Factors affecting biting fly harassment of feral horses (Equus caballus) on a barrier island Author names and Institutional affiliation have been deleted Feral horses in harem bands on Assateague Island National Seashore, Maryland were observed in June and August 2000 to determine what behavioral and ecological factors affect the intensity of biting fly harassment. Fly counts and frequencies of comfort movements were recorded during focal animal samples, as well as data on sex, group size, habitat type, temperature, humidity, wind speed, and behavior. The number of biting flies on the horse was affected by horse sex, habitat, temperature, and group size. The number of comfort movements a horse made could be predicted by habitat, temperature, wind speed, group size, number of horses within one body length, and individual identification. Males had more flies on them than females. Fly numbers and comfort movements were highest in scrub habitat followed by dunes and then marshes. Fly numbers and comfort movements increased with increasing temperature and decreased as group size increased. Comfort movements decreased as both wind speed and the number of horses within one body length increased. The number of comfort movements made by a horse was found to be a reliable indicator of fly numbers. Thus the intensity of biting fly harassment is dependent upon a number of intrinsic and extrinsic physical, social, and ecological factors. Introduction Hamilton’s (1971) ‘selfish herd’ model of how spatial relationships among individuals can reduce predation may also apply to animals subjected to ectoparasitism by biting flies (Mooring and Hart, 1992). Several studies have found that animals tend to group together when biting fly density or harassment is high (Bergerund, 1974; Schmidtmann and Valla, 1982; Rutberg, 1987; Rubenstein and Hohmann, 1989). Encounter-dilution effects (Mooring and Hart, 1992) assume that in order for an increase in the number of potential hosts (group size) to dilute the risk of ectoparasitism on any one of them, the increase in number of hosts cannot result in an equal or larger increase in parasite numbers. Three studies of feral horse populations have found that per capita fly numbers decreased with increasing social group size (Duncan and Vigne, 1979; Rutberg, 1987; Rubenstein and Hohmann, 1989). In addition to aggregating into groups, animals may take other measures to lessen the degree of biting fly harassment. These measures include body movements designed to remove or flush flies from the body (Hughes et al., 1981), lying down to reduce exposed surface area (Espmark and Langvatn, 1979), increasing physical activity (Breyev, 1964, Khumunen, 1968, and Thomson, 1971, 1973 cited in Espmark and Langvtan, 1979), and moving to different habitat types (Duncan and Cowtan, 1980). Biting fly intensity can have serious consequences. A study in New York found that a dairy cow may receive as many as 4,000 fly bites per day, resulting in a loss of as much as 0.5 L of blood. Cases of horses being fatally wounded by severe insect harassment have also been reported (Webb & Wells, 1924, cited in Keiper & Berger, 1982). Reindeer will sometimes refuse to stand still to nurse their young when fly harassment is intense (Breyev, 1964, Kuhmunen, 1968, and Thomson, 1971, 1973 cited in Espmark & Langvatn, 1979). On Assateague Island, horses that remain on the southern portion of the island during the summer, where flies are more numerous, have spend significantly less time foraging during the day (Powell, 2000). Warble flies cause infections resulting in an estimated 20-70 kg per year growth deficit in growing calves (Gunderson, 1945; Campbell et al., 1973). Biting flies also transmit a number of diseases including equine infectious anemia, equine encephalitis, anthrax, swamp fever, and a variety of trypanosome diseases (Askew, 1971; Chvala et al., 1972). In this study we sought to determine what behavioral and ecological factors predict biting fly intensity. We hypothesized that per capita biting fly intensity would be lower in larger groups of horses and on horses standing in close proximity to one another. Biting fly intensity was hypothesized to be higher when temperature and humidity were high and wind speed was low (Tashiro & Schwardt, 1949; Rockel & Hansens, 1970a). We also expected that biting fly harassment would be more intense on stationary as opposed to moving animals. Finally, we expected that biting fly intensity would be highest in marshes, followed by scrub habitats, and finally beaches and dunes. Our study was conducted on Assateague Island, a 56 km long barrier island running along the coasts of Maryland and Virginia, U.S.A. Assateague supports a feral horse population of about 170 individuals. There are five common species of tabanid flies on Assateague: green-headed horse flies (Tabanus