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Conservation attempts to preserve species as well as their genetic diversity (Schonewald-Cox et al. 1983; Avise 1996). Roads can have a major impact on genetic diversity, primarily through altering gene flow (Fankel & Soule 1981; Forman et al. 2003; Conard & Gipson 2006; Jackson & Fahrig 2011). The potential barriers created by a road can increase genetic drift when a population is fragmented (Wright 1931). A second impact of roads on wildlife is mortality caused by impacts with motor vehicles. Some researchers have noted that populations are influenced by vehicle induced mortality (VIM; e.g. Caro et al. 2000; Bujocek et al. 2011). There are only a few studies that have investigated the possibility of selection in relation to VIM. Bujocek et al. (2011) investigated the possibility of selection by VIM in birds. The potential of effects of VIM on selection in mammals has been studied in red foxes (Vulpes vulpes; Baker et al. 2007), gray wolves (Canis lupus; Whittington et al. 2004), and black bears (Ursus americanus; Brody & Pelton 1989) by assessing movement patterns. Several studies have investigated the effects of roads on fragmentation and gene flow (see Balkenhol & Waits 2009; Jackson & Fahrig 2011). If selection is influenced by VIM, this could exacerbate the fragmentation effects roads have on a population and further reduce gene flow within the population (Wright 1931). However, if VIM is random, genetic drift may not be affected by this source of mortality.

The objective of this study was to investigate the importance of selection on age class and sex in relation to VIM. Although there are other characters that affect selection within a population (e.g. sex, pelt color, aggressiveness, foraging), selection could be occurring if certain age groups or sexes are subjected to higher rates of VIM than other groups. Raccoons (Procyon lotor) make excellent organisms to test this hypothesis. The species is urban-adapted and has higher densities in both suburban and urban areas than many other species (Gehrt 2003; Rosatte et al. 2010; Graser et al. 2012), making them more likely to come into contact with roads. These higher densities have been implicated in increased VIM (Conard & Gipson 2006). Additionally, raccoons killed by vehicles are large enough to be relatively easily detected while driving, reducing accidental exclusion from collection.

This study was conducted in Harrison County, Texas, where raccoon roadkill specimens were collected from the U.S. Highway 80 corridor between Hallsville and Marshall, TX, including the urban areas of both cities. This road varies from a two-lane to a four-lane highway surrounded primarily by forested areas with interspersed pastures. Collection occurred from 1 April 2012 to 1 October 2013. The road was driven twice daily (morning and afternoon) during the duration of the study to increase the chance of detecting most of the raccoon road mortalities occurring daily and nightly along the route. Sex of the raccoon was recorded on site. The mandible, with teeth, was removed for assessment of age via tooth-wear. Raccoons were aged using categories of Grau et al. (1970; Age I 0-14 months, Age II 15-38 months, Age III 39-57 months, Age IV 58-86 months, Age V [greater than or equal to]87 months).

A Kruskal-Wallis test was used to compare age differences between male and female raccoons. Age class data were tested for randomness using a Goodness of Fit test comparing the distribution of specimen age classes to a Gaussian distribution generated using Excel (Microsoft 2013) based on the collected data. All remaining data were analyzed using SPSS (IBM 2012). A second test for randomness of specimen age classes was made by comparison to a known raccoon population from a mesic hardwood forest located in a suburban setting 50 km north of Memphis, TN (Ladine 1995). The authors acknowledge that a disparate population is not ideal for comparison to population parameters of the current study, since geographic variability among populations of raccoons is known to occur (see Gehrt 2003). However, lack of data from a more proximate population was unavailable, and the second test allows a comparison of age classes to a known population in a similar habitat and climate. Previous studies (Case 1978; Rolley & Lehman 1992) indicated that VIM is not a good indicator index of population densities, therefore density estimates were not attempted for this study.

