Denning Ecology of Female American Black Bears In
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DENNINGECOLOGY OF FEMALEAMERICAN BLACK BEARS IN SOUTH CENTRALLOUISIANA DWAYNEA. HIGHTOWER,'School of Forestry, Wildlife, and Fisheries, Louisiana State University AgriculturalCenter, Baton Rouge, LA 70803, USA, email: [email protected] ROBERT 0. WAGNER,1School of Forestry, Wildlife, and Fisheries, Louisiana State University AgriculturalCenter, Baton Rouge, LA 70803, USA, email: [email protected] RICHARD M. PACE, 111,2U.S. Geological Survey, Biological Resources Division, Louisiana Cooperative Fish and Wildlife Research Unit, Louisiana State University, Baton Rouge, LA 70803, USA Abstract: We examinedthe denningecology of female Americanblack bears(Ursus americanusluteolus) in the upper(Inland) and lower (Coastal) AtchafalayaRiver Basin of southcentral Louisiana. We attempted101 den visits of 35 female bearsduring February-March, 1992-2000. We found dens on 28 of 56 (50%) Coastal and 44 of 45 (98%) Inlandattempts. We determinedreproductive status on 62 (61 %) den visits, but our success differedbetween Coastaland Inlandareas (43% and 84%,respectively). The distributionof littersizes differedbetween areas. We did not detect any cub abandonment. Use distributionamong den types (e.g., tree, snag, groundnest, slash, stump, log, and excavated) differed between areas. We found no evidence that the distributionof den security classes differed between parturientand barrenfemales. Using the median location errorof field trialswith test transmitters(206 m) as our criterion,we classified bearsas stationaryduring the denningseason if the distancesof 3 consecutive locations (a location set) from their centroidwere each <206 m. Of 29 dennedbears, 24 had >1 stationarylocation set (classified as denned),and 16 of 117 bearseasons outside of the denningperiod contained >1 stationarylocation set. We classified 10 of 23 Coastalfemales, 6 of 19 Coastalmales, 2 of 4 Inlandfemales, and 1 of 1 Inlandmales of previously unknown status as denned. Based on movement data, many Coastal bears, including adult females, remainedactive or only bedded for shortperiods duringwinter. Ursus 13:11-17 (2002) Key words: Americanblack bear,denning, litter size, location constancy,Louisiana, reproduction, Ursus americanus luteolus Denning by black bears is hypothesizedto be an adap- Denning is not, however, compulsory for all bears in tation to decrease stress during periods of inclement some mild climates. Excludingparturient females, which weather and food shortage (Lindzey and Meslow 1976, must den to give birth,bears may remain active if suffi- Johnson and Pelton 1980a), and is when parturitionoc- cient food is available(Graber 1990). Winter-activebears curs (Alt 1983). After enteringdens, bears cease all in- have been reportedfor manysoutheastern black bear popu- gressive and eliminative functions and enter a lethargic lations (Taylor1971, Hellgrenand Vaughan 1989, Wood- or deep sleep (Nelson et al. 1973). Disturbanceduring ing and Hardisky1992, Weaverand Pelton 1994). These the denningperiod may result in cub abandonmentor re- observationsindicate that bears in milder climates may duce the survivalprobability of adultbears by increasing have winter behavioraloptions not available to bears in energy expenditure when food resources are scarce colder climates, where food may be more limited. Popu- (Linnell et al. 2000). Because managersperceive denned lation-specific knowledge of bear behavior in winter is bears,especially parturientfemales, to be vulnerable,ad- criticalto understandthe seasonal needs of bears and for- equate denning habitatand structureshave been empha- mulateappropriate management strategies. sized as importantin managingblack bear habitat (Johnson To aid in conservationplanning for the threatenedLoui- and Pelton 1981, Weaverand Pelton 1994, Linnell et al. siana black bear (U. a. luteolus; U.S. Fish and Wildlife 2000). Service 1992), we investigatedthe denning habitat,den- Researchersin the southeasternU.S. have reportedblack ning frequency,and reproductiverates of bears in 2 iso- bears using a wide varietyof den structuresincluding trees, lated populations within the Atchafalaya River Basin, standingsnags, groundnests, logs, stumps,piles of woody Louisiana. Our objectives were to document den struc- debrissuch as logging slash, andexcavated areas beneath tures used, determineif parturientfemales used more se- or human trees,logs, debris, structures(Johnson and Pelton cure dens than other females, document reproductive Smith and 1981, 1985, Hellgren Vaughan1989, Weaver output,and classify wintermovement behavior (e.g., sta- and Pelton 1994). Of these, tree dens are consideredthe tionary or moving). Because of habitat differences be- most secure (Johnson and Pelton 1981). Hence, bears tween study areas (coastal swamps and mesic islands tree dens using are presumedto realize increasedsurvival versus inland bottomlandhardwood forests), known dif- for themselves and their offspring (White et al. 2001). ferencesin movementdynamics (Wagner 1995) andmor- tality (Pace et al. 2000) between and 1 populations, reported Presentaddress: Quantitative Ecological Services, Inc., 200 differencesin den use among Americanblack bear Willow Lake LA popu- Lane, DeRidder, 70634, USA lations in the southeasternU.S., we of 2 Presentaddress: Northeast Fisheries Science Woods comparedaspects Center, between our Hole, MA 02543, USA, email: [email protected] denning ecology study populations. 