MOVEMENTS OF THE WHITE-TAILED DEER AND THEIR RELATIONSHIP WITH PRECIPITATION IN NORTHEASTERN MEXICO

Joaquín Bello, Sonia Gallina and Miguel Equihua

Joaquín Bello. Doctor in Sciences, Instituto de Ecología, A.C., Mexico. Professor, Universidad Juárez Autónoma de Tabasco, Mexico. Address: Km 0,5 Carretera Villahermosa-Cárdenas, Villahermosa 86039 Tabasco, Mexico. e-mail:   joaquin.bello@cicea.ujat.mx

Sonia Gallina. Doctor in Sciences, Universidad Nacional Autónoma de México. Researcher, Instituto de Ecología, A.C., Xalapa, Veracruz, México.

Miguel Equihua. Ph.D., University of York, UK. Researcher, Instituto de Ecología, A.C., Xalapa, Veracruz, México.

Resumen

Mejores estrategias de manejo pueden ser diseñadas mediante la evaluación a mediano y largo plazo del efecto de las variaciones anuales e interanuales de las condiciones ambientales en el comportamiento del venado. Se analizó la relación del tamaño del ámbito hogareño y las distancias de desplazamiento del venado cola blanca (Odocoileus virginianus texanus) con la distribución y cantidad de precipitación en el Rancho San Francisco, noreste de México. Se evaluó también las diferencias entre épocas, sexos y años. El área de estudio comprende 1000ha, con alta disponibilidad de agua (34 fuentes de agua/km²). Se estimó el ámbito hogareño y las distancias de desplazamiento en tres épocas biológicas: reproductiva, posreproductiva y crianza, durante cuatro años (sep 1994 a ago 1998). El tamaño del ámbito hogareño fue mayor en 1997 (258 ±18ha) cuando se registró la mayor cantidad de lluvia. Se encontraron diferencias en el tamaño del ámbito hogareño entre sexos, siendo mayor para los machos (234 ±14ha) que para las hembras (193 ±14ha). Durante 1998, el tamaño del ámbito fue mayor en la crianza que en la reproductiva por retraso de las lluvias. La distancia total promedio desplazada por día fue mayor en 1997 (7016 ±354m), año en que la mayor distancia desplazada fue en la crianza (8300 ±640m). Encontramos que cuando incrementó la precipitación aumentó la distancia diaria recorrida. La alta variabilidad estacional y anual en la precipitación, que influye directamente en la disponibilidad de alimento y los requerimientos de agua, a pesar de la existencia de un programa de manejo de agua, puede explicar los cambios en los movimientos entre años.

Summary

Better management strategies can be designed through the medium and long-term evaluation of how inter- and intra-annual variations in environmental conditions affect deer behavior. The relationship between home range size and distances traveled by white-tailed deer (Odocoileus virginianus texanus) and precipitation were analyzed, both in terms of distribution and quantity, in the San Francisco Ranch, Northeastern Mexico. Possible differences between seasons, sexes and years were evaluated. The study area has a surface of 1000ha and high water availability (3.4 troughs/km²). Home range size and distances traveled were estimated during three biological seasons: reproductive, post-reproductive and fawning, over a period of four years (Sept 1994 to Aug 1998). Home range size was largest in 1997 (258± 18ha), when the highest precipitation was registered. Differences in home range size were found between sexes, it being larger for males (234± 14ha) than for females (193± 13ha). In 1998, the home range was larger in the fawning season than in the reproductive season because of a rain delay. The average total distance traveled daily was longest in 1997 (7016 ±354m). In the same year, the longest distance traveled was during the fawning season (8300 ±640m). When precipitation increased, an increase was found in the daily distance traveled. The high variability in monthly and annual precipitation factors that directly influenced food availability and water requirements, despite the high water management program, may explain changes in deer movements between years.