Data from 33 raccoons were collected. Sex ratio of the collected raccoon specimens was slightly biased toward females 0.83:1.0 (male: female). However, there was no significant deviation from a 1:1 ratio ([X.sup.2]=0.1366, P=0.7116). Sex ratio of collected raccoon specimens in this study differed from some mark-recapture studies (Kaufmann 1982; Sanderson 1987; Ladine 1995; Gehrt & Fritzell 1996b). However, female biased sex ratios are not unknown in raccoons (Gehrt & Fritzell 1996a; 1998), and the difference between this study and previously published data may be an artifact of small sample size. Further study and a larger sample size will be required to determine if a female biased sex ratio in VIM could exist for this population.

A Kruskal-Wallis test indicated no significant difference in ages between sexes (H=0.037, P=0.843). The mean age class of raccoons was between II and III. No age class V individuals were collected. The distribution of the age class of raccoons (number of raccoons collected: 8:14:8:3, for Age Classes I, II, III and IV, respectively) did not deviate significantly from the expected Gaussian distribution ([X.sup.2]=0.042, P=0.9998) nor from the distribution found in Ladine (1995; [X.sup.2]=0.123, P=0.9972). Therefore, age and sex were not deterministic of VIM in this population of raccoons.

No Age Class V individuals were collected. Additionally, animals were collected throughout an entire year with no increases in any age class collected, except for Age Class I individuals. These raccoons are young-of-the-year individuals and are usually present in larger numbers during the late summer to fall (July-October) when they are becoming independent of mothers (see Lotze & Anderson 1979; Gehrt 2003). There were two incidences of VIM involving an adult female and a young raccoon. The timing of the increased collection of Age Class I individuals in the current study could be related to novel experience at roads without the presence of the mother at weaning.

Alternatively, VIM of raccoons observed in the present study may be attributed to speed and density of traffic. Raccoons are primarily nocturnal with the greatest amount of activity between 2300 h and 0100 h (Ladine 1997; Gehrt 2003). The timing of raccoon activity at night may make it difficult for drivers to detect raccoons and avoid collisions. With one exception, all raccoons, regardless of age, were found on stretches of road with posted speeds between 88 and 105 km/h. The exception was an Age Class I raccoon located on a road with a posted speed of 56 km/h, but with a higher traffic density (Ladine pers. obs.). Previous studies have found a positive correlation between vehicle speed and VIM as well as traffic density and VIM (Case 1978; Rolley & Lehman 1992; Caro et al. 2000).

Predation has been used to assess selection by comparing predation to VIM in songbirds (Bujocek et al. 2011). However, adult raccoons have few predators, and the known predators in the study area, bobcats (Felis rufus), coyotes (Canis latrans), and great-horned owls (Bubo virginianus) (Whitney & Underwood 1952; Sanderson 1960; Johnson 1970) most likely occur at low densities. Therefore, due to lack of known rates of predation and few other known sources of induced mortality apart from vehicles and hunting (Stuewer 1943; Whitney & Underwood 1952; Sanderson 1960; Johnson 1970), this type of comparison was not possible in this study. Estimates of individual fitness such as mass-to-girth ratio and kidney fat indices could elucidate whether individual fitness contributes to VIM. Mass-to-girth ratio has been shown to be a poor indicator of fitness in raccoons (Ladine 1998); however, there is a correlation between kidney fat index and fitness in raccoons (Johnson 1970). None-the-less, the state of decomposition of specimens in this study made these methods unreliable as an estimator of fitness relative to VIM.

Roads result in fragmentation of habitat for a considerable number of species. However, for meso-carnivores, fragmentation of habitat appears to be minimal (Oxley et al. 1974; Caro et al. 2000) and roads do not serve as a major barrier to separate populations. Nevertheless, populations of meso-carnivores can become fragmented by other means and VIM could become important in altering genetic diversity. Additional studies are needed to determine the effect of VIM on meso-carnivore populations and occurrence of any VIM-induced genetic drift.


We thank J. C. Cone and M. Miller for their constructive comments on this manuscript. We also thank D. Brooks for notifying us of animals along the roads that were used in the study. Finally, we thank our reviewers for their constructive comments on this manuscript.


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Michelle Ray and Troy A. Ladine

Department of Biology and Chemistry East Texas Baptist University Marshall, TX 75670

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Author:Ray, Michelle; Ladine, Troy A.
Publication:The Texas Journal of Science
Article Type:Report
Date:Dec 1, 2017

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