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Spatial and genetic organization of the island spotted skunk, Spilogale gracilis amphiala.

Mammals are characterized by a general pattern of female philopatry and male-biased dispersal and express varying degrees of sociality, from solitary to highly social (Greenwood, 1980; Dobson, 1982; Liberg and von Schantz, 1985). Female philopatry is expected to generate spatial clusters of related females, which can result in the formation of social groups, and studies from a variety of mammalian species have documented that related females often live in close proximity (e.g., Armitage, 1996; McEachern et al., 2007).

Carnivores are no exception to this pattern (Gompper and Wayne, 1996; Ratnayeke et al., 2002), but they are on the less social end of the mammalian spectrum (Bekoff et al., 1984). Nonetheless, even solitary carnivores are expected to follow the typical mammalian pattern of females establishing home ranges close to their natal origins and often sharing space with female kin (Waser and Jones, 1983). Evaluating this expectation has been hindered by the difficulty of capturing solitary carnivores and monitoring them closely enough to observe their spatial movements and kin relationships (Ratnayeke et al., 2002). Moreover, of the few studies conducted, some have revealed that not all populations of carnivores follow the typical matrilineal pattern (Schenk et al., 1998; Schaeff et al., 1999; Blundell et al., 2004).

The western spotted skunk (Spilogale gracilis) is an elusive species, and little is known of its spatial or social organization (Dragoo, 2009). One subspecies, the island spotted skunk (S. g. amphiala), is restricted to two of the Channel Islands, Santa Cruz and Santa Rosa, off the coast of southern California. We used trapping, radiotelemetry, and genetic analysis of microsatellite variation to investigate spatial organization and its relationship with genetic relatedness in island spotted skunks on Santa Cruz Island. We hypothesized that the island spotted skunk would be solitary (Dragoo, 2009) and exhibit a matrilineal spatial structure resulting from female philopatry and male dispersal (Greenwood, 1980; Waser and Jones, 1983). We expected that relatedness would be negatively correlated with geographic distance for females but show no correlation for males, that neighboring skunks would show little overlap in space use (Bekoff et al. 1984), and that, because of the benefits of kin selection (Hamilton, 1996), same-sex pairs of neighboring skunks would be more likely to share space if they were more closely related.

Santa Cruz Island (34[degrees]0'N, 119[degrees]45'W) is located within Channel Islands National Park and lies 40 km off the mainland south of Santa Barbara, California. The island is ca. 3-11 km wide and 39 km long, with a total area of ca. 25,000 ha. Its climate is maritime Mediterranean, with pronounced wet (December-April) and dry (May-November) seasons. Research on the island was conducted from August 2003-September 2004 at two sites 7.5 km apart. The site at Valley Anchorage (370 ha), located at the eastern end of the Central Valley of the island, was characterized by open grassland, coastal sage scrub, and chaparral. The site at Willows Drainage (410 ha), located on the southern side of the island, had steeper topography and more barren slopes and supported mostly open grassland and chaparral, with little coastal sage scrub.

Adult skunks were live-trapped in Tomahawk single-door box traps placed throughout each study site. We attempted to capture all skunks at Valley Anchorage via repeated trapping throughout the site, but we may have missed some skunks at Willows Drainage because of lower intensity of trapping and inaccessibility of some areas. We collected a 2-mm biopsy of tissue from an ear of each skunk and stored the sample in 95% ethanol for subsequent genetic analysis. We fitted 40 skunks with radio-collars (Holohil Systems, Carp, Ontario). Island spotted skunks are nocturnal, so we determined spatial locations of skunks using a combination of radiotelemetric triangulation during the night, radiotelemetric homing on skunks in dens during the day, and locations of traps in which skunks were captured. Details of methods of trapping and radiotelemetry are provided in Jones et al. (2008).

