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Ranging behaviour and habitat utilisation of the Malayan wood rat (Rattus tiomanicus) in an oil palm plantation in Johore, Malaysia

A. P. Buckle**, T. H. Chia+, M. G. P. Fenn* and M. Visvalingam+ *Zeneca Agrochemicals, Fernhurst, Haslemere, Surrey GU27 3JE, UK and TCM Bioscience Research Station, 16th Mile Jalan Ayer Keroh, Lock Bag No. 7006 GPO 75990, Malacca, Malaysia

The ranging behaviour and habitat utilisation of Rattus tiomanicus, a serious pest of plantation crops in Southeast Asia, was studied extensively for the first time using radio-telemetry in an oil palm estate in Johore, Malaysia. Range size calculated by the minimum convex polygon (MCP) method reached an asymptote after between 55 and 70 telemetry fixes for both sexes. MCP estimates of the range sizes of R. tiomanicus males (mean 1696 m 2, SEf491.6, II = 6) and females (mean 390 m2, SEk72.3, n = 6) differed significantly (t = 2.634, P = 0.045). The ranges of perforate females (mean 504 m2, SE+ 113.3, n = 3) were larger than those of lactating females (mean 275 m2, SE* 12.5, n = 3) but the difference was not significant, probably because of the small sizes of the samples. Range sizes, particularly of males, were substantially larger when estimated in this study by radio-telemetry than when examined by previous authors using the capture-mark-recapture procedure. Habitat utilisation was studied by recording the occupancy of the following habitat elements at the time of a fix: burrows, palm crowns, ground vegetation, inter-row frond piles. A total of 49.6% of all radio fixes of rats were made in the frond piles. This element of the oil palm habitat was probably used both as a site for nest construction and for moving around protected, to some extent, from predation. The remaining fixes occurred almost equally among the other three elements, which were used for resting (mainly burrows and tree crowns), feeding (vegetation and tree crowns) and moving about the habitat (vegetation). These observations are discussed in relation to the design of experiments to test rat control measures in oil palm plantings and the practical control of R. tiomanicus in this crop. 0 1997 Elsevier Science Ltd

Keywords: Rattus tiomanicus; oil palm; telemetry; Malaysia; rodent control; radio-tracking; range size

them almost to the complete exclusion of other The Malayan wood rat (Rattus fiomanicus, Miller; rodent species. In this habitat, it is arguably one of formerly Rattus jaforensis, Bonhote) is a major pest in the most thoroughly researched tropical rodent pests, plantations of oil palm (Hueis guineensis, Jacq.) in by virtue of the work of Wood (1984) and Wood and Malaysia, where it inflicts important economic losses Liau (1984a,b). if populations are not controlled (Wood, 1976, 1994). The range of movement of R. tiomunicus was first It is frequently, too, the most serious rodent pest of observed in relation to the transmission of the tick- Malaysia’s extensive cocoa plantings (Lee and borne disease scrub-typhus (Harrison, 1958). As the Kamarudin, 1990). The distribution of R. tiomunicus importance of the species as a pest of oil palm plant- is known to extend to southern Thailand and the ings was recognised, a knowledge of the movement of Indonesian island of Sumatra (Wood, 1976), although R. tiomunicus in this crop was required for the its pest status in those places is not as well development of effective control measures (Wood, documented. 1971). In both of these studies, rat movement and Harrison (1957) regarded the natural habitat of R. home ranges were deduced by the capture-mark- tiomunicus in Malaysia to be ‘scrub edge’; it being recapture (CMR) technique. We used radio-tracking rarely found either in mature forest or in open, grass- as a measure of efficacy in field trials of rodenticide land situations. Habitation of this biome appears to applications for rat control in oil palms (Chia et al., have effectively ‘pre-adapted’ the species for life in 1990). During these experiments, the radio-tracking oil palm plantations (Wood, 1994) and it often infests of rats in an untreated control plot provided an opportunity to observe for the first time, the ranging *Author to whom correspondence should be addressed behaviour and habitat utilisation of R. tiomunicus

