How far do animals move? Routine movements in a tortoise

ADRIAN HAILEY' Department of Zoology, University of Thessaloniki, Thessaloniki, Greece 540 06

Received February 12, 1988

HAILEY, A. 1989. How far do animals move? Routine movements in a tortoise. Can. J. Zool. 67: 208 -215. Although many aspects of the ecology and physiology of animal movement have been well studied, little is known about the

distance moved during routine activity. To examine this problem, daily movements of adult tortoises Testudo hermunni were measured by thread trailing. The activity season was about 210 days per year; individual tortoises were active on 95 % of days from April to June, but only on 49% of days in the dry months, July-October, probably because of a digestive bottleneck caused by dry food. Daily movements ranged from 1 to over 450 m, with a daily mean over a year of 80 m in males and 85 m in females; there was significant variation between months. Daily movements were 5 - 10 times less than those of similarly sized mammals. Individuals were active on average for 140 days per year, moving a total of about 12 km. Routine movements occurred within home ranges of 1.8 ha. The intensity of use of the home range was thus 12 km11.8 ha, equivalent to annual sampling of a strip 1.5-m wide. Food plants were approached over distances of 1 -2 m, so that on average all points within the home range were sampled for food once per year; mates were detected over much larger distances. Tortoises used many refuges spread throughout the home range, and rarely returned to the same refuge. Most refuges were insubstantial and gave only concealment and shade; only one-third provided physical protection. This pattern of refuge use is permitted because tortoises have an armoured carapace, the transport of which is estimated to be energetically less expensive than daily return to an environmental refuge.

HAILEY, A. 1989. How far do animals move? Routine movements in a tortoise. Can. J. Zool. 67 : 208 -215. L'Ccologie et la physiologie des dkplacements des animaux ont CtC CtudiCes sous de nombreux aspects, mais il existe peu

d'informations sur la distance parcoume par les animaux au cours de leurs activitCs de routine. Les mouvements quotidiens de tortues Testudo hermanni adultes ont CtC mesurCs au moyen d'un fil d'Ariane. La saison d'activitC a CtC CvaluCe h environ 210 jours par annCe; les tortues ont CtC actives 95 % des jours d'avril h juin, mais seulement 49% des jours au cours des mois secs, de juillet h octobre, probablement h cause d'un blocage digestif causC par de la noumture skche. Les dkplacements quoti- diens allaient de 1 m h plus de 450 m, ce qui Cquivaut h une moyenne journalikre annuelle de 80 m chez les males et de 85 m chez les femelles; il y avait une variation significative de la distance parcoume d'un mois h l'autre. Ces dkplacements sont de 5 - 10 fois moins importants que ceux de mammifkres de taille semblable. En moyenne, les tortues Ctaient actives environ 140 jours par annCe et se dCpla~aient sur une distance totale de 12 km. Les dkplacements de routine avaient lieu au sein d'aires vitales de 1,8 ha. L'intensitC des dCplacements dans l'aire vitale Ctait donc de 12 km11,8 ha, ce qui Cquivaut h un Cchantillon- nage annuel d'un bande de 1,5 m de largeur. Les tortues parcouraient des distances de 1-2 m pour s'approcher des plantes leur servant d'aliments; elles visitaient donc en moyenne tous les points de leur aire vitale une fois par annCe pour trouver leur noumture. Les partenaires Ctaient dCtectCs h des distances beaucoup plus CloignCes. Les tortues utilisaient plusieurs refuges rCpartis dans toute leur aire et retournaient rarement deux fois au meme refuge. La plupart des refuges n'Ctaient pas de vCri- tables abris et n'offraient que l'ombre et le camouflage; seulement un tiers des refuges utilisCs offraient une vCritable protection physique. Ce phknomkne est possible parce que les tortues sont munies d'une carapace armCe dont le transport est plus Ccono- mique Cnergiquement que le retour quotidien h un abri offert par le milieu.

[Traduit par la revue]

Introduction The area used during routine activity of animals (the home

range) has been widely studied, and analytical reviews are available for most groups of terrestrial vertebrates (e.g., Harvey and Clutton-Brock 198 1). In contrast, the distance moved during routine activity has been almost entirely neglected: the most recent substantial review of movement ecology does not mention this aspect (Swingland and Green- wood 1983). Distances moved have been neglected because of the difficulty of recording exactly where an animal has been. Radio tracking allows animals to be located at will (Amlaner and MacDonald 1980), but the path followed between radio fixes cannot be determined. In some situations movements can be followed by thread trailing (Breder 1927). Although this method may appear to be a poor alternative to radio tracking, and is difficult to use in some habitats (Chelazzi and Francisci 1979), it provides data not obtainable except by elaborate auto- matic tracking systems (Deat et al. 1980).

