Author: Nixon, Charles M
Date published: April 1, 2010
(ProQuest: ... denotes formulae omitted.)
How social groups form and maintain stability through time has not been well documented in the Midwest Agricultural Region. Permanent cover is scarce and fragmented in this Region, and female whitetails exhibit high levels of dispersal or migratory behaviors manifested in response to high spring population densities and limited parturition sites prior to agricultural crop maturation (Sparrowe and Springer, 1970; Vercauteren and Hygnstrom, 1994; Brinkman et al, 2005; Nixon et al, 2007, 2008). Social bonding among ruminants appears to have evolved as a means to better compete for resources and to aid in avoiding predation (Weckerly, 1999) . Family groups develop when the inclusive fitness of offspring that remain sedentary exceeds that of offspring that disperse from their natal range (Emlen, 1994).
White-tailed deer society is organized as groups of related deer that are led by a dominant matriarch surrounded by various relatives of both sexes that share portions of the matriarch's home range (Nixon et al, 1991; Porter et al, 1991; Miller and Ozoga, 1997). As dominance seems to be a function of age in white-tailed deer (Townsend and Bailey, 1981), the key to obtaining a dominant position is survival to an adequate age (Porter et al, 1991). Females must live long enough on a stable home range to produce offspring that perpetuate landscape tenure, with die matriarch and daughters able to fend off competing conspecifics.
Female fawns learn about their female relatives as early as the first month of life and this familiarity continues throughout life (Schwede et al, 1994; Tang-Martnez, 2001). Social inheritance of a home range among females evolves because selection should favor minimal dispersal of females, allowing grouping to reduce predation risk (Nelson and Mech, 1981). Proliferation of these social groups located on the most secure home ranges maintains the highest number of deer closest to the safest landscape (Nelson and Mech, 1981 ; Ozoga et al, 1982; Mathews and Porter, 1993; Mathews et al, 1997).
In this paper we were concerned with the effects of social rank on individual characteristics such as longevity and reproductive success, rather than how social rank was obtained at the time of capture (through dyad encounters won and lost) . Dominant and subordinate rank refers to the female social position in the population as a whole based on years of deer observations on the study areas, often from shortly after birth until death. We compare the frequency of association, survival, fawn recruitment and dispersal rates for known dominant and subordinate females and their offspring on three study areas in Illinois where female dispersal is a prominent characteristic of population dynamics. We hypothesize that social group longevity is the result of individual social behaviors and landscape stability that combine to produce long-term occupancy of a particular location.
Deer were captured using rocket-propelled or drop nets (Hawkins et al, 1968) on areas located in west-central (WC, 1990-1993), northern (NO, 1990-1993) and east-central (EC, 1980-1985) Illinois (Fig. 1). Each study area contained a mix of private and public lands and included a forest-dominated public park. These areas were surrounded by privately owned farms dominated by row crops and were hunted Oct.-mid Jan. (archers) and Nov.early Dec. (firearm hunters) under "any sex" regulations. In addition, portions of die park on the NO area were hunted by archers and portions of the WC area were hunted by both firearm and archery hunters. Only the EC area was closed to all hunting during our studies (Fig. 1).
Deer were aged as fawns (<12 mo), yearlings (13-23 mo), 2 y old females (24-35 mo), or adults (≥36 mo) on the basis of tooth replacement and wear (Severinghaus, 1949). Deer that were <3 y old when captured could be aged by observation of the premolars and incisors on live deer. These known aged deer enabled us to assign our observations to a particular age class each year.
Males were marked with ear streamers, metal ear tags or numbered cattle-type plastic ear tags (Y-Tex Corp., Cody, WY) and selected males with radio collars (Wildlife Materials, Carbondale, IL; Telonics Inc., Mesa, AZ; Advanced Telemetry Systems, Inc., Isanti, MN). Females were marked with plastic collars bearing reflective numbers or with radio collars. Radios were placed on deer (both sexes) within 24 of the 25 social groups led by a dominant female and within 14 of the 17 known groups led by a subordinate female. Radios were functioning until field work ended in 19 of the 25 dominant groups and within eight of the 17 subordinate groups.