Resumo

Melhores estratégias de manejo podem ser desenhadas mediante a avaliação a médio e longo prazo do efeito das variações anuais e interanuais das condições ambientais no comportamento do veado. Analisou-se a relação do tamanho do âmbito caseiro e as distâncias de desplazamento do veado cauda branca (Odocoileus virginianus texanus) com a distribuição e quantidade de precipitação no Rancho San Francisco, nordeste do México. Avaliou-se também as diferenças entre épocas, sexos e anos. A área de estudo compreende 1000ha, com alta disponibilidade de água (34 fontes de água/km²). Estimou-se o âmbito caseiro e as distâncias de deslocamento em três épocas biológicas: reprodutiva, pós-reprodutiva e criação, durante quatro anos (sep 1994 a ago 1998). O tamanho do âmbito caseiro foi maior em 1997 (258 ±18ha) quando se registrou a maior quantidade de chuva. Encontraram-se diferenças no tamanho do âmbito caseiro entre sexos, sendo maior para os machos (234 ±14ha) que para as fêmeas (193 ±14ha). Durante 1998, o tamanho do âmbito foi maior na criação que na reprodutiva por atraso das chuvas. A distância total média deslocada por dia foi maior em 1997 (7016 ±354m), ano em que a maior distância deslocada foi na criação (8300 ±640m). Encontramos que quando incrementou a precipitação aumentou a distância diária recorrida. A alta variabilidade estacional e anual na precipitação, que influi diretamente na disponibilidade de alimento e os requerimentos de água, apesar da existência de um programa de manejo de água, pode explicar as mudanças nos movimentos entre anos.

KEYWORDS / Home Range / Odocoileus virginianus / Precipitation / Water Management /

Received: 25/02/2004. Accepted: 05/25/2004.

In order to survive, animals must satisfy cover, food, and water requirements within a given area. According to Burt (1943) home range is defined as "the area in which an individual normally eats, reproduces, raises its young, rests, and moves about." Home range size depends on intrinsic characteristics of an individual (physiological stage, sex, age) as well as on extrinsic characteristics such as vegetation type, quantity and quality of available food, concealment cover, availability of free-flowing water and other environmental factors such as temperature, humidity, and rainfall (Ockenfels et al., 1991; Kroll, 1992).

The distance traveled by white-tailed deer (Odocoileus virginianus) has been little-studied despite the fact that it is a behavior that serves to satisfy requirements for survival. Migratory movement has been detected in males that separate from their mothers (Nelson, 1998), as well as wandering from paths and other nocturnal movements during the hunting season (Kilgo et al., 1998). Some studies have also examined the distances that deer keep from water sources and how this varies according to physiological stage, sex, body size, and season (Rautenstrauch and Krausman, 1989; Fox and Krausman, 1994; Boroski and Mossman, 1996). However, no known research has addressed the long-term relationship between deer movement and environmental conditions, such as variations in rainfall, that influence food resource availability.

A long-term study of home range size and distances traveled by the white-tailed deer in arid and semi-arid regions is important. In these areas, where temperatures top 40ºC and annual rainfall does not reach 400mm, it is unknown how individuals respond to intense heat and variations in rainfall as time goes by. The valuable outcome of researching this response is to allow the definition of better management strategies, both for the species and its habitat. It has been determined that the adaptability of white-tailed deer to high temperatures in arid regions is inferior to that of the mule deer (Odocoileus hemionus), as the former’s thermoneutral limit is 30°C (Ockenfels and Bissonette, 1984).

The importance of a resource may vary with both site and season (Kie and Thomas, 1988). In the case of water, a critical habitat element in arid environments, deer can travel outside their home range in search of sources such as dams or troughs (Rautenstrauch and Krausman, 1989). Nevertheless, in the early rainy season deer are found furthest from water sources (Maghini and Smith, 1990). This lower dependence on water is due principally to an increase in the abundance and amount of available food, mainly plants. Thus, deer have a larger diversity of food and water options (Terr, 1984; Henry and Sowls, 1980). During this period, there is also increased availability of shrub and herb species, vital elements in the deer diet that are dependent primordially on precipitation (Davis, 1990). A relationship has been found between rainfall and fluctuations in deer density (Henry and Sowls, 1980) as well as water sources and their behavior (Boroski and Mossman, 1996), demonstrating that they do modify deer distribution patterns. The influence of free-flowing drinking water and climatic conditions such as rainfall on deer movement is little-known, due to difficulties as to how to separate the effect of each factor on behavior (Rosenstock et al, 1999).

