Characteristics of natal and non-natal kit fox dens in the northern Chihuahuan Desert

Transcription

1 Great Basn Naturalst Volume 59 Number 3 Artcle Characterstcs of natal and non-natal kt fox dens n the northern Chhuahuan Desert Penny J. Rodrck Unversty of Wsconsn Madson Nancy E. Mathews Unversty of Wsconsn Madson Follow ths and addtonal works at: Recommended Ctaton Rodrck Penny J. and Mathews Nancy E. (1999) Characterstcs of natal and non-natal kt fox dens n the northern Chhuahuan Desert Great Basn Naturalst: Vol. 59: No. 3 Artcle 6. Avalable at: Ths Artcle s brought to you for free and open access by the Western North Amercan Naturalst Publcatons at BYU ScholarsArchve. t has been accepted for ncluson n Great Basn Naturalst by an authorzed admnstrator of BYU ScholarsArchve. For more nformaton please contact scholarsarchve@byu.edu.

2 Great Basn Naturafut 59(3) C:l999 pp CHARACfERSTCS OF ATAL AND NO -NATAL KT FOX DENS N THE NORTHERN CHHUAHUAN DESERT Penny J. Rodrckl and Nancy E. Mathews 2 AUSTRACf.-We studed kt fox den characterstcs on the northern McGregor Range of Fort Blss Mltary Reserva ton New Mexco durng :l.D.d Twenty ra.do-collared kt foxes used 132 dfferent dens ncludng 16 natal dens. Kt fox dens were located prmarly n creosote-domnated habtat found n relatvely flat well-draned terran. Natal dens were vrtually ndstngushable from non-nalal dens; however natal den entrances were tuller than mm-n«tal den entrances. Entrances found at all dens 'were orented more frequently to\'1rard the northwest and southeast. Kt foxes used more new dens durng the breedng (January-February) and pup-rearng season (:ay-jlly) than durng gestaton. Key words: Vulpes macrots neomexcana ktjqx dens hobl(jt use ChhuahUJn Desert sols. G1S. Ecology ofthe desert kt fox (Vulpes macrots 1l6011l xcanw) n the northern Chhuahuan Desert has not been prevously studed. The regon ncludes southeastern New Mexco and southwestern Texas and s recognzed by some bologsts as a zone of sympatry between the kt fox and swft fox (V. vewx vewx). Some queston remans as to whch speces nhabts ths area (Rohwer and Klgore 1973 Thornton and Creel 1975 Dragoo et al Mercure et al. 1993). The U.S. Fsh and Wldlfe Servce (FWS) has concluded that the swft fox s declnng throughout ts range and determned ts lstng warranted but precluded due to other hgher prorty speces (U.S. Federal Regster 1995). t s. now lsted as a speces of concern. Because the kt fox s closely related to the swft fox and appears to have smlar behavors baselne ecologcal data for kt fox may contrbute to swft fox conservaton efforts. Prevous studes on the effects of mltary act.tes on San Joaqun kt foxes (Y. m munca) n Calforna demonstrated few drect adverse mpacts (Berry et a Reese et a. 1992). Controlled publc access ncludng huntng and trappng restrcted vehcle traffc. and lmted land development on several western mltary reservatons may preserve sutable habtat necessary to snstan fox populatons. Although authorzed mltary actvtes such as mssle frngs vehcle traffc and feld exercses may negatvely mpact some areas sutable for kt fox dens such dsturbance may postvely alter vegetaton mportant to prey populatons. The prmary concern on mltary lands however s destructon of sutable dennng habtat by vehcles or constructon. Kt and swft foxes use underground dens for escape cover. protecton from envronmental condtons and rasng pups (Seton 1925). These foxes are 2 ofthe only cands to use dens year-round and they typcally use a subset as natal dens n whch to rase pups (Morrell 1972). Egoscue (1975) suggested kt fox populatons may be lmted by avalable dennng habtat. Whle ths has not been defntvely confrmed the kt fox's strong dependence on den stes throughout the year s crtcal to ts survval. We undertook our study to further understand kt fox dennng behavor and to desclbe characterstcs of natal dens non-natal dens and surroundng dennng habtat on the northern McGregor Range of Fort Blss Mltary Reservaton New Mexco. Recognton ofdfferences n den trats and dennng habtat mal' assst the mltary n land-use decsons. STUDY AREA AND METHODS Our study area encompasses approxmately 53 km 2 n the northern porton of McGregor Range on Fort Blss Mltary Reservaton n south central New Mexco. Fort Blss les n the Tularosa Basn surrounded by the San Andres Frankln and Orgau Mountans to the Departmcnt of\\'1d.lk Ecoll:lg)' Unr.eltr of15consn-mn 226 P.usd1 Labs Lnden Dr. Madloon. W Deplll'lment ow'ldjfe E<:clog. t:nyefttymwlxxlnsn--.