Social Position. - Female social position was determined from observations of marked individuals made throughout the duration of our field studies. For dominant females, observations averaged 74 ± 9.2 (S. E.) for an average of 3.7 ± 0.4 y after tagging. For subordinates, observations averaged 42 ± 5.6 observations for an average of 2.4 ± 0.3 y. We included only those observations for which we had a complete count of deer present. Behaviors that indicated dominance or subordination within each group included aggressive encounters won or lost, leadership among related deer, leadership of large groups of related and unrelated deer in winter when Illinois deer group together after breeding and crop harvest, and duration of ownership of stable home ranges that included successful parturition sites. Dominant females were those that were consistently successful in dyad encounters (100% within related female members of the group as well as most encounters with unrelated neighbors), were leaders among related females in traveling between bed sites and feeding areas, and that led larger groups of deer in winter when such groups were common on all three areas. Females were considered subordinate when they were always observed alone (except for their fawns), frequently exhibited migratory or dispersal behaviors, and were frequently the loser in dyad encounters with other mature females.
Associations. - Associations among yearling, 2 y old, and older daughters with their mother was compared seasonally using Chi-square likelihood ratios. Seasons were postpartum (Jun.-Sep.), breeding (Oct.-Nov.), and winter-spring (Dec-May), months when there were significant changes in deer movements and behavior. When multiple comparisons of observations among ages were made, two-tailed tests were adjusted using the Bonferroni Correction, resulting in significance levels of 0.012 or 0.017, the level used dependant on the number of comparisons made for the same age class (Holm, 1979) .
Longevity. - We compared longevity between dominant and subordinate females using one-way analysis of variance. We knew the dates of birth (assumed to be 1 Jun.) and death for all the females used in the analysis.
Recruitment. - Fawn recruitment was determined from fawn searches on the EC and NO areas, and from daily (EC and WC areas) and weekly (NO area) observations of fawns (marked and unmarked) of our marked females. For subordinate females located away from the refuges, we used both observations and deliberate attempts to walk up and observe radio-marked females and their associates once or twice each year. We used two dates to measure recruitment success; 1 Oct. as the date of weaning and 1 Jun. when fawns reached independence at 1 y of age (Clutton-Brock et al, 1982) . We based these dates on behavior of marked fawns with very litde nursing observed after 1 Oct. and observations of fawnsyearlings seen alone after 1 Jun. as well as the extensive dispersals and migrations of fawns 10-12 mo old (Nixon et al, 2007).
Dispersal Behavior. - Dispersal was defined as a one-way movement at least 5 km away from the natal range. Dispersals occurred in late spring-early summer for both sexes and Oct.Nov. for males only. We used Chi-square likelihood ratio tests to examine differences in dispersal rates between fawns of dominant-led and subordinate led social groups.
Social Position. - There were 25 females (three yearlings, 1 1 2 y old females, 1 1 adults when captured) observed to be dominant members of the population on the three study areas (EC = 9, NO = 7, WC = 9). Of these 25 females, 12 were known leaders of others within their social group, and in winter, leaders of mixed sex social groups of up to 40 deer. Thirteen additional females were sedentary on stable home ranges for extended periods (up to three generations during our study) that produced large fawn crops. These females were associating with other females at the time of capture and likely inherited a stable home range which they occupied during our studies. No dominant female was a known orphan.
Groups led by a dominant female averaged 5.2/y on the EC study area (n = 9 deer, range 2-12), 4.0 on the NO area (n = 7, range 2-7) and 4.4 on the WC area (n = 9, range 3-7). These differences in group sizes were not significantly different (P > 0.05) . These numbers somewhat reflected the impact of hunting within portions of the study areas, from total protection on the EC refuge, to the heaviest hunting pressure on the WC area. We base this assumption on the higher losses due to hunting found on the WC and NO areas compared to the EC area. A total of 27 marked deer were killed on the WC area during the study, 12 of whom were members of a dominant-led study group. Twenty-one deer were killed on the NO area by archers and 14 of these were members of a study group. There were no hunting related deaths within the boundaries of the EC refuge during our study, although several members of our study groups were killed by hunters on lands adjacent to this refuge.
There were 17 females (two fawns, seven yearlings, five 2 y old females and three adults when captured) observed to be subordinate members of the population (EC = 13, NO = 3, WC = 1 ) . Three of these females were known orphans (known deaths of marked mothers of marked fawns) , without kin support and who evidenced no settled home range on the study areas. These three deer were migrators, moving away from a study area in the spring and returning in the fall. Three others were also migrators but were not known to be orphans. Five of the subordinate females dispersed away from the study areas and remained away until death. The remaining six females were sedentary on the study areas, were isolated by location (on the edge of the study areas in a mix of refuge and hunted landscapes) and were never observed with other females except their own fawns. These ranges featured seasonally unstable conditions including a high percentage of row crops and frequently flooded bottomland forests. One of these females on the EC refuge appeared to be driven from an established parturition site (3 y of successful parturition) by a very aggressive dominant marked female who suddenly appeared on the site. The resident female moved to a new site, had a fawn and then wandered until killed by a hunter. Another EC female lost her parturition site when it was converted to row crops after 3 y of successful parturition and fawn rearing. She moved off refuge, fawning in a small woodlot where she was killed by a hunter during the first hunting season after moving away from the refuge. This female was apparently unable to select a new parturition site on the EC refuge already occupied with established mature females.
