Sexual Dimorphism in Endangered Jemez Mountains Salamanders (Plethodon neomexicanus)
Sex ratio is a key demographic characteristic indicative of the condition of populations. Despite over 70 yr of study, researchers have not fully evaluated morphological characteristics that differentiate sex in Jemez Mountains Salamanders (Plethodon neomexicanus; federally endangered). Populations of this endemic salamander, which are distributed in north-central New Mexico, have undergone declines in the past two decades. We assessed morphological characters of 160 preserved P. neomexicanus specimens, evaluated our ability to infer sex in the field, and tested our ability to determine sex on a subset of preserved specimens. In preserved salamanders with body length (i.e., postcloaca snout–vent length, SVLp) ≥ 55 mm, females exhibited greater total length, trunk length, tail length, and cloaca length, and males exhibited greater precloacal tail width, head length, head width, and head height. We documented weakly female-biased size dimorphism. Females with SVLp ≥ 52 mm had cloacal rugae, whereas males with SVLp ≥ 51 mm had distinct papillose tissue in the cloaca and a cloacal cleft. In an evaluation of 30 preserved specimens, we correctly inferred sex in 97% of salamanders by cloacal characters alone. Of 29 adult salamanders captured in the field, we confidently inferred the sex of 27 individuals (16 females, 11 males) with SVL ≥ 44 mm. Thus, sex of most individuals can be correctly inferred in the field by cloacal characters. This information will aid researchers in better understanding population trajectories of this endangered species.ABSTRACT
Nonmetric multidimensional scaling (NMDS) ordination for three data subsets using snout–vent length measured to the posterior edge of the cloaca (SVLp) cutoffs of (A) ≥45, (B) ≥55, and (C) ≥60 mm. Orange (female) or blue (male) dots represent the location of individual salamanders by sex in multidimensional space defined by morphological measurements. Orange (female) or blue (male) ellipses are 95th percentile confidence intervals (standard error) around centroid means for females versus males. Biplot arrows extend in the direction of increasing values for each variable.
Scatter diagrams of relationships between snout–vent length measured to the posterior edge of the cloaca (SVLp) and morphological measurements for all specimens (n = 160). Orange lines represent the trend line for female salamanders (n = 104) and blue lines represent the trend line for male salamanders (n = 56) using least-squares linear regression. Gray shading is the 95% confidence interval of the fitted values.
High-resolution images of the cloacae of (A) female and (B) male Plethodon neomexicanus taken in the Museum of Southwestern Biology (Department of Biology, University of New Mexico, Albuquerque, New Mexico, USA).
Presence of cloacal traits for preserved specimens of Plethodon neomexicanus for (A) females (n = 102) and (B) males (n = 56), and (C) percent of samples within each sex exhibiting each trait. Sample size was 102 for females for all characters except cleft, which could not be evaluated for one female.
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