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American Society of Mammalogists

Spatial Segregation in Long-Nosed Bats, Leptonycteris nivalis and Leptonycteris curasoae, inMexicoAuthor(s): Hctor T. AritaSource: Journal of Mammalogy, Vol. 72, No. 4 (Nov., 1991), pp. 706-714Published by: American Society of MammalogistsStable URL: http://www.jstor.org/stable/1381831

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SPATIALSEGREGATIONIN LONG-NOSED BATS,LEPTONYCTERIS NIVALIS AND LEPTONYCTERISCURASOAE,

IN MEXICO

HiETOR T. ARITADepartment f WildlifeandRangeSciences,Schoolof ForestResourcesnd Conservation,University f Florida,Gainesville, L 32611Presentaddress:FloridaMuseumof NaturalHistory,University f Florida,Gainesville, L 32611ABSTRACT.-Analysisf distributionalatterns f batsof the genusLeptonycterisn Mexicoshoweda spatial egregationf the twospecies.Data on climateandvegetationweregatheredfor 43 localities f L. nivalis and 269 of L. curasoae eportedn a recent axonomicevision fthe genus.A statisticalnalysisusinga gridof quadrats f 1"of latitudeandlongitude howed

nosegregationf the twospeciesat themacrogeographiccale.However, ignificant ifferenceswere foundbetween he altitudeandmeanannual emperaturef localities f the twospecies;L. nivalisoccursn higherandcoolerplaces hanL. curasoae.No significant ifferencesxist ntermsof meanannualprecipitation;othspeciesoccur n drierzonesof the country.L. nivalisoccursmorefrequentlyn areaswithpine(Pinus)-oakQuercus) ndtropicaldeciduousoreststhanexpectedby chance,whereasL. curasoae s foundmainly n areaswithtropicaldeciduousand thorn orests.Batsof the genusLeptonycteris re the mainpollinatorsf somespeciesofmagueyplants Agave), ndtherangesof thebatscoincidewith thatof certain peciesof Agave,suchas A. angustifolia mezcalplant),A. salmiana(pulqueplant),andA. tequilana tequilaplant).

Basedon the competitive-exclusion principle, two species with identical or similar ecologicalrequirementscannot occur in the same place at the same time. When two closely relatedspeciesare sympatric, several mechanisms potentially can differentiate their niches and permit theirdistributionalranges to overlap. One such ecological mechanism is spatial segregation. Speciescan segregatewithin a given geographic areaaccordingto differencesin habitat,verticalstratum,or altitude, or by presenting a checkerboard distribution n the case of archipelagoes(Diamond,1975, 1986).

Jones(1965)and Findley (1969) foundsegregationof certainsympatricspeciesof vespertilionidbatsalongaltitudinalgradientsin Arizonaand New Mexico.Koopman(1978)and Graham(1983)presented several examples of altitudinal segregationof closely related species of the Andes ofPeru. Start and Marshall(1976) observed segregation by habitat in two species of Macroglossusthat occur sympatrically in Malaysia;M. minimus forages in coastal zones, feeding mainly onthe flowers of Sonneratia sp., whereasM. sobrinus selects flowers of wild banana plants (Musasp.) that grow inland. Handley (1967) and Bonaccorso(1979) documented several examples ofsegregationof sympatricbats by habitat and vertical stratumin Neotropicalforests. Some otherexamples of segregationof bat species along gradientsof altitude or habitat were discussedbyHumphrey and Bonaccorso(1979).

Big (Leptonycteris nivalis) and little (L. curasoae) long-nosed bats are sympatric in centraland northernMexico (Arita and Humphrey, 1988). They are similar morphologically,and thedifference in size is slight; the body-massratio L. nivalis: L. curasoae is 1.42:1 (from means of43 L. nivalis and 13 L. curasoae males from Morelos and Distrito Federal). The existence oftwo species in North America was recognizedwhen Davis and Carter(1962) reviewed the genus.The main food of both species is pollen and nectarof the flowersof some tropicaland subtropicalplants,especially cacti (Cactaceae)and certainspeciesof Agave (Alvarezand Gonzilez-Q., 1970;Gardner, 1977;Quirozet al., 1986).BakerandCockrum(1966)suggestedgeographicand ecologicseparationof these two species in North America, L. nivalis occurringprimarilyin the highlandsof eastern Mexico and L. sanborni (=L. curasoae) occupying lower elevations in western andJ. Mamm., 72(4):706-714, 1991 706