Group size led by subordinate females averaged 2.4 deer/y in EC Illinois (n = 12 females, range 1-4) and 2.5/y in NO Illinois (n = 3, range 2-3) . Group size for the single female from the WC area averaged only 2 deer/y. These group sizes were not significantly different among areas.
Assoaations. - The association between dominant mothers and their female offspring remained quite high throughout life, particularly in winter, but with close association (seen feeding or bedded togetiier) declining with advancing age of the offspring (Table 1). In summer, females >2 y old were observed less often with their mother than yearling females (χ^sup 2^ = 35.9, P < 0.0001). During the breeding season yearling females were also observed with their mother much more often than older females (P < 0.0001) (Table 1). Females ≥3 y spent the same amount of time with their mother in summer as did 2 y old females but were observed less often with her during the remaining months (P < 0.0001) (Table 1). Older females also associated with other relatives more often than 2 y old daughters and granddaughters. In fall and winter, however, as groups began to associate more together, 2 y old daughters were seen more often with their mother than were older daughters (Table 1) .
Excessive mortality and movements away from the natal range by female offspring of subordinates reduced the opportunities for observing extended associations. Fourteen of the 19 fawns of subordinates dispersed or migrated away from the study areas to areas where observations were more difficult (private lands with limited access) . For 24 marked fawns of these subordinate females (10 female, 14 male), nine of 10 females dispersed/migrated away from their mother and as did 11 of 14 males. The remaining four marked fawns were killed before reaching 1 y old and independence.
Longevity. - For 25 dominant females, 20 survived to the end of our studies. Five females were killed before the end of our observations and in each case, the oldest marked daughter replaced the dead matriarch in the social group. Dominant females survived significandy longer than did subordinate females (F = 7.8, df = 1,40, P = 0.008) Table 2). This difference in female longevity was related to the higher hunting losses and highway accidents experienced by subordinate females. Subordinate females were frequently located off refuge in areas featuring limited protective cover and environmentally unstable features such as frequently flooded bottomland forests or a high proportion of row crop agriculture, exposing them to higher mortality.
Longevity of fawns born to dominant and subordinate females was nearly identical for both sexes (Table 2) . Second generation fawns of dominant females (too few of diese deer were available from subordinates) survived into breeding age and helped maintain the integrity of the social group (Table 2).
Recruitment. - Fawn recruitment was also affected by the social position of the mother. The number of fawns alive at weaning age (1 Oct.) was not significantly different (F = 3.5, df = 1,42, P = 0.06), but the number of fawns alive at 1 y old was significantly higher for dominant mothers compared to subordinate mothers (F = 13.8, df = 1,42, P = 0.001) (Table 2).
The effect of dominant female presence on female fawn breeding rates has been extensively examined (McCullough, 1979). We examined the proportion of fawn breeders observed with live fawns in late summer and fall and found no indication that fawn breeding was suppressed within these groups on the EC and NO study areas. It is possible fawn breeders may have conceived and then aborted a fetus, but this is rare in well nourished whitetails (Verme, 1962) . However, fawn breeding was infrequent throughout the years of our studies on the WC study area. Total estimated female density on the WC area was higher (...) areas (Nixon el al, 1994). Yet our data for dominant social groups indicated that group size was somewhat smaller on the WC refuge. These data may indicate a social effect suppressing female fawn breeding on this study area but was not manifested on the EC and NO areas.
Breeding rates for female second generation fawns born within dominant led groups were also high on the EC and NO study areas (EC =11, 64% breeding; NO = 3, 66% breeding) with no second generation female fawns breeding on the WC refuge.
Our data do show a sex ratio difference for offspring of dominant mothers that relates to the age of the dominant breeder. For females <3 y old, 72% of fawns born were female (n = 25 breeders, 68 live fawns, X^sup 2^ = 6.6, df = 1, P < 0.05.); for breeders 2:3 y old the sex ratio was nearly equal (49% female for 69 fawns) . Fawns born to subordinate females regardless of breeding age averaged 50% female (n = 38 fawns) .