\f;ull'> 215 RUHcll L:lbs. 1631) Lr>deTl Dr Mlldll. '153;00. ('n!ll1\lno

3 254 CHEAT BASN NATURALST [Volume 59 west Otero Mesa and Hucco Mountans to the cast and Sacramento Mountans to the uorth McGregor Range ncludes 288 km 2 of transton zone from the northern Chhuahuan Desert to the Sacramento Mountans. Elevaton ranges from 900 to 1800 m. The area s ard to semard wth varable precptaton of 8-25 em per year (Gle and Grossman 1979). Vegetaton conssts of creosote (Larrea trdentata) and tarbush (Flourensa cemua) flats grasslands predomnantly characterzed by burrow grass (Scleropogon brevffolus) nterspersed wth cactus (Opunta spp.) and yucca (Yucca spp.) mesqute (Prosol's glandulosa) dunes and saltbush (Atrplex canescence) dunes. Mltary actvtes on McGregor Range nclude the frng of 7.62-mm coax machne guns 25-mm Bradley flghtng vehcles Stnger and Avenger mssles and HAWK and Patrot mssles. These muntons are launched from statonary frng postons nto desgnated mpact areas on north McGregor Range. Greatest mltary use occurs annually durng 3 wk n May and June. Prmary actvtes durng ths perod nclude mssle frngs and feld tranng. Vehcles whch range from 4-person Jeeps to 28 ton Patrot launchers utlze only mproved desgnated roads. Approxmately 3500 personnel are nvolved n off-road feld tranng exercses prmarly on Otero Mesa. The range s also used September through December for frng mssles nto desgnated mpact areas. Kt f()xcs were lve-trapped usng snglegate wre-box traps and ftted wth rado collars durng late January through early June 1994 and late January through Februmy 1995 (Zoellck and Smth 1986 Whte et a ). raps were bated and opened for 4 nghts. Faxes were handled wthout usng mmoblzaton drugs when possble. vvhen mmoblzaton became necessary a 5: 1 mxture of ketamne hydrochlorde (Ketaset 4.0 mgjkg) and acepromazne maleate (Promaee 0.25 mg/kg) was admnstered ntramuscularly. All foxes were ftted wth numbered car tags. Adults were ftted wth a 70-g rado collar equpped wth an actvty sensor and released at the trap ste. We dentfed known kt fox dens by tracl<ng rado-collared ndvduals to dens 3-6 tmes per week 31.anuary-14 July 1994 and The number of dens used by foxes was determned for each of3 seasons: breedng (31 January-28 February) gestaton (1 March-30 Aprl) and pup-rearng (1 May-14 July). Wcckly den use was standardzed usng length of tme n each breedng season (breedng 2.9 wk gestaton 8.7 wk pup-re'rrng 10.7 wk) Seasonal descrptons of den use were based on observatons of resdent kt fox behavor. We used 2 methods to analyze den usc. Frst we examned the rate of den ste change by comparng average number of dens used per fox ncludng prevously nhabted dens revsted by the same fox between years and among the 3 seasons usng 2-way ANOVA. Second we examned the seasonal rate ofnew den use hy comparng only the number ofnew dens occuped by each fox also usng 2-way ANOVA. f the season X year nteracton was not statstcally sgnfcant data were combned and a -way ANOVA was used to compare den use among seasons. Analyses were consdered statstcally sgnfcant fp < 0.05 Habtat condton at each den ste was assessed n a 5-m-radus crcle (78.5 m 2 ) centered over the central den entrance durng June and July of each year. n a network of dens wth many openngs the entrance that appeared to have most use was classfed as the central entrance. Caved-n entrances were not talled. A