Crop Protection 1997 Volume 16 Number 5 467 Behaviour and habitat of the Malayan wood rat: A.P. Buckle et al. using this powerful technique. The results of those chosen to ensure that the transmitters’ weights were studies are presented here. They indicated that R. not a burden disproportionate to the animals’ size. Of tiomanicus, and in particular, males, range further course, such selection of individuals for study means than was previously thought. The implications of that the information obtained on their behaviour was these observations for the management of this pest, not typical of either immature animals or those in and for those investigating novel control measures, poor physical condition. After the standard are discussed. processing described earlier, each animal was trans- ferred into a doubled polythene bag containing a paper tissue soaked in analytical grade diethyl ether. Materials and methods At the onset of deep anaesthesia, the animal was Study area removed from the bag and fitted with a miniature radio-transmitter weighing 3-5 g. The transmitters The study was conducted at a commercial, estate were attached by crimping, at the back of the neck, in Johore, Malaysia, (2”07’N,103”15’E), which com- two pieces of thin gauge wire which protruded from prised 1993 ha of oil palm fields. The nature of the either side of the body of the transmitter. The wire oil palm agro-ecosystem is described by Wood (1984). loop so formed served as both attachment collar and The climate in the study area was typical of the antenna for the transmitter. Care was taken to attach humid tropics, with 1761 mm (93 days) of rain falling the wire collars neither too loosely nor too tightly and in the year of the study (1989) and 1920 mm (98 some practice in this operation was undertaken in the days) in the year preceding it. The study was laboratory before the field work began. conducted, however, during the ‘dry season’ months All radio-telemetry equipment was purchased from of January, February and March. The average Wildlife Materials Inc., Route 1, Carbondale, IL day-time temperatures were 27-29°C. 62901, USA, via a local agent (Cornintel Malaysia A field near the centre of the estate was chosen for Sdn. Bhd.). The transmitters used were model the study. The palms in this field were 8 years old SOM-1038-LD and the receiver was model and approximately 3 m in height (to the fruit TRX-1000S. The equipment operated between 163.00 bunches), with a full canopy of overlapping fronds and 163.01 MHz, with different transmitters being between palms. An assessment of the population distinguished by frequency (mean separation density of R. tiomanicus was required prior to radio- tracking. Therefore, a CMR study was conducted in a 6.27 kHz) and pulse rate (range 0.6-1.6 Hz). 5 x 10 palm block (0.3 ha) at the centre of the selected field following the method described by Chia Radio-tracking et al. (1990). The results of this study indicated that, estimated by the Lincoln Index method (Wood, Tagged rats were tracked on foot using a hand-held, 1984) about 130 R. tiomanicus inhabited the block; a collapsible, three-element Yagi antenna. During population density of 433 rats ha-i. intensive tracking sessions the position of each of the 12 animals was determined (a ‘fix’), in rotation, every hour. As the terrain became more familiar, the Rat capture and radio-tagging interval between fixes was reduced to about 45 min. Rats were captured for radio-tagging in the traps Similarly, at the start of the study, the position of the used previously for population density estimation. animal was confirmed by causing sufficient disturb- These locally-available drop-door cage traps were ance to obtain a sighting by torch-light, but this made from 2.5 x 1.25-cm galvanised wire mesh and became unnecessary in the later tracking periods. measured approximately 30 x 12 x 12 cm. Traps were A grid of 15 x 15 palms (1.64 ha), with the trapping set, two to a palm, in the weeded circles at the bases area at its centre, was marked out using fluorescent of palms and baited with a slice of banana about spray paint on the trunks of the palms. The spatial 1.5 cm thick. The trapping took place on four consec- location of each animal was recorded to the nearest utive nights (23-26 January 1989). Traps were 5 m using the marked palms as reference. The inspected in the early morning and re-set after positions of the rats were also recorded in relation to inspection and processing of any rats caught. Nearly that element of the oil palm habitat (rat burrow, all captures occurred at night. ground vegetation, piles of old palm fronds and palm Captured rats were run into a black cloth bag and crowns) being utilised at the time of the fix. weighed to the nearest gramme. New captures were Radio-tracking was carried out intensively over marked by toe-clipping to give a unique number by either the first or the second half of the night for 5 sex. The location of the site of capture and the consecutive nights between 30 January and 3 general and sexual condition of the animals were February 1989. Approximately 10 hxes were obtained noted and the rats were either released at the site of for each rat during each of these sessions. Five capture or anaesthetised for the attachment of a further tracking sessions, in which five or six fixes per radio-transmitter. rat were obtained, were conducted before midnight Twelve mature rats, six males with scrotal testes, during 5 further nights between 15 February and 24 three lactating females and three perforate but March 1989. The daytime locations of the rats were non-lactating females, were selected as subjects for determined on 19 occasions between 31 January and radio-tracking. All these rats were in excellent overall 24 March 1989. condition, with no overt signs of either injury or Radio-tracking data were analysed using Statistical disease. All weighed more than 80 g; the body weight Analysis System (SAS) programs (Anonymous, 1988)

