Scent Marking by Male Caribou: an Experimental Test of Rubbing Behavior
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Scent marking by male caribou: an experimental test of rubbing behavior Craig A. Adams1, R. Terry Bowyer24, Jan E. Rowell2, William E. Hauer3, & Jonathan A. Jenks3 1 College of Veterinary Medicine, Washington State University, P.O. Box 647012, Pullman, WA 99164, USA. 2 Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA. J Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B, Brookings, SD 57007, USA. 4 corresponding author ([email protected]). Abstract: We studied scent marking by adult male caribou (Rangifer tarandus) during rut in September 1998 at the Large Animal Research Station in Fairbanks, Alaska, USA. We used an experimental approach involving two captive groups of two males each to test for effects of social status, tree size, texture, and scent on rubbing behavior by cari• bou. Dominant males did not rub more often or for a longer duration than subordinates. Caribou rubbed trees with smaller diameters more often than large-diameter trees. Males preferred trees with bark for rubbing to those trees with their bark removed prior to the experiment. Caribou exhibited no preference for posts with pine-oil applied compared with posts without that aromatic scent. We hypothesize that rubbing of trees by male caribou is related to synchronization or priming of estrus in females, but more research is needed to test that potential function of scent marking. Key words: behavior, caribou, dominance, Rangifer tarandus, rubbing, rut, scent marking. Rangifer, 21 (1): 21-27 Introduction barking and rubbing of shrubs and trees (Bowyer Scent marking is a common behavior among et al, 1994 for review). mammals (Ralls, 1971), and has been particularly The rubbing of trees (i.e., scent-posting) has well described for the species of ungulates been described for an array of cervids including (Coblentz, 1976; Gosling, 1985). Glands involved territorial species such as fallow deer (Dama and putative pheromones released also have dama; Massei & Bowyer, 1999) and roe deer been identified for some species (Quay & Müller- (Capreolus capreolus; Johansson et al., 1995), as Schwarze, 1970; Müller-Schwarze et al., 1978a, b; well as nonterritorial species including mule deer Mossing & Damber, 1981; Mossing & Kallquist, (Odocoileus hemionus; Bowyer, 1986), white- 1981; Atkeson & Marchinton, 1982; Flood, 1989). tailed deer (O. virginianus; Nielsen et al., 1982; The Cervidae possess a rich repertoire of scent- Miller et al., 1987; Benner & Bowyer, 1988; marking behaviors that often involve deposition Oehler et al., 1985), North American elk (Cervus of urine, scraping the ground, wallowing, and elaphus; Bowyer & Kitchen, 1987), and moose Rangifer, 21 (1), 2001 21 31 m 30m (Alces alces; Bowyer et al, 1994). The physical characteristics of trees and their aromatic proper• ties are thought to play a role in determining which trees cervids select for scent marking HOLDING (Benner & Bowyer, 1988; Bowyer et al, 1994; PENS FOR .12 m Oehler et al, 1995; Massei & Bowyer, 1999). The OTHER MALE CARIBOU function of rubbing by cervids remains uncertain, ° 6.8 m but probably relates to dominance or reproduc• tive status of individuals that perform those Î/o behaviors (Bowyer et al, 1994; Oehler et al, POLES —• 1995, Massei & Bowyer, 1999). The rubbing of trees by reindeer and caribou (Rangifer taran- dus) has been described (i.e., antler rubbing, EXPERIMENTAL PEN Espmark, 1964; bush-thrashing, Bergerud, 1974; head rubbing, antler thrashing, Pruitt, 1966, Fig. 1. Overview of the pen used for observations of Muller-Schwarze et al, 1979), but accounts of scent marking by male caribou, Large Animal this behavior are not as detailed as for other Research Station, University of Alaska Fairbanks, Fairbanks, Alaska, USA. Circles represent place• North American cervids. In addition, preferences ment of posts. The observation tower (T) and for and against particular species of trees to rub the gate (G) through which the animals entered have been determined in the field for many the trial pen from the holding pens are indicat• ungulates, but few experimental studies of scent ed. The pen was partially divided by a steel- marking exist for cervids, and none for rubbing paneled fence. by caribou. This lack of an experimental approach has lead to controversy over which other males for the entire 19-day duration of the characteristics of trees are most important in study. Four posts, which were offered in each determining rubbing behavior among the treatment for scent marking by caribou, were Cervidae (Benner & Bowyer, 1988; Oehler et al, anchored solidly into four buried steel pipes 1995). within the enclosure. The pipes were spaced 6.8 We conducted an experiment using a captive m apart, were 12 m from the corner of the pen, herd of caribou to test hypotheses about scent and were arranged to offer an unobstructed view marking (rubbing) by