Comparison of Morphological versus Molecular Characters for Discriminating between Sympatric Meadow and Prairie Voles

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ABSTRACT.-

Prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) voles are morphologically very similar species of rodents that are often found sympatrically. To assess the reliability of morphological characters typically used to differentiate between these two species, we compared the concordance of species identification based on morphological characteristics with identifications based on a species-specific difference within the avpr1a gene. We found that intraspecific variation in morphological characteristics resulted in erroneous or ambiguous species identification in the field (generally ≤5%) as well as for preserved specimens (up to ~45%). Our data suggests that genotyping putative M. ochrogaster and M. pennsylvanicus at their avpr1a locus may be warranted for some individuals to ensure accurate species identification.

INTRODUCTION

In most ecological or behavioral studies, accurate species identification of the animals is critical. In the field, mammalogists typically use a combination of external morphological characters, such as pelage coloration and patterning, the length of the tail relative to the body, the length of the tail relative to the hind foot, the number of plantar tubercles and the number of mammae to distinguish between species (e.g., Gottschang, 1981; Hall, 1981). However, in some cases, external morphology may be unreliable, either because the two species in question are extremely similar in appearance, or due to intraspecific variation in supposedly diagnostic morphological characters. For example, in a region of sympatry in the eastern United States, subspecies of Peromyscus maniculatus (deer mouse) and P. leucopus (white-footed mouse) are so morphologically similar that accurate species determination in the field is almost impossible, and a sizeable literature is devoted to methods for distinguishing between the two species (e.g., Rich et al, 1996; Bruseo et al, 1999; Lindqtiist et al, 2003). Where external morphology is unreliable, dental characteristics may allow the determination of species (e.g., Hall, 1981). However, the observation of dental characters typically requires sacrificing the animal, making the use of dental characters unsuitable for studies in which the observation of individuals through time is necessary, such as many field studies that utilize catch-mark-recapture techniques. Thus, reliable identification of all individuals for some sympatric species may be extremely difficult in certain field situations.

When the identification of morphologically similar sympatric species in the field is difficult because of intraspecific variation or interspecific overlap in external morphology, species-specific differences at the molecular level may provide a more accurate means for identifying species. For example, Aquadro and Patton (1980) demonstrated that speciesspecific differences in salivary amylase electromorph phenotypes could be used to unambiguously distinguish between morphologically similar Peromyscus leucopus and P. manimlatus. More recently, several DNA-based techniques have utilized species-specific nucleotide polymorphisms to identify a variety of species of mammals (Beriy and Sarre, 2007; O'Reilly et al, 2007; Moran et al, 2008). These molecular diagnostics have proven extremely useful in cases where it is difficult or impossible to distinguish between morphologically similar species in the field.

Microtus ochrogaster (prairie voles) and M. pennsylvanicus (meadow voles) are morphologically similar species of arvicoline rodents that occur sympatrically in some parts of the midwestern United States (Reich, 1981; Stalling, 1990). Numerous field studies have examined the ecology, demography and social behavior of these two species in areas of symapatry (e.g., Krebs, 1977; Getz et al, 2001, 2005), and positive species identification is critical. Unfortunately, intraspecific morphological variation may confound the ability of researchers to reliably identify the species of some individuals in the field using external characters as diagnostics (e. …