'Present address: Department of Physiology, The Medical College of St. Bartholomew's Hospital, Charterhouse Square, London, EClM 6BQ, U.K.

This study used thread trailing to investigate seasonal and sexual variation of routine movements in the tortoise Testudo hermanni. The main aim was to quantify daily movements in this reptile, to compare these data with the few available for mammals of similar size, and to provide an objective measure of ecologically significant realized performance. A secondary aim was to examine the distance moved in relation to the area occupied, and so to quantify the intensity of use of the home range.

Methods

Site details Tortoises were trailed in two areas (sectors 1 and 7) of dry heath

vegetation at Alyki, northern Greece. Wright et al. (1988) provide a map and description of the main heath, and details of sector 1, the salt works heath, are given by Stubbs et al. (1985). Dry heath vegetation was a sparse cover of herbaceous plants on firm sandy soil, in the drier parts of the heath. There were scattered bushes of Crataegus sp. (hawthorn) and low-growing, dense clumps of Ruscus aculeatus, Artemisia, and Rubus (bramble), especially in sector 7. The two areas were chosen because dry heath proved to be the vegetation type most suitable to thread trailing after trials in other sectors, and because the

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tortoises in these sectors were well known from a simultaneous study of population dynamics.

Thread trailing Only adult tortoises were studied; straight carapace length ranged

from 14 to 18 cm and weight, from 0.5 to 1.2 kg. Boxes holding thread were constructed from a 0.5 mm thick aluminium sheet and were fixed to the fifth central scute, which was roughened with a saw blade, using quick-setting plastic cement (Fast Chemicals, Thessa- loniki). The box pointed backwards, and was tilted up at an angle of 40" from the horizontal (similar to the position shown in Fig. 2 of Marlow and Tollestrup, 1982). The tortoise was cooled with water, because the cement set exothermically. A reel of white cotton was held horizontally in the box by a wire spindle and was covered with the bottom of a plastic bottle (AVA detergent), necessary to protect the cotton when the tortoise pushed under cover. The thread passed through a 7 mm diameter hole; results were more reliable if the thread fed from the bottom of the reel. Cotton spools (Papillon, Athens) with 450 m of thread were used in the first few months, but the manufac- turer replaced these with 360-m reels which were used for most of the study. The complete box, with cement and a full cotton reel, weighed about 60 g, that is, from 5 - 12 % of the weight of an adult tortoise. The effect of carrying this weight on tortoise activity is not known.

Protocol Measurements were made between July 1985 and August 1986. In

the first months tortoises were fitted with boxes as they were encoun- tered. Later on, boxes were fitted to new individuals only when too few of the old animals were relocated. Boxes were attached to 78 tortoises in total.

Almost all tortoises were active when first found, and the first movement to a refuge was ignored. Each tortoise was then located during the period of daily inactivity for 12 days or until it was lost. Tortoises were located at midday from May to September, when daily activity was bimodal, and in the early morning or evening at other times. Daily movement in May - September thus covered one evening and the following morning. Movements were paced out (1 pace = 0.95 m in dry heath vegetation) and plotted on detailed maps of sectors 1 and 7 produced with measuring lines. The current refuge occupied by each tortoise was marked with a plastic bottle on a cane. Cotton reels were replaced if less than 150 m of thread remained; this could usually be done without removing the tortoise from its refuge. After the experiments, thread boxes were removed from all the tortoises located (August - November 1986).

Analysis Trailed tortoises were often lost after the thread broke or ran out

after long movements. All of these incompletely recorded movements were included in the calculation of the percentage of days on which a tortoise was active. When calculating the mean distance moved per day, incomplete movements were included only if they were longer than the average of the individual's other movements in that month. This did not substantially affect the results reported; exclusion of all incomplete movements would cause negative bias as long movements were more likely to be interrupted. Thread breakages caused by grazing sheep in sector 7 in spring meant that no data were collected there in April and May.