We used Ranges 6 (Kenward et al., 2003) to determine spacing of centers of ranges and overlap of home ranges between skunks. For each skunk, we determined a rangecenter using the harmonic mean of all locations; for individuals known only from their initial capture, the location of the trap was used as their range-center. We used the 95%-fixed-kernel method, with least squares cross-validation, to calculate annual home ranges for those skunks with [greater than or equal to]25 locations, with each location separated by [greater than or equal to]4 h. We then determined the percentage of overlap of home range for all adults with overlapping or adjacent home ranges, with a modification of Cole's coefficient of association (Kenward, 2001): coefficient of overlap = 2 x ([Overlap.sub.1] x [Area.sub.1])/([Area.sub.1] + [Area.sub.2]), where the home-range [area.sub.x] of skunk x has a proportion of [overlap.sub.x]. Because data were not normally distributed, we used nonparametric tests in JMP (version 4.0; SAS Institute Inc., Cary, North Carolina) to assess the effect of site (Wilcoxon rank sums test) and dyad type (male-male, female-male, and female-female; Kruskal-Wallis nonparametric one-way analysis of variance) on overlap of home range. These nonparametric results should be interpreted with caution due to lack of independence among measures of overlap (i.e., some skunks were represented in multiple dyads).

Genomic DNA was extracted from samples of tissue from ears using either the Qiagen DNeasy 96 Tissue Kit or the Promega Wizard SV 96 Genomic DNA Purification System. We then examined genotypes at the eight microsatellite loci described by Floyd et al. (2011) except for rTt-2, for which our skunks were monomorphic. Polymerase chain-reaction (PCR) amplifications were made in 10-[micro]L reactions (0.5-1.0 ng/[micro]L, DNA, IX Roche FastStart Taq buffer, 0.05 units/[micro]L Roche FastStart Taq DNA polymerase, 2-3 mM Mg[Cl.sub.2], 0.16-0.2 mM dNTPs, 0.1 [micro]g/[micro]L BSA, and 0.25-0.5 [micro]M of each primer) on either an MJ Research, Inc PTC-100 or a DNA Engine Dyad thermocycler. Conditions for amplification began with 4 min at 95[degrees]C, followed by 40 cycles of denaturing for 30 s at 95[degrees]C, annealing for 25-35 s at 50-55[degrees]C, and extending for 30 s to 1 min at 72[degrees]C, with a final extension of 10-30 min at 72[degrees]C. Products resulting from PCR were diluted with 10-30 [micro]L of 98%-formamide loading buffer, denatured for 6 min at 92[degrees]C, separated on 5% denaturing polyacrylamide gels, stained with fluorescent dye, and visualized with a Molecular Dynamics Fluorlmager 595.

For the microsatellite data, Bonferroni-corrected tests for Hardy-Weinberg equilibrium and linkage disequilibrium were conducted using GENEPOP 4.0 (Raymond and Rousset, 1995). After utilizing CONVERT 1.31 (Glaubitz, 2004) to convert the data files, STRUCTURE 2.2 (Pritchard et al., 2000) was used to confirm that both study sites represented the same single interbreeding population; all subsequent analyses treated them as such. Because STRUCTURE assumes Hardy-Weinberg equilibrium, we also did the analysis without the locus that failed to meet this assumption (Lut832), but a single population was still indicated. Finally, pairwise-relatedness-coefficients that did not assume Hardy-Weinberg equilibrium (Queller and Goodnight, 1989) for all possible pairings of skunks were generated using SPAGeDi 1.2 (Hardy and Vekemans, 2002). Relatedness coefficients range from 1 for genetically identical individuals to 0 for unrelated individuals, although negative values can occur when individuals are less related than random individuals (Hardy and Vekemans, 2002).

We used range-centers to calculate pairwise geographic distances between skunks. To assess correlations between relatedness and geographic distance, Mantel tests (Mantel, 1967) in XLSTAT (version 2008.5.01, Addinsoft, New York, New York) were conducted using the Pearson correlation coefficient and 10,000 permutations. To assess correlations between overlap of home ranges and relatedness, use of the Mantel test was precluded because of missing cells or nonsquare matrices. Hence, we first used Spearman's rank correlation to measure the degree of association between the two variables. We then tested the null hypothesis of no significant correlation with a nonparametric permutation test in Resampling Stats for Excel (version 3.2, Resampling Stats, Inc., Arlington, Virginia), using 10,000 iterations (Ratneyeke et al., 2002; Stoen et al., 2005).

We trapped 115 adult skunks (85 males, 30 females) and had sufficient radiotelemetric locations to delineate home ranges for 27 skunks (14 males, 13 females). Coefficients of overlap of home ranges for adults ranged from 0.0-69.1%, with a mean of 22.8% (SI. = 2.21, n = 90). Mean overlap was 24.7% for male-male dyads (SI. = 3.42, n = 38), 19.5% for female-female dyads (SE = 7.97, n = 7), and 21.7% for male-female dyads (SE = 3.14, n = 45). We found no significant effects of site ([chi square] = 1.57, df = 1, P = 0.21) or dyad type ([chi square] = 1.75, df = 2, P= 0.42) on overlap of home ranges.

