Wallowing Behavior of American Bison (Bos bison) in Tallgrass Prairie: an Examination of Alternate Explanations.
|Article Type:||Statistical Data Included|
McMILLAN, BROCK R.
COTTAM, MICHAEL R.
KAUFMAN, DONALD W.
|Publication:||Name: The American Midland Naturalist Publisher: University of Notre Dame, Department of Biological Sciences Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Earth sciences Copyright: COPYRIGHT 2000 University of Notre Dame, Department of Biological Sciences ISSN: 0003-0031|
|Issue:||Date: July, 2000 Source Volume: 144 Source Issue: 1|
|Geographic:||Geographic Scope: United States Geographic Name: United States Geographic Code: 1USA United States|
BROCK R. McMILLAN 
Division of Biology, Kansas State University, Manhattan 66506
ABSTRACT.--Wallowing is a common behavior of American bison (Bos bison). Past explanations and current hypotheses suggested for wallowing behavior include grooming behavior associated with shedding, male-male interaction (typically rutting behavior), social behavior for group cohesion, play behavior, relief from skin irritation due to biting insects, reduction of ectoparasite (tick and lice) load and thermoregulation. We monitored circannual and circadian patterns of wallowing frequency by American bison during 1996-1997 in the tallgrass prairie region of eastern Kansas. Wallowing activity increased from April to late June or July (during 1996 and 1997, respectively), decreased during midsummer, peaked again in September, decreased from September to October and then remained low from November to March. Diurnally, wallowing was low in early morning, increased to a peak in early afternoon and then decreased during mid afternoon and evening. Within the herd adult males wallowed more frequently than adult female s and both adult males and females wallowed more frequently than yearlings. We observed behaviors that were consistent with all of the hypotheses previously suggested to explain wallowing behavior by bison. Based on our observations we suggest that the alternate explanations for wallowing behavior are not mutually exclusive. However, only the relief from biting insects hypotheses was consistent with both the circannual and circadian patterns of frequency of wallowing by American bison.
Wallowing is a common behavior of many ungulates, but its function is not consistent among species. For example, wallowing by the domestic pig (Sus scrofa) and great Indian rhinoceros (Rhinoceros unicornis) is a thermoregulatory behavior (Pal and Bhattacharyya, 1986; Vestergaard and Bjerg, 1996). Wallowing by wapiti (Cervus elaphus) and male European bison (Bison bonasus) during the rut is a male-male conflict social behavior (Struhsaker, 1967; Geist, 1982; Cabon-Raczynska et al., 1987). Wallowing by male European bison at other times of the year and by females is a grooming behavior (Cabon-Raczynska et al., 1987) and wallowing by the tamaraw (Bubalus mindorensis) may be for relief from biting insects (Momongan and Walde, 1988). In contrast to these species for which primarily one, explanation has been suggested for wallowing behavior, several explanations have been proposed and argued for wallowing behavior of American bison (Bos bison).
Wallowing by American bison consists of several different behaviors. Most commonly, bison lie down and roll repeatedly onto their sides (McHugh, 1958; Lott, 1974; Reinhardt, 1985). Additional behaviors associated with wallowing that we observed were repeated forward lunging or neck and belly rubbing and face rubbing while lying upright in wallows. Bison also just laid without any movement. However, these alternative behaviors were nearly always initiated with the side roll (McHugh, 1958; B. McMillan, pers. obs.). Most recently, it has been argued that wallowing by American bison is a grooming behavior that reduces tick load (Mooring and Samuel, 1998). However, Mooring and Samuel (1998) conducted behavioral observations during October-April and likely missed the majority of wallowing activity, which occurs during summer months. Wallowing by bison has been attributed to relief from skin irritation during shedding or social behavior associated with group cohesion (Reinhardt, 1985). Although a quantitative exami nation, Reinhardt (1985) studied a bison herd (13-26 individuals) with only one adult bull in a 40 ha enclosure; and therefore, behavior likely did not reflect that of free-ranging animals in herds with more natural sex ratios and age structures. Wallowing by bison has been described as a behavior for relief from biting insects (Soper 1941; Lott, 1974). Alternatively, both Lott (1974) and McHugh (1958; 1972) argued that male bison wallowed much more than female bison and the behavior was associated with male-male conflict and tension during the rut. McHugh (1958) also suggested that wallowing was a play behavior of young animals. However, most of these reports are from anecdotal observations with no rigorous quantitative examination.