It has been suggested that the extreme drought conditions characteristic of summer and autumn in arid zones of the southern United States and northern Mexico may explain fluctuations in white-tailed deer populations (Henry and Sowls, 1980; Brown, 1984). Furthermore, as its temporality is irregular in dry zones such as Southern Texas (Davis, 1990), the influence of drought on individuals may also vary, requiring long-term monitoring to determine what modifications occur both in terms of individual behavior and populations level (Rosenstock et al., 1999). The long-term monitoring of deer movement in areas where drinking water is not a limiting factor will lead to an understanding of how rainfall variability may influence movements despite the existence of water sources.

On this basis, it is assumed that home range size and distances traveled will be larger in years with heavy rainfall, as resource availability increases; despite the high temperatures, such conditions should permit the deer, a selective species in terms of food, to travel larger distances in search of foods that best fulfill its requirements without risking water loss from evapotranspiration. Thus, the home range size and distance traveled by the Texan white-tailed deer in xerophilous shrubs of northeastern Mexico is analyzed in relation to the amount and distribution of rainfall, environmental temperature and other factors such as sex, physiological stage, and year.

Study Area

This study was conducted at the San Francisco Ranch, property of Ducks Unlimited of Mexico A.C. (DUMAC), located between the municipalities of Lampazos, Nuevo León, and Progreso, Coahuila (Figure 1). The ranch extends over 1000ha, is enclosed in a deer fence, has 32 artificial troughs and 3 small dams, and an estimated deer population of 80-100 individuals. The climate is semi-arid with mean annual rainfall under 400mm and a mean annual temperature of 21ºC. The rainy season, although variable, tends to run from May to Sept (Figure 2). Yearly rainfall was 131.5, 294.5, 357 and 230.3mm in 1995, 96, 97 and 98, respectively. In the four years studied precipitation did not reach the mean recorded for the area (400mm), and variation among years was only 38%. The rainfall recorded each month was, in contrast, markedly variable during the four year period, although 1997 was the year with the most unusual pattern: the largest precipitation corresponded to Mar and May, before the rainy season had presumably begun (Figure 2). Vegetation in the zone is a xerophylous brushland, with high species richness as two physiographical provinces meet: the coastal plains of the Northern Gulf and the Great North American Plains (Briones, 1984). There are seven plant associations in the region, Hilaria grassland and several types of brushland: Opuntia (1%), Leucophyllum frutescens (11%), Flourensia cernua (6%), Acacia-Prosopis (54%), Prosopis (15%), and Acacia-Celtis (10%) (Bello et al., 2001).

 

Methods

A total of 18 deer, 9 females and 9 males, were captured using drop nets between Sept 1994 and Nov 1996. Subsequently, each deer was fitted with a color coded collar containing a transmitter that emitted specific signals of 150-154MHz. Individuals were located using TR-2 y TR-4 reception equipment (Telonics), with H-type antennae and SUUNTO compasses. Two persons positioned at fixed georeferenced stations took simultaneous readings hourly for continuous 24h cycles, thus carrying out 2-3 cycles per trip for each deer. Two to three trips were made per sampling season, these corresponding to the deer’s physiological stages: reproduction or mating (Nov-Feb), post-reproduction or gestation (Mar-Jun), and fawning (Jul-Oct). The sampling period ran from Oct 1994 to Sept 1998.

Deer location coordinates were obtained in UTM units using the TRÍPOLI program (Laundré, 1990), considering a magnetic deviation of 9.15º. To estimate average error polygon for the locations, readings were taken on five collars at known locations in the area. Six persons, unaware of collar locations, took six readings of each collar from each station. The error polygon for each collar was calculated with the LOCATE II program (Nams, 1990), while the overall error was estimated as the mean size of the error polygon. We used the CALHOME program (Kie et al., 1994) to obtain the home range size of each individual per season. The minimum convex polygon model (MCP) was applied to 95% of the data in order to reduce the risk of overestimating activity area. This model has the disadvantage of being influenced by outliers (Beier and McCullough, 1990) but is relatively robust when used with autocorrelated data, and accuracy increases as N increases, even when autocorrelation is increased (Swihart and Slade, 1985). Home range was calculated per season in order to produce trustworthy estimates of its size, despite the loss of degrees of freedom.

Estimated in meters, distances traveled were considered straight movements between successive deer locations. For each individual, mean distance traveled and distance covered per day were calculated for successive readings. Distance was also estimated for the three seasons during all four years. Data regarding minimum, mean, and maximum temperature as well as precipitation for all four years were obtained from the climatological station located at the Venustiano Carranza Dam, located approximately 30km from the study area.