Genetic analyses included all 115 individuals, with missing genotype data averaging 1%. No linkage disequilibrium was found (21 tests, P [greater than or equal to] 0.05); however, one locus (Lut832) was not in Hardy-Weinberg equilibrium (P < 0.01). Observed heterozygosities for loci ranged from 0.348-0.704, with a mean of 0.556. The mean relatedness among all individuals was -0.01 (SI) = 0.345, n = 3,489 dyads) within study sites and -0.02 (SI) = 0.338, n = 3,066) between sites. Mean relatedness within sites was 0.00 for male-male dyads (SI) = 0.353, n = 1,887), -0.04 for female-female dyads (SI) = 0.325, n = 238), and -0.02 for male-female dyads (SI) = 0.337, n = 1,364).

We found no significant correlation between relatedness and geographic distance for females at Valley Anchorage (r = -0.04, P = 0.54, n = 210) and Willows Drainage (r = -0.01, P = 0.97, n = 28), even when females from the two sites were combined to increase the spatial scale and improve statistical power (r = 0.03, P = 0.58, n = 406). Males at Willows Drainage also showed no significant correlation (r = 0.01, P = 0.85, n = 561), but the correlation was significantly negative for males at Valley Anchorage (r = -0.11, P < 0.01, n = 1,326). We found no significant correlation between relatedness and overlap of home ranges for males at Valley Anchorage (r = -0.21, P= 0.32, n = 23) and at Willows Drainage (r = 0.04, P = 0.88, n = 15) or for females when data were combined for both sites to improve statistical power (r = 0.02, P = 1.00, n = 7).

Mammals on islands might show a number of demographic and behavioral differences compared with mainland species, including higher densities, constrained dispersal, and increased overlap between adjacent territories (Stamps and Buechner, 1985; Adler and Levins, 1994). Island spotted skunks expressed at least one of those characteristics, with densities of populations at Valley Anchorage (19 adults/[km.sup.2]) and Willows Drainage (9 adults/[km.sup.2]) that were extraordinarily high for carnivores (Jones et al., 2008). The skunks we captured exhibited a sex ratio biased toward males. A male bias for captured skunks has been reported previously for Santa Cruz Island (Crooks et al., 2003) and for mainland spotted skunks as well (Doty and Dowler, 2006) and might reflect differential trappability instead of a biased sex ratio in the population (Doty and Dowler, 2006). Consequently, actual densities of adults at our study sites might have been even higher.

Our estimates of overlap of home ranges for island spotted skunks are consistent with expectations and indicate that home ranges of neighboring pairs of adults are largely non-overlapping. However, our mean overlap (23%) is toward the upper end of the range of values reported for other solitary mammals (5-31%; Oli et al., 2002; Burton and Krebs, 2003; Innes et al., 2009; Maher, 2009; Jesmer et al., 2011). Further, we did not radio-track all skunks at each study site. Consequently, actual overlap among skunks may have been somewhat higher, perhaps reflecting the pattern of increased territorial overlap thought to be a characteristic of island vertebrates (Stamps and Buechner, 1985).

The spatial clustering of related females is well known for social mammals, but recent research has revealed that an association between spatial proximity and relatedness also may occur in solitary species because of female philopatry (Ratnayeke et al., 2002; McEachem et al., 2007; Maher, 2009), with females more likely to share space with relatives (Ratnayeke et al., 2002; Moyer et al., 2006; Maher, 2009; Innes et al., 2012). Contrary to these findings, we found no evidence of an association between relatedness and spatial proximity of females in island spotted skunks. Relatedness of female skunks was not negatively correlated with geographic distance, even at the larger spatial scale of combined study sites. Relatedness also was not positively correlated with overlap of home ranges. Hence, our findings join those of the few other studies that have shown a lack of spatial proximity or space sharing by female relatives among solitary mammals (Schenk et al., 1998; Burton and Krebs, 2003; Blundell et al., 2004).

