Matson's
Laboratory LLC
US Post Office address:
PO Box 308, Milltown MT 59851 USA
Physical address (for UPS, FedEx, and other
couriers:
8140 Flagler Road, Milltown MT 59851 USA
Email address: gjmatson@montana.com
Skeletochronology
service has been discontinued.
Matson's
no longer offers histological sectioning of frog or amphibian
toe specimens for skeletochronology. A description of our
lab processing method for preparing thin sections of amphibian
or reptilian toes for skeletochronology is available upon
request.
Aging
Amphibians and Reptiles
Methods
A
toe clipped from an amphibian or reptile may be used for
SKELETOCHRONOLOGY. Transverse histological sections taken
through the midpoint of the toe phalanx are stained with
hematoxylin. The femur bone of the turtle, and a small quadrant
of the alligator femur can also be processed for age analysis.
Applications
Circumferential
annual layers are present in the sections, and are counted
to determine age. There are two layers formed during the
annual growth cycle: The "zone" is a thick layer
of cellular bone, formed during the season of greatest somatic
growth. The "Line of arrested growth" (LAG) is
a thin, acellular layer that is darkly stained by histological
dyes.
Spotted frog toe transverse section, approximately 150x
Histological Sections
The
segment of the amphibian/reptilian phalanx that is suitable
for aging by counting annual layers is very short. Matson's
conducted trials to determine the correct sectioning point.
Phalanges from 5 spotted frogs were serially sectioned.
The sections showing the full complement of annuli in fully
developed bone were located within a segment approximately
0.3mm on either side of the exact midpoint. Closer to the
epiphyses, the thinner-walled and younger bone lacked the
annuli formed earlier in life. Very near the epiphyses,
the phalanx is mostly cartilage and entirely lacking in
annuli.
Because
of these results, Matson's narrowly defined the sectioning
point. First, the phalanx midpoint is identified using visible
joints as indicators. The sectioning point is then selected
to begin just proximal to the midpoint. We recommend that
a series of 8 sections be taken at intervals that extend
distally, and pass entirely through the midpoint.
Complications
Few,
if any species-specific, standardized age analysis models
have been described and tested using known age specimens.
Past experience at our laboratory suggests that not only
each species but also each population may require a unique
model. Visibility of the bone layers used as age indicators
varies greatly among species. The number of annuli present
in the toe and femur sections may be affected by bone growth
dynamics and resorption (a poorly understood but normal
process of bone loss and replacement). Turtle femur sections
prepared in this laboratory show clear evidence that the
innermost annuli are being lost because of resorption. In
spite of this complication, analysis of the turtle femurs
(from roadkills) resulted in age approximations considered
to be useful by researchers.
Recommended Specimen Handling
The
excised amphibian toe specimen should be as long as possible
to ensure the presence of a whole phalanx. Standardize the
excision point so it is the same for all specimens. The
turtle femur specimen should have all excess tissue removed
and then be allowed to dry. Place the toe or femur in a
clean paper envelope where it can dry. If specimens are
to be shelved for months or years before analysis, store
them frozen.
What Researchers Should Expect
Annuli
of different species vary in their distinctness and complexity.
Indistinct and complex LAG (having more than a single annual
component) are important sources or error. Ages estimated
by skeletochronology may give useful approximations of the
population's age structure. However, aging errors are expected
to be common and researchers should take every opportunity
to test precision, with duplicate specimens, and accuracy,
with known age individuals.
Further Studies Needed
Standardized
age analysis models are needed for all species of amphibians
and reptiles. The models should describe characteristics
of annuli, where to begin the count, the age corresponding
with the first visible annulus, and sources of aging error.
Evaluation
of amphibian and reptile aging methods need to be conducted
using specimens of known age.
Bibliography
Matson's
has compiled a bibliography, available upon request, for
skeletochronology of terrestrial amphibians and reptiles.
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