nigrovittatus and T. lineloa), deer flies (Chrysops fuliginosus and C. atlanticus) and a stable fly, Stomoxys calcitrans (Rockel & Hansens, 1970b). Females of these species lay eggs in wet areas, and the larvae develop in the soil. On Assateague, this primarily takes place in the marshes where the adults emerge and reach maximum numbers in late July through mid-August (Morgan & Lee, 1977). Female flies seek hosts and engorge themselves with the host’s blood in order to produce eggs (Hughes et al., 1981). Methods We observed feral ponies at Assateague Island National Seashore on a total of 15 days from 8-16 June and 14-28 August, 2000. We conducted the study on an approximately 18 kilometer portion of the island that had been developed for both a national and state park. This portion of the island contained beach/dune, bayberryscrub/pine forest, and salt marsh habitats. A more complete description of the flora and fauna can be found in Higgins et al. (1971). The Maryland portion of the island supports a population of approximately 170 feral horses, approximately 140 of which inhabited the study area during the sampling period. Our study did not concentrate on specific horses, but rather we sampled all possible individuals in the study area; we made an effort to sample any horse only once per day. If a horse was sampled multiple times in a day, the samples were taken at least 3 hours apart. Horses were identified using drawings provided by the National Parks Service. In most cases, groups of horses could be approached to within 5-10m; otherwise, binoculars were used to conduct observations. Focal sampling of horses within a group was done in random order. The horse’s identification, color pattern (bay, sorrel, or pinto), habitat type, behavior, group size, number of horses within one body length of the focal, and any important weather data (e.g. wind gusts or rain) were recorded. On each side of the horse, three counts of flies of any kind on the horse were taken at 20-second intervals during a one-minute sample. Mosquitoes were not counted because of the difficulty seeing them. During the next minute, all comfort movements (snorts, stomps, head sways, bites, muscle twitches) intended to dislodge flies were counted and totaled. During the third minute, three additional fly counts were taken as described previously. This procedure was repeated for the other side of the horse. Data from both sides of the animal were averaged to produce one fly count and one value for the number of comfort movements per minute per horse sampled. All samples were collected between 1030 and 1930 hours. Weather data were obtained for sampling periods from the National Parks Service weather station on Assateague. Temperature, humidity, and wind speed data were recorded every hour by the station. Proc Mixed (SAS Stat module, SAS Stat User’s Guide Version 8, SAS Institute Inc, Cary N.C.) was used to perform a repeated-measures general linear model predicting mean number of comfort movements and the square root-transformed mean number of flies on the horse during the sample based on the following. Fixed effects included wind speed, temperature, humidity, group size, number of horses within one body length of the focal horse, habitat type (marsh, scrub, or dunes), behavior (play, graze, or stand), color (pinto, sorrel, or bay), and sex. Horse identification was included as a repeated measure. The covariance structure did not display any particular or unique type, although compound symmetry was considered and tested. The data did not display compound symmetry; it was not perfectly balanced – typical of data collected without strict control of experimental units and/or treatments. Small sample size (N=93) in concert with a relatively large number of experimental subjects (ID, N=55 individual horses) prevented horse ID from qualifying as a predictor. The full model (all variables above included) was run using maximum likelihood estimation on components of variance. Non-significant variables were then excluded and the remainder put through the same mixed model using REML (restricted maximum likelihood estimation). No improvement was noted in this secondary model, whether or not the non-significant variables were omitted from the analysis. The Z-test for the hypothesis that ID=0 was negated in all cases (p < 0.001). This may imply a non-zero effect of ID on the significance of the fixed-effect parameters. Tukey tests were performed when habitat type, behavior, and sex were found to have significant effects. Tukey tests were not performed when group size and number of horses within one body length of the focal were found to be significant effects because the number of tests was too large relative to the number of degrees of freedom. Means were inspected visually to identify these effects. Means are presented ± SE. Comfort movements as indicators of flies To determine whether comfort movements could be used as reliable indicators of fly numbers, we conducted a simple linear regression analysis of comfort movements using fly numbers as the independent variable. We also included fly numbers in the mixed model with the other factors that were found to have significant effects on comfort movements. Results In the majority of the samples (82%, n=93) the focal animal was grazing. The focal animal was standing (17%) or playing (1%) in the rest of the samples. Since only one sample included an animal that was playing, only the effects of standing versus grazing were determined in the behavior analysis. The horses were in the marsh in 37.5% of the samples, the scrub/pine forest in 38.5% of the samples, and in the dunes/beach area in 24% of the samples. Relative humidity averaged 78.6% (range: 57-100%); the average temperature was 23.9°C (range: 17.2-29.4°C). Wind speed averaged 15.4 km/hour (6.4-22.5 km/hour). Fly counts The mixed model ANOVA found significant effects of sex (F1,53=11.30, p=0.001), habitat type (F2,13=8.79, p=0.004), behavior (F1,12=5.96, p=0.031), temperature (F1,33=8.54, p=0.006), and group size (F1,33=15.24, p=<0.001) on mean fly number. There was also a significant effect of the repeated measure Horse I.D. (z=6.56, p<0.001). Male horses had more flies on them than female horses (males: 3.880.48, females: 1.940.25). Grazing horses (3.300.36) had more flies on them than standing horses (1.760.49). Fly numbers were highest in the scrub habitat followed by the dunes and marshes (Figure 1). The difference in fly counts between marsh and scrub habitats was significant (Tukey test t=-3.38, 13 df, p=0.002). There was no significant difference in fly numbers between marsh and dune habitats (Tukey test t=-0.70, 13 df, p=0.498). The difference in fly numbers between scrub and dune habitats approached significance (Tukey test t=1.84, 13 df, p=0.088). Fly numbers increased with increasing temperature (Figure 2) and decreased with increasing group size (Figure 3). Horse color was not a significant predictor of fly numbers (p>0.05). Comfort movements The mixed model ANOVA found significant effects of habitat type (F 2,13=5.05, p=0.024), temperature (F1,32=15.80, p<0.001), wind speed (F1,32=14.01, p=<0.001), and number of horses within one body length of the focal (F1,32=4.69, p=0.038) on mean number of comfort movements. The effect of group size on comfort movements approached significance as well (F1,32=2.95, p=0.095). Horse I.D. also had a significant effect (z=6.63, p<0.001). As with fly numbers, comfort movements were highest in scrub habitat, followed by marsh and dunes (Figure 1); however, only difference in comfort movements between scrub and dune habitats was significant (Tukey test t=2.68, 13 df, p=0.019). Comfort movements increased with increasing temperature and decreased with increasing wind speed (Figure 4). Comfort movements decreased with increasing numbers of horses within one body length of the focal animal (Figure 5) as well as with group size. Horse color was not a significant predictor of comfort movements (p>0.05). Comfort movements as indicators of flies In the linear regression, fly counts were a very significant predictor of comfort movements (F1,94=25.60, p<0.001, =9.02, adjusted R2=0.206). Fly count also had a significant effect on comfort movements when included in the mixed model with the significant factors listed above (F1,31=4.34, p=0.046). Discussion Male horses had more flies on them than females. Similar results were found in a previous study of the Assateague horses (Rutberg, 1987) and in a study of the horses on Shackleford Banks, North Carolina (Rubenstein & Hohmann, 1989). In general, metabolic rates of males are higher than those of females (Altmann & Dittmer, 1968), and metabolic rate may be linked to carbon dioxide (CO2) emission (Smythe & Goody, 1972). Biting flies are drawn to baits with a carbon dioxide source (Wilson et al., 1966; Knox & Hays, 1972); therefore, more CO2 production by males may lure more flies to them. We found support for our hypothesis that per capita fly harassment would decrease with increasing group size and increasing numbers of close (i.e. within one body length) neighbors. Similar results have been found in other horse studies (Duncan & Vigne, 1979; Rutberg, 1987; Rubenstein & Hohmann, 1989). Several studies have shown that ungulates will clump together when biting fly harassment is severe (Bergerund, 1974; Schmidtmann & Valla, 1982; Rubenstein & Hohmann, 1989). Since this study was conducted during the summer when biting flies are generally most numerous, we are unable to demonstrate that the horses in our sample reduced their inter-personal distances in response to flies (but see Rutberg, 1987), but we did observe that the horses would form tightly-clumped groups, often standing with their flanks touching one another. Very commonly groups of horses would stand side by side in opposite orientations, such that one horse’s face was flanked by a neighbor’s tail. Thus the tail movements of one horse would also help to flush flies off the face of its neighbor. Movement in these groups was fluid, with animals continuously trying to occupy spots in the middle, where both of their sides would be protected. The horses would take shelter on the dunes and beaches (where winds and breezes were strongest), even standing in the surf, or they would migrate long distances to narrower portions of the island (where perhaps oceanic and bay breezes would sweep the entire landscape) to escape flies. Habitat shifts and migrations in response to biting insects like this have also been documented in the Camargue horses (Duncan & Cowtan, 1980). Fly numbers and comfort movements increased with increasing temperature as predicted, but we found no significant effect of humidity on either measure. Fly numbers and comfort movements actually show a slight decrease with increasing humidity (data not shown); however, this effect may be due in part to the effect of rain. Samples were not collected during heavy rain, but some were collected during light rain. It is impossible to distinguish rainy periods from very high humidity periods in the meteorological data from the weather station, so some of the high humidity readings might actually reflect rain. If we exclude samples when humidity readings exceeded 80%, then both measures of fly intensity show a general increase with increasing humidity. Though we did not find a significant effect of wind speed on fly numbers, we did see that comfort movements decreased with increasing wind speed. We expected that fly harassment would be greater for animals that were standing still as opposed to those that were moving. In fact we found that the opposite was true. Horses that were grazing (and thus moving) had more flies on them than horses that were standing still resting. We believe that this is due to the fact that as horses were grazing, they were flushing flies from the substrate through the movements of their limbs and snouts, whereas resting horses were not disturbing the surrounding vegetation. Findings from studies of red deer (Cervus elaphus) support this hypothesis (Espmank & Langvatn, 1979). Similarly, we found that flies and mosquitoes bothered the observers less when they were sitting or standing still as opposed to walking through the marshes. We expected that fly harassment would be greatest in marshes, followed by scrub and dune habitats since marshes are where the flies lay eggs and where most adults emerge from the soil. However, fly counts were significantly higher in scrub habitat, and comfort movements did not differ between marsh and scrub, though movements were significantly more frequent in these habitats compared to dunes. One explanation for these results is that the scrub habitats provide more “cover” for flying insects in the form of vegetation to rest on during periods of high wind or rain. Dunes and beaches likely have fewer flies because they provide the least amount of cover for insects because vegetation or other features are very sparse, and wind speed is generally higher on the beach than inland (see also Rubenstein & Hohmann, 1989). . Horses likely flush more insects as they move through scrub habitats since the vegetation is higher and more dense. This could explain why we saw more flies on the horses here than in other habitats. We were able to identify a statistically significant effect of individual on fly numbers and comfort movements; however, it is not clear what the causal factor is given that other individual characteristics (sex, color) were included in the model. Numbers of repeated samples from the same individual did not differ greatly across horses, and the sample is relatively large (55 individuals), so it is unlikely that observations from a few outstanding individuals are influencing the data. One factor could be the age of the animal, given that metabolic rate changes with age (Altmann & Dittmer, 1968), Finally one of our goals was to assess the validity of using comfort movements as a proxy measure of fly harassment, given that fly counts are generally more difficult to do for a number of reasons. Simple regression and mixed model analysis both indicated that fly numbers were significant predictor of comfort movements. In addition to reflecting the number of flies on a horse, comfort movements likely also reflect the number of flies surrounding (and potentially harassing) the horse, which would be difficult to obtain via visual counts. We therefore suggest the use of comfort movements to assess biting fly intensity. In conclusion, biting fly harassment of feral horses is influenced by intrinsic and extrinsic physical, social, and ecological factors. Harassment by biting insects shapes feral horse ecology, specifically spatial relationships of individuals and habitat use. Reduction in biting fly harassment is potentially one factor selecting for group living in this population. Acknowledgements This section has been deleted References Altmann, P. & Dittmer, D. (1968). Metabolism. Federation of American Societies for Experimental Biology, Bethesda. Askew, R. (1971). Parasitic insects. Elsevier, New York. Bergerund, A. (1974). The role of the environment in the aggregation, movement, and disturbance behavior in caribou. In: The behavior of ungulates and its relation to management, V. Geist & F. Walther, eds. International Union for the Conservation of Nature, Morges, Switzerland, p. 552-584. Campbell, J.; Woods, W.; Hagen, A.F. & Howe, E.C. (1973). Cattle grub insecticide efficacy and effects on weight gain performance of feeder calves in Nebraska. Journal of Economic Entomology 66, p. 429-432. Chvala, M; Lyneborg, L. & Moucha, J. (1972). The horse-flies of Europe. Publication 498 of the Entomological Society, Copenhagen. Duncan, P. & Cowtan, P. (1980). An unusual choiceof habitat helps Camargue horses to avoid blood-sucking flies. Biological Behavior 5, p. 55-60. Duncan, P. & Vigne, N. (1979). The effects of group size in horses on the rates of attack by blood-sucking flies. Animal Behavior 27, p. 623-625. Espmark, Y. & Langvatn, R. (1979). Lying down as a means of reducing fly harassment in red deer (Cervus elaphus). Behavioral Ecology and Sociobiology 5, p. 51-55. Gunderson, H. (1945). Effect of cattle grub treatment on weight gains in beef cattle. Journal of Economic Entomology 38, p. 398-399. Hamilton, W.D. (1971). Geometry for the selfish herd. Journal of Theoretical Biology 31, p. 295-311. Higgins, E.A.T.; Rappleye, R.D. & Brown, R.G. (1971). The flora and ecology of Assateague Island. Agricultural Experimental Station Bulletin 172. University of Maryland, College Park. Hughes, R.D.; Duncan, P. & Dawson, J. (1981). Interactions between Camargue horses and horse flies (Tabanidae). Bulletin of Entomological Research 71, p. 227-242. Keiper, R. & Berger, J. (1982). Refuge-seeking and pest avoidance by feral horses in desert and island environments. Applied Animal Ethology 9, p. 111-120. Knox, P. & Hays, K. (1972). Attraction of Tabanus spp. (Diptera: Tabanidae) to traps baited with carbon dioxide and other chemicals. Environmental Entomology 1, p. 323-326. Mooring, M & Hart, B. (1992). Animal grouping for protection from parasites: selfish herd and encounter-dilution effects. Behaviour 123, p. 173-193. Morgan N. & Lee, R. (1977). Vegetative barriers influence flight direction of saltmarsh greenheads. Mosquito News 37, p. 263-268. Powell, D.M. (2000). Evaluation of effects of contraceptive population control on behavior and the role of social dominance in female feral horses, Equus caballus. Unpublished doctoral dissertation, University of Maryland, College Park. Rockel, E. & Hansens, E. (1970a). Emergence and flight activity of salt-marsh horseflies and deerflies. Annals of the Entomological Society of America 63, p. 27-31. Rockel, E. & Hansens, E. (1970b). Distribution of larval horseflies and deerflies of a New Jersey saltmarsh. Annals of the Entomological Society of America 63, p. 681-684. Rubenstein, D.I. & Hohmann, M.E. (1989). Parasites and social behavior of island feral horses. Oikos 55, p. 312-320. Rutberg, A. (1987). Horse fly harassment and the social behavior of feral ponies. Ethology 75, p. 145-154. Schmidtmann, E. & Valla, M. (1982). Face-fly pest intensity, fly-avoidance behavior (bunching) and grazing time in Holstein heifers. Applied Animal Ethology 8, p. 429-438. Smythe, R.. & Goody, P. (1972). The Horse. J.A. Allen, London. Tashiro, H. & Schwardt, H. (1949). Biology of the major species of horse flies of central New York. Journal of Economic Entomology 42, p. 269-272. Tashiro, H. & Schwardt, H. (1953). Biological studies of horseflies in New York. Journal of Economic Entomology 46, p. 813-822. Wilson, B.H.; Tugwell, N.P. & Burns, E.C. (1966). Attraction of tabanids to traps baited with dry ice under field conditions in Louisiana. Journal of Medical Entomology 3, p. 148-149. Figure 1: Least-squares mean number of flies/horse (A) and comfort moves/minute (B) in different habitat types. Bars with different superscripts are significantly different. Figure 2: Mean number of flies/horse as a function of temperature. Figure 3: Mean number of flies/horse as a function of group size Figure 4: Mean number of comfort movements/minute as a function of temperature (A) and wind speed (B). Figure 5: Mean number of comfort movements/minute as a function of the number of horses within one body length of the focal horse. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5