468 Crop Protection 1997 Volume 16 Number 5 Behaviour and habitat of the Malayanwood rat: A.P. Buckle et al.

recorded ftxes. Wood (1971) observed some individuals to change their home sites entirely during studies using the CMR technique. All range size estimates (apart from time-of-night comparisons, see below) are therefore made only on the first 70 fixes recorded for each animal between 30 January and 10 March 1989.

Range size and sex

./* Table I shows the asymptotic minimum convex A0 l’ polygon (MCP) range estimates over 70 locations for ‘1’ I I I I 0. 20 40 60 80 the 12 individual rats together with their sexual Number of fixes status. Estimates from the grid area (GA) and Figure 1. The relationship between range size and the number of maximum length (Mx.L) methods are included for fixes used to determine range size for male R. tiomanicus in an oil comparison. palm plantation The mean MCP range size estimates and standard errors (SE) for males and females were 1696 m2 + 490 and 390 m2f 72 respectively; a difference which is on an IBM personal computer. The sizes of the home significant (t = 2.634, P = 0.045). The mean range ranges of rats were calculated by minimum convex sizes for perforate and lactating females were polygon (MCP), grid area (GA) and maximum range 504 m2 + 113 and 275 m2 + 12. The latter difference is length (Mx.L) methods, using a 5-m grid cell size (see not stat&ically significant (t = 2.01, P = 0.17), but this Fenn, 1989; Harris et al., 1990). is probably due to the small sample size and large variance, since, as can be seen from Table 1, the minimum range size of all perforate animals (350 m2) Results exceeds the maximum range size of all lactating Range size asymptotes and sample size animals (300 m’). Calculations of range size of individual animals should be based on subjects for which there are suffi- cient fixes for a plot of range size against number of Range size and time-of-night fixes to reach an asymptote. Figures 1 and 2 show For logistical reasons, there were only 2 nights when such plots of range size for male and female R. the locations of rats were determined in the second tiomanicus in oil palm, based on minimum convex half of the night (midnight to dawn). The impression polygon estimates, which produce smoother asymp- gained on those occasions was that the animals totes than the other methods (Harris et al., 1990). covered the majority of their ranges in the first half It is clear by inspection of Figures 1 and 2 that of the night, so that range sizes based on locations asymptotes are reached for both sexes between 55 obtained between dusk and midnight should provide and 70 lixes. The sharp increase in range size a reasonable estimate of total range size. between 71 and 80 fixes is due to a gap of a minimum To test this assumption, Table 2 gives the home of 12 days between successive tracking sessions, range sizes for the rats calculated by three different during which time several animals’ ranges had shifted methods over either the first (27 fixes) or the second in location, although probably not in actual size, (23 fixes) half of night from 30 January to 3 February leading to an artificially large range size based on all 1991, compared with the total calculated range size based on all 50 locations. The range sizes calculated by the minimum convex polygon method show that, overall, the area covered in the first half of the night 600 was 77.6% of the area covered over the whole night.

