Percent of small to
large intestine 38.2 43.4 47.5 49.2 55.2 55.3 55.4 60.9 63.7
-----------------------------------------------------------------
From the differences noted in the skeleton, in the entire visceral mass,
and in the shape and position of the liver it appears that as a saltator
becomes more specialized skeletally, there is a concurrent compacting
and aligning of the viscera into a more or less bilaterally balanced
mass. It seems that this alignment is for a stabilization in leaping. It
seems reasonable that the individual that has a loose and unconsolidated
visceral mass, or in which the viscera or at least the heaviest part of
the viscera is relatively unilateral, would be thrown slightly off
balance at the end of the jump. This would place the animal at a slight
disadvantage before being able to make the next jump. Howell (1944:40)
comments on the fact that kangaroo rats often land off balance, "owing
apparently to clumsy use of the tail." Possibly the unilaterality of the
visceral mass plus a shorter tail and a more clumsy use of that organ
accounts for the off balance landings which Howell has observed.
The skeleton, particularly of the appendages, shows the most
modification, ranging from a relatively generalized to a specialized
condition. Skeletal indices, as established by Howell (1944:199) have
been used in estimating the amount of such specialization.
These indices are obtained by dividing the length of one segment of a
limb by the length of another segment and are expressed in percentages.
The Femorotarsalmetatarsal and Cranial indices are not from Howell
(_loc. cit._).
The Humeroradial index (radius/humerus ×100) in the generalized animal
is theoretically 100 because the humerus and radius are of the same
length. In kangaroo rats, which are saltators, the index rises to more
than 100 owing to the lengthening of the radial component.
The Intermembral index (humerus and radius/femur and tibia ×100) in a
generalized animal is theoretically 100, but, as Howell (1944:205)
points out, the index in generalized mammals is probably nearer 75. If
the hind leg elongates at the expense of the forelimb the animal will be
a better saltator and the skeletal elements will yield a lower
intermembral index.
The Femorotibial or Crural index (tibia/femur ×100) expresses the
development of the tibia as an adaptation to the saltatorial habit and
in generalized animals would be expected to be 100. As an adaptation to
saltation the tibia would elongate at the expense of the femur and the
index would be more than 100. The degree of divergence from 100 would be
an expression of the degree of saltatorial ability.
The Tibioradial index (radius/tibia ×100) in the generalized animal also
would be expected to approximate 100 but it is doubtful if any living
mammals, except brachiating kinds, yield an index of more than 75. In
saltators, the index is low because of the elongation of the hind
appendages, whereas the forelimbs do not change their length or are
shortened.
The Femorotarsalmetatarsal index (tarsometatarus/femur ×100) in the
generalized condition would be less than 50 and an index approaching 100
would indicate a specialization for saltation owing to the elongation of
the tarsometatarsal elements.
The Cranial index (breadth across bullae/length of skull ×100) reflects
the development of the auditory or mastoid region of the skull as an
adaptation for more acute hearing and possibly for more delicate
balance. In heteromyids, the generalized condition would be represented
by an index of 50 or less, and as the width across the bullae increases,
the index rises toward 100.
[Illustration
FIGS. 2-10. Showing the compacting of the visceral mass; liver at
the top, small intestine and caecum at the bottom. All figures
approximately × 1.
FIG. 2. _Dipodomys ordii inaquosus_, [M], adult, no. 23365, KU; 7
mi. W Fallon, Churchill County, Nevada; trapped 27 October, 1945.
FIG. 3. _Dipodomys panamintinus mohavensis_, [M], adult, no.
22094, KU; 1-1/2 mi. N Mojave, Kern County, California; 3
February, 1948.
FIG. 4. _Dipodomys heermanni morroensis_, [M], adult, no. 22082,
KU; S side Morro Bay, 4 mi. S Morro, San Luis Obispo County,
California; 25 January, 1948.
FIG. 5. _Dipodomys ingens_, [F], adult, no. 22069, KU; 25 mi. SW
Mendota, San Benito County, California; 2 February, 1948.
FIG. 6. _Dipodomys agilis perplexus_, [F], adult, no. 22091, KU;
1-3/10 mi. N Monolith, Kern County, California; 3 February, 1948.
FIG. 7. _Dipodomys venustus sanctiluciae_, [M], adult, no. 22071,
KU; 1-1/2 mi. S Jolon, Monterey County, California; 26 January,
1948.
FIG. 8. _Dipodomys spectabilis spectabilis_, [F], adult, no.
22110, KU; 5 mi. NE Willcox, Cochise County, Arizona; 19 January,
1948.
FIG. 9. _Dipodomys merriami merriami_, [F], adult, no. 23366, KU;
E side Carson Lake, Churchill County, Nevada; 2 October, 1945.
FIG. 10. _Dipodomys deserti deserti_, [M], adult, no. 23364, KU;
15 mi. WSW Fallon, Churchill County, Nevada; 3 November, 1945.]