Absolute size of head and body, tail, hind-foot and ear are useful in
distinguishing subgenera and subspecies and to some extent in
differentiating species.
The length of head and body is large to medium in _Proechimys_ and
medium to small in _Trinomys_. The tail is long to medium in _Trinomys_
and short in _Proechimys_. The longest tail, 242 mm, is found in _P. i.
denigratus_, and the shortest tail, 123 mm, in _P. g. steerei_. The
relative length of tail also provides gradients or clines.
In every species, males surpass females in average size. Nevertheless,
the largest animals are usually females. How this paradoxal fact is to
be accounted for, I am not sure, but it may be that the animals grow as
long as they live and that females have more chances to survive longer
since the care of the young keeps them closer to shelter.
_Color._--Upper parts vary from Buckthorn Brown to Ochraceous-Buff. Dark
color ordinarily is correlated with an environment of higher degree of
humidity and light color with lower humidity. However, species may be
found in similar conditions of humidity but differing in color.
_Proechimys albispinus albispinus_, for example, a light-colored form,
is found in areas where the rainfall averages 1,000 to 1,500 mm of
annual precipitation, in the isohygra of 80 per cent relative humidity.
These conditions actually are similar to those where _P. dimidiatus_, of
darker color, is found. The subspecies _albispinus_, however, ranges
mostly over a dry area and the fact that it occurs also in a moist area
without appreciable change in color is difficult to explain.
Insular populations are usually darker or richer in color than
corresponding continental populations. On a small island, uniformity of
environment and inbreeding may be responsible for an accumulation of
characters for richness of color.