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November 1991 ARITA-SPATIAL SEGREGATIONIN LONG-NOSED BATS 707

central Mexico. Since then, many localities have been added to the known distribution of Lep-tonycteris in Mexico, allowing more rigorous analysisof Bakerand Cockrum's(1966) idea. Theobjectiveof thispaper is to use available information to test the hypothesisthat spatial segregationallows coexistence of two species of nectar-feeding bats with similar ecological requirements.Also examined herein is the association between bats of the genus Leptonycteris and certainspecies of maguey plants (Agave). Bats of the genus Leptonycteris are known to be the primarypollinatorsof magueyes (Eguiarte and Buirquez,1987; Howell, 1979; Howell and Hartl, 1980;Howell and Roth, 1981;Martinez del Rioand Eguiarte,1987). If a close associationexists betweenthese plants and bats, matching distributional ranges are to be expected for both groups oforganisms.Furthermore,if spatial segregation occurs between the two species of Leptonycteris,an association of each species of bat with a particular species of Agave might be predicted.

MATERIALS NDMETHODSReview of the literaturepertaining to the distributionof the genus Leptonycteris has been hindered bynomenclaturalproblems. In a recent taxonomic review of the genus, Arita and Humphrey (1988) reportedthat many museum specimens labeled as L. nivalis were misidentified, and that this species has a morerestricted distribution than formerly believed. To avoid problems with erroneous distributionalrecords, Irelied only on localities reported by Arita and Humphrey (1988), who examined specimens of L. nivalisfrom 43 localities and of L. curasoae from 269 localities (Fig. 1).To test the hypothesisof geographic segregationof long-nosedbats, Mexico was divided into 218 quadratsof one degree of latitude and longitude. All localities of Leptonycteris in Mexico reported by Arita and

Humphrey (1988) were located on a map and assignedto theirrespective quadrat.A two-by-twocontingency-table analysiswas employed to test the null hypothesis that the distributions of L. nivalis and L. curasoaein Mexico are independent.For each locality, I compiled meteorological data from Garcia (1981). These data included geographiccoordinates, altitude, mean annual precipitation, and mean annual temperature. Most of the stations hadrecords for both temperatureand precipitationfor >10 yearsof observations,but a few lacked informationfor one of these variables. I first performed a Shapiro-Wilktest for normality to analyze the frequencydistribution of altitude, temperature,and precipitationof the localities of both species. All distributionsweresignificantlydifferent froma normal distribution(P < 0.01), and I usedonly nonparametric estsin subsequent

comparisons.Long-nosedbats are consideredmigratoryspecies(AlvarezandGonz~alez-Q.,970;Easterla,1973;Haywardand Cockrum, 1971;Quirozet al., 1986), so seasonal movements in altitude or latitude of one orboth speciescould obscure differencesbetween the species. To analyze the extent of migration, I divided localities ofeach species into dry-seasonlocalities (recordsfrom November to May) and wet-season localities (records

from June to October). Although this division could be considered arbitrary,it correspondswith the ap-proximate duration of the two main seasonsin most places in Mexico (Garcia, 1981). Wilcoxon two-sampletests were performed to compare for each species the localities of the two seasonsin terms of altitude, meanannual temperature,and mean annual precipitation.Subsequently,I used additional Wilcoxon two-sampletestscomparingthe localities of both speciesto test the hypothesisthat differencesin any of the three variablesindicated spatial segregationof the two bat species.I also compared the distributionalrange of the two bat species in terms of vegetation. I mapped localitiesof Leptonycteris on Rzedowski's(1978) vegetation map and tallied the number of localities of each speciesof bat in each of the eight major types of vegetation: tropical rain forest; semi-evergreen tropical forest;tropicaldeciduousforest;thornforest;grasslandor savanna;desert or chaparral;pine (Pinus)-oak (Quercus)forest; and cloud forest. I compared the frequency of each bat species in each vegetation type with a nulldistribution based on the proportionof Mexican territorycovered by each of the eight types of vegetation.Deviation from the null distribution would suggest a nonrandom distribution of Leptonycteris species interms of vegetation types, demonstratinga selection of some of the available environments. I also used abinomial test to compare the observed number of localities with the number of localities expected to fall ina given type of vegetation if the distributionswere random.Distributionaldata presented by Gentry (1982) for Agave species were used to compare the distributionsof these agaves in Mexico with distributionsof both species of long-nosed bats, looking for a geographicassociationsupportingthe hypothesisof an ecological relationshipbetween the plants and one or both of thebat species.Specialemphasiswas given to three magueyes that are sourcesof traditionalbeveragesin Mexico:Agave angustifolia, mezcal plant, A. salmiana, pulque plant, and A. tequilana, tequila plant. I tested the