12 Ursus 13:2002 STUDYAREA deltaic plains (Evans et al. 1983). Most of the land area, We worked with 2 populations of bears located in 2 excluding the salt domes and areas protected by levees, areas of south central Louisiana, referred to as Inland was inundatedthroughout most of the year. The water (approx460 km2in size) and Coastal (approx570 km2in table was generally <0.2 m above the surface. Drier sites size; Fig. 1). The Inland population was located prima- in the swamp and scrub habitatsgenerally were those ar- rily within the MorganzaSpillway, a floodway developed eas surroundingisolated groupsof trees. Detailed habitat by the U.S. Army Corps of Engineersto release floodwa- descriptionswere provided by Nyland and Pace (1997). ters of the Mississippi River into the AtchafalayaBasin. Forestedhabitats within the floodway were primarilybot- tomlandhardwood stands managed for timberproduction. METHODS Forest managementprescriptions, especially even-aged We capturedbears in leg snares or culvert trapsfollow- management,may have influenced den tree availability. ing the methods detailed in Johnson and Pelton (1980b). Agriculturalfields within the Spillway were adjacentto We trappedfrom April to Septemberduring 1992-99, with the levees and LouisianaHighway 10, which bisected the trappingsessions ranging from 3 to 6 months. Trapping Spillwaynear its northernend. Seasonalflooding occurred effort varied among years and between study areas, with throughoutthe Inlandarea but was not severe during the Coastal receiving more effort during several years. Cap- term of this study. Large areas of slightly higher eleva- turedbears were immobilized with eithera 2:1 mixtureof tion were dry throughoutwinter, and flooded areas were ketamine hydrochlorideand xylazine hydrochloride(4.4 generally <0.5 m deep. mg/kg) or a 1:1 mixture of tiletamine hydrochlorideand The Coastal population was in an area consisting of zolazepam(6.0 mg/kg). Bears were uniquelymarked with small bottomland hardwood stands, baldcypress-water ear tags, lip tattoo, and, beginning in 1997, with a passive tupelo(Taxodium distichum-Nyssa aquatica) swamps, and injectabletransponder (AVID Incorporated,Norco, Cali- coastal marshesthat varied along salinity gradientsrang- fornia, USA). A first premolarwas extractedfor cemen- ing from fresh to saline (U.S. Army Corps of Engineers tum annuli aging (Matson's Laboratories,Incorporated, 1974). Wetlandscrub habitatsoccurred in the transition Milltown, Montana,USA), and radiocollarswith leather zone between swamp and marsh. Bottomlandhardwood spacing materialwere attachedto bears >2 years old (Ad- stands were restrictedto the borders of sugarcanefields vanced TelemetrySystems, Incorporated,Isanti, Minne- protectedfrom flooding by levees, and along spoil areas sota, USA; Telemetry Systems, Incorporated,Mequon, nearwaterways. Coastalhabitats were highly interspersed Wisconsin, USA; Telonics, Incorporated,Mesa, Arizona, and most containedopen water, sloughs, bayous, and ca- USA). nals influencedby tidalfluctuations (Chabreck 1970). The We attemptedden visits for adult and subadultfemale Coastal area also contained 4 salt domes, characterized bears during Februaryand March from 1992 to 2000 to by upland cover types, rising >30 m above surrounding replaceradiocollars, document den location and structure, and determine reproductive status. Conditions permit- ting, bearswere immobilized in dens and the young sexed and weighed. We listened for nursing cubs at den sites we were not able to enter. To assess cub survival, se- lected litters after 1995 were radiotagged following the protocol of Higgins et al. (1997). Females were assigned to reproductiveclasses at den visits or from observations made after den emergence: solitary adult female, solitary subadultfemale, adult female with cub (born in current winter),subadult female with cub, adultfemale with year- ling, and subadultor adult female of unknown condition. We tested for differences between areasin the proportion of den visits resulting in den observationsusing a X2test and for differences between areas in female reproductive conditionusing