From past reports and current observations there are at least seven possible explanations or hypotheses, which may not be mutually exclusive, for wallowing behavior. Explanations for wallowing behavior are: (1) a nonsocial behavior that provides relief from skin irritation during shedding, (2) a nonsocial behavior that provides relief from skin irritation caused by biting insects, (3) a nonsocial behavior for defense from ectoparasites (ticks and lice), (4) a social behavior associated with group cohesion, (5) a social behavior associated with male conflict and the rut, (6) a play behavior and/or (7) a thermoregulatory behavior. We predicted that circannual and circadian patterns of relative frequency of wallowing by bison would help elucidate which of the alternate hypotheses best explains wallowing behavior. For example, if wallowing is in response to skin irritation from shedding, wallowing frequency should be greatest at our study site in Kansas in late spring at the peak of shedding (B. McMillan, pers. obs.), be relatively constant throughout the day and similar in frequency in males and females. If wallowing is in response to skin irritation from biting insects, wallowing frequency would peak both in early summer (July) and late summer (September) with peaks in abundance of biting insects (Mailen, 1940; Kunz and Cunningham, 1977; Davis and Sanders, 1981; A. Broce, pers. comm.). In addition, daily wallowing would peak in midafternoon when ambient temperature and insect activity both peak, except during July-August when the activity of biting insects exhibits a bimodal distribution with decreased activity during the hottest time of the day. Finally, if irritation from biting insects is the cause, males would wallow more than females because biting insects prefer male ungulates over females (Dobson et al., 1970; Christensen and Dobson, 1979; A. Broce, pers. comm.). If wallowing is to reduce ectoparasite load, wallowing activity should peak at the time of greatest infestation rates or in early summer (May or J une) if in response to the American dog tick (Dermacentor variabilis) or the lone star tick (Amblyomma americanum) and peak in early and late winter if in response to the winter tick (D. albipictus) or lice, respectively (D. Mock, pers. comm.). If wallowing is a social behavior for group cohesion, frequency of wallowing would remain relatively constant throughout the year and throughout the day. If wallowing is a social behavior due to male-male conflict associated with the rut, frequency would peak in midsummer with the rut, remain relatively constant throughout the day and only males would participate. If wallowing is a play behavior, frequency of wallowing should be greater for juveniles and subadults than for adults (McHugh, 1958) and frequency would be greater at dusk, when play behavior is most common (McHugh, 1958), than at other times of the day. If wallowing is for thermoregulation, wallowing would peak with high summer temperatures and high daily temperatures and be more common when the soil in wall ows is relatively wet.
In this study we test for differences in frequency of wallowing in different months and seasons of the year, at different times during the day, between males and females and between adults and yearlings. We use the patterns observed in frequency of wallowing to test the hypotheses posited to explain wallowing behavior by American bison.
Study site.--We studied wallowing behavior of American bison on the Konza Prairie Research Natural Area in northeastern Kansas. Konza Prairie is a 3500-ha tallgrass prairie research area owned by The Nature Conservancy and managed for research by the Division of Biology at Kansas State University. A herd of approximately 200 bison is maintained in a 1000-ha fenced area on the site (McMillan et al., 1997). Bison were present on the study area throughout the year without supplemental food. In addition, culling maintained relatively natural herd structure during our study. For example, the sex ratio was approximately 0.7:1 (male:female) with strong representation of all age classes except old ([greater than or equal to]10 y) males, which have been removed from the herd due to their unpredictable nature.
Annual wallowing frequency.--We monitored wallow use by bison from April 1996 to November 1997. Frequency of wallow use was measured during a 4- to 6-d sampling period every other week from April to September of both years. During other months of the years (October-March), frequency of wallowing was examined during a 4- to 6-d sampling period approximately every other month.
During each sampling period we placed a small wooden marker (11 cm tall, 1 cm wide) in an upright position in the center of all wallows that could be seen from 14 km of roads throughout the site. Wallows that had standing water were not used because bison tended to avoid them, and the markers would have been under water. Subsequently, wallows were checked every other day for either 4 or 6 consecutive d. We assumed that at least one wallowing event had occurred if the wooden marker was either missing or broken, unless there was obvious evidence otherwise (e.g., trampled). This measure of wallowing frequency was likely conservative because more than one bison could have, and likely did on many occasions, wallowed within a single wallow when a marker was broken during
the 2-d sampling period. Wallows with broken or missing markers were recorded and the markers were replaced. Relative frequency of wallowing activity for each 2-d period was calculated by dividing the number of wallows that were used by the total number of wallows initially marked. We used analysis of variance (PROC GLM; SAS Institute Inc. 1990) to test for significant differences in use of wallows among sampling periods. We used regression analysis to test for a relationship between frequency of wallowing and ambient temperature. All proportional data were arcsine-square root transformed before statistical analysis.
