Neophobia field experiments 7 1 Introduction

Experiments on neophobia have been carried out almost exclusively in laboratory or at best in semi-wild conditions whether the stimulus be food (eg Mitchell et al., 1911 \ Partridge, 1981) or objects (eg Cowan, 1977; Greenberg, 1983). Under such conditions where it is possible to control characteristics of individuals and their environment, it has been possible to examine a number of factors contributing to individual and group variation in neophobia. Using controlled conditions for feeding experiments, Mitchell (1976) investigated strain differences in wild and domesticated rats; and Cowan ( 1977) looked at differences between species of the genus Rattus with reference to influence of

commensal ism. In addition, influences of a variety of ecological factors have been investigated in a number of rodents including dietary efficiency in mice and voles

(Partridge, 1981), nutritional status in voles (Partridge and Maclean, 1981) and ecological plasticity of warblers (Greenberg, 1983; 1984; 1990).

Studies in the laboratory can reveal a great deal about the feeding behaviour of mammals (reviewed in Domjan, 1977). However, they may also prove misleading if animals are forced to made inappropriate choices in artificial surroundings or in situations which would not occur in the wild (Rozin, 1968). Experiments on wild populations do not suffer from these kinds of problems. However, probably because of logistic difficulties, experiments on neophobia in the wild are very rare. They have mostly been carried out on species of agricultural interest such as various birds (Rabinovitch, 1968) and rabbits (Bell,

1975). Emphasis of these expenments has usually been on populations rather than individuals. However, knowledge of the causes of indiv idual differences in neophobia is important because it should be informative about the ecological significance of the

behaviour and the practical implications for population control could be great if there were control-resistant behaviours (Reidinger and Mason, 1983).

There appears to be significant variation in individual levels of neophobia within most species studied (Mitchell, 1976; Greenberg, 1990; Bell, 1975). This variation was studied in rabbits by Bell ( 1975). A number of pre-feeds resulting in optimal uptake of excess bait were applied to an area of New Zealand farmland. The bait was carrot dyed with Rhodamine B (see Cowan, etal., 1984). Poison bait was then applied in the same manner, causing 75% mortality. All surviving rabbits observed were shot. Distnbution of rhodamine showed that all individuals which were poisoned had consumed the pre-feeds (n=242/242) whilst nearly all which were shot had not (n=60/63): this was not due to differential access to food.

Bell‘s work did not classify rabbits other than avoider/non-avoider. Fraser (1985) tagged wild rabbits in New Zealand belonging to known social groups and presented them

carrot bait in specially-drawn furrows. Bait was largely avoided for two days or so but had all gone by day 6. The rate of uptake was constantly high during the next pre-feed, at the

end of which poison baits were applied. Fraser reported possible changes in space-use during these experiments, and low activity levels. He considered that there was no effect of sex or social status on observed individual differences in reaction. However, there is no statistical analysis and the metrics used did not appear sensitive. In a second experiment Fraser placed potentially aversive objects (golf balls) in the home range of the rabbits surviving the first experiment. These also apparently caused avoidance and shifts in space use, but again there was no statistical analysis.

In field experiments involving food, it is almost impossible to control for the dietary status of individuals or populations. Avoidance of objects would not be expected to be influenced by dietary considerations. In studies on species which have been subject to control measures it is possible to envisage that new objects might have some ecological significance (Cowan, 1977): even when their ecological significance is not apparent, novel objects have caused avoidance (Greenberg, 1990).

The experiments described in this chapter were intended to produce measures of neophobia in a wild population of rabbits where individuals and their interactions were known. Knowledge of individuals would make it possible to look for biologically

meaningful correlates with any observed behaviour. By using objects rather than foods as the aversive stimuli, it was hoped to minimize differences between individuals which were related to dietary status. One field experiment (experiment 1) was carried out in August of

1988 at Bridgets Farm near Itchen Abbas, Hampshire and a second in April 1989 (experiment 2).

7 . 2 . 0 First neophobia field experiment 7 .2.1.0 M ethods

For 4 days in August prior to experiment 1, the population was observed to see that social groups and space-use recorded throughout the year was consistent at the start of the neophobia experiments. Observations were made of tagged individuals present during daylight hours from 1600h to 2000h. The hide was occupied at 1500h to allow the

disturbance to be reduced before the start of observations. Scan samples of the population were made at 15 minute intervals.