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708 JOURNALOF MAMMALOGY Vol.72,No. 4hypothesis f an association etween L. curasoaeand A. angustifoliaby usinga binomialdistributionclassifyingocalities f batsasinsideoroutside herangeof theagave.I madea qualitativeomparisonfthe distributionsf L. nivalisand A. salmiana,but noquantitativeest couldbe performed ecauseof thesmallsample.

RESULTSOf the 218 1*quadratsthat include Mexican territory,24 include localities of L. nivalis and78 include L. curasoae. Sixteen quadrats include localities of both species. A two-by-two con-

tingency-table analysis demonstrateda significantpositive association between the two speciesof bats on a macrogeographicscale (x2 = 11.2, d.f. = 1, P < 0.001, Fig. 1).For L. nivalis, I found no significantseasonal variationin altitude (Wilcoxon two-sampletest,P = 0.56), mean annualtemperature(P = 0.78), or mean annualprecipitation(P = 0.16). Similarresultswere obtained for L. curasoae (P = 0.17 for altitude, P = 0.41 for temperature,and P= 0.32 precipitation). These results indicate that no seasonal pattern can be detected at themacrogeographiclevel. Because no seasonalvariation can be shown, I combined data for eachspecies without regard to time of year.Most localities at which L. nivalis has been collected are between 1,000 and 2,200 m elevation(Fig. 2a), but L. nivalis can reach higher elevations;Saussure(1860) described the type localityof this species at the snow line on the Pico de Orizaba, and Koestner(1941) reported a colonyof L. nivalis in the Cerro Potosi, Nuevo Le6n, at 3,780 m. In contrast,L. curasoae, occurs atlower elevations (Fig. 2a). Mostlocality records for thisspecies are

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November 1991 ARITA-SPATIAL SEGREGATION IN LONG-NOSED BATS 709

1120W 1040W 960 W 88 W

240 N

32?N--.nivaohs

32 N

6N6

...... f

24 o0N .,aa ..

tl W

160N- '0N

1120W 1040 W 960 W 88?W

{.s rtf ~ ;9~.N.../ 5

FIc. 1.-Distribution of: a, Leptonycteris nivalis and b, L. curasoae in Mexico, the latter superimposedon the distribution of Agave angustifolia. L. nivalis alsooccurs in a few localities in Texasand New Mexico;L. curasoae also occurs in Arizona,New Mexico, and some parts of Central and South America. Data onLeptonycteris from Aritaand Humphery (1988);distributionof A. angustifolia adapted from Gentry(1982).

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710 JOURNALOF MAMMALOGY Vol. 72, No. 4

500-

6 40 L.nivoiszn=41U 0ED L. curasooeO 30,2 1 n =269LL ;P

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November 1991 ARITA-SPATIAL SEGREGATIONIN LONG-NOSED BATS 711

2520 L. n/va/is

P < 0.00151S Observed

"] ExpectedF-

00LL-S140-

20n P

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712 JOURNALOF MAMMALOGY Vol.72,No.4