r The maximum range-length method indicates that, on 500 - average, 95.8% of the maximum length was achieved in the first half of the night. However, both these “5 400 - approaches concern the coverage of the range rather

8 than occupancy, so that, for example, an animal ‘Z 300 ~ circumnavigating the perimeter of its range in the %I first half of the night would appear to have occupied 9 cr: 200 - its whole range when it had not. The grid area method is calculated on the occupancy of ‘grid cells’ 100 - (5 m2), and, by this method, the average range use in I,*’ the first half of the night was 75.8% of the total I I I I occupied. 0* 20 40 60 80 Number of fixes The minimum convex polygon and grid area methods are therefore in close agreement, indicating Figure 2. The relationship between range size and the number of fixes used to determine range size for female R. tiomanicus in an that about 22% of the range is not covered or oil palm plantation occupied in the first half of the night. However, as

Crop Protection 1997 Volume 16 Number 5 469 Behaviour and habitat of the Malayan wood rat: A.P. Buckle et a/.

Table 1. Home range sizes of individual male and female R. tiomanicus calculated by the minimum convex polygon, grid area and maximum length methods using the first 70 radio-telemetry fixes for each animal

Sexual status Animal no. Min. conv. poly. (mZ) Grid area (mZ) Max. length (m)

Mature male 39 1213 600 69.6 48 938 575 60.2 53 2612 950 91.2 65 813 575 61.0 70 3725 600 134.6 72 875 450 53.8 mean f SE 1696k490 625 + 28 78.4 f 12.4 Perforate female 56 350 350 30.4 80 438 300 35.4 99 725 575 39.0 mean f SE 504*113 408 + 85 34.9k2.5 Lactating female 30 263 325 33.5 35 300 375 46.1 60 263 250 39.1 mean&SE 275 + 12 317+36 39.6f3.7

Minv. cow. poly = minimum convex polygon

discussed in the previous section, these comparisons individuals in subsequent tracking sessions in the first are made when the calculated range size is increasing half of the night. as a function of increased fix numbers. Thus the apparent 22% increase due to the inclusion of fixes Habitat- utilisation from the midnight to dawn data may only reflect this effect, rather than a real difference in ranging The percentage utilisation of the four habitat behaviour. This hypothesis is supported by the obser- elements revealed by radio-tracking fixes is shown in vation that all cells occupied by animals in the two Table 3 separately for the periods dawn to dusk, dusk tracking sessions conducted in the second half of the to midnight and midnight to dawn. Overall, regard- night, were found to be occupied by the same less of time of day, most fixes (43.0-52.1%) were

Table 2. Mean home range sizes (*SE) for R. tiomanicus calculated by three different methods for the first and second halves of the nights from 30 January to 2 February 1989

Dusk-midnight Midnight-dawn Dusk-dawn

Sexual status MCP GA Mx.L MCP GA Mx.L MCP GA Mx.L

Mature males (n = 6) 1156k434 658k39 63.9 + 15.5 640 + 195 263 + 49 44.Ok6.7 1513+547 479*77 65.7) 15.7 Perforate females (n = 3) 396+48 308k30 29.Ok2.1 284 + 177 200&52 26.3 + 3.3 4715110 358_+60 31.3k15.7 Lactating females (n = 3) 213k13 233 + 22 27.25 1.1 123k44 192+36 26.9k4.6 267k4.0 267 2 33 29.4 + 2.3 All animals mean k SE 730 k 245 300+24 46.0 + 9.2 425 + 121 229 k 29 35.3k4.5 941+314 396k48 48.Ok9.2