For the experimental treatments, four sites were chosen where aversive objects spaced equally along the hedgerow would impinge most equally on ranges of the largest number of rabbits. The potentially aversive objects (below) were left from 1500h to 2000h on each of 8 successive days at one of these sites as shown in Table 7.1. As the interval

between the centres of test sites was 25m, and as these were selected carefully, each rabbit should have been exposed to one object per night. The potentially aversive stimuli (novel

objects) were:

i/ ‘Tyres’. A large car tyre was arranged on each comer of a 10 x 5m rectangle with a fifth in the centre. This arrangement was constructed at a point with the long side parallel to and 5m from the vegetation containing the rabbit burrows.

ii/ ‘Posts’. Five farm fence posts, about 2m long by 10cm diameter were placed horizontally in a similar arrangement to the tyres. As the centre of the posts were placed over the points corresponding to the tyres, the whole arrangement was slightly larger (but less tall) than the tyres.

iii/ ‘Radio’. As an alternative to the two visual stimuli, the third object was a radio receiver detuned to produce white noise. This was made a non-visual stimulus by digging a small depression in advance of the experiments and placing the radio in it. Cut grass was then placed over the radio flush with the surface. The radio was set at such a volume that it could be heard from 10m away.

iv/ Control. Control was having nothing at the test site, but the area was walked over to create a similar amount of disturbance as caused by setting out the aversive stimuli. Table 7.1. Design of the first neophobia field experiment.

GROUP (no. 1(6) 2(3) 3(4) 4 (4 )

individuals)

replicate 1 TYRES RADIO CONTROL POSTS

POSTS TYRES RADIO CONTROL

CONTROL POSTS TYRES RADIO

RADIO CONTROL POSTS TYRES

replicate 2 TYRES POSTS CONTROL RADIO

POSTS CONTROL RADIO TYRES

RADIO TYRES POSTS CONTROL

CONTROL RADIO TYRES POSTS

7 . 2 . 1 . 1 Classification o f social rank and trappability

Rabbits were classified into social ranks ( l=dominant, 2=non-dominant) in chapter 4. Trap-shyness is known in a number of mammal species, and can be related to a variety of factors probably including genetics as well as a host of experiential factors. It was reasonable to assume from the trapping schedule that rabbits were quite equally exposed to the risk of trapping. The number of captures of each animal as an adult up to the start of the neophobia experiments were used as the starting point of the ranking. These were standardized by the number of months that each animal was available for trapping. Iri the event of ties, the ranks were separated on the number of captures after the experiments. 7 . 2 . 1 . 2 Analytical methods

The positions of all rabbits for all scans were plotted onto graph paper. The measure of avoidance was taken as the minimum distance of an animal from a test object (on control evenings, from the position where an object would have been). This was measured on the activity map and the recorded as the dependent variable object distance.

Observations relating to agonistic and sexual encounters between subjects were excluded: these rarely affected the minimum distance recorded.

On a number of occasions rabbits failed to appear at all during daylight hours. In an approach suggested by R. Sibly, their distance from object was taken as the distance from their burrow to the nearest object point: this was justified because non-appearance was not random with respect to treatment (Table 7.2). Rabbits were only included in the analysis if they appeared on a minimum of 6/8 evenings. C o n tr o l d is ta n c e for each individual in each replicate was defined as the minimum distance which a rabbit was from the test site on its control evening (ie in the absence of an object). A v o id a n c e d is ta n c e of all rabbits from the objects was calculated by subtracting c o n tr o l d is ta n c e from the each o b je c t d is ta n c e in each replicate for each rabbit.

Preliminary analysis showed that a number of factors could have contributed to neophobic response. An appropriate way to determine the individual importance of these was multiple regression (Sokal and Rohlf, 1981). It was not possible to enter all variables, as many were classification variables, and class III sums squares were appropriate (SAS,

1985) ; classification- compared to continuous variables greatly reduce the number of degrees of freedom , as do class III sums of squares compared to class I. Thus only the following 6 independent variables were added to the models: classification variables

id e n tity (individual identity), o b je c t (test object not including control), re p lic a te , s e x and ra n k (dominant or subordinate, also non-adult <5 months) and the continuous variable

c o n tr o l d is ta n c e . The dependent \ ariable was square root a v o id a n c e d is ta n c e (see above),

as this transformation increased the multiple R2, and gave normal residuals.