The area occupied was defined by drawing straight lines between peripheral locations to form a convex polygon, that is, one with no internal angle greater than 180". An example of this method is shown in Fig. 2. The analysis excluded movements on the first day of trail- ing, when tortoises were significantly more active (see Results). The area was measured by tracing the polygon and counting squares over graph paper. The repeatability of defining and measuring areas in this way was checked by re-analyzing the data for May 1986 (1 2 animals), a year after the original analysis. The original mean was 0.457 ha; the repeated measure was 0.458 ha. The mean absolute (unsigned) differ- ence between the two analyses for each individual was 0.01 17 ha, 2.6% of the mean area. This difference was small compared with the natural variability of the area occupied; the standard deviation was

between 50 and 100% of the mean in most months (see Results, Table 1). The simple measurement technique was therefore adequate to analyze these data. Cumulative areas occupied were measured for tortoises trailed in more than 1 month. The area occupied in each month was superimposed on a single map of the study area, and a convex polygon was drawn around the total area and measured as above.

Statistical analysis followed Sokal and Rohlf (1 98 1). The data were grouped for G and x2 tests so that no cell contained fewer than five observations.

Results Activity

The activity season of Testudo hermanni at Alyki was between late March and early November, a period of about 210 days. Within this period, at least some of the population was active on sunny days, or when there were periods of sun- shine interspersed with cloud cover (changeable days in the sense of Avery, 197 1). These are termed "days of good weather" below. Days falling within the activity season, but on which the whole tortoise population was inactive because of bad weather, were excluded from analyses. These were mostly dull days without sunshine, but included a few changeable days when there was a strong cold wind.

Data from tortoises that were trailed for 7 or more days were used to test for short-term effects of trailing on activity using Friedman's rank method for randomized blocks (individuals were treated as blocks). For each individual, days 1 - 7 were ranked in order of distance moved, including days when a tortoise was not active as movements of 0 m. The test involved summing these ranks across individuals, for each day. There was significant variation over days 1-7 for both males and females in both spring (April -June) and in the dry months, July -October. This variation was due to increased activity on day 1 and, in July -October, on day 2 (Fig. 1). However, after removing the data from day 1, there was no significant varia- tion over days 2 - 7 in any of the four groups. Data from the 1st day of trailing were therefore excluded from all further analyses .

Individual trailed tortoises were inactive on many days of good weather, when other members of the population were active, and so activity and movement were analyzed sepa- rately. Activity was measured as the percentage of days of good weather on which a trailed tortoise moved 4 m or more. Movement was measured as the average distance moved per day of activity. The few recorded movements of 1-3 m were treated as inactivity, because short movements wholly within large hawthorn bushes or brambles were not detectable, and would be a source of bias in future comparisons between habitats.

The average number of days of good weather per month during the study and the percentage of these on which indi- vidual trailed tortoises were active are shown in Table 1. There were no significant differences between the activity levels of the sexes in any month. The sexes were therefore combined for analysis of variation between months, which was highly significant (Kruskal- Wallis test, H = 92.8, df = 6, P < 0.001). Individual trailed tortoises were active on 95 % of days of good weather in spring, compared with 49% in the dry months July -October.

Spring measurements were made in 1986, a year of normal rainfall; others were made in 1985, a dry year. Tortoises were trailed in August 1986 as a check for variation between years.

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CAN. J . ZOOL. VOL. 67, 1989

JULY - OCTOBER

SPRING

Day after trailing

FIG. 1. Short-term effects of trailing on activity in males (0) and females (a) in the months July -October (A; 47 males, 41 females) and in spring (B; 24 males, 24 females). For each individual, the first 7 days of trailing were ranked in order of distance moved, to test for association using Friedman's xZ method; the average rank in all indi- viduals is shown for each day. There was significant variation of activity in the first 7 days in all groups (all with 6 df): July -October, males xZ = 36.6, P < 0.001, females xZ = 13.8, P < 0.05; spring, males x2 = 20.5, P < 0.005, females x2 = 14.5, P < 0.025. How- ever, variation over days 2 -7 was not significant in any group (5 df); July -October, males x2 = 7.23, P > 0.1, females xZ = 3.30, P > 0.5; spring, males x2 = 9.96, P > 0.05, females x2 = 10.6, P > 0.05.