MCP = minimum convex poygon; GA = grid area; Mx.L = maximum range length

Table 3. Habitat utilisation by time of night/day and sex for R. tiomanicus in an oil palm plantation. Data given are the percentage of fixes, for each category of sexual condition (n = 6, 3 and 3 for mature males, perforate females and lactating females, respectively), out of the first 70 radio-telemetry fixes for each individual animal

% of fixes

Time period Sexual condition Burrow Fronds Vegetation Tree crowns No. of fixes

Dawn to dusk Mature male 14.4 55.6 25.6 4.4 90 Perforate female 31.1 28.9 22.2 17.8 45 Lactating female 55.6 44.4 0 0 45 mean + SE 33.7 f 14.7 43.Ok9.5 15.9k9.8 7.4 + 6.6 180 Dusk to midnight Mature male 9.7 58.0 16.9 15.4 195 Perforate female 7.2 25.8 26.8 40.2 97 Lactating female 5.3 72.6 5.3 16.8 95 mean f SE 7.4* 1.6 52.1 k 16.9 16.3+ 7.6 24.1 k9.9 387 Midnight to dawn Mature male 14.8 56.3 11.9 17.0 135 Perforate female 11.8 16.2 22.1 50.0 68 Lactating females 1.4 77.1 4.3 17.1 70 mean f SE 9.3k5.0 49.9 k 21.9 12.8f 6.3 28.0 + 13.5 273

470 Crop Protection 1997 Volume 16 Number 5 Behaviour and habitat of the Malayan wood rat: A.P. Buckle et al. made in the piles of old fronds stacked in the palm usage of the different habitat elements either inter-rows. These are used by R. tiomanicus both as according to sex or sexual condition of the animals. sites for above-ground nests and, when rat-runs were There was, in fact, considerable variation in established among them, as a means of moving about behaviour both among and between groups. For the palms protected in some measure from predation. example, among the perforate females, one animal At night, the second most frequented element of the (no. 56) was found to spend the majority its time habitat was the tree crowns (24.1 and 28.0%). The among the vegetation, while another (no. 80) was rats went into the palm crowns to feed on the oil never found in that habitat element. Similarly, palm fruit. Occasionally, nests were also made by burrows were the least-used element by most male R. animals among the frond rachis bases high in the tiomanicus, but almost 45% of the fixes of one male palms and this probably accounted for the presence animal (no. 65) were below ground. of some animals in the crowns during the day. Although R. tiomanicus feeds predominantly on fruit in oil palm plantations, other food is required to Discussion supplement this diet (Wood, 1976; Wood and Liau, 1984a). The animals therefore forage for insects, A knowledge of the way in which pest rodents utilise molluscs and other invertebrates among the ground the habitats they infest is useful to those attempting vegetation of the palm fields. For this reason, and to apply established rodent control techniques. For because they were intercepted while moving about example, when control measures are based on roden- the fields under the cover that this vegetation ticides, the disposition of bait placements, the provides, many fixes (12.8-16.3%) occurred in that optimum area of rodenticide application, the speed of habitat element. R. tiomanicus is not a strongly fosso- recovery of pest populations after treatments and, rial species but, from the number of fixes made when hence, the frequency of control operations, are all rats occupied burrows, particularly as resting sites influenced to some extent by population density, rat during the day, more time is spent below ground by home range sizes, dispersal patterns and habitat utili- R. tiomanicus than was previously thought (see sation. For the same reasons, researchers also Wood, 1984). It was not possible to determine during frequently seek an understanding of the biology of this study whether the burrows had been constructed rodent pest species to aid the development of new by the animals using them or by another agency (e.g. management strategies (e.g. Chitty and Southern, individuals of the more fossorial species Rattus argen- 1954; Wood, 1984; Wood and Liau, 1984a,b). In the tiventer, Robinson and Kloss). Certainly, little sign of latter case, however, rodent ranges also affect the size fresh digging activity was observed. and shape of experimental plots and the distances The percentage of fixes in which individual animals required to separate them, both from one and other were found in each of the four habitat elements is and from untreated areas. Similar parameters will given in Table 4. Most of the rats were, at some stage, also affect the susceptibility of rodents to predation detected in all of the habitat elements but some and therefore have an impact in biological control showed quite strong preferences for certain habitat programmes, such as those putatively based on barn types within their ranges. This probably reflected owls (Duckett, 1982; Lim et al., 1993). mainly the position of the nest site(s). For most Little empirical information on the spatial animals these were among the frond piles in the palm behaviour of R. tiomanicus in oil palm plantings has inter-rows. Although it is apparent that some been published since the work of Wood (1971). He individuals preferred the tree crowns, burrows and determined home range dimensions using data vegetation for nesting. derived from CMR programmes and expressed them Statistical analysis of the data given in Table 4 as maximum range diameters (equivalent to the Mx.L showed that there was no consistent pattern in the value used in this paper). In his study Wood (1971)