(Easterla, 1973) and some parts of Coahuila (Easterla,1972; Wilson et al., 1985). My analysisconfirms a positive association between the two Leptonycteris species at the macrogeographicscale.Assuggested by BakerandCockrum(1966),long-nosedbatssegregatein an altitudinalgradient,with L. nivalis occurringat higher and cooler localities than L. curasoae (Fig. 2). Both speciesare found more frequently in dry areas of the country, with annual rainfall 4,000 mm.Long-nosed bats do not occur in those partsof the country (Fig. 2), suggesting that bats of thegenus Leptonycteris are adapted to arid conditions,as discussedby Koopman(1981). The twospeciesof long-nosedbat occurat localitieswith differenttypesof vegetation (Fig. 3). Asobservedby Baker and Cockrum (1966), the most common habitat for L. nivalis is the transitionzonebetween tropicaldeciduous forest andpine-oak forest.Conversely,L. curasoaeis morefrequentlyfound in places characterizedby tropical and subtropical dry forests.The mezcal plant and L. curasoae occur in almost identical areas;many of the localities ofA. angustifolia listed by Gentry (1982) are close to collection sites of the little long-nosed bat.This geographic association,and the close plant-pollinator relationshipdemonstratedbetweenL. curasoae and other species of Agavaceae (Eguiarteand Buirquez,1987;Howell, 1979;Howelland Hartl, 1980; Howell and Roth, 1981) might suggest that L. curasoae could be a pollinatorof wild mezcal plants. However, the possibility exists that the positive geographic associationdemonstratedherein may be only a consequence of both the bat and the agave occurringchieflyin arid partsof the country; only direct field observationscould confirman ecological associationbetween these species. A. salmiana is a bat-pollinated plant (Martinezdel Rio and Eguiarte,1987), but the identity of the species that pollinatesthis maguey remainsunknown.On the basisof the habitats in which the plant grows, that pollinatorcould be L. nivalis, but this must beconfirmed by field studies.The two species of long-nosedbats seem to have similarfeeding habits and foraging strategies.Alvarez and Gonzilez-Q. (1970) found that both species of Leptonycteris depend heavily onplants of the genera Agave, Myrtillocactus, Ceiba, Ipomoea, and Bombax, and that the onlydietary difference between the two species was the relative proportionof pollen of these plantsin their diet. Given the altitudinal segregationof long-nosedbats, it is possiblethat even if thetwo species of Leptonycteris feed on nectar and pollen of the same genera of plants, as dem-onstratedby Alvarez and Gonzilez-Q. (1970), they could specializeon differentspecies;it seemsthat L. curasoae exploits lowland species of magueyes and L. nivalis feeds on highland species.

Diamond (1986) found spatial segregationto be the most frequent mechanism allowing pairsof bird species to exist sympatricallyin New Guinea;among such pairsof species,he found littleevidence of ecological segregation in feeding habits or body mass. Diamond (1986) found thatpairsof bird species in New Guinea with body-massratios

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November 1991 ARITA-SPATIAL SEGREGATIONIN LONG-NOSED BATS 713

a nivel macrogeogrifico. Por el contrario,se encontraron diferencias significativasen cuanto aaltitud y temperatura media anual entre las localidades de las dos especies. L. nivalis habitazonas mas altas y frias que L. curasoae. No se encontraron diferencias entre la precipitaci6nmedia anual de las localidades de las dos especies; ambas ocupan las partes mis secas del pais.L. nivalis se encuentraen areascon bosque de encino y coniferaso con bosque tropicalcaducifoliocon mayor frecuencia que la esperada al azar, mientras que L. curasoae se encuentra princi-palmente en losbosques tropicalcaducifolioy espinoso.Losmurcidlagosdel g6neroLeptonycterisson los principales polinizadoresde algunos magueyes (genero Agave), y sus patronesde distri-buci6n coinciden con los de algunas especies de magueyes de importancia comercial, como elmaguey mezealero (A. angustifolia), el maguey pulquero (A. salmiana) y el maguey tequilero(A. tequilana).

ACKNOWLEDGMENTSI thank S. R. Humphrey for continuoussupportof my researchon long-nosedbats. C. Martinezdel Rio,D. E. Wilson, and an anonymousreviewer provided good ideas and stimulatingcriticism. This researchwasmade possible by scholarshipsfrom the Consejo Nacional de Ciencia y Tecnologia and the UniversidadNacional de Mexico. This is a contribution of the Program for Studies in Tropical Conservation and acontributionof the JournalSeries, Florida AgriculturalExperiment Station,Gainesville, Florida.

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Submitted 6 October1988.Accepted3 December 990.