Trailed tortoises were active on 63% of days in August 1986 compared with 43 % in August 1985. These values are signifi- cantly different (Mann- Whitney test, UZ0.16 = 231.5, P < 0.05, two-tailed), showing that activity was lower in the dry August. However, activity in August 1986 was still substan- tially lower than in spring 1986, confirming the seasonal variation.

wooden boards were used occasionally. The category "coastal" includes refuges in drift line debris, in banks of sand above the strand line, or in patches of sea holly (Eryngium maritimum) or other maritime plants. Continuous occupation of a single refuge was only counted once, but returns to a single refuge after movement were counted separately. The following refuge types provided good protection: Ruscus aculeatus, Crataegus sp., brambles, burrows, salt marsh plants (Salicornia and Halimione), and artificial cover. These "substantial" refuges made up only 35.8% of the total (Table 2). Nearly two-thirds of refuges provided little physical protection, only shade and concealment.

The types of refuge occupied by males and females were broadly similar (Table 2). The main difference was that females were more likely to use grass and less likely to use herbs than males. The difference was statistically significant, because of the large sample sizes. It is not thought to be eco- logically significant, however, as grass and herb refuges offered similar levels of cover and protection. The use of sub- stantial refuges was similar between the sexes; these accounted for 35.5 % of the total in males and 36.1 % in females. Males and females were therefore pooled in subsequent analysis of variation between areas and seasons, as approximately equal numbers of the sexes were trailed in each area and month.

Seasonal patterns of refuge use were considered only in sector 1 as no data were collected in sector 7 in spring (because of grazing livestock). There was a statistically significant difference between refuges used in spring and in the dry months (Table 2). This difference appeared to be ecologically significant, as it involved refuges offering different levels of cover. Refuges in grass were especially common in spring, when substantial refuge types made up only 24.1 % of the total. Crataegus and Ruscus were more often used in the dry months, when 38.5 % of refuges were substantial.

Refuges used in July -October differed significantly between the two areas (Table 2), reflecting the availability of cover. There were no brambles and less Crataegus in area 1, but more artificial cover around the salt works. The burrows in area 1 were produced by suslik ground squirrels (Citellus citel- lus). Tortoises widened burrows for their own use, but did not dig burrows in area 7 where there were no susliks.

Pattern of refuge use Most tortoises occupied a different refuge after each move-

ment, and refuges were spread throughout the home range (Fig. 2). A total of 670 complete movements between refuges was recorded, only 9.0% of which were returns to the same refuge. This proportion did not differ significantly between males (10.6% of 357) and females (7.0% of 313, G = 2.7, P > 0.05); the sexes were therefore pooled. There was no significant difference between area 1 (10.7 % of 159) and area 7 (13.9% of 201) in the months July-October (G = 0.85, P > 0.05). However, tortoises were more likely to return to the same refuge in the months July -October (results for area 1 above) than in spring (area 1, 4.8% of 310, G = 5.3, P < 0.05). Return to the same refuge was significantly associated with refuge type (Table 2). Tortoises were most likely to return to burrows, followed by Crataegus bushes, which were major

Inactivity: types of refuge used features of the landscape. Tortoises also frequently returned to Tortoises occupied a wide diversity of refuges; most were coastal refuges, as the coastal heath was rather open with little

shallow scrapes beneath vegetation of various types, the choice of cover. greater part of the tortoise's body being above the surface Tortoises remained inactive in their refuges on many days in (Table 2). Burrows and artificial cover such as scrap metal and the dry months; was this pattern associated with the type of

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TABLE 1 . Activity of trailed tortoises in different months

DGW* Male activity (%) Female activity (%) U

April (1986) May (1986) June (1986) July (1985) August

1985 1986

September ( 1 985) October (1985)

NOTE: In addition to the months shown, there were on average 10 days of good weather in March and 7 in November. Activity is measured as the percentage of days of good weather on which trailed males and females moved (& 1 SD) with the number of individuals and the average number of days on which they were followed, excluding day 1 , given in paren- theses. U is the result of a two-tailed Mann-Whitney test comparing the sexes in each month. ns, P > 0 .05 .

*Average number of days of good weather, 1984- 1986.

