wildfact.com/forum/attachment.php?aid=649 Muscular Structure and Morphology in Bears Document: #2
April 26, 1998
Muscle mass and muscular force analysis in brown bears of varying
ages
Dr. MartinAndrews of Michigan State University (Prof. of Zoology, M.Sc. in Zoology, 1989)
In this study, four brown bears of the male sex, each from the Yellowstone National Park
and respectively aged 3, 6, 11, and 17, were acquired to determine the rate of muscular growth,
or possible muscular deterioration, and to analyze the amount of muscle fibers (IIa/x, IIb, Ia, Ib)
that produce force. All bears were sedated and weighed with an electronic weight scale. The
youngest of them all, the threeyear old specimen, weighed 124.8 kilograms, the sixyear old
subadult weighed in at 213.9 kilograms, the elevenyear old adult weighed 228.3 kilograms, and
the last specimen, the seventeenyear old elderly bear, weighed in at 212.7 kilograms. All four
bears were measured a month after their hibernation ended, resulting in all specimens displaying
body mass indexes ranging from 4.7% to 11.6%. The initial hypothesis provided that the two
eldest bears possessed significantly more muscle deterioration per body weight and significant
reduction of muscular force.
ERL levels are a great indication in the slowtwitch fibre buildup and deterioration rate of
muscle mass. As the rate of enzyme reproduction, or protein substrate reproduction, increases,
the rate of Type I muscle fibre buildup increases and the rate of muscular deterioration decreases
proportionally. In the younger bears, ERL levels were moderate between the threeyear and
1
sixyear old specimens. As expected, the elevenyear adult possessed ERL levels averaging
6.94% lower than those of the younger and older specimens. However, the ERL levels of the
eldest bear were significantly surprising: 13.22% greater than the average ERL levels of the
elevenyear old specimen. In this case, right after hibernation , ERL levels seem to increase
2
drastically in postprime adult bears, and young bears seem to have greater ERL levels than
prime adult bears.
Thus, it should logically follow that slowtwitch fibres, Type Ia and Ib fibres, are greater
percentagewise in the three, six, and seventeenyear old bears than in the prime elevenyear old.
By analyzing the bears’ muscles with a muscle density scanner, the amount of fasttwitch and
slowtwitch muscle fibres can be differentiated due to different muscle densities (Figure 1).
1 Enzyme reproduction levels
2 After hibernation, bears are at a minimal percentage for BMI% and display higher ERL levels than in the
fall months.
Muscular Structure and Morphology in Bears Document: #2
April 26, 1998
Figure 1: The muscle fibers of each bear calculated by the muscle density scanner
AGE 3 6 11 17
FastTwitch
Type IIa/x(%)
14.7% 12.1% 2.1% 8.7%
FastTwitch
Type IIb(%)
19.0% 20.7% 36.9% 15.8%
Total
FastTwitch
Fibre(%)
33.7% 32.8% 39.0% 24.5%
SlowTwitch
Type Ia(%)
41.5% 40.6% 34.2% 35.6%
SlowTwitch
Type Ib(%)
24.8% 26.6% 26.8% 39.9%
Total
SlowTwitch
Fibre(%)
66.3%
As expected, the two youngest bears had just as much, if not slightly more, slowtwitch
Type I fibre percentages than the elevenyear old bear. Conclusively, the oldest bear possessed
the highest percentage of Type Ib per body weight, yet experienced a major reduction in Type
IIa/x and IIb fibres unlike all other bears. This specimen also had the lowest body mass index of
the four bears in comparison. Since, at equal masses, Ib fibres achieve a greater maximal force
output than all other fibres, the oldest grizzly can conclusively provide the greatest muscular
force out of the other bears. The elevenyear old bear, due to its overaccumulation of powerful
IIb fibres, can output a force at the fastest rate possible out of the other bears: the two young
specimens are somewhere between the powerstrength range of the two older specimens.
Though, muscle volume doesn’t universally reflect maximum muscular force. The
muscle volume of the oldest specimen seemed to be slightly depleted among its shoulder,
forequarter, neck, and calf regions. However, most of the muscles that comprise this specimen’s
muscle mass are greater in maximal force production than in other bears; assuming a great ratio
between Type Ib fibres and the rest of its muscle composition. The prime elevenyear old had a
considerable muscle volume in the aforementioned areas due to the accumulation of fasttwitch
IIb fibers; younger bears possessed a more even distribution of IIa and IIb fibres rather than a
Muscular Structure and Morphology in Bears Document: #2
April 26, 1998
considerable IIb/IIa fibre ratio found in the prime and postprime specimens; their muscle
volume was similar to that of the postprime specimen.
Muscle mass is similar between the prime bear and the sixyear subadult bear, with
average muscle mass percentage levels differing at a rate of 1.38±0.24%. The muscle mass of the
three year old bear was relatively low due to its low total weight, and the muscle mass of the
oldest specimen concurred with the relatively high amounts of Ib fibres: the densest muscle
fibres discovered in mammalian and reptilian bodies so far. Thus, the expected muscle weight of
the postprime aged bear should exceed that of the prime bear, even though the muscle volume is
slightly higher in the prime aged bear. The maximum percentage of Ib fibres in the postprime
bear slightly, by 0.9%, exceeds the maximum percentage of IIa/x and IIb fibres in the prime bear,
thus producing this result. Muscle density is farexceeding within the oldest specimen also due to
the overaccumulation of dense Ib fibers.
In conclusion, no significant evidence provided any leads towards increased muscle
strength deterioration or muscle mass deterioration in postprime brown bears, yet evidence had
been found of increased muscle volume deterioration in postprime brown bears as. The brown
bears who were experimented on were shortly released back into their respective locations within
Yellowstone National Park after the study commenced. These findings suggest that brown bears,
if not all bear species as a whole, are able to retain their muscle mass and maximum muscle
strength well past their prime years, unlike most other animals.
Credits to:
Dr. MartinAndrews (head of this study, and professor at Michigan State University.)
Matt Powers (alumni from MSU in 1997, master’s degree in zoology and bachelor’s
degree in computer data systems.)
Travel Rills (student at MSU, bachelor's degree in zoology.)
Mary Gillesy (student at MSU, bachelor's degree in zoology.)
Adam Leesok (student at MSU, associate’s degree in computer systems engineering,
volunteered for the weighing of bears analyzed.)