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Twenty-year Home-range Dynamics
of a White-tailed Deer Matriline

Discussion


Our finding that females from a single matriline occupied the same summer range during a 20-year period, some concurrently and others separately in time, is not unexpected. Hawkins and Klimstra (1970) demonstrated a high frequency of association between related females, and a low female dispersal rate, both of which we also observed (Nelson and Mech 1981; Nelson 1993). We later documented high philopatry by five other female fawns, one followed to 5 years old, and by nine other adult females followed for a minimum of 3-8 years (Nelson and Mech 1984, 1987). Tierson et al. (1985) reported that yearling and 2-year-old females established home ranges overlapping and adjacent to their mothers' ranges, as did Nixon et al. (1991) for older females as well. Recently, Aycrigg and Porter (1997) also reported a 15-year record of philopatry by one female. These findings predict a high degree of relatedness and spatial stability among adjacent females. The spatial persistence of the matriline in our study and the evidence from other studies carried out in different environments suggest that matriarchal behavior and philopatry are deep-seated traits in the genome, the long-term products of evolution.

The related females we studied showed much spatial overlap in their locations, although they maintained exclusive sites during fawn rearing. The literature emphasizes aggression as the mechanism causing spacing, but observations of the tolerance of maternal females towards other deer in the absence of their fawns suggest that either aggression is limited to the vicinity of the fawn and (or) that deer mutually avoid each other rather than actively search for potential intruders (Dasmann and Taber 1956; Hirth 1977; Robinette et al. 1977; Nixon et al. 1991). Whatever the mechanism, after 4-6 weeks fawns begin to travel with mothers, and social tolerance by maternal females increases (Dasmann and Taber 1956; Hirth 1977; Robinette et al. 1977).

Aycrigg and Porter (1997) observed exclusive ranges only by females 5-year-old females and considered them to be a result of social dominance. However, it is unclear how their finding is affected by the lower frequency of older females than of more abundant younger ones and the use of 50% home-range polygons in their spatial analysis. Furthermore, evidence for a mechanism for exclusive range use other than during early fawn rearing is lacking.

Except for one great-granddaughter's small range expansion and new fawn-rearing area, our matriline's other females used sites within the matriarch's original range, and three of them used the same fawn-rearing sites of their mothers before them, but only after their mothers had died or also changed sites. Ozoga et al. (1982) documented successive annual use of the same fawning site by a matriarch, as well as shifting site use by her nearby daughter. Nixon et al. (1991) also observed a female's fawning site change between years but they attributed this to the presence of an aggressive female.

Obvious individual differences in site use existed between members of our matriline, although they used a common area. A granddaughter and great-granddaughter in our study appeared to abandon their use of different halves of the original matriline's range. For one female, this could have been partially or entirely the result of home-range expansion, but this does not explain why the result is the same for the other female. Conceivably, the presence and fawning territoriality of other maternal females also played a role in those dynamics.

The effects of deer density on matriarchal range dynamics should be manifested most directly through female dispersal. Theoretically, high density should create competition for space and resources that should precipitate higher female dispersal, thus disrupting matriline dynamics. However, the evidence does not support this notion. Two studies that examined dispersal at high densities (27-38 deer/km²) found dispersal rates comparable to dispersal in our low-density (1-3 deer/km²) population, and in both, matriarchies and (or) matrilineal ranges were observed (Hawkins and Klimstra 1970; Nixon et al. 1991). One of those studies reported a higher female dispersal rate, but a more precise dispersal rate measured in a subsample of known mother-fawn pairs was much lower and comparable to our dispersal rate (Nixon et al. 1991). In addition, the study showed no relationship between changing densities and dispersal rate, which was also observed in matriarchies of red deer (Cervus elaphus; Clutton-Brock et al. 1982).

Our results on spatial dynamics are from only one matriline, and two semicaptive matrilines produced the results of Ozoga et al. (1982) on spatial dynamics. Thus, it would be premature to generalize these findings. Many factors besides maternal territoriality may influence where females establish fawn-rearing sites (Nixon et al. 1991). Wolf predation and extremely low deer density are relevant factors in our study, as are the barrier of a fence and a much higher deer density in Ozoga et al.'s (1982). In our study, a spatial vacancy due to predation and the absence of nearby conspecifics could have precluded dispersal or a small range expansion that was otherwise imminent if social pressure influences either process. However, our results do suggest that home-range expansion by free-ranging philopatric female white-tailed deer is an exceedingly slow process. Furthermore, the one female that expanded its range actually vacated a previously used portion of original home range of the matriarch.

Based on the individual spatial dynamics they observed, Ozoga et al. (1982) concluded that deer range expansion depends upon the future production and survival of female progeny. Porter et al. (1991) similarly proposed that deer populations expand spatially through small incremental home-range additions by female progeny which overlap with family ranges. They likened this process to the overlapping petals of a rose. The ages and home-range boundaries of females of unknown, but presumed, relatedness constituted the basis of their model. Our previous and present results confirm the validity of their model.

While the rose-petal metaphor is heuristically appealing, the complexities of home-range expansion which we observed indicate that such a model oversimplifies the reality of deer home-range dynamics, and does not adequately explain the rapid range extension of northern white-tailed deer in historic times (Halls 1984). However, the distances associated with natal dispersal are at least 1-2 orders of magnitude greater than that attributed to the rose-petal metaphor and do explain rapid range extension. Our findings leave no doubt that such extension of deer range occurred primarily through short- and long-distance natal dispersal (Hawkins and Klimstra 1970; Nelson 1993), most of which was probably accomplished primarily by yearling males and to a small extent by less frequently dispersing yearling females (Downing and McGinnes 1976). During the rapid extension of the northern deer range in historic times, these males plus the occasional female positioned near the edge of deer range would have been the primary individuals colonizing new inhabitable areas, probably at a linear rate of 40-50 km per year (Nelson 1993).


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