Dispersal behavior. - Dispersal rates for fawns of both sexes born to subordinate females were significantly higher compared with fawns born to dominant females (males - X^sup 2^ = 9.2, df = 1,53, P = 0.002; females- X( = 18.4, df = 1,66, P < 0.001) (Table 2). The lower dispersal of males born to dominant females was unexpected given the high rate of dispersal for all males after family breakup in the spring. This higher dispersal rate for fawns of subordinates contributed to the difficulty of maintaining a stable social group.
There was an effect of progression of female birth on dispersal rates for marked fawns born to dominant females. One hundred percent of first born marked females remained on their natal range compared to 68% of later born females (n = 18 female fawns of first birth, 41 females born later; X^sup 2^ = 11.0, df = 1,57, P = 0.0001). This difference may be the result of first born females usurping the limited parturition ranges available prior to agricultural crop maturation in central and northern Illinois. Because we did not always know when our dominant females assumed their social standing, this effect may or may not be entirely the case. This effect was not significant for males, with 55% of first born males remaining sedentary (n = 9) compared with 46% of later born males (n = 35) (P > 0.05).
The role of female dominance in controlling population status as a whole has received little attention in Illinois. The scattered nature of sufficiently large habitats promoting female land tenure also promotes high dispersal/migration behavior in females and dispersal in males born later to dominant females and for nearly all births from subordinate females. These movement behaviors effectively buffer hunting pressure on smaller, less stable habitats. Yearlings are continually moving from high quality habitats to areas less stable where they are subject to numerous life threatening situations (Nixon et al, 1991, 2007).
High female dispersal rates from the EC and NO areas and hunting losses from all three study areas did not reduce female occupancy, as first born females almost always remained on their mother's range. If the matriarch died, the oldest living daughter became the leader of the social group. Later born females showed a mixed response, with some dispersing and others remaining sedentary. More crowded conditions on limited habitats apparently promoted female dispersal from theses areas (Campbell et al, 2004; Nixon et al, 2007).
The selection of a stable, secure home range is vital to individual reproductive success. McLoughlin et al (2007) found that a home range offering nutritious foods and adequate cover was significantly related to lifetime reproductive success for female roe deer ( Capreolus capreolus) in France. This was also true for white-tailed deer in Illinois.
There is no evidence that dominance is a heritable trait for white-tailed deer (Barrette, 1993) . However, body size and mass and perhaps intelligence are heritable and may aid in establishing dominance (Barrette and Vandal, 1986; Holekamp and Smale, 1991; Hyland et al, 2004; Sheldon and West, 2004; Taillon and Cote, 2006). Fawns of dominant females inherit by association a stable home range free of undue human and predator harassment and occupied by a mature, well adapted matriarch capable of leading relatives through the rigors of daily living (McCullough, 1979; Nelson and Mech, 1981). As an example, Etter et al, (2002) observed that dominant females timed road crossings to lulls in traffic patterns while yearlings without kin support were often killed crossing roads in the Chicago metro area.
In contrast, subordinates, whether emigrants from other areas without female kin, females that were orphaned before 1 y old, females that were physically impaired at any age, or fawns of migrating or dispersing females that also evidenced wandering behaviors, did not usually have access to home ranges that were free of environmental problems. These females lost proportionally more fawns than did dominant females, a result also reported for penned deer in Michigan (Ozoga et al, 1982). However, subordinates were allowed to feed on our capture bait sites where aggression from dominants was usually short lived, similar to behavior observed for female Roosevelt elk (Cervus elaphus Roosevelt) (Weckerly, 1999). Food resources are dispersed and abundant in Illinois, and subordinates were generally well fed. Life span and fawn recruitment was reduced for our marked subordinate females, while dispersal rates were higher as these females had difficulty in occupying landscapes offering protection and safe parturition. Subordinates were generally younger than dominants, and older females usually breed earlier than do younger females (McCullough, 1979; Taillon and Cotes, 2006; Haskell et al, 2008) . Earlier breeding produces fawns with a competitive advantage over fawns born later in the year and helps perpetuate land tenure and continued social dominance between generations.
Whether or not females should invest in sons (Trivers and Willard, 1973) or daughters (McCullough, 1979; Caley and Nudds, 1987; DeGayner and Jordan, 1987) remains unproven for white-tailed deer (Hewison and Gaillard, 1999). We found younger dominant females produced significantly more daughters than older dominants or subordinate females, both of whom produced nearly a 50:50 sex ratio of offspring. This resulted in a more rapid increase in the number of females available for the social group. Reproductive success was significanüy greater for red deer females above the median dominance rank, as more calves were born and survived to 1 y (Clutton-Brock et al, 1986). This was also true for our marked females. (Mech et al, 1991) reported that the benefits in increased survival and body condition of well fed fawns before and after parturition extended well into the life span of the female offspring. This was likely also the case with dominant Illinois females.