Two-way ANOVAs were employed to determine whether or not home range size and distances covered, in successive readings and per day, varied with sex, year, and season; when significant differences were noted, the SNK test (Student-Neuman-Keuls) was applied a posteriori to identify the factor responsible for the differences (Zar, 1996). To find out if a relationship existed between home range size/distance traveled by deer and precipitation/maximum temperature, a regression analysis was applied per season for all four years. The maximum temperature was used as it was over 30º, the deer’s thermoneutral limit (Ockenfels and Bissonette, 1984).

Results

During the four years of this study, females were located 1239, 1350, 1181 and 819 times (in 1995, 96, 97 and 98, respectively) and males were located 1317, 897, 913 and 453 times. To estimate home range size per season for each individual, a minimum of 70 locations was used. Between 4 and 9 individuals were followed during each season, as some collars stopped working, others were lost, and some deer died. The error polygon was estimated as 0.7ha and did not affect estimations of deer movement significantly.

Home range

Female ranges were smaller (0= 193 ±13ha) than those of males (0= 234 ±14ha), with significant differences evident (F= 4.61, P=0.035). Differences among sexes were noted in 1997 (females 0= 200 ±25ha and males 0= 316 ±25ha), while range size was similar for the other years (Figure 3). Upon application of a new analysis that excluded data for 1997, there were no differences in the size of the area used by each sex (F=0.55, P=0.46). Significant differences were found between years (F=3.34, P=0.024); in 1997 mean range size was the largest (258 ±18ha) and in 1998 it was the smallest (174 ±20ha). No significant differences in home range size were found between seasons (F=0.29, P=0.75), although season-year interaction was significant (F=2.49, P=0.032). The differences noted were between the fawning and reproductive seasons of 1998 (Figure 4). Comparison of the physiological periods of different years showed that only during the reproductive season of 1998 was home range size smaller (106 ±33ha) than in the other three years (over 200ha).

Distances traveled

Distances traveled per day (Figure 5a) were significantly different between years (F= 20.218, P<0.001), the largest corresponding to 1997 (0= 7016 ±354m). In terms of distance covered per day, no significant differences were found between sex and seasons (F=0.723, P=0.398 and F=2.333, P=0.106, respectively). Although significant differences in season-year interaction were detected (F= 2.42, P=0.037), deer traveled more during the three seasons of 1997 than during the other years. Furthermore, in terms of seasonal movement, in 1997 deer moved more during the post-reproductive (0= 7422 ±640m) and fawning seasons (0= 8300 ±640m) as compared to the reproductive season (0= 5326 ±555m), while during the other years, seasonal distances were similar (Figure 5b).

Relationship between precipitation and temperature

A significant relationship was found between precipitation and distance traveled per day (R²=0.129, P=0.002); as the former increased, so did the latter. This was not true, however, for maximum temperature (R²=0.004, P=0.594). No relationship was found between precipitation (R²= 0.034, P=0.115) and maximum temperature (R²= 0.00554, P=0.529) with the home range size per season. Significant differences were found in precipitation per season (F=4.54, P=0.017), being highest during fawning (0= 34.8 ±6.3mm) as compared to reproduction (0= 8 ±6.3mm), and the season-year interaction was not significant (P= 0.177). High seasonal variation in rainfall was detected, particularly during the post-reproductive season of 1997, when a noteworthy change in rainfall was recorded (0= 52 ±12.6mm), a similar amount to that of the rainy (fawning) season (Figure 6). There were also differences in precipitation during the fawning seasons of all years, the highest occurring in 1996 (0= 53.1 ±12.6mm) and the lowest in 1995 (0= 15.8 ±12.6mm).

Discussion

Only in 1997 males had larger home ranges than females. Such differences have been noted in other studies (Ockenfels et al., 1991; Kroll, 1992). The differential response of the sexes to environmental changes may be due to physiological requirements and some authors have suggested that males respond more quickly to such changes than females (Relyea et al., 2000). In 1997, when the highest rainfall occurred during the dry period (post-reproductive season, 52mm), males occupied the largest home ranges. In periods with high precipitation, deer have both a larger quantity and diversity of resources available (Terr, 1984; Davis, 1990), allowing them to be more selective. During seasons of unusual abundance, they can choose species of greater nutritional value (Murden and Risenhoover, 1993). In particular, males begin to recover their physical conditions after the season of greatest energy expenditure has ended, a season characterized by substantial stress and weight loss in males due to the mating process (reproduction; Kroll and Koerth, 1996).

