In his excellent taxonomic treatment of the tree squirrels of Mexico
and Central America, Nelson (Proc. Washington Acad. Sci., 1:15-110,
2 pls., May 9, 1899) recognized three subspecies of red-bellied
squirrels, _Sciurus aureogaster aureogaster_ F. Cuvier, _Sciurus
aureogaster hypopyrrhus_ Wagler, and _Sciurus aureogaster frumentor_
Nelson. In his lists of specimens examined, Nelson (_op. cit._:42 and
44) assigned certain specimens from "mountains near Santo Domingo"
and Guichicovi in Chiapas, and Catemaco in Veracruz, to _S. a.
aureogaster_, and other specimens from the same localities to _S. a.
hypopyrrhus_. I originally attempted to study (identify to subspecies)
the series of animals from only three places, but it became evident
that a more extensive study was indicated.
The locality whence the holotype of _Sciurus aureogaster aureogaster_
was obtained is unknown. Because certain specimens from Altamira,
Tamaulipas, closely resemble Cuvier's figure of the type, Nelson
(_op. cit._:41) subsequently designated Altamira as the type locality.
Miniatitlan, Veracruz, was designated by Nelson as the type locality
of _S. a. hypopyrrhus_ because Wagler's description of the type of
that subspecies fitted so well certain of Nelson's specimens from that
place.
_Sciurus a. hypopyrrhus_ was said by Nelson (_op. cit._:43 and 44) to
differ from _S. a. aureogaster_ in darker color, thinner pelage, much
stiffer and more shining dorsal hairs, slenderer tail with black
predominating, larger and proportionately narrower skull with larger
auditory bullae, each bulla being "slightly constricted just in front
of middle." _Sciurus aureogaster_ varies greatly in intensity of color
and in color-pattern. Fully 30 per cent of the specimens examined are
in some degree melanistic and approximately 20 per cent of them are
completely so. Others are more or less brown; the brown dulls the
usually rufous parts. In many specimens this brown is well distributed
even in the otherwise grizzled areas; in some specimens it is evenly
distributed and in others it is in patches. Indeed, scarcely any two
"normally" colored specimens are alike. Typically, the intense rufous
color characteristic of the underparts in both _S. a. aureogaster_
and _S. a. hypopyrrhus_ is also present on the costal region and
shoulders. Even this distribution of color is highly variable; some
specimens (for example No. 23948 KU, from 3 km. E San Andres Tuxtla,
Veracruz) show no rufous dorsally and others (for example No. 19307
KU, from 20 km. W Piedras Negras, Veracruz) have the rufous extending
over the legs, sides, and almost all of the dorsum from the shoulders
to the rump except (in some) for an interrupted median strip of
grizzled gray. It is true that specimens from Miniatitlan are darker
than those from Altamira, but this seems not to be significant
taxonomically, because examination of series from other localities
provides no evidence of geographic variation in color except,
possibly, in the frequency of melanism. A series of 13 specimens
(Univ. Kansas) from 7 and 8 km. WNW Potrero, Veracruz, for example, is
quite as dark as topotypes of _S. a. hypopyrrhus_ from Miniatitlan,
although the localities of capture are approximately in the center of
the geographic range of _S. a. aureogaster_. In short, there seems to
be no way to distinguish _S. a. hypopyrrhus_ from _S. a. aureogaster_
on the basis of color. An unusual amount of variation exists, but it
seems to occur at random. Fixing type localities of the two subspecies
at the places of origin of certain specimens which in color fit the
original descriptions is meaningless because selected specimens or
series from almost any place in the geographic range of the species
would qualify as approximate color-duplicates of the types.
My findings agree with those of Nelson in that skulls from Miniatitlan
average longer and narrower than those from Altamira, but this seems
not to be significant taxonomically because the series from Altamira
is, to judge from the material I have seen, somewhat shorter and
broader cranially than is "average" for the alleged subspecies _S. a.
aureogaster_. For example, series from Metlaltoyuca in Puebla, 3 km. E
Axtla in San Luis Potosi, 8 km. NW Potrero and 20 km. NW Piedras
Negras in Veracruz, although obtained from localities well within the
geographic range of _S. a. aureogaster_ (as outlined by Nelson), all
more closely resemble the "topotypes" of _S. a. hypopyrrhus_ in
cranial measurements than they do "topotypes" of _S. a. aureogaster_.