TABLE 2. Occupation of different types of refuge

Area 1 Area 7 Percent Percent

Overall Males Females Apr. -June July - Oct. July - Oct. remaining returning (% of n ) (% of n ) (% of n ) (% of n ) (% of n ) (% of n ) 2 24 h to same refuge

Grass Herb Artemisia Crataegus Ruscus Coastal Bramble Juncus Burrow Artificial Salt marsh n

*P < 0.001 (ns, P > 0.05) .

refuge? An association would indicate that refuge type influenced activity, or that the probability of remaining inactive on subsequent days influenced a tortoise when choos- ing a refuge. This possibility was examined by comparing ,the percentage of tortoises that remained inactive on ,the first day of occupation, for each type of refuge. There was no signifi- cant association in July -October (Table 2); too few tortoises remained inactive in spring to allow comparison between refuge types.

Movement Males and females moved similar mean distances in each

month (Table 3), and the sexes were therefore pooled for sub- sequent analysis. There was no significant difference between distances moved in August in 1985 and 1986 (U18,16 = 168, P > 0.2, two-tailed); the data were pooled. There was signifi- cant variation between months (H = 36.6, df = 6, P < 0.001), both sexes showing low movement in October. Move- ments were also low for males in May and June and for females in May and September.

The frequency distributions of daily movements of each sex in spring and in the dry months are shown in Fig. 3. These

distributions were compared by x2 tests, after grouping cate- gories with few data so that there were nine groups in each distribution. There was significant variation between the four distributions (x2 = 50.30, df = 24, P = 0.0013), various combinations of which were then compared (all with 8 df). There was no significant difference between the sexes in spring (x2 = 10.44, P = 0.23) or between males in the two seasons (x2 = 14.97, P = 0.060), but there was a difference between the sexes in summer (x2 = 16.94, P = 0.03 1) and between females in the two seasons (x2 = 20.48, P = 0.0087).

There were some notable seasonal and sexual differences in the frequency of short ( < 20 m) and long (> 200 m) daily movements, which were tested as 2 x 2 tables (1 df). Short movements were equally common in males (16.7 %) and females (13.9 % , x2 = 0.93, P = 0.34), but were less frequent in spring (10.6%) than in the dry months (20.4%, x2 = 13.3, P < 0.001). Long movements were less frequent in males (3.7%) than females (9.0%, x2 = 8.51, P = 0.0035), but were equally common in spring (7.6%) and in the dry months (4.8%, x2 = 2.20, P = 0.14). The average of all complete movements was 80 m in males and 85 m in females.

There was considerable variation in distances moved per day

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TABLE 3. Daily movements and area used by trailed males and females in each month

Daily movement (m) Area used (ha)

Males Females U Males Females U

April May June July August

1985 1986

September October

NOTE: The table shows the distance travelled for days on which tortoises moved, and the area used during the trail- ing period of about I week, all f I SD. Sample sizes as in Table 2. U is the result of two-tailed Mann- Whitney tests comparing the sexes in each month; *, P < 0.02; ns, P > 0.05.

SPRING JULY - OCTOBER

Doi ly movement lkml

FIG. 3. Frequency histograms of the length of daily movements. All complete daily movements are shown, except those on the 1st day of trailing. (A) Males, spring (x = 81 m, n = 202). (B) Males, July- October (x = 78 m, n = 205). (C) Females, spring (x = 95 m, n = 193). (D) Females, July -October (x = 75 m, n = 173).

tive pairs were used; for example, the correlation was between months 1 and 2, and 2 and 3, but not 1 and 3. There was significant correlation for females (r = 0.43, n = 38, P < 0.01) but not for males (r = 0.22, n = 42, P > 0.1).

The total distance moved per year can be calculated from the data in Tables 1 and 3. This calculation assumed that activity and distances moved in March and November were the same as those in October, as suggested by sighting frequencies. On average, there were 210 days of good weather per year; males and females were active on 137 and 143 days, and moved 11.7 and 12.1 krn, respectively.

FIG. 2. Movements of male No. 1695 trailed in 5 months (September 1985, and April, May, June, and August 1986). Note that the refuges occupied (0) were spread throughout the home range. The smaller centre of activity was around a pool. The star shows the place of hibernation in winter 1985 - 1986. A convex polygon has been drawn around the cumulative home range; area = 1.68 ha. The movement labelled " 1" was omitted from analysis as it occurred on the 1st day of one trailing period; movements on day 1 of other months were in the centre of the home range.