adult males during rut. We from the 3-5-m high observation tower located tested the following null hypotheses to gain immediately NE of the study pen (Fig. 1). Posts insights into the potential function of scent mark• that were placed in pipes were approximately 2 ing in this arctic ungulate: 1) dominance status m tall when measured from the ground, and has no effect on rubbing behavior; 2) males do were changed for each scent-marking trial; each not select particular size-classes of trees to rub; post was used only once. A circle with a radius 3) texture of trees do not affect selection by of 1 m was painted on the ground around each males; and 4) an aromatic scent has no effect on post to help quantify behaviors associated with which posts are rubbed. scent marking. On 21 September 1998, following velvet shed• ding and onset of rut, four adult male caribou Materials and methods were assigned randomly into two groups of two We conducted research at the Large Animal individuals each. The research was partitioned Research Station, University of Alaska Fairbanks, into 12 trials: two groups of males in two treat• Fairbanks, Alaska, USA (64°52'N, 147*51'W) ments with three replicates. For the first treat• where captive caribou, originating from the ment, four birch trees (Betula papyrifera) without Porcupine herd in northeastern Alaska, are main• branches and with diameters 2.5, 5, 7.5, and 10 tained for research purposes. The study pen was cm, respectively, as measured at the end not 2385 m2 (Fig. 1) and was constructed of solid placed in the ground, were located in pipes ran• steel and plywood panels 1.8 m in height. Test domly. Those posts were available to a group of animals were isolated visually from females (80 two caribou for 24 h. During that period, animals m away) with solid fence panels and from the were observed for two, 3-h periods, from 22 Rangifer, 21 (1), 2001 approximately 1500 to 1800 h Alaska Standard sented together (i.e., the two spruce trees and Time, when the animals were first introduced two pieces of lumber were next to each other), into the pen, and again the next morning from but their position in the pipes was arranged ran• 0700 to 1000 h. All observations were made and domly. A second set of poles with the same char• recorded by C. A. Adams, who viewed caribou acteristics was erected for the next group of ani• from the observation tower, using all-occurrences mals. Posts were cut on the day of the trial. One sampling (Altmann, 1974). Dominance status complete replicate took 4 days. Each replicate within each group was established via behavioral was repeated two more times for a total of 12 observations prior to and during the experiment; days of observations. antler threats were used most often to establish During trials, animals had access to water but and maintain dominance relations between pairs there was no feeder, which potentially could bias of males. Once established, dominance status of the amount of time spent in one part of the males was consistent throughout the duration of enclosure. Male cervids reduce food intake dur• our study, as noted for other caribou by Barrette ing rut (Bowyer, 1981; Miquelle, 1990; Suttie et & Vandal (1986). al, 1992), and absence of food for 24 h was Activities occurring within the 1-m circle deemed humane. All aspects of this research around each post were divided into seven cate• were approved by an Institutional Animal Care gories. We defined an approach without intent as and Use Committee at the University of Alaska occurring when an animal came within 1 m of a Fairbanks, and were in keeping with guidelines post but did not stop or stare at the post. An established by the American Society of approach with intent was similar except that the Mammalogists for experimental research on animal faced the post for >ls and also may have mammals (Animal Care and Use Committee, slowed or stopped within the 1-m circle. 1998). Approaches with intent were further subdivided Data were analyzed with Mests and %2 analy• into flehmens (Estes, 1973), sniffs, nibbles, and ses (Zar, 1996). The two-sample 2-test for pro• licks if associated with those other behaviors. portions (Remington & Schork, 1970) was used Rubbing (scent posting) occurred when an ani• to compare behaviors that were sampled with mal performed some combination of those replacement (i.e., repeated behaviors by the behaviors in addition to scraping or thrashing the same animal or dominance class of animals); post with the antlers and rubbing a preorbital thus, these samples were not pseudoreplicates. gland or its forehead on the post or tree. Such For other analyses, however, our domain of events have been described elsewhere as «bush inference is our experimental area. During trials thrashing" (Bergerud, 1974), «head rubbing», and caribou broke some posts; consequently, expect• ••antler thrashing" (Pruitt, 1966; Muller-Schwartze ed values used in the %2 analyses were adjusted et al, 1979).