11,2, 8, 6, 5, 1, 4, 3, whilst the ranking using Grassia’s average dominance values was 9,12, 7, 10, 2, 5, 11, 8, 6, 4,1, 3. The correlation between rankings was (r = 0.93, P < 0.01). The average competitiveness obtained by Grassia’s technique is not an index of aggression because it was poorly correlated with the number of aggressive acts given (r = 0.07). The two analyses showed that Mare 1, which was the most socially attractive (PCP4), also had the highest incidence of significant specific dominance associations. This horse avoided few horses when seeking a place to feed, and was infrequently avoided itself (Table V). It was thus exposed to a high degree of social contact, which gave it a higher risk of conflict and a high rank in average competitiveness. The general and specific dominance, and the general values derived from the technique of Grassia (1978), appear to be useful statistics for comparing individuals. They can be used for ranking individuals in any gregarious social groupings of animals. Likewise, the specific association statistics are suggested to be of general value in studies of social relationships in any species. There have been several models developed to describe social dominance hierarchies (Rapoport, 1949; Landau, 1951), but none that quantify the differences between individuals. Grassia’s technique and the principal component analysis appear to have potential for quantifying social differences between individuals in gregarious species of animals, and for constructing social profiles of the individuals.

25 ACKNOWLEDGEMENTS

The authors are indebted to Messers. D. Stevens and S.R. Wallace for the careful recording of the animals’ behaviour.

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