Refuge-seeking and pest avoidance by feral horses in desert and island environments

Applied Animal Ethology, 9 (1982/83) Ill-120 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

REFUGE-SEEKING AND PEST AVOIDANCE DESERT AND ISLAND ENVIRONMENTS

BY FERAL

111

HORSES IN

RONALD R. KEIPER’ and JOEL BERGER* ‘Department of Biology, Pennsylvania State University, Mont Alto, PA (U.S.A.) Tonservation and Research Center, National Zoological Park, Smithsonian Institution, Front Royal, VA (U.S.A.)

(Accepted for publication 12 November 1981)

ABSTRACT Keiper, R.R. and Berger, J., 1982. Refuge-seeking and pest avoidance by feral horses in desert and island environments. Appl. Anim. Ethol., 9: 111-120.

The hypothesis that insect harassment of feral horses results in the use of “refuge” sites to minimize pest pressures was examined. Patterns of resource exploitation were identified in two populations of feral horses (Equus caballus), one in an insular mountain range in the deserts of Nevada and the other on a barrier island off the Atlantic Coast of Virginia and Maryland.

Although physical features, amount and distribution of free water, and insect densities differed between the 2 study sites, horses displayed similar seasonal and daily patterns of habitat use. In the island horses, the rate of tail swishing/min was used as a crude measure of insect harassment. This behavior occurred least at beach and bay sites. These sites, in addition to the ocean, were used most during the summer, particularly between 10.00 to 16.00 h.

In the desert population, the horses grazed in meadows early in the day when it was cooler and pest densities were lower. Movements shifted during the day, and horses rested on higher slopes and ridges primarily during the afternoon.

Two new refuges for energy conservation were observed, viz. inshore water and snow patches. Resting occurred often in each of these areas.

INTRODUCTION

Large mammals represent rich sources of protein and are ideal hosts for many species of biting and blood-sucking flies. Harassment of ungulates by insects has been documented for both Perissodactyls and Artiodactyls inhabiting diverse environments such as the Arctic tundra (Kelsall, 1968; Espmark and Langvatn, 1979; Geist, 1978), true islands (Darling, 1937), barrier islands (Ford and Keiper, 1979), and African highlands (Joubert, 1972).

When pest densities are great, 300-500 cm3 of blood per day may be consumed from large-bodied ungulates (Tashiro and Schwardt, 1953). In addi-

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o 1982 Elsevier Scientific Publishing Company

112

tion to blood loss, other problems inflicted on the hosts, in a proximate sense, include infection, ecto- and endoparasitism, and disease transmission.

Animals that are constantly harassed interrupt foraging and resting activities. If such interruptions are frequent or severe enough, they may be detrimental to the reproductive success of the host. For instance, reindeer (Rangifer tarandus) occasionally fail to maintain proximity to, or even nurse, their young (see Espmark and Langvatn, 1979), or they cease foraging (Marsden, 1976). Horses have been fatally wounded when harassment by insects was severe (Webb and Wells, 1924).

In this paper, we report quantitatively the selection of sites of “refuge” from biting insects used by feral horses (Equus caballus) inhabiting a desert mountain range and an oceanic barrier island. We hypothesized that, if insects harassed horses, discernible patterns of movement would occur whereby pest pressures would be minimized. We also describe previously unknown strategies for avoiding insects based upon features unique to the desert and barrier islands’ ecosystems.

METHODS

Study periods and locutions The first study-site was Assateague lsland, an 81-km long barrier island located off the Atlantic Coast of Maryland and Virginia. The major types of breuilingulata) on sand vegetation include American beach grass (Ammophila dunes, American three square (Scirpus americanus) and various sedges in the inner-dune zone in the middle of the island, and saltmarsh cordgrass (Spartina spp.) in the marshes along the bay. A shrub zone, composed of bayberry (Myricu pennsylvunicu) and marsh elder (Iva fruscens), and a narrow band of Loblolly pine (Pinus tuedu), is also present on higher regions of the island. Data were collected in 1978 and 1979. The population consisted of about 65 animals during the study period.