Area occupied The activity of most trailed individuals was restricted to a

small home range. However, one female made a long, straight movement before she was lost, and was apparently a transient (Kiester et al. 1982). Some trailed tortoises in sector 7 were found nearer to the coast in summer (see Wright et al. 1988); this change was achieved by a small shift of the home range rather than a migration between separate seasonal ranges (Swingland and Lessells 1979). No migratory movements were observed for nesting (Gibbons 1986) or hibernation. The spatial patterns of routine movement were similar to those of

(Fig. 3). Was this variation partly due to differences between individual tortoises? This question was examined for tortoises trailed in 2 or more months, by testing for correlation between the distance moved in different periods of trailing (Fig. 4). The data were normalized as the percentage of the mean for each month (from Table 3), to correct for seasonal variation. For tortoises trailed in 3 or more months, only data from consecu-

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HAILEY 2 13

MALES A B FEMALES

g o - 2 0: 0 100 200 0 100 200

> u

Average doily movement, month x (%)

FIG. 4 . Correlation between the average length of daily movements of individual tortoises trailed in different months. Distances were normalized as a percentage of the mean daily movement for all indi- viduals in that month, from Table 3. (A) Males, r = 0.22, n = 42, P > 0.1. (B)Females, r = 0 . 4 3 , n = 38, P < 0.01.

the box turtle, described in detail by Stickel (1950), except that hibernation occurred within the home range in T. hermnni. An example of an individual trailed in several months is shown in Fig. 2. As these patterns are not of general interest, a full description will be given elsewhere in an account of the home range of T. hermnni.

The cumulative area occupied increased with the number of months trailing (Table 4), without significant differences between the sexes. The average home range of tortoises trailed in 4 or more months was 1.8 ha. The area covered within the trailing periods (about 7 days) was 0.35 ha in males and 0.38 ha in females, averaged over the whole year, and there were no significant differences between the sexes in most months (Table 3). Adult T. hermnni thus cover about 20% of their home range in a week.

Discussion

Length of routine movements Thread trailing was used to record movement of chelonians

in previous studies (e.g., Stickel 1950; Chelazzi and Francisci 1979). However, these authors were concerned mainly with the spatial pattern of movement and orientation, and did not fully report the distances moved. Miles (1976; Miles et al. 1981) evolved the thread-trailing method for studies of move- ment by rain forest mammals, independently of the work on Chelonia. These data and subsequent work by Greegor (1980) and by Berry et al. (1987) are the most comparable with the present study.

Medium-sized mammals move between 5 and 10 times fur- ther than tortoises each day (Table 5). This reflects the low performance capability (Bennett 1982) and energy turnover (Nagy 1987) of ectotherms. A particularly interesting compar- ison is between T. hermnni and the armadillo Chaeto- phractus, two animals of similar size (1 kg), cruising forager life-style (widely foraging but slow moving), and defence (amour); the armadillo moved 6.5 times as far as the tortoise per day.

Movements of anuran amphibians (Taigen and Pough 1985) and lizards have been studied by following the activity of focal individuals. Some of these small ectotherms move surprisingly large distances compared with the mammals trailed by Miles et al. (1981). For example, assuming that the daily activity

TABLE 4 . Cumulative area (mean f 1 SD) occupied by tortoises trailed in 1 or more months

Cumulative area occupied (ha)

No. of months Males Females U

NOTE: Number of tortoises is given in parentheses. Sexes were compared using two- tailed Mann-Whitney U-tests; ns, P > 0.05.

period was 5 h, half of the lacertid lizard species studied by Huey and Pianka (1981) moved more than 1 kmlday. These measurements, however, take no account of possible periods of inactivity other than short pauses.

Intensity of home range use The home range of adult T. hermnni at Alyki was about

1.8 ha, similar to that found by Swingland et al. (1986) for this species in France. The home ranges were not exclusively occupied. The population density of tortoises in the areas studied here was about 25 -40/ha, excluding juveniles (Stubbs et al. 1985), so that at any time the home range would also contain 40 - 70 other individuals.

Measuring both the location and the length of movements allows the intensity of use of the home range to be quantified. Intensity of home range use is one source of the unexplained variation in analyses of home range and energy requirements (e.g., Mace et al. 1983); it has apparently not been considered before. A simple, useful measure is the distance moved per year (12 000 m) divided by the annual home range size (18 000 m2), that is, 0.67 m-I in adult T. hermnni. This statistic would be suitable for comparative studies when other data become available. However, as a single measurement, it is more easily interpreted as a reciprocal; in T. hermnni it is equivalent to sampling a strip 1.5-m wide once per year. This value can be compared with the width of the transect that the animal actually samples, to estimate the number of times the home range is examined per year. This width will depend on what is being searched for, as shown by the following general observations of active tortoises; more precise estimates will be the goal of future studies at Alyki.