The reduced dispersal behavior for males born to dominant females was not expected. If male dispersal in spring is a result of incest avoidance and fall dispersal by yearling males prevents mating competition, the abundance of females in our dominant social groups should have increased male dispersal (Shaw et al, 2006; Long et al, 2008). However, unrelated females were also more abundant on the study areas compared to the surrounding private farms and would provide males with increased breeding opportunities away from their social group. Marked yearling males that did not disperse usually left their natal range in the fall until breeding concluded when they often returned to some portion of their mother's range for the winter (Nixon et al, 1994).
White-tailed deer are very adaptable, able to quickly adjust to changing environmental conditions. Selection of an environmentally stable home range enables ungulate females to live long enough to rise in the social hierarchy and to develop a kin related social group that ensures generational longevity on these ranges (Thouless and Guinness, 1986). Where permanent vegetation is finite (row crops offer no cover in winter, forests cover < 1 0% of the landscape in many counties of central and northern Illinois) , much can happen to weaken or destroy this effort. Matriarchs may be killed by hunters, die on highways, or may be crippled enough to reduce social status and are continually tested by other females for space. Two females (EC - 503; NO - 180) were dominant until ages 3 and 5 y, respectively, when both were crippled by hunters. Both females subsequendy became loners, isolated from their female relatives.
Protection of these stable habitats appears to be important in maintaining the current high populations in the agriculture dominated landscapes of the Midwest. Because of high dispersal behavior of females in this region, deer population control using localized removal programs will need to adopt an annual removal strategy. Adjacent social groups, whether led by dominant or subordinate females, will continually provide dispersing deer of both sexes to repopulate these areas. To date there have been no studies in the Midwest to examine if localized control of deer numbers will work where female dispersal behavior exceeds 10% (Campbell et al., 2004; Porter et al., 2004) . (Porter et al., 2004) found that culling in excess of 75% was necessary when dispersal behavior was >25%, a rate recorded in some years on our EC and NO study areas (Nixon et al., 2007). The amount of landscape offering females an environmentally stable home range in order to perpetuate social bonding is often limited in much of Illinois, perpetuating high dispersal behavior in both sexes.
Acknowledgments. - This study would not have been possible without the able assistance of R. Koerkenmeier, J. M. Nelson, M. Challand, S. Rueff, M. E. Billman, R. Sawtelle, M. Nelson, W. Iko, J. Cruise, R. Conover, B. Martin, S. White, D. Tazik, J. Dewalt, D. Dillow, G. Potts, R. Seimers, J. Brown, M. Schmierbach, J. Legare, C. Nixon, J. Nixon, J. McNamara, J. Bothwell, R. Harrison and J. Ver Steeg in capturing deer. J. Seets, Illinois Natural History Survey, assisted in many ways. D. Bowman, J. Assell and J. Sandine, site superintendents of Robert Allerton Park, Siloam Springs State Park, and Shabbona Lake State Recreation Area, respectively, provided study areas and logistic support. We also thank the numerous landowners who allowed us access to their farms. This paper is a contribution (in part) of Federal Aid in Wildlife Restoration project W-87-R, Illinois Dept. Natural Resources, U.S. Fish and Wildlife Service, and the Illinois natural History Survey cooperating.
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SUBMITTED 27 FEBRUARY 2009 ACCEPTED 27 JUNE 2009
CHARLES M. NIXON1
Illinois Natural History Survey, Champaign 61820
PHILIP C. MANKIN2
Department of Natural Resources and Environmental Sciences, University of Illinois, 1101 West Peabody, Urbana
DWAYNE R. ETTER
Michigan Department of Natural Resources, P.O. Box 30444, Lansing 48909
LONNIE P. HANSEN
Missouri Department, of Conservation, UlO South College Ave., Columbia 65201
PAUL A. BREWER
Illinois Department Natural of Resources, 1660 West Polk Ave, Charleston 61920
JAMES E. CHELSVIG
Forest Preserve District of Cook County, 536 North Harlem Ave., River Forest, Illinois 60305
Illinois Department of Natural Resources, 4295 North 1000th St., Newton 62448
JOSEPH B. SULLTVAN
RR #5, Box 201A, Mt. Sterling Illinois 62353
1 Corresponding author: 19 Westwood, Monticello, Illinois 61856; e-mail: email@example.com
2 Present address: 5 Steele A Way, Brooksville, Maine 04617