Home range size was similar for the two sexes during years in which precipitation was low, except during the rainiest year (1997). During the present study period, deer behaved differently from those observed by Relyea et al. (2000), who showed that males and females had similar home ranges in highly productive habitat types, while males had larger ranges than females when productivity was low. In years with high rainfall in the study area, habitat conditions improved as the amount and diversity of food increased, and both males and females preferred habitat types that offered the largest availability of food (Bello et al., 2001).

Females had smaller home ranges during the reproductive and post-reproductive seasons of 1998. Although these seasons tended to have the lowest rainfall of the study period, in 1998 they were markedly lower during these seasons, with temperatures reaching over 30ºC; under these conditions, deer preferred habitats in the study area that provided high cover (Bello et al., 2001) or a reduction in activity level (Ockenfels and Bissonette, 1984). Such environmental factors also affect the availability of food and cover (Davis, 1990), vital elements for female survival. Furthermore, the post-reproductive season is among the most critical times in terms of water requirements, as the gestation process is nearing its end and milk production for lactation can be affected (Yagil et al., 1986). The extreme conditions of 1998 may explain the low abundance of fawns born in the study area that year (Soto-Werschitz et al., 2000).

In 1997 home range size and distances traveled were larger. That year took place the heaviest precipitation of the study period (357mm). Furthermore, it began early and in an anomalous fashion increased exceptionally during "drought" months (Mar-May), considered the post-reproductive season. These factors may have favored the availability of food in the study area, vital to the deer because during initial antler growth, a nutritionally rich diet helps increase antler size as well as total body weight, elements that provide greater access to females and more mating success (Verme and Ullrey, 1984). In 1998, when it did not begin to rain until August, food availability was lower; this may have led to a foraging strategy that maximized energy intake for vital functions (Kie, 1999). Both male and female deer reacted by reducing the size of their ranges. Furthermore, the few fawns that were born were lost, either to predation or death by other causes (Soto-Werschitz et al., 2000), a situation that has been observed in other sites under similar circumstances (Leopold and Krausman, 1991).

Deer moved larger distances per day as rainfall increased, the largest corresponding to the post-reproductive and fawning seasons of 1997. Both the amount and distribution of precipitation influence plant production and, as a result, habitat productivity (Kie and Thomas, 1988; Kroll, 1992), a phenomenon that is more noticeable in areas with little rainfall. In turn, such conditions affect the area necessary for individuals to meet their physiological needs, as resource availability is not homogeneous throughout the year nor in different habitat types (Orians and Wittenberger, 1991). Nevertheless, when resource availability is high, deer spend more time searching for and selecting food that is high in nutrients (Murden and Risenhoover, 1993), thus influencing their habitat preferences in the study area, especially during years with high precipitation (Bello et al., 2001).

The largest home range size during 1997, and the relationship between precipitation and longest distances covered per day during the post-reproductive and fawning seasons of the same year, indicate that deer respond behaviorally to changes in habitat conditions. These conditions depend upon the amount and distribution of rainfall. Therefore, rainfall is a factor that must be taken into account for the definition of management strategies to conserve or favor the species. As the animal-habitat relationship is dynamic, it is advisable that longer-term research be directed at understanding how deer behavior relates to environment, especially in regions such as the study area, where climate is highly variable.

ACKNOWLEDGMENTS

The authors thank Simón Ortiz for field assistance and Christian Delfín Alfonso, Nora Delia López, Carlos Contreras, Salvador Mandujano, Alejandro Pérez Arteaga and Rosa Elena Sánchez-Mantilla for gathering data. Juan Carlos Serio provided comments that improved the manuscript. This study was made possible by the financial support of CONACYT through projects #03270 and #225260-5-2480PB, with logistical support provided by Ducks Unlimited of Mexico (DUMAC). The authors are also grateful to the Mexican Fund for the Conservation of Wildlife for granting Joaquín Bello scholarship #D-O-97/021.

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