Daily wallowing frequency.--During June-July 1997 we investigated wallowing behavior of individual bison to assess differences between the sexes and different age categories (calf, yearling and adult). It is possible that the relative frequency of wallowing among age and sex categories of bison varies throughout the year. However, we assumed that wallowing activity of bison during June-July was representative of the entire year for this part of the study. We divided the day into seven 2-h intervals beginning at 600 h and ending at 2000 h. All intervals could not be sampled each day. Therefore, we selected the time intervals in a stratified random incomplete-block design so that throughout the study period we had similar sample sizes for each 2-h interval.
We sampled wallowing activity during each interval by driving the access roads of the site until bison were located. Once located, we recorded sex and age of each individual exhibiting wallowing behavior. In addition, we recorded the total number of individuals in each sex/age category (male or female and calf, yearling or adult) in the group. We included wallowing events that were initiated while the data were being recorded (approximately 3- 5 mm of observation). Then we drove until another group of animals was located and followed the same protocol. We continued with this method for the entire 2-h interval sampled.
We corrected for differences in sample size among sex/age categories within a 2-h interval by calculating a wallowing index (WI). The WI we used was equal to the number of wallowing events observed in a particular sex/age category divided by the total number of animals in the group from the same sex/age category. For example, if 3 adult females and 1 calf were wallowing when we located a group of bison composed of 10 adult females, 2 adult males, 2 yearling males and 5 calves, then the wallowing index for adult females would be 3 (number of adult females wallowing)/10 (number of adult females in the group) or 0.3. Therefore, we had a wallowing index value for each sex/age category observed during each sampling period. Because we were not confident that we located all of the calves present in the larger groups observed, we included only yearlings and adults in our analyses. We used analysis of variance (PROC GLM; SAS Institute Inc. 1990) followed by LSD (least squared difference) to test for significant ([alp ha] [less than or equal to] 0.05) differences in wallowing frequency between sexes and ages and among time periods. In addition, we used regression analysis to obtain a model that best fit the daily pattern in wallowing frequency that we observed. All proportional data were arcsine (square root) transformed before use in statistical analyses.
Wallow temperature.--To determine if thermoregulation was a plausible hypothesis to explain wallowing behavior, we measured soil temperature in wallows. We took measurements of ambient air temperature 1.5 m above the soil surface, temperature at the soil surface and soil temperature at depths of 1, 3 and 5 cm in wallows and in adjacent undisturbed prairie. In addition, we shaded areas in the wallows by either blocking the sun with a piece of cardboard or by one of us laying down on the soil surface and then monitored temperature and rate of temperature change when wallows were not exposed to direct sunlight. All temperature measurements were taken during early afternoon on full sunlight days in July 1997. We chose this time of the day because surface temperature of the soil is greatest. Measurements at all 5 depths were taken simultaneously in each treatment (wallow and undisturbed prairie) by using thermocouples connected to StowAway data loggers. Data loggers were downloaded directly to the computer for st atistical analysis.
Annual wallowing frequency.--Proportion of wallows used by bison averaged (0.31 [+ or -] 0.07 (x [+ or -] SE) over all sample periods and ranged from 0.03 [+ or -] 0.07 in April 1996 to 0.59 [+ or -] 0.07 in September 1996. Relative frequency of wallowing by bison varied significantly among sampling periods ([F.sub.21,25] = 4.40, P [less than] 0.001) and peaked twice, during July and September, in both years (Fig. 1). In addition, frequency of wallowing activity was positively related to average maximum ambient air temperature during each sampling period ([R.sup.2] = 0.80, P [less than] 0.001).
Daily wallowing frequency.--Although wallowing occurred throughout the day, activity differed significantly among time intervals ([F.sub.6, 385] = 3.48, P [less than] 0.001). Bison wallowed more during the afternoon than during the morning and evening intervals and the regression model that best fit the data was a polynomial (Fig. 2) Both males and females and adults and yearlings exhibited wallowing behavior throughout our June-July observation period. However, males wallowed 1.7 times more frequently than females ([F.sub.1,385] = 4.15, P = 0.04; Fig. 3) and adults wallowed nearly 4 times more frequently than yearlings ([F.sub.1,385] = 37.9, P [less than] 0.001 Fig. 3).