Food plants were usually approached over distances of about 1-2 m. The tortoises therefore sampled the food resources of any point within the home range on average about once per year, that is, on a similar time scale to that of vegetation renewal. This pattern could reflect systematic exploitation of the food in the home range, but this explanation is unlikely in view of the large number of other tortoises in the same area. The movement patterns of males and females were similar, so that the need to search for mates had no effect on the level of activity of males (although there is indirect evidence that males use more open microhabitat, where they would find females more easily; Hailey et al. 1988). Males approached rustling sounds similar to those made by females over distances of 10-20 m, thus sampling the home range about 10 times per year in terms of encounters with mates.

Refuge use Tortoises used many different refuges spread throughout the

home range, and most of these refuges gave little physical

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TABLE 5. Mean length of daily movements of mammals compared with that of the tortoise

Distance Group @/day Reference

Didelphis marsupialis Opossum 800 Miles et al. 1981 Chaetophractus vellerosus Armadillo 520 Greegor 1980 Marmosa nudicaudatus Opossum 5 10 Miles et al. 1981 Philander opossum Opossum 440 Miles et al. 1981 Proechimys guyannensis Rodent 420 Miles et al. 198 1 Mallomys rothschildi Rodent 420 Berry et al. 1987 Testudo hermanni Tortoise 80 Present study

protection. Use of flimsy refuges was possible because tortoises carry a secure refuge around with them, in the form of the armoured carapace. Withdrawal into the shell is one of the main defensive responses of T. hermanni, used especially at lower body temperatures (Hailey and Theophilidis 1987). What are the costs of the alternative strategies of carrying the carapace or returning to a single refuge after foraging in the surrounding area?

The mineral part of the carapace and plastron accounts for 19% of body weight in adult female T. hermanni, so that the ratio of armour to body less armour is 0.23 (Hailey and Loum- bourdis 1988). In tetrapods, the energetic cost of carrying loads is a simple fraction of the cost of locomotion (Taylor et al. 1980), i.e., carrying a load of x% of body weight increases the cost of transport by x%. Therefore an unarmoured tortoise should be able to move 23% further than an armoured one for the same cost, about 20 m extra per day.

The cost of returning to a single secure refuge can be calcu- lated in the same units. A home range of 1.8 ha is equivalent to a circle of radius 76 m. The average distance from any point within a circle to the centre is about two-thirds of the radius (empirical result). An unarmoured tortoise with the same home range would therefore have to make an extra journey of about 50 m to return to a single refuge after each period of activity. Carrying armour is therefore energetically cheap compared with returning to a single refuge.

Physiological ecology of tortoises The most striking feature of the activity of T. hermanni was

that individuals were inactive on half the days in the dry months, July -October. This was probably due to a digestive bottleneck caused by dry food, compared with the fresh vege- tation available in spring. In the giant tortoise Geochelone gigantea, Coe et al. (1979) have shown that the gut transit time of dry vegetation is much greater than that of moist food. This is important when considering strategies of foraging and physiological performance. In particular, it suggests that in summer, processing and not foraging limits intake in T. her- manni; the ionic balance of the food may also be important (Nagy and Medica 1986).

Tortoises moved variable distances on days when they were active. Similarly wide variation in daily activity has been found in the tortoises Geochelone carbonaria and G. denticu- lata in Amazonian forest (Moskovits and Kiester 1987). These authors note that high variability of activity is a consequence of the low energy requirements of ectotherms (Huey 1982). In T. hermanni, some of this variability was accounted for by differences between individual females (but not between indi- vidual males). The next goal for the physiological ecology of performance (Hailey and Davies 1988) is to show how far such

differences of realized performance are correlated with physio- logical variation between individuals.

Acknowledgements

This study was carried out during a European Science Exchange Program fellowship from the Royal Society and the National Hellenic Research Foundation. I am grateful to Professor M. E. Kattoulas, Dr. N. S. Loumbourdis, and other staff members in Thessaloniki for providing facilities and help during my work in Greece. I also thank Dr. I. R. Swingland for help in the Ecology Research Group, University of Kent at Canterbury, and Drs. R. A. Avery and M. R. K. Lambert for useful comments on the manuscript.

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