Data from the first experiment were contributed by 5 dominant males (M44, M27, M I84, M208, M223), one subordinate male (M195), 7 dominant females (F30, F34, F35, F4I, F93, F 182, F193) and 4 subordinate females (F33, F 134, F154 and F224).

7 . 2 . 2 . 0 Results of the first experiment

Some individual rabbits did not appear at all in daylight hours on 26/136 rabbit/days; significantly non-random with respect to treatment (Table 7.2).

T ab le 7.2. Numbers of rabbits failing to appear during daylight hours, divided into non-appearances when different objects were present (or no object, ^control). %2(df=l) was caculated between control and the object with the smallest niunber of non-appearances (=radio) to test the smallest object-treatment difference.

R e p lic a te ty re p o st radio c o n tro l P

1 5 4 5 0 (control-radio)

2 5 4 2 1

TOTAL 10 8 7 1 4.5 <0.05

7 . 2 . 2 . 1 A v o id a n c e o f n o v e l o b je c ts

Within replicate, distribution of distance of rabbits from each object did not deviate significantly from normality by Kolmogorov-Smimov one sample tests for each object.

By paired sample t-tests, distance from tyres and posts was seen to be significantly different to c o n tr o l d is ta n c e on first presentation (Table 7.3). C o n tr o l- and a v o id a n c e d is ta n c e (see above) were not distributed significantly differently from normal.

T able 7.3. Results of paired samples t-tests of mean distances of all rabbits from object positions

r e p .l

re p .2

io control distance.** P<0.01, * P<0.05.

object m ean distance/m (S.E .) *(16) P

t>Te 10.6(1.4) -3.62 0.002** post 9.84(1.8) -2.88 0.011* radio 6.71(.66) -0.87 0.397 control 5.80(1.1) t\TC 8.08(79) 1.65 0.119 post 7.17(1.1) 0.72 0.480 radio 6.25(95) -0.30 0.766 control 6.54(1.0)

Differences in avoidance of the three objects were also investigated by paired samples t-tests of a v o id a n c e d is ta n c e { o b je c t d is ta n c e minus c o n tr o l d is ta n c e ) of each object in each

replicate. Results are presented in Table 7.4, showing that at first presentation, avoidance of tyres was significantly greater than that of radio.

T able 7.4. Results of paired samples t-tests of difference in avoidance of objects in each replicate. ** P<0.01, * P<0.05.

r e p .l

re p .2

object mean avoidance *(16) P

distance/m (S.E.) t\T e/p o s t 4.82(1.3)/4.04(1.4) 0.85 0.409 t>Te/radio 4.82(1.3)/0.91(1.0) 2.75 0.014* p o st/ra d io 4.04(1.4)/0.91(1.0) 1.92 0.072 t}T c/p ost 1.53(.93)/.62(.86) 0.78 0.444 t\Te/radio 1.52(.93)/-.30(.99) 1.02 0.325 p o st/r a d io 0.62(.86)/-.30(.99) 1.56 0.139

Individual reactions to aversive objects

Spearman’s rank correlations were used to test the relationships between individual avoidances of each object. Mean a v o id a n c e d is ta n c e was ranked from high to low for each

object, and correlations calculated for each possible pair of objects. This was done for the first- and both replicates. Individual avoidances of objects were positively correlated, significantly so for two object pairs in the first replicate (Table 7.5).

T ab le 7.5. Results of Spearman's rank correlations investigating the within-rabbit relationship between avoidances of the 3 objects (n=17, d.f 16) over one and two replicates. ** P<0.01, * P<0.05.

o b je c ts Spearm an’s r 2 tail Spearm an’s r 2 tail

both replicates _P firs t replicate _P

TYRE POST 0.40 =0.10 0.62 <0.01**

TYRE RADIO 0.38 >0.10 0.58 <0.02*

POST RADIO 0.32 >0.10 0.25 >0.10

1 . 2 , 2 . 2 A tte n u a tio n o f a v o id a n c e

Taking all rabbits together, all novel objects were avoided more on the first presentation than the second (Figure 7.1). There was little difference in mean coTitrol

F ig u re 7.1. Mean distance from test positions in the presence of each novel object by ail rabbits in each rephcate of the first experiment (with 95% c.l. marked as +) F irs t experim ent

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