Wallow temperature.--Soil temperature within wallows (mean of all depths was 36.5 [+ or -] 0.32 C) was significantly warmer than that in adjacent areas (35.4 [+ or -] 0.32 C) not denuded of vegetation ([F.sub.1,181] = 4.8, P = 0.01). Within wallows, temperatures varied significantly with depth from surface ([F.sub.4,181] = 10.1, P [less than] 0.001). Temperature at the soil surface was the warmest (40.2 C), but temperature rapidly decreased with depth and, at 3 cm, soil temperature (35.6 C) and ambient air temperature (35.0 C) were similar. In addition, when one of us laid in the wallow, temperature at the soil surface and at 1-cm depth dropped so that no difference could be detected at any depth in less than 30 sec.
American bison wallow throughout the year, but frequency of wallowing behavior peaked during late-June or early-July and September of each summer. These observations were not distributed as expected based on our prediction that wallowing was in response to skin irritation associated with shedding. Most shedding on Konza Prairie occurred during April and May and was completed by mid-June (B. McMillan, pers. obs.). It has been suggested that bison both wallow and rub on trees to relieve irritation associated with shedding (Reinhardt, 1985; Coppedge and Shaw, 1997). We observed wallowing by bison during the time of shedding, perhaps to relieve irritation associated with shedding. However, because frequency of wallowing peaked well after all shedding was completed, shedding was probably not the primary factor influencing the pattern of wallowing frequency that was observed.
Based on observations of rutting activity (B. McMillan, pers. obs.) and the time that most calves were born on Konza Prairie (Towne, 1999; G. Towne, pers. comm.), rutting activity in our bison herd peaked in early to mid-August. Wallowing was suggested to be a social behavior of male bison primarily and associated with male-male conflict during the rut (McHugh 1958, 1972; Lott, 1974). Wallowing was apparently associated with male-male conflict during the rut (Coblentz, 1976; B. McMillan, pers. obs.). Consistent with earlier reports (McHugh, 1958), these were, in fact, some of the most noticeable wallowing events because they were typically accompanied by aggression and fighting. In addition, we found that males wallowed significantly more than females. However, wallowing by females was common and occurred throughout the study. In addition, wallowing peaked both before and after the rut during both years of the study. Therefore, it is unlikely that wallowing is primarily a social behavior associated with male -male conflict during the rut.
Young bison 'play' more often than adults and wallowing frequently occurs with play behavior (McHugh, 1958). On several occasions we observed pairs of young bison that exhibited play behavior when they chased each other. During a few of these play events one of the individuals began to wallow. However, wallowing by young bison was relatively uncommon during our study. In addition, wallowing frequency at dusk, the peak time of play behavior (McHugh, 1958), was significantly less than the frequency of wallowing earlier in the day. We conclude that play behavior was not the primary cause of the circannual or circadian patterns in frequency of wallowing.
Our observations indicated that some wallowing was a social behavior possibly associated with group cohesion (Reinhardt, 1985). For example, on one occasion five adult females came, single file, to a wallow and one at a time excreted small amounts of urine into the wallow and then briefly rubbed their neck on the soil surface. However, these 'social' behaviors were infrequently observed. Social behaviors may account for some low level of wallowing throughout the year, but it is unlikely that either of the circannual or circadian patterns in frequency of wallowing resulted primarily from social interactions.
Frequency of wallowing peaked during early afternoon and during summer, which was generally consistent with the hypothesis that wallowing is a thermoregulatory behavior. However, wallowing activity decreased during the hottest month (August) of the year and did not occur in wallows with standing water. This difference in temporal patterns of wallowing and temperature argues against thermoregulation as the primary cause of wallowing. Bison have very little hair on their bellies during the summer and they did lay in wallows for extended periods of time during the heat of the day (B. McMillan, pers. obs.). If wallowing is for thermoregulation during summer, then soil temperature of wallows needs to be cooler than the body temperature of bison. Surface temperatures of soils in wallows that were exposed to direct sunlight during the heat of the day were considerably warmer (40 C) than both the body temperature of bison (37 C) and the surface temperatures of soils that were not denuded of vegetation (36 C). Howeve r, soil temperature in wallows was at or below body temperature in less than 30 s after an animal laid down in the wallow. Therefore, it is likely that conductive heat transfer occurred from the belly of bison to the soil that would not have occurred if the bison laid on grass-covered (insulated) soil. However, it is unclear whether a few degree difference between body temperature and soil temperature results in conductive heat transfer substantial enough to cool a large animal.