Table 4. Habitat utilisation by individual R. #iomanicus in an oil -palm plantation. Data given are mean percentages (*SE) by sexual condition for the first 70 radio-telemetry fixes for each individual animal

% of fixes

Sexual condition Animal no. Burrow Fronds Vegetation Tree crowns

Mature males 39 1.5 68.2 10.6 19.7 48 5.9 67.7 16.2 10.3 53 4.5 40.3 17.9 37.3 65 44.8 20.9 31.4 3.0 70 4.4 69.6 17.4 8.7 72 14.9 71.6 10.5 3.0 mean f. SE 12.7k6.7 56.4 + 8.5 17.3k3.1 13.7 +5.4 Lactating females 30 22.1 47.1 7.6 23.5 35 15.9 68.1 4.4 11.6 60 2.9 91.3 0.0 5.8 mean + SE 13.6k5.6 70.0 * 12.8 3.9k2.2 13.8 +5.2 Perforate females 56 14.5 20.3 60.9 4.4 80 4.6 16.7 0.0 78.8 99 22.4 29.9 10.6 31.3 mean + SE 13.9k5.1 22.3 + 3.9 24.3 k 18.7 39.6521.5

Crop Protection 1997 Volume 16 Number 5 471 Behaviour and habitat of the Malayan wood rat: A.P. Buckle et al. found that range diameters varied from 0 m (i.e. all supports the observations of Wood (1984). Rats captures at the same trapping location) to 82.3 m, but sometimes occupy subterranean burrows (either the means for males (29.3 m) and females (24.4 m) constructed by themselves or by some other agency), did not differ markedly. For both sexes, the maximum but spend the majority of their time among the piles range length derived from radio-tracking (Table I) of old fronds stacked in the palm inter-rows, with the was larger than the analogous figure estimated by remainder spent equally in the palm crowns or trapping and, in the case of male R. tiomanicus, the among other vegetation. The high frequency of fixes difference was substantial. This is expected if, as of rats in frond piles emphasises the relative import- suggested by some authors, the probability that a ance of this habitat component. Consideration has rodent will reach the extremities of its range during a been given to the disposal of these fronds, either by CMR programme is reduced by the likelihood that it burning or shredding, as this might reduce the will enter a trap before it does so. Wood (1971) also capacity of oil palm plantations to support rat infesta- suggested that edge effects may cause range sizes tions (Macdonald and Fenn, 1994). However, beyond derived from trap data to be underestimates. An the expense of these operations, removal of the frond earlier trapping study in a natural grassland scrub piles is detrimental in palm agronomy because it habitat showed R. tiomanicus range lengths to exposes soils to water loss through increased evapora- average between 73 m and 100 m (Harrison, 1958). tion, to erosion and to potential colonisation by The observation that R. tiomanicus ranges more weeds, which would compete with the palms for widely than previously shown has implications for the nutrients, minerals and water. Therefore, the disposal methods used in the evaluation of rodenticides in oil of the old fronds for improved rat management is palm. Chia et al. (1990) recommended the use of presently impractical. Also, the ability of certain study plots of at least S-10 ha for this purpose but individuals observed in this study (nos 6.5, 56 and 80, researchers continue to publish results from plots of Table 4) to exist in oil palm trees with little recourse much smaller size. The efficacy of rodenticide treat- to the frond piles suggests that R. tiomanicus may be ments is considerably reduced by edge effects in small able to adapt to their absence. plots. One such effect is that a relatively large Barn owls @to alba) have been recommended as number of rats forage partly off the plot and are then biological control agents in oil palm plantations not exposed constantly to the treatment. For because they feed almost exclusively on pest rodents example, on a square plot of 4 ha (e.g. Chung, Sim (Duckett, 1982). No observational studies have been and Balasubramanium, 1994), only an estimated 5% conducted of barn owl predation behaviour in the oil of males and 40% of females will range only within palm agro-ecosystem. The ratio of male to female R. tiomanicus is usually 1:l in oil palm plantings (Wood the treated area. These numbers will be even smaller and Liau, 1984b). However, Lim et al. (in press) if plots are of an irregular shape. A further effect is found female rats to predominate at an estate where produced by the relatively large number of animals barn owls were established. They speculated that the that enter small treated plots from neighbouring, shift in sex ratio was brought about by the selective untreated areas to feed on baits. This further dilutes predation of male rats by owls, which occurred the effect of the treatment. The separation of plots is because, as in most rodent species, males range more also an important consideration in multi-plot experi- widely than females and are more at risk to preda- mental designs. An obvious requirement is that tors. The finding reported here, that the ranges of rodents living in an area in which the effect of one adult male R. tiomanicus, observed by radio-tracking, treatment is to be measured should not be exposed to are more than four times larger than females’ ranges, another. The maximum range length in the present supports this suggestion. One possible effect of this study was 134.6 m (Table I) and this is the distance sex ratio bias in favour of females is to increase the that theoretically should separate assessment areas potential rate at which the predated population can from those receiving a different treatment. Undoubt- make good the loss of individuals caused by owls edly, some animals will range further (see Wood, (Lim et al., 1993). 1971) but, in the present study, most ranges were less than 100 m long and this may be a more appropriate distance for separating trial plots. Acknowledgements Rats fitted with radio-collars were selected so that their likely ranges were widely spaced. This was done The authors are grateful to Dr K. J. Ward for assist- to make tracking easier and to avoid disturbing ance with statistical analysis of results and to an resting tagged rats as others were being tracked. anonymous referee for helpful comments on an Therefore no estimate of the amount of range earlier draft of the manuscript. overlap could be made directly from radio-tracking data. However, the observed mean range sizes of 1696 m2 for males and 390 m2 for females (Table I) References permit 1 ha of oil palm to accommodate fewer than five rats of each sex without overlap. The estimated Anonymous(1988) SASISTAT User’s Guide? Release 6.03. Statis- population density of 433 R. tiomanicus per hectare tical Analysis System Institute Inc., Cary, NC, USA in this study therefore implies a very considerable Chia T. H., Buckle A. P., Visvalingam M. and Fenn M. G. P. degree of range sharing in this species. (1990) Methods for the evaluation of rodenticides in oil palm The data on habitat usage by R. tiomanicus plantations. Proceedings of the Third International Conference on provided by the present radio-tracking study largely Plant Protection in the Tropics, 20-23 March 1990, Genting

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Highlands, Malaysia, pp. 19-26. Malaysian Plant Protection Yusof et al., pp. 458-465. 9-14 September 1991, Kuala Lumpur, Society Malaysia, Palm Oil Research Institute of Malaysia Chitty D. and Southern H. M. (eds) (1954) Control of Rats and Lim J. L., Visvalingam M., Buckle A. P., Ward K. J. and Fenn M. Mice, 3 Vols. Clarendon Press, Oxford G. P. (in press) Prey selection by barn owls (Zjto alba) in an oil palm plantation in Malaysia Chung Cl. F., Sim S. C. and Balasubramanium R. (1994) Effects of bait size on rat control in mature oil palm plantations. Proceedings Macdonald D. W. and Fenn M. G. P. (1994) The natural history Fourth International Conference on Plant Protection in the Tropics, of rodents: preadaptations to pestilence: In Rodent Pests nnd Their 28-31 March 1994, Genting Highlands, Malaysia, pp. 1466149. Control. eds A. P. Buckle and R. H. Smith. DD. 1-21. CAB Inter- .& Malaysian Plant Protection Society national, Oxon, UK Duckett J. E. (1982) Barn owls (Tyto &a) - a proven natural Wood B. J. (1972) Sources of reinvasion of oil palms by the wood predator of rats in oil palm. In The Oil Palm in Agriculture in the rat (Rattus tiomanicus). In Crop Protection in Malaysia, eds R. L. Eighties, eds E. Pushparajah and P. S. Chew, pp. 461-472. Incor- Wastie and B. J. Wood, pp. 146-165. The Incorporated Society of porated Society of Planters, Kuala Lumpur Planters, Kuala Lumpur Fenn M. G. P. (1989) Ecology of the Norway Rat, Rattus notve- Wood B. J. (1976) Vertebrate pests. In Oil Palm Research, eds R. gicus. on Lowland Farms in England. D.Phil. Thesis, University of H. V. Corley, J. J. Hardon and B. J. Wood, pp. 395-418. Elsevier, Oxford, UK Amsterdam

Harris S. S., Cresswell W. J., Forde P. G., Trewhella W. J., Wood B. J. (1984) A long term study of Ruttus tiomanicus popula- Woolard T. and Wray S. (1990) Home-range analysis using radio- tions in an oil palm plantation in Johore, Malaysia. I. Study tracking data - a review of problems and techniques particularly methods and population size without control. J. Appl. Ecol. 21, as applied to the study of mammals. Mammal Rev. 20, 97-123 445-464 Wood B. J. (1994) Rodents in agriculture and forestry. In Rodent Harrison J. L. (1957) Habitat of some Malayan rats. Proc. Zool. Pests and Their Control, eds A. P. Buckle and R. H. Smith, pp. Sot. Land. 28. 1-21 45-83. CAB International, Oxon, UK Harrison J. L. (1958) Range of movement of some Malayan rats. Wood B. J. and Liau S. S. (1984a) A long term study of Rattus .I. Mammal. 39, 190-206 tiomanicus populations in an oil palm plantation in Johore, Lee C. H. and Kdmarudin K. A. (1990) Rodent species associated Malaysia. II. Recovery from control and economic aspects. J Appl. to cocoa in Peninsular Malaysia. Proceedings Third International Ecol. 21,465-472 Conference on Plant Protection in the Tropics, 20-23 March 1990, Genting Highlands, Malaysia, pp. 2-7. Malaysian Plant Protection Wood B. J. and Liau S. S. (1984b) A long term study of Rattus Society tiomanicus populations in an oil palm -plantation -in Johore, Malavsia. III. Bionomics and natural regulation. J. ADDI. Ecol. 21. Lim J. L., Fenn M. G. P., Buckle A. P. and Visvalingam M. (1993) . . 473-495 Prey selection by barn owls (Tyto alba) and its impact on rat control in oil palm plantations. Proceedings 1991 PORIM Inter- Received 9 September 1996 national Palm Oil Development Conference - Agticulture, ed. B. Accepted 20 January 1997

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