den \vas classfed as a natal den based on presence of pup scat or observaton ofpups at the den ste. Mean and standard error were determned for physcal characterstcs ncludng heght and wdth of the central entrance and number ofentnmces at each den. We categorzed surroundng habtat as creosote grass mesqute tarbush saltbush or a combnaton of these types based on vsual assessment of predomnant vegetaton. dentfyng and countng each stem wthn the crcle assessed densty of surroundng vegetaton. Relatve percent cover was estmated at 4 ponts randomly placed along a 5-m radus n each cardnal drecton. At each pont cover was estmated usng a 0.8-m 2 crcular samplng pont frame. Vegetaton was classfed as shrub grass cactus or yucca. Percent cover of ltter lchen and bare sand was also estmated. The 4 cover measurements were averaged for analyss. Mean and standard error of stem counts wthn the samplng radus were determned to compare vegetaton densty surroundng dens. Mean percent of total vegetatve cover was determned n addton to mean percent ofcover for shrub grass yucca cactus

4 1999] KT Fox DENS 255 sand ltter and lchen We compared all varables between natal and non-natal dens usng t tests and consdered analyses statstcally sgnllcantfp < 010 A glohal postonng system was used to determne Unversal Transverse Mercator (UTM) coordnates at 94 den stes UTM coordnates were nput nto a geographc nformaton system (GS; Arcnfo) and plotted on a sol survey map. Land slope and aspect at each den ste were determned usng dgtal topographc maps n GS format. The proporton of dens prescnt wthn each sol type slope and aspect class was analyzed relatve to avalablty of those classes n the 53-km 2 study area usng ch.-square analyss. Den openng orentaton was classfed as north northeast east southeast south southwest west or northwest and tested usng a ch-square test. Dens wth mssng data were censored from analyses for that varable but remaned n the analyss for remanng varables. RESUlSrs We captured 20 kt foxes (14 females 6 males) durng 1680 trap-nghts n 1994 and Durng the study perod we observed 7 dfferent mated pars Based on 480 radolocatons 116 non-natal and 16 natal dens were located Mean number of den ste changes dd not dffer wthn seasons between years (P = 084) When each year was comhned the rate of den ste changes dd not vary seasonally (P = 028; Fg 1) The numher ofnew dens used wthn each season dd not dner between years (P = 099) Thereforc years were combned and we found that kt j(jx used more new dens durng breedng and pup-rearng seasons (P = 005; Fg 2) Natal dens dd not dffer from non-natal dens wth few exeeptons (Table 1) Natal den entrances were taller than non-natal den entranees (P = 001) Overall den cntranee heghts were sgnfcantly larger than wdths (P < 001) Vegetatve eharacterstcs dd not dffer between natal and non-natal dens although mean percent vegetatve cover tended to be greater around natal dens (P = 0.10) Cactus speces were more abundant around non-natal dens (P = 002) Kt faxes on McGregor Range denned more frequently than expected n creosote habtat assocatons relatve to avalablty (P < 0.01; Fg 3) Small sample szes precluded statstcal mlalyss ofhabtat c1flerenees between natal and non-natal dens. Kt foxes denned more f'equently than expected wthn the Mmbres 'lome sol seres assocaton n relaton to ts avalablty (P < 001) Nnety-four percent (N = 88) of kt fox dens were located wthn ths seres whch s well draned and occurs throughont lowlands Slope s less than 5% and Hoodng may occur perodcally. Predomnant vegetaton assocated wthn ths seres s creosote bush. Most sol wthn ths assocaton s 0.70 aw! j Q Z < DAD! Q < 0.20 [ n.o _ ''''' lreedng GESTATON PUP.REARNG BREWNG GESTATON PUP-REARNG f'g. 1. Mean and standard error numher of den ste changes per week by kt foxes durng breedng gestaton und pup-rearng seasons n the northern Chhllahuan Desert ort Blss Mltary Reservaton New Mexco 1994 <md Fg. 2. Mean and standard error number of new dens used per week by kt foxes durng breedng geslaton Hnd puprearnp; seasons n the northern Chhuahuan Desert Fort Blss Mltary Reservaton New Mexco 1994 and 1995