Although increased density often correlates with increased philopatry and kin associations (e.g., Cutiera et al., 2005; Randall et al., 2005), this pattern may break down at very high densities where local competition among kin can promote dispersal (Armitage, 1975; Hamilton, 1996). Further, the competition resulting from high densities might promote increased dispersal distances (Waser, 1985). Thus, unusually high densities in populations and resultant high rates of dispersal out of our study sites could explain why we failed to detect the expected matrilineal structure. Similar results have been noted in other high-density populations of mammals (e.g., Matocq and Lacey, 2004). As proposed by Hamilton (1996), intermediate densities in populations may be best suited for maintaining spatial clusters of kin and promoting kin-based sociality (McEachem et al., 2007).

Alternative explanations for a lack of a matrilineal spatial organization seem unlikely. The male-biased sex ratio in skunks in our study suggests that some adult females may have been present but not studied; however, missing females would affect our results only if the uncaptured females themselves were a biased subset, such as if we consistently failed to capture one female of each dyad of closely-related females. Limited dispersal, a trait sometimes found in mammals on small islands because of the physical constraint of boundaries of the island (Adler and Levins, 1994), might preclude the formation of a gradient of decreasing relatedness with distance. However, Santa Cruz Island is a large island, almost 500 times larger than the typical home range of an island spotted skunk (52 ha; Jones et al., 2008). Limited dispersal also might result if high densities create a social fence that discourages dispersal (Hestbeck, 1982; Matthysen, 2005). However, we did not detect the high levels of relatedness among individuals within study sites that should result from extensive philopatry.

Although we found no evidence of a matrilineal spatial structure, we did find evidence of a patrilineal structure at one study site, Valley Anchorage, where relatedness among males was negatively correlated with distance. The explanation for this pattern is obscure, but it may be related to density, which was about twice as high at Valley Anchorage as at Willows Drainage. Dispersal in spotted skunks has not been studied, but it is possible that density influenced differences in dispersal between sexes, as has been found for kangaroo rats (Dipodomys spectabilis; Jones et al., 1988). Hence, perhaps both sexes dispersed beyond the study site at Willows Drainage, but only females did so at Valley Anchorage. Another possibility is that male skunks form coalitions that bring relatives together. Male coalitions have been reported for raccoons (Procyon lotor), perhaps in response to a high density of females (Gehrt and Fritzell, 1998).

Our results of spatial organization of the island spotted skunk are consistent with those of other species of solitary mammals, but our findings of no matrilineal spatial structure, along with evidence of a patrilineal structure at one site, are not. Generality of our findings is uncertain because the island spotted skunk is an insular endemic subspecies, and island forms may express differences in density, behavior, and dispersal compared with the mainland (Adler and Levins, 1994). Nonetheless, our study advances our understanding of the spatial and genetic structure of an insular endemic carnivore, the first such study of which we are aware, and perhaps also sheds light on the influence of high densities, at levels rarely achieved on the mainland, on spatial and genetic structure.

This research was made possible through the support and guidance of E. Aschehoug, S. Morrison, D. Garcelon, the staff of the Institute for Wildlife Studies, L. Laughrin, the staff of the University of California Natural Reserve System, and the members of the Genomic Variation Laboratory at the University of California, Davis. We thank S. Krause, A. Blackford, and B. Burkholder for assistance in the field, N. Willits for statistical consultation, and C. Floyd for sharing his primers and expertise in the genetics of S. gracilis. Financial assistance was provided by grants from The Nature Conservancy and the Santa Cruz Island Travel Fund. The study was conducted under a Memorandum of Understanding between the University of California, Davis and the California Department of Fish and Game (expiration on 31 December 2007).


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Submitted 14 February 2012. Accepted 3 February 2014.

Associate Editor was Marlis R. Douglas.


Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA 95616 (KLJ, DHW)

John Muir Institute of the Environment, University of California, Davis, CA 95616 (MBM)

Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, CO 80523 (KRC)

Museum of Southwestern Biology, MSC03 2020, University of Neto Mexico, Albuquerque, NM 87131 (JWD)

Department of Animal Science, University of California, Davis, CA 95616 (BM)

* Correspondent:
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Author:Jones, Krista L.; Van Vuren, Dirk H.; McEachern, Mary Brooke; Crooks, Kevin R.; Dragoo, Jerry W.; Ma
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Date:Dec 1, 2013
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