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James
Morgan
Research Training Program, 2004
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Abstract
Morphological variation in the forefeet of shrews (Family Soricidae) of the genus Cryptotis has been previously used to help understand taxonomic and evolutionary relationships. However, few skeletons of shrews are available for study. I used digital x-rays to closely observe the forefeet of 101 dried skins in which the bones of the forefeet were preserved. These specimens included eight taxa from four species-groups that are hypothesized to represent distinct evolutionary lineages within the genus Cryptotis: C. p.parva and C. p.floridana (C. parva-group); C. nigrescens and C. mera (C.nigrescens-group); C. meridensis (C. thomasi-group); C. mexicana, C. goldmani, and C. goodwini (C. mexicana-group). The left forefoot of each specimen was digitally x-rayed, the resulting image edited, and the bones measured using Photoshop computer software. For this study,
I focused primarily on the bones of digit III because digit III of all
specimens appeared most variable. This variation is useful in distinguishing
among species and groups of species. For example, C. meridensis
has a long metacarpal relative to all other taxa. The length of the
distal phalanx relative to the length of the middle phalanx distinguished
three groups of species. In four species (C. p.parva, C. mexicana,
C. goldmani, C. goodwini), the distal phalanx is less
than 90% the length of the middle phalanx. In C. meridensis,
the distal phalanx is greater than 100% but less than 120% and in C.
mexicana, C. goldmani, and C. goodwini, the distal
phalanx is more than 150%.
Each taxon can be distinguished based on morphological characteristics of the forefeet. The taxa shows variation in the forefeet and can be placed in groups based on similarities. Some results were as follows:
Relative to head and body length C. p. floridana has a moderate sized distal phalanx (Table 2), and relative to total digit length, it has a moderate sized distal phalanx (Table 1). Relative to distal phalanx length, the claw of C. p. floridana is long (Table 5) because the distal phalanx is moderate in size compared to other species. In relation to distal phalanx, the claw is longer than all species with distal phalanges longer than C. p. floridana. Therefore, C. p. floridana has a long claw, relative to distal phalanx length. In all other taxa, claw length increases with distal phalanx length (Table 5).
The different taxa studied showed little variation within species but more variation among species. Many individuals were placed in groups with species sharing similar characteristics but others were in a species of their own. For example, C. meridensis has a very long metacarpal relative to other taxa. In comparison, the other taxa have relatively moderate to short metacarpals. Members within the original groupings have different forefeet structure than other members of the same group. I was able to place different species from different groups into the same group because of similar forefeet structure and characteristics. The four original groups were compared to the other groups to yield an overall sense of variation within and among species. Members within the same proved to be similar but surprising members within different groups also proved to share similar characteristics. This will help better understand the relationships within and among species as well as understand the interaction between shrews and their environment.
Many thanks to S. J. Raredon, who provided x-ray equipment, the National Museum of Natural History for providing shrew specimens, Dr. Neal Woodman, Mary Sangrey, and the Smithsonian Staff for guiding me through an unforgettable summer. This research was
supported by a grant from the National Science Foundation Research Experience
for Undergraduates program, Award Number DBI-0243512. Want to ask a question? Visit the Message Board Virtual
Poster Session Message Board Research
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X-rays
were taken using the Kevex-Varian Digital X-ray system. The bones of
Digit III were measured in Photoshop 6.0 using a coordinate grid, where
(x1,y1) represents the coordinates of point A (Figure 2), which is the
base of the bone being measured, and (x1,y2) represents point B, which
is the most distal point of the bone. C = (x1,y1)-(x1,y2), represents
the length of the bone in mm. D is the width (0.52 mm) of the pin used
for scale. 