5 256 GREAT BASN NATURALTST [Volume 59 TABLJ< 1. Physcal and vegetatjve chantder.tcsof natal and non--natal kt fox dens on Fort Blss Otero County New Mexco Non-ontnl dens Natal dells Characterstc (mean ±s:d N (mean +sx) N P-values Mc<m den entrance heght (ern) w.n (0.4) (1.5) F \can den entrance \vdth (em; 18.7 (0.4) (Ol! 16 {)18 Mean number (>fden entrances 1.9 (0.2) (0.:1) 16 0.:39 :Mean percent shrub cover 11.4 (1.5) (0.3) 15 0.'14 Vlcan percent yucca cover 0 SO 1.3 (0.3) Mean percent vaetu!> cover 0.1 (0.1) SO l.02 Mean percent grass cover 196.\ 1 1.) '3(40\ '<.1\.) Mean penent sand cover _'2.D (1.6) (4.0) Mean percent ltter c:over 11..) (1.3) (2.1) \oleun percent lchen COver 5.0 (1.0) (1.9) Mean percent total vegetatve cover 31.'1 (1.4) o 38.6 (.5.1) Shruh densty (stems P(l' 7.'L5m2.) 46.2 (4.7) (1.8) Grass densty (stems per 78.5m 2 ) 49.9 (4.7) (10.3) Yucca densty (stems per 7R5m 2 ) 0.3 (0.2) :3 (0..3) Cactus densty (stems per 78.5m 2 ) 1.7 (0.'1) (0.4) slty loam or slty clay loam n hgher elevatons and sandy loam n lower elevatons (Derr 1981). There were several sol assoeatons wthn the study area n whch no dens of rado-collared kt foxes were located. 'These assocatons have steeper slopes rang.ng from 5% to 20% are more rocky and may have been unsutable for den excavaton. Eghty-fve percent of the study area had mnmal slope (0 to 1 degree). Kt fox dens \vere located n expected proportons wthn these slope classes (P = 0.93). Kt {(]X dens occurred more frequently than expected (P = (U)3) on slopes orented to the northwest. Both natal and non-natal dens opened more frcqnently to the southeast and northwest (P < 0.01). DSCUSSON 'Ve found no seasonal varaton n average number of kt fox den ste changes. These results are consstent wth prevous studes of seasonal kt fox den use on a mltary nstallaton n Calforna (Heese et a!. 1992). However kt foxes used more ne\v dens durng breedng and pup-rearng seasons. Egoscue (1956) suggested that durng the breedng season kt foxes may vst many occuped and unoccuped dens stes possbly surveyng for potental dens or a prospectve mate. Egoscue (1956) also noted that on several occasons natal dens have been abandoned suddenly and the pups moved to a new den. Movement between natal dens has been attrbuted to predator avodance local depleton offood or ntolerable flea levels (Egoscue 1956). Such behavors may have contrbuted to the greater number of new dens used per fox durng these seasons. Kt foxes often den wth a mate or n a socal group; therefore there may be a lack of ndependence tn den use among ndvdual foxes. \Vthl1 ths study no groups of3 or more rado-collared foxes were located and pared rado-collared kt foxes were found n the same den n only 20% of total locatons. Although we dd not account for autocorrelaton n our analyss. \ve recognze that we may have underestmated the average number of new dens due to the presence of mated pars n our sample. OUf results suggest that natal and non-natal dens are qute smlar on McGregor Range. The only dstngushable feature s central den entrance heght. Furthennore a keyhole shape (e.g. greater heght than dth) characterzed all den entrances. 'Ve beleve ths shape may allow quck entrance by kt foxes whle mpedng entrance ofpredators such as coyotes (Cans latrans) or badgers (Taxdea taxus; Egoscuc 1962 Beny et a! Reese et al. 1992). Dens provde escape cover throughout the year but are especally mportant durng the pup-rearng season when pups are most vulnerable. Kt foxes selected stes most frequently n creosote habtat assocatons. Dennng habtat n general was characterzed by relatvely short patchy vegetaton. These results concur wth