Mooring and Samuel (1998) suggested that wallowing may be a grooming behavior to reduce infestation of winter ticks because wallowing frequency was greater in October than during November-March at their study site in Canada. We found a similar pattern in wallowing frequency during October-March in Kansas. However, wallowing during this time was much lower than during the other months (May-September) of the year. In addition to winter ticks, the American dog tick and the lone star tick are the other common species in eastern Kansas (D. Mock, pers. comm.). Wallowing should be greatest during April-June if it is to reduce infestation by these other species. Likewise, if wallowing is to reduce infestation by lice, frequency should be greatest during the late winter (D. Mock, pers. comm.). Wallowing occurred during the time of greatest tick and lice infestation, but only at low frequency. Although the patterns of wallowing frequency that we observed were similar to the pattern observed by Samuel and Mooring (1998 ), our results were not consistent with the idea that wallowing is a grooming behavior to reduce infestation by ticks or lice.
From our observations, it is likely that wallowing (especially, side rolling) was related primarily to relief from biting insects (Soper, 1941; Lott, 1974). Abundance of biting flies generally peaks twice each summer; typically once early summer (late June or July) and again in late summer (Mailen, 1940; Kunz and Cunningham, 1977; Davis and Sanders, 1981; A. Broce, pers. comm.). Likewise, these flies are also most active and spend more time on the thinner skin of the sides and belly during early afternoon in early and late summer (Morgan, 1964; A. Broce, pers. comm.). During the summer months we commonly saw hundreds of biting horn flies (Hamematobia irritans) on each bison. Patterns of circannual and circadian frequency of wallowing, therefore, were consistent with the annual and daily cycles of abundance of biting insects. Likewise, the pattern of males wallowing more than females was consistent with the preference for males shown by biting insects (Dobson et al., 1970; Christensen and Dobson, 1979). Yearl ings unexpectedly wallowed less than adults. A possible determinant of the large difference in frequency of wallowing between adults and yearlings is that adults typically have very little and very short hair on their backs and sides behind their front shoulders and on their rump during the summer months (typically through September), whereas yearlings typically have relatively thick hair over their entire body (B. McMillan, pers. obs.). The relief-from-biting-insect hypothesis could be easily tested, experimentally, by applying a topical insecticide to populations of animals and then comparing frequency of wallowing between treatment and control populations. However, relatively isolated populations are needed for this experiment because treatment of a subgroup of individuals within a population effectively controls all insects in the population (Harvey and Brethour, 1980; Haufe, 1986). Therefore, we could not do this experiment with the single population of bison on Konza Prairie.
It was apparent from our observations that wallowing consisted of a suite of behaviors and that several of the explanations suggested may be correct to varying degrees (i.e., the alternate explanations are not mutually exclusive). We observed wallowing behaviors that were consistent with each of the hypotheses. Further, it is possible that wallowing at different times of the year may be due to different causes. However, only the relief-from-biting-insects hypothesis was consistent with all the predictions we made with respect to the circannual and circadian patterns in frequency of wallowing that we observed. Therefore, we suggest that relief from biting insects provided by wallowing is the primary cause of the April-October increase, bimodal peaks and decrease in wallowing frequency that we observed. Although our observations on the temporal pattern of wallowing frequency provide supporting evidence for the relief-from-biting-insects hypothesis and against several of the other hypothesis, our observations d o not rule out the influence of other factors on wallowing by American bison. For example, wallowing occurred at a low level throughout the year. This low level of wallowing may have been for group cohesion, grooming, comfort or some other factor we have not considered. Experimental manipulations in which biting insects are reduced to low levels are needed to assess whether wallowing is reduced when insect irritation is reduced. If so, then relief from biting insects would be implicated as the primary factor driving the circannual and circadian patterns of wallowing frequency that we ob-served for American bison.
Acknowledgments.--We thank J. Blair, J. Boyer, D. Hartnett, E. Finck, A. Knapp, M. Mooring and D. Van Vuren for comments on earlier versions of this manuscript. A. Broce and D. Mock from the Entomology Department at Kansas State University provided valuable information on the life history of biting flies, ticks and lice that parasitize bison. Our study was conducted at the Konza Prairie Research Natural Area, a preserve owned by The Nature Conservancy and operated by the Division of Biology, Kansas State University. Partial support for our project was provided by National Science Foundation (NSF) grant BSR-90-11662. M. Cottam received support for his participation in our project through the NSF funded (Grant DBI-9531310) Research Experience for Undergraduates (REU) program. This is contribution number 00-152-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan.
(1.) Present address: Department of Biological Sciences, Minnesota State University Mankato, Mankato, Minnesota 56001
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