The second study-site was the Granite Range, situated in the northwestern portion of the Great Basin Desert in Nevada. This insular range is approximately 40 km long by 11 km wide and phytogeographically it is typical of other north-south ranges in the Great Basin Desert (see Billings, 1978; Harper et al., 1978). Elevations range from 1292 m to 2786 m. General habitat features include: sagebrush (Artemisia spp.) and saltbush (Atriplex spp.) at lower elevations; cottonwoods (Populus fremontii) in canyons; junipers (Pinus juniperous) at intermediate elevations; wet meadows and small relict populations of aspen (P. tremuloides) forests at high elevations.

The current population size was about 125. Data were collected on this population during June and July 1979.

Blood-sucking flies of the family Tabanidae (Diptera) are common throughout the United States, being represented by over 470 species in 27 genera (Philip, 1947). In the Granite Range, the family is represented by

113

Tabanus aegrotus, Tabanus atratus, Chrysops furcata and Chrysops fulvaster (Philip, 1947), while on Assateague there are 5 common tabanids; Tabanus nigrovittatus and T. lineola are green-headed horse flies, Chrysops fuliginosus and C. atlanticus are deer flies, and Stomoxys calcitrans is the stablefly (Rockel and Hansens, 1970a).

In both areas, tabanid eggs are laid in wet areas (saltmarshes on the western edge of Assateague and wet lowland meadows in the Granite Range) and larvae develop in the soil. Adults emerge from the marsh vegetation primarily during the months of June, July and August, with maximum population levels on Assateague occurring in late June and mid-August (Morgan and Lee, 1977).

The daily activity-pattern of Tabanid flies was greatly influenced by weather conditions and vegetation. At temperatures below 22°C fly activity was negligible, but it increased as the temperature increased (Tashiro and Schwardt, 1949). Fly activity also increased as humidity increased (Tashiro and Schwardt, 1949) and as cloud-cover decreased (Schulz et al., 1975), but decreased as wind velocity increased above 8 kmph (Rockel and Hansens, 1970).

Although tabanids are strong fliers, at wind velocities over 8 kmph the flies rest in marsh vegetation or in wooded areas (Sheppard, 1977). Since the flies generally fly only at heights of l-l.5 m, belts of tall vegetation tend to act as barriers to tabanid movement and tend to keep the flies in marsh areas (Rockel and Hansens, 1970b).

Definitions and data collection Grazing included ingesting vegetation as well as walking in search of food.

Resting is relatively motionless behaviors (standing or lying) exceeding 2 min duration (Berger, 1977; Keiper and Keenan, 1980).

In the Granite Range, with its sharp altitudinal gradients, many or all of the habitats could be viewed simultaneously.

Data on activity patterns were sampled at 3-min intervals. On Assateague Island, the flatness of the terrain precluded sampling from all sites at the same time. Focal animal sampling (Altmann, 1974) was therefore used to monitor activity patterns. These were recorded over the course of a day by following a band and scoring data on check sheets at 1-min intervals. Fourteen all-night watches were performed (see Keiper and Keenan, 1980).

The Assateague data included only bands that had minimal contact with humans and equal opportunities to use inshore sites.

RESULTS

Although equid foraging strategies may show basic similarities, it should not be unexpected that habitat exploitation occurs quite differently and results in maximizing the use of physical features for comfort movements.

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Mud and dust bathing, rubbing against trees (and rocks?) as well as occupying shady or windy locations has been reported by Klingel(l968,1974) and Joubert (1972), with a few exceptions for all African equids; the same has been found for feral horses by Berger (1977), Duncan and Cowtan (1981), Ford and Keiper (1979) and Tyler (1972).

Insect refuges

on Assateague

Island

In order to examine the hypothesis that discernible patterns of movement occur in response to pests, refuges were identified through indirect estimates of pest densities. The frequency of tail swishing per minute was used as an indirect measure of pest densities, since Hughes and Duncan (1977, in Duncan and Vigne, 1979) demonstrated that tail swishing increased with insect harassment. Randomly-selected animals were observed for 60-s intervals.

Pest densities differed between habitats. Horses on bays, beaches and mudflats averaged 30.9 tail swishes/min (n = 59) over all hours of the day, but averaged 54.8 tail swishes/min (n = 89) when inhabiting marshes, dunes and innerdunes. It also appeared that insect activity increased positively with ambient temperature.

Such a relationship was suggested by data on tail swishes/minute averaged over all hours of the day. For rainy days, tail swishes averaged 19.7/min (n = 6); for cloudy days, tail swishes averaged 27.4/min (n = 42); for sunny days, tail swishes averaged 45.l/min (n = 51).

These differences were significant (one-way ANOVA: F = 8.93; df = 2.96; P < 0.01). Fly activity was reduced not only when the sun was obscured but also when winds exceeded 16 kmph.

Since certain areas were characterized by lower insect densities, we next identified the way(s) in which these areas were used. The horses on Assateague Island showed seasonal variations in habitat exploitation.

Grazing occurred primarily in the saltmarsh, on the dunes, and in grassy areas of the inner dune zone. Resting occurred on the beach, in inshore water, and on the mudflat portions of the innerdune zone. Resting areas were characterized by a lack of vegetation, conditions which apparently resulted in higher wind velocities and fewer biting insects. If shores and beaches were sought by horses as refuges, we reasoned that they would be used more in the summer than at other times of the year, especially because insect densities were greatest then. The fact that beaches and water received virtually no winter use yet were exploited heavily during the summer (Table I) suggested that littoral features influenced the seasonal movements of horses.

Two further points support the idea that beaches and bays were used when insect densities were greatest in other areas. First, beaches and bays were utilized significantly more often between the 6-h period from 10.00 to 16.00 h than the 18-h period between 16.00 and 10.00 h (X2 = 25.24; P < 0.001). These data were based on the frequency of observations of horses occupying all areas at the beginning of each hour. Second, for any 6-h period throughout the day, the minimum number of minutes spent grazing/h

115 TABLE

I

Utilization of different habitats on Assateague Island (expressed in percent of total observation time).

Horses were observed about 450 h.

--Habitat Band

Winter

T K L

-__

Size Beach

1 6 15

Average

Summer

Winter

MiUSh

Innerdune

Dune

Summer

Winter

Summer

Winter

Summer

Winter

Summer

Bay

0 0 2.9

21.9 3.6 4.1

0 0 0

2.9 13.5 8.2

41.3 42.1 17.6

14.3 34.5 17.5

26.8 42.1 29.4

29.3 8.3 8.2

31.9 15.8 50.0

31.5 40.0 61.9

0.9

9.9

0

8.2

33.7

22.1

32.8

16.3

32.6

44.5

occurred from 22.00 to 04.00 h (X = 28.5; n = 140). This mean for minimum grazing/h did not differ significantly (T test) from that for the period between 10.00 and 16.00 h (X = 31.2; n = 450), which was when horses used the bays. Both of the above measures were made from June to August, and they indicate that grazing occurred at about the same rate when insects were presumably the greatest (10.00-16.00 h) and approaching the minimum (22.00+4.00 h). However, each of the above means is significantly lower than the grazing averages for the other 18-h periods of the day (T tests: for 04.00-22.00 h X = 34.7, n = 570, P < 0.05; for 16.00-10.00 h X = 36.2 , n = 750, P< 0.02). Thus, grazing was lowest during a 6-h day and night period. During the former beaches and bays were used, whereas they received essentially no night usage.

Insect refuges in the Granite Range In June when the study began 3 snow patches, ranging in size from 260 m* to 900 m*, existed. These patches occurred on slopes near ridge crests at the highest elevations and they represented a mere fraction of the total area available to horses. Although the snow melted from all sites by mid-July, these patches and the areas peripheral to them appeared important to horses, since such regions were used about 15% of the time horses were observed (Fig. 1). These areas were designated snow refuges. The area8 near a snow refuge were divided into those adjacent to it (i.e. the snow had melted leaving only soil or newly sprouted grasses) and the snow itself. The hypothesis that sex differences occurred in activity patterns within a snow refuge was examined by comparing time spent foraging versus time spent resting (standing in this case; Table II). A test for the equality of means with high variances indicated that sex differences within the categories (a) time spent standing in snow, (b) time spent standing peripherally to snow, and (c) time spent foraging peripherally to snow were not significant. In fact, the only significant difference within a category occurred between foals and parous females (T, = 3.74; T,fit. = 0.18; P < 0.001) for snow standing.

116 SLOPES

.

PLAINS

MEADOWS 4

B

7

LOWER c

MlOOLE

IOP

SNOW

RIOGE

PATCHES

CRESIS

0

A

B

c

D

I N

7 /

6

: R E

s 1 I N E

Fig. 1. The relationships between activity patterns and temporal exploitation of various refuge sites by Granite Range horses. A, B, C and D equal successive 2-h time-periods, beginning with 08.00 h. Percent is expressed as the observation time per 2-h period divided by total observation time during the same 2-h period (see text for further explanation).

TABLE II The time-budgeting of activities at a snow refuge in the Granite Range. Time is expressed in hours. The disparity between the number of foals and parous females resulted from failure to observe foals continuously -__ Peripheral standing Peripheral foraging Snow standing

Male Parous females Foals

N

X

S

N

X

s

N

X

S

7 12 8

1.29 1.53 0.37

2.09 0.95 0.19

7 15 12

1.29 1.62 1.57

1.67 1.29 1.22

7 15 12

0.28 0.31 0.33

0.04 0.05 0.04

Since sex differences in snow or peripheral standing were not significantly different, these data were pooled and compared to time spent foraging peripherally to snow. The use of snow patches differed significantly from areas adjacent to it (T, = 3.01; Tcdt. = 2.08; P < O.OOl), indicating that snow refuge-zones were areas used most often for resting; foraging was infrequent.

Selection

of refuge zones

The activity patterns of Granite Range horses were monitored at different sites. These included: snow patches - areas of snow accumulation (see

117

above); meadows - regions with heterogeneous mixtures of grasses and other monocots in association with streams, springs, or seeps; sagebrush plains flat regions characterized

by Artemisia; slopes - areas of sage-covered topographic relief commencing where plains terminated; ridge crests - the apices of slopes.

Data were collected on the use of these sites only when all areas could be viewed simultaneously from vantage points. Observational points were moved at random around the highest slopes throughout the study. Hence, the probability of observing horses at any site was equal. A total of 70 h was spent recording these data. Slopes were divided into the top one-third, middle one-third, and the bottom-third.

Data on the resting or foraging activities of individual horses were pooled so that a measure of band activity could be derived. Such an approach seemed justified since band members were involved in homogeneous patterns (i.e. all engaged in the same activity; II = 228) significantly more often than they were in heterogeneous patterns (n = 62; 2 = 6.78; P < 0.001). Comparisons could then be made between bands rather than individuals. It also appeared logical to measure band activity since the “leading”, wandering or foraging (and/or resting) movements of one individual seemed to influence the subsequent activities of other band members.

Most foraging behavior occurred in meadows and about 75% of it occurred in the morning (Fig. 1) when, presumably, it was cool. In contrast, resting behavior was most frequent on ridge crests, the highest portions of slopes, and at snow patches (Fig. 1). Remarkably little time was spent foraging in these latter 3 sites.

Shade was unavailable for use by Granite Range horses simply because trees were infrequent above 2200 m. Horses spent most of their time above this elevation. Other studies of horses reported shade utilization (Berger, 1977; Salter and Hudson, 1978).

DISCUSSION

Insect avoidance

and thermoregula

tion

In both the Granite Range and Assateague Island, our data suggest that refuges were primarily used for protection from biting insects, In the Granites, the refuge sites utilized most frequently were ridge crests, upper slopes and snow patches. These areas were characterized by higher wind velocities and lower temperatures, conditions that yield lower pest densities. Because of these environmental conditions, it could be argued that horses select such sites more to minimize heat loads than to reduce insect harassment.

Two inferential arguments tend to support the insect refuge idea. First, if horses sought to lower their body temperatures a gradual increase in their distribution on higher slopes could be predicted, since ambient temperature is inversely related to altitude. The expected differences in the use of the middle portions versus the top-third of slopes should only be small, since

118

differences in ambient temperature should also be gradual. Nevertheless, disparities in the use of these sites were dramatic (Fig. 1); the top-third of slopes received almost 4 times as much use.

The second argument in favor of the insect avoidance idea is that horses rested almost 14 times as much as they grazed at these sites (Fig. 1). If such refuges were not used almost exclusively for avoiding insects, horses should have grazed more often.

It could also be argued that, if horses were bothered by insects, they should have utilized meadows more in the morning when ambient temperatures were lowest. Since Tashiro and Schwardt (1949) and Joubert (1972) suggested positive correlations between temperature and insect activity, horses could indeed maximize food intake by grazing early in the day and prior to intense insect activity. The fact that horses grazed almost 3 times as much in the morning (than in the afternoon) (Fig. 1) supports the idea that movements were not random and reflected a strategy of insect avoidance.

Although we have argued in favor of insects as the primary factor affecting the use of refuges in the Granite Range, we do not exclude a multifactoral explanation for the temporal and spatial distribution of horses there.

Almost certainly, the avoidance of heat stress interplays with pest avoidance tactics in some refuges. For instance, snow patches are cool and few insects are associated with them. Horses could have remained cool and avoided insects by remaining in them.

On Assateague Island, refuge sites primarily served to reduce pest harassment, although thermoregulatory benefits were also likely to be derived.

Beaches and mudflats provided some relief from biting flies, as evidenced by reduced tail swishing, but they did not reduce heat loads. Temperatures on these sites were similar to surrounding non-refuge areas and horses resting on beaches and mudflats were exposed to the direct heating effects of the sun.

If Assateague horses sought refuge from higher temperatures rather than from insects, the animals should have utilized the shade available in the Loblolly forest or the salt water itself. However, the tree and shrub zones received no use for resting and beaches were used more often than the bay.

The fact that horses used the bay and drank salt water indicates they were, in fact, heat stressed to some, as yet undetermined, degree. Under “normal” conditions, horses drink from fresh water sources located in the center of the island.

Studies in other environments clearly implicated pests as the driving force in refuge-seeking behavior. Duncan and Cowtan (1981) did not detect a correlation between the use of a refuge (i.e. a barren patch of ground) and ambient temperature in Camargue horses, but they found an inverse relationship between tabanid number at different sites and their usage by horses.

Also, horses in the Camargue lowered their body temperatures by standing more in the shade than in the sun, yet they selected denuded sites in the sun for resting (Duncan and Cowtan, 1979). These data indicate that insects rather than thermal considerations were the primary force in refuge selection.

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Comfort-seeking tactics in other species Many species of ungulates exhibit comfort movements, probably stemming from insect pests (Leuthold, 1977). Species inhabiting temperate and Arctic environments, such as caribou, red deer (Cervus eluphus), and musk ox (Ouibus moschatus), experience great discomfort from biting insects during the summer months. These species apparently combat insect harassment in a manner similar to Great Basin horses, since they also rest on windswept ridges and in snow (Kelsall, 1975; Marsden, 1976). Mule deer (Odocoileus hemionus) have been observed lying in snow patches for 3 h continuously during the summer above 2600 m in the Santa Rosa Mountains, Nevada (J. Berger, 1981, personal observation).

Likewise, elk (C. canadensis) bathed and rolled in snowbanks during the summer (Altmann, 1956). Thus, it appears than when environmental conditions are broadly similar, ungulates demonstrate a range of behavioral responses that parallel each other, whether avoiding insects or increasing thermoregulatory efficiency.

Although the barrier island and the desert mountain ecosystems are unlike in many physical features, horses showed similar patterns of refuge exploitation. Those on Assateague Island used mudflats, beaches and salt water to avoid insects; in addition the latter yields considerable thermoregulatory benefits. In the Granite Range, horses migrated to higher elevations, stood in snow when available, and used ridges at other times.

ACKNOWLEDGEMENTS

This research was supported by a Smithsonian Post-doctoral Fellowship (to Berger), the Bureau of Land Management, and by a grant from the National Park Service (to Keiper). We also offer special thanks to the following people: Christen M. Wemmer, Yaden Stickley, Paul Jancar, Nick B. Hunter, and Stamatis M. Zervanos.

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