6 1999] Krr Fox DENS _ 2 'j' j 1(lj ] _non-natal natal l:! Fg. 3. Number of kt fox dens by habtat ty-p n the northern Chhuahuan Desert Fort Blss :.\Hltary Reserva ton New Mexco 1994 and prevous studes examnng kt fox habtat ut lzaton throughout ther range (Egoscue O'Farrell 1987 Zoel1ck et al 1989). Egoscue (1962) stressed the mportance of creosote habtat for den stes n the Great Basn and suggested that kt faxes utlze adjacent sandy dune habtat n Utah for foragng Both kt and swft faxes have demonstrated an ablty to adapt to open habtats ncludng creosote flats and grasslands wth low and sparse vegetaton. These adaptatons may allow small foxes to more effcently detect predators such as coyotes and bobcats (Fels rufus; Zoel1ck et al 1989). We found cbaracterstcs of sol and terran used as dennng habtat smlar to those reported n prevous studes. Kt and swft foxes typ cally den n loose sandy and welldraned sols (Hall 1946 Cutter 1958 Egoscue 1962 O'Neal et al 1987 Zoellck et al Reese et al. 1992). Past studes ndcate dens are often located on gentle to moderate slopes (Egoscue 1956 Morre Berry et al. 1987). Kt fox dens were located on slopes wth a northwest aspect more than expected and den openngs were more frequently orented to\vard the northwest and southeast. Prevous studes suggest that drecton of den openng may reflect tle prevalng orentaton of slopes n the ' 8 -'3 ro :HA1JlTAT T'PES area (Berry et al. 1987). Whle aspect may can fer specal advantages for dens drecton of den openng may be related to ste-specne condtons such as vegetaton dranage or clmate. Den orentaton for red fox (V. wpes) on Assateague sland Maryland s often asso cated wth local prevalng wnd drectons (Krm et al 1990). Cheescmore (1969) found tbat most arctc fox (Alopex lagopus nnutus) den entrances had a southerly easterly or westerly orentaton possbly ndcatng a pre terence for a warmer exposure. f orentaton s related to wnd and mcroclmate t s possble that kt foxes orent ther dens toward a cooler northern exposure. We dd not however assess any mcroclmatc varables. Recognton of kt fox dens and dennng habtat may be useful to the mltary n makng land-use decsons and assessng potental mpacts of actvtes 011 the envronment. Prevous studes ofthe effects ofmltary actvtes dscovered that vehcles had damaged several dens but kt faxes were not trapped nsde the dens (Berry ct al. 1992). On our study ste we dd not observe drect mpacts resultng from mltary actvty on any surveyed dens. AddtollHlly there was no evdence of damage to dens from lve-fre exercses. However we had lmted access to desgnated mpact zones to fully document effects ofsuch use n ths area. Berry et a!. (1992) suggest that numerous dens stes provde adequate shelter and protecton for kt faxes whle allong for the destructon of several dens wthout adversely mpactng fox populatons. Because kt foxes change dens at a constant rate throughout most of the year and we do not know why they change ther den stes we vew dens as mportant to populaton survval. None should be consdered surplus. Further because few trats dstngush natal from nonnatal dens we recommend that all dens be consdered potental natal dens. Although mltary actvtes do not adversely mpact den stes on our study area off-road actvtes hold the greatest potental for nadvertent mpacts to dens or sutable dennng habtat Careful consderaton should be gven to ntensty of offroad actvtes n potental kt or swft fox habtat ACKNOWLEDGMENTS We thank P. Hansen and many dedcated feld techncans for ther assstance. D. Bash