Conversely, specimens from that part of the range of _S. a.
hypopyrrhus_ most remote from the range of _S. a. aureogaster_
(Montecristo, La Venta, and Teapa, all in Tabasco) more closely
approximate the Altamiran series in cranial size and proportions than
they do the Miniatitlan material. Therefore, my data contradict the
statement of Nelson (_loc. cit._) that the skulls of _S. a.
hypopyrrhus_ are larger but proportionately narrower than those of
_S. a. aureogaster_. The constriction of the auditory bullae alluded
to by Nelson as being present in _S. a. hypopyrrhus_ is also present
in _S. a. aureogaster_, occurring in both subspecies in varying
degrees without correlation with geographic distribution.
Actually, the only concrete evidence of geographic variation that I
can detect in these animals is a slight increase southwardly in the
frequency and degree of melanism, a kind of variation that is unworthy
of taxonomic recognition in this species. It seems best, then, to
regard the name _Sciurus aureogaster hypopyrrhus_ Wagler as a synonym
of _Sciurus aureogaster aureogaster_ F. Cuvier.
Nelson (_op. cit._:45) stated that _S. a. frumentor_ "Differs
strikingly from typical _aureogaster_ in having well-marked nuchal and
rump patches of yellowish brown or rufous brown; the underparts gray
or gray washed with rufous; tail heavier and more bushy; pelage
softer.... Skull indistinguishable from that of typical
_aureogaster_." I have examined 22 specimens from Jico, 7 from Las
Vigas (the type locality), and one from 3 km. E Las Vigas, all in
Veracruz. These include the type and paratypes of _S. a. frumentor_.
Part (probably 7 specimens) of the series from Jico was referred by
Nelson (_op. cit._:46) to _S. a. frumentor_ and he thought, or knew,
these specimens to have been taken _above_ Jico. The remaining
specimens labelled as from Jico were referred to _S. a. aureogaster_.
I am unable to find fault with the characterization of _S. a.
frumentor_ insofar as color or skull are concerned. I cannot verify to
my own satisfaction the presence of "heavier" and bushier tail and
softer pelage. The characters considered to be diagnostic of _S. a.
frumentor_ are distributed in an interesting geographic pattern the
genetic import of which is not wholly clear. One specimen (No. 23945
KU) of the two available from 3 km. SW San Marcos, Veracruz, a
locality on the coast approximately 50 miles north of Las Vigas, is
indistinguishable from topotypes of _S. a. frumentor_ except for
slightly lighter-colored grizzled parts. The second specimen (No.
23946 KU) from the same locality, although a subadult in worn pelage,
shows the color and striking dorsal pattern of _S. a. frumentor_ and
the ventral color of _S. a. aureogaster_. The dorsal pattern of _S. a.
frumentor_ is found also in the three specimens from San Carlos and
Plan del Rio, Veracruz (Nos. 11082, 11083 and 8278), Chicago Nat.
Hist. Mus., respectively. (The two specimens from San Carlos were
referred to _S. a. frumentor_ by Elliot, Field Columb. Mus., Zool.
Ser., vol. 8, Publ. no. 115:128, February 9, 1907.) Nevertheless,
although the essential morphological characters of _S. a. frumentor_
occur sporadically in other populations, the animals from the higher
elevations above Jico and Las Vigas are notably homogeneous, differ
collectively from surrounding populations, and occupy a logical
geographic range. Therefore _S. a. frumentor_ is retained as a tenable
subspecies, and the animals from the vicinity of San Marcos, and from
San Carlos and Plan del Rio are referred to _S. a. aureogaster_.
Incidentally, Nelson (_op. cit._:45) remarks that he saw no melanistic
specimens of _S. a. frumentor_. This is not strange because melanistic
specimens could not be identified anyway.
[Illustration: FIG. 1. Geographic distribution
of _Sciurus aureogaster_.
1. _Sciurus aureogaster aureogaster._
2. _Sciurus aureogaster frumentor._]
The names, absolute synonyms, and geographic ranges of the two
subspecies of _Sciurus aureogaster_ here recognized are as follows: