THE JOURNAL OF EXPERIMENTAL ZOOLOGY 280:97–99 (1998)

© 1998 WILEY-LISS, INC.

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An Automated System, Based on Microchips,for Monitoring Individual Activity in WildSmall Mammals

GIACOMO DELL’OMO,1 RICHARD F. SHORE,2* AND HANS-PETER LIPP1

1Institute of Anatomy, University of Zurich-Irchel, CH-8057 Zurich,Switzerland

2N.E.R.C., Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton,Huntingdon, Cambridgeshire, PE17 2LS, United Kingdom

ABSTRACT A new battery-operated system based on microchips has been developed for de-tecting free-moving, wild small mammals. An electronic identification unit connected to a portabledata-logger can simultaneously accommodate up to eight detector antennae, which are positionedin the habitat of the species under study. The system can operate even in extreme weather condi-tions and has the capacity to distinguish and record individual mammals entering burrows orvisiting artificial feeding stations. Detection events are stored in a data-logger and subsequentlycan be edited and analysed by many common software packages. A pilot study tested the systemby monitoring the visits of three species of small mammals to artificial feeding stations. Furtherpossible applications are discussed. J. Exp. Zool. 280:97�99, 1998. © 1998 Wiley-Liss, Inc.

*Correspondence to: Richard Shore, ITE Monks Wood, AbbotsRipton, Huntingdon, Cambridgeshire, PE17 2LS, United Kingdom.E-mail: r.shore@ite.ac.uk

Received 15 May 1997; accepted 21 August 1997.

The use of implantable, passive-integrated tran-sponders (PIT), also called microchips, has in-creased rapidly in this decade for both laboratoryand wild animals (Elbin and Burger, ’94). Micro-chips are useful for individual identification andlast for the lifetime of the animal (Thomas et al.,’87). They are also suitable for implanting in avariety of species of different taxonomic groups(Elbin, ’91). Microchip use has been promoted bythe International Union for Conservation of Na-ture (’90); furthermore, marking of zoo animalsand endangered species by microchips has beenendorsed by the Convention of InternationalTrade of Wild Fauna and Flora (CITES; CBSGNews, ’92). Microchips were recently used for thefirst time to monitor runway activity of Microtusochrogaster and M. pennsylvanicus, (Harper andBatzli, ’96). Although this landmark study col-lected data automatically from free-living rodentsby means of microchips, the authors concludedthat ‘‘further development of PIT technology toprovide more reliable, weatherproof, and portableequipment should be encouraged.’’ In the presentpaper, we describe a portable, weatherproof, bat-tery-operated system for automatic detection offree-living rodents with implanted microchips.The utility of this system was demonstrated in apilot field study of the activity of free-living woodmice Apodemus sylvaticus, bank voles Cleth-

rionomys glareolus, and field voles Microtusagrestis at artificial feeding stations.

DESCRIPTION AND OPERATIONOF THE SYSTEM

An electronic decoder equipped with battery-backed RAM and a real time clock (SPD-02415;RS Biotech, Finedon, Northants, UK) is housedin a waterproof box and can be operated by a 24-volt DC power supply. Two car batteries will op-erate the system for up to eight days, dependingon the ambient temperature. The SPD unit is con-nected to a multi-plexer (MUX) that drives up toeight separate antennae by relay activation. Theantennae, which can be any shape, detect thetransponders and are connected to the MUX by10-m cables, which allows an area of more than300 m2 to be covered (Fig. 1). The MUX sequen-tially interrogates each of the eight antennaeports for 300 msec, thus completing a cycle of alleight ports in approximately 2.5 sec. No time de-lay occurs between repeat cycles. A maximum of1,000 transponder detection events can be stored

98 G. DELL’OMO ET AL.

in the RAM. In addition, linkage to a PC com-puter by a serial communication cable assures vir-tually unlimited data storage capacity.

When a microchip-bearing animal passes overan antenna, the transponder is detected and itscode, together with the date and time, is recorded.Detection events for each antenna are storedseparately in the data-logger, and files, consist-ing of a set of events, are stored either in theRAM or transferred in ASCII format to a PC,where they can be edited by most common soft-ware packages. Replicated detection events areignored for a fixed period that can be set incre-mentally in 5-sec steps. This is important as itprevents the RAM being filled rapidly when asingle animal remains in close proximity to anantenna for a prolonged period.

MONITORING THE ACTIVITY OF WILDSMALL MAMMALS

The ability of the automated SPD recording sys-tem to monitor the activity of free-living rodentswas tested at eight artificial feeding stationsplaced at intervals along the circumference of a300-m2 circular area (Fig. 1). The habitat in halfof the area was deciduous woodland with littleground cover, whereas the habitat in the otherhalf was tall, rank grassland. Two feeding sta-tions were located in the woodland, four were on

the woodland edge, and the remaining two sta-tions were in the grass. All the stations were sup-plied ad libitum with peanuts and wheat for therodents but were covered by inverted plastic milkcrates to exclude rabbits, deer, and most birds. Asingle 300 × 300 mm square antenna was posi-tioned on the ground under the crate at each feed-ing station to monitor the access route to the food.The antenna could detect every transponder mov-ing directly across it or within 120–140 mm of it.

After initial set-up of the feeders, two lockedopen Longworth live traps were positioned at eachfeeding station. Apart from being restocked withfood, the traps and feeding stations were then leftundisturbed for 10 days to allow the animals todiscover and habituate to them. The traps werethen opened and six individuals of wood mice,bank voles, and field voles were captured for thestudy. Using a sterile hypodermic needle, a singlemicrochip was inserted subcutaneously in theback of each individual; animals were releasedimmediately afterwards. The automated record-ing system was subsequently activated, and datawere collected continuously for 34 days.

The activity of the three rodent species at thefeeding stations was recorded by the automatedsystem (Fig. 2). The difference in activity patternsbetween the almost exclusively nocturnal woodmouse and the more diurnal bank and field voleswas particularly clearly illustrated. When data foreach separate antenna were examined (data notshown), species differences in habitat use werealso indicated; wood mice were detected at all thefeeding stations, whereas bank and field voleswere only detected at the feeding stations in therank grassland where ground cover was good.

DISCUSSIONAn automated microchip recording system pro-

vides a versatile technique for studying intra- andinterspecific relationships and habitat use in sym-patric small mammals and other vertebrates. Thepilot study demonstrated the potential of thesystem as a tool to investigate activity patternsand habitat preferences, the recordings confirmingdata that have been collected by other labour-in-tensive means, such as capture-mark-release stud-ies and radio-tracking (Corbet and Harris, ’91).Antennae placed at burrow entrances and alongrunways would also allow investigation of socialbehaviour and associated activity patterns. Thesystem allows data collection for a large numberof individuals and is suitable for field conditionsin various habitats. Performance at temperatures

Fig. 1. Schematic diagram of the layout of the SPD auto-mated recording system. Eight antennae (black circles) actas detectors and are connected to a multi-plexer (large blackrectangle) by 10-m cables. The multi-plexer is linked to theSPD electronic decoder (small black rectangle) which can beconnected to a PC (hashed line).

MICROCHIP MONITORING OF ACTIVITY IN RODENTS 99

as low as –10°C was satisfactory, the only draw-back being a shorter battery duration.

Another advantage of this system is the poten-tial for field studies in access-limited, remote ar-eas. If the data-logger is linked to a PC by a mobileGPS phone, data can be transferred remotely viae-mail from the field site to the laboratory. “On-line” monitoring would reduce the number of visitsto the field site to set up and battery replacementtrips. Use of solar-powered rechargeable batter-ies would further reduce maintenance. Other fu-ture refinements will include increasing thecoverage area, possibly by using radio-transmit-ted signals between the antennae and the decoder,and the incorporation of food dispensers, traps,and video cameras into the system.

ACKNOWLEDGMENTSThis work was supported by grant NF 31-

7497.93 and 31-46691.96 to H.-P. Lipp from theSwiss National Foundation for Scientific Researchand by a European Environmental ResearchOrganisation (EERO) post-doctoral fellowship toGiacomo Dell’Omo.

LITERATURE CITEDCBSG News (1992) Use of coded microchip implants for mark-

ing live animals in trade. CBSG News, 3:21–22.Corbet, G.B., and S. Harris (1991) The Handbook of British

Mammals, 3d ed. Blackwell Scientific Publications, Oxford.Elbin, S.B. (1991) Recommendations for standardised tran-

sponder implantation sites. CBSG News, 2:6–7.Elbin, S.B., and J. Burger (1994) Implantable microchips for

individual identification in wild and captive populations.Wildl. Soc. Bull., 22:677–683.

Harper, S.J., and G.O. Batzli (1996) Monitoring use of run-ways by voles with passive integrated transponders. J.Mammal., 77:364–369.

International Union for Conservation of Nature and NaturalResources/Captive Breeding Specialist Group, WorkingGroup on Permanent Animal Identification (1990) Electronic“tattoos” offer accurate, versatile animal identification.CBSG News, 1:17.

Thomas, J.A., L.H. Cornell, B.E. Joseph, T.D. Williams, andS. Dreischman (1987) A transponder chip used as a perma-nent tag for sea otters (Enhydra lutris). Mar. Mammal Sci.,3:271–274.

Fig. 2. Total number of visits to artificial feeding stationsby microchipped wood mice, bank voles, and field voles over34 continuous days of recording. The visits were made by sixindividuals of each species (data for each individual not shownseparately).