7 258 GREAT BASN NATURALST [Volume 59 provded GS analyss for the sols and habtat analyss. Logstc support was provded by Fort Blss Mltary Reservaton. Specal thanks to B. Russell K. von Fngel; T Bashore and A. Warren. The Unted States Army Constructon Engneerng Research Unt and the Natonal Bologcal Servce's Texas Cooperatve Fsh and Wldlfe Research Unt Texas Tech Unversty Lubbock Texas provded research support. The Texas Coop Unt s jontly supported by the Wldlfe Management nsttute Texas Parks and Wldlfe Depaltment and Texas Tech Unversty. The Department of Wldlfe Ecology Unversty of Wsconsn-Madson Madson Wsconsn and the Max McGraw Wldlfe Foundaton supported preparaton of the Hnal report. We thank Laura Myers for her nsght and assstance n addton to J. Scrvner and many addtonal revewers for ther recommendatons. Specal thanks to Chrs Wllams for hs dedcated support effort and sh'uflcant contrbuton to the statstcal analyss <.md manuscrpt revsons. LTEHATURE CTED BE:HY WH. TP. O'FAHELL TT. KA'ro AND P.M. MCCUE Characterstcs of clem; used by radocollared San Joaqun!ct f<lx (Vulpes 1nacmtW mutca) Naval Petroleum Heserve #1 Kern County Calforna. US. Department of Energy Topcal Report EG&GEM Santa Barbara Operatons Report EGG pp. BEnHY W.-. we. STANDLEY AND T.P O'FAHHELL Effects of mltary-authorzed actvtes on the San Joaqun kt fox (Vulpes velox mutca) at Camp Roberts Army Natonal Guard Tranng Ste Calforna. U.S. Department of Energy Topcal Report EG&G/EM Santa Barhara Operatons Report EGG pp. CHEt';SEMOHt: D.L Den ecology of the arctc fox n northern Alaska. Canadan Journal of Zoology 47: CUTrEH WL Dennng of the swft fox n northern Texas. Journal of Mammalogy 39: Dmm P.S Sol survey of Otero area New Mexco: parts of Otero Eddy and Chaves countes. Unted States Department of Agrculture Sol Conservaton Servce. 244 pp. DHAC;OO J.w. J.R. ClOATl TL. YATES AND TP O'FAH HELL Evolutonary and taxonomc relatonshps hetween North Amerc:un ard land foxes. Journal ofmammulogy 71: EGOCUE H.J Prelmnary studes of the kt fox n Utah. Journal of Manunaloh'Y 37: coco EcolOh'Y and lfe hstoly of the kt fox n (boe!e County Utah. Ecology 43: Populaton dynamcs of the kt fox n western Utah. Bulletn of the Southern Calforna Academy ofscences 74: CfLE LH. AND R.B. CROSSMAN The desert project sol monograph: sols and landscapes of a desert regon astrde the Ro Grande near Las Cruces New Mexco. USDA and Sol and Conservaton Servce Bulletn October HALL E.R Mammals of Nevada. Unversty of Calforna Press Berkeley. 710 pp. KRlM P.M. TL. BASHORE AND C.L KRKLAND JR Den ste characterstcs and food habts of the red fox (Vulpes vulpes) on Assateague sland Maryland. Vrgna Journal ofscence 41: MEHCURE A. K. RALLS K.P. KOEPFL AND R.K. WAYNE Genetc subdvsons among small cands: mtochondral DNA dfferentaton of swft kt and arctc foxes. Evoluton 47: MORHELL S Lfe hstoly of the San Joaqun kt fox. Calforna Fsh and Game 58: O'FAHRELL 1:P Kt fox. Pages n M. Novak J.A. Baker M.E. Obbard and B. Malloch edtors Wld furbearer management and conservaton n North Amerca. Mnstry of Natural Resources Ontaro Canada O'NEAL G.T. JT FLNDERS AND WE CLARY 1987 Behavoral ecology of the Nevada kt fox (Vulpes macmts nevadenss) on a managed desert rangeland. Pages n H.H. Geneway edtor Cur rent mammalogy. Volume. Plenum Press New York REESE E.A. WC. STANDLEY AND WH. BERRY Hab tat sols and den use by San Joaqun kt fox (Vulpes velox mutca) at Camp Roberts Army Natonal Guard Tranng Ste Calforna. U.S. Department of Energy Topcal Report EG&G/EM Santa Barbara Operatons Report EGG Pl'. ROHWER SA AND D.L. KLGORE JR nterbreed ng n the ard-land foxes (Vulpes velox and V. macrots). Systematc Zoology 22: SETON E.T Lves of game anmals. Volume L Cats wolves and faxes. Doubleday Doran and Company Garden Cty NY 640 Pl'. THORNTON W.A. AND C.G. CREEL The taxonomc status of kt faxes 1exas Journal of Scence 26: U.S. FEDERAL REGSTER. 16 June Endangered and threatened wldlfe and plants: 12 month admnstratve fndng on petton to lst swft fox as endangered. Volume 80 No WHTE PJ. T.]. KREEGER U.S. SEAL AND JR. 'fester Pathologcal response of red fox to capture n box traps. Journal ofwldlfe Management 55: ZOELLfCK B.W AND N.S. SMTH Capturng desert kt fox at dens wth box traps. Wldlfe Socety Bul letn 14: ZOELLCK B.W NS SMTH AND R.S. HENRY Habtat use movements of desert kt fox n western Atzona. Journal ofwldlfe Management 53: Receved 19 December 1997 Accepted 26 August 1998