First published online August 8, 2008
Journal of Experimental Biology 211, 2559-2565 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018119
The ontogeny of aerobic and diving capacity in the skeletal muscles of Weddell seals
S. B. Kanatous1,*,
T. J. Hawke2,
S. J. Trumble3,
L. E. Pearson1,
R. R. Watson4,
D. J. Garry5,
T. M. Williams6 and
R. W. Davis7
1 Department of Biology, Colorado State University, Fort Collins, CO 80523-1878,
USA
2 School of Kinesiology and Health Science, York University, Toronto, ON, Canada
M3J 1P3
3 Department of Biology, University of Michigan Flint, Flint, MI 48502,
USA
4 Biological Sciences Department, California State Polytechnic University,
Pomona, CA 91768, USA
5 Lillehei Heart Institute, University of Minnesota, Minneapolis, MN 55455,
USA
6 Department of Ecology and Evolutionary Biology, University of California at
Santa Cruz, Santa Cruz, CA 95060, USA
7 Department of Marine Biology, Texas A&M University at Galveston,
Galveston, TX 77551, USA
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Fig. 1. (A) The percentage of fast-oxidative fibers (Type IIA) increased
significantly in the swimming muscle as seals matured and increased their dive
durations. Histogram showing the change in fiber type composition (% of total
fiber number), in the swimming (longissimus dorsi) muscles of Weddell seals as
they mature. (B) The mitochondrial volume densities in the swimming muscles
decreased as the seals matured. Histogram showing the decrease in the volume
density of mitochondria in the swimming muscle of Weddell seals as they
mature. Values are means ± s.e.m. (N=6). * denotes
significantly different from the pups;  denotes
significantly different from adults (P<0.05, ANOVA).
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Fig. 2. Plot of muscle mitochondrial volume density against body mass in athletic
(filled circles) and sedentary terrestrial mammals (small dots) and pinnipeds
(open circles). The linear relationship [y=6.75–1.34(log
x), r2=0.70] was generated from the volume
densities of the vastus medialis, a primary locomotory muscle, from various
terrestrial mammals ranging in size from the dwarf mongoose to the steer
(Kanatous et al., 1999 )
[terrestrial data from Hoppeler (Hoppeler et al., 1987)]. In contrast to the
adult Weddell seals, whose mitochondrial volume densities are similar to that
predicted for a sedentary terrestrial mammal of comparable size, juveniles and
pups have mitochondrial volume densities similar to those of athletic
terrestrial mammals and shorter-duration divers of similar size.
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Fig. 3. The concentration of myoglobin in the swimming muscles of different age
classes of Weddell seals. The concentration of myoglobin was significantly
higher in the swimming muscles of juveniles compared with either pups or
adults. In addition, myoglobin concentration was also significantly higher in
adults compared with pups. Values are means ± s.e.m. (N=6).
* denotes significantly greater than pups;
denotes significantly greater than adults (P<0.05, ANOVA).
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Fig. 4. Histogram displaying the changes in the expression of calcium regulatory
and responsive proteins that may influence the expression of myoglobin. Adult
seals had the lowest relative protein expression and were significantly
different from the juvenile in all but one (calsequestrin) of the proteins
measured. Protein concentrations were similar in pups and juveniles in all
proteins measured except myoglobin, which was significantly greater in the
juvenile than either the pup or adult. Values are means ± s.e.m.
(N=6). * denotes significantly different from pups;
denotes significantly greater than adults
(P<0.05, ANOVA).
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Fig. 5. Summary reveals the ontogeny of skeletal muscle adaptations that enable
long deep dives in Weddell seals. As newborn pups, Weddell seals have an
extremely high aerobic capacity, similar to that found in terrestrial animal
athletes and short-duration divers. However, this enhanced aerobic capacity is
not an adaptation towards diving but is due to their high fat diet and the
need to offset thermoregulatory costs associated with using their lanugo
(natal fur) for insulation in the extremely harsh environment of Antarctica.
As the pups begin to dive and mature into juveniles, their skeletal muscles
begin to transform. As juveniles, they initiate the development of
fast-oxidative fibers and significantly increase their intramuscular stores of
oxygen in the form of oxymyoglobin. As they continue to mature and increase
their diving capacity, Weddell seals increase their percentage of Type IIA
fast-oxidative fibers in their skeletal muscles. In addition, their skeletal
muscles transform to a more sedentary state in order to maintain low levels of
aerobic metabolism under the hypoxic conditions associated with long-duration
diving. Similar to what has been found in terrestrial mammals; the results of
our subtractive hybridization analysis indicate that these changes in skeletal
muscle metabolic potential are regulated by calcium signaling and its
downstream mediator, calcineurin.
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© The Company of Biologists Ltd 2008
