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(Received 2 September 1986)
Abstract

Nine skulls of the rare Oryzomys argentatus
are compared to 109 skulls of the six races of
O. palustris. Mahalanobis distance is greater
between O. argentatus and all Floridian forms
of O. palustris than the Floridian forms are
from each other. In a canonical discriminant
analysis, two models grouping O. argentatus
with one or both of the insular races of O.
palustris (sanibeli and planirostris) were shown
by the Roy’s Greatest Root statistic to fit the
data less well than a model in which O.
argentatus was regarded as distinct. A
one-way ANOVA and Duncan's Multiple
Range Test on the variation in nasal bone
proportions show that there are two
significantly different groups of these
Oryzomys (p < 0.05): all O. palustris together
and O. argentatus alone. We hypothesize O.
argentatus originated on the Lower Keys in
the late Sangamon and underwent selection
for character divergence in sympatry with O.
palustris during the Wurm.

© Copyright 1986 by the Peabody Museum of
Natural History, Yale University. All rights reserved.
No part of this publication, except brief quotations
for scholarly purposes, may be reproduced without
the written permission of the Director, Peabody
Museum of Natural History.

ISSN No. 0-912532-00-9

Postilla Number 198

30 December 1986

Relationships of the
Silver Rice Rat
Oryzomys argentatus
(Rodentia: Muridae)

Numi C. Goodyear and
James D. Lazell, Jr.

Key Words

Oryzomys argentatus, Florida Keys,
speciation, morphometrics, discriminant
analysis, silver rice rat.

Introduction

Oryzomys argentatus, the silver rice rat, was
originally described on the basis of two
specimens (Spitzer and Lazell 1978). The work
was criticized by Humphrey and Barbour
(1979) and Barbour and Humphrey (1982) who
felt that the use of ratios was not appropriate,
and that the species was probably invalid.
Because of a lack of data they did not
substantiate these claims; they stated that the
issue was moot because the taxon was
probably extinct. Since, we have found that O.
argentatus is extant on at least nine of the
Lower Florida Keys (Goodyear, in press).
Because the original sample size was too
small for a statistically significant analysis of
differences between O. argentatus and O.
palustris, we present herein additional data
corroborating our earlier results. We compare
nine silver rice rat skulls with 109 skulls from
the six races of O. palustris. The possibility of
a close relationship between O. argentatus
and the other insular forms, O. p. sanibeli and
O. p. planirostris is especially considered. Two
skulls are shown in Figure 1.

2 Silver Rice Rat

Postilla 198

Fig. 1

Skulls of male rice rats in dorsal view. Right,
Oryzomys argentatus, YPM 4667, Raccoon Key,
Monroe Co., Florida. Left, Orzyomys palustris
sanibell, YPM 4670, Sanibel Island, Lee Co., Florida.
Bar = 1 cm.

Specimens Examined and
Related Abbreviations

Abbreviations for museums in which

specimens are housed are as follows:

AMNH American Museum of Natural History,
New York

ANSP Academy of Natural Sciences,
Philadelphia

FSM Florida State Museum, Gainesville

NCSM North Carolina State Museum, Raleigh

MCZ Museum of Comparative Zoology,
Harvard University

USNM US National Museum of Natural
History, Smithsonian Institution

YPM Peabody Museum of Natural History,
Yale University

Not all specimens could be used for all

characters.

Oryzomys argentatus. YPM 4664-9; USNM
514994-5; AMNH 256405-6; FSM 16366.

3 Silver Rice Rat

O. p. palustris. YPM 4406-7; ANSP 11870,
11875; MCZ 1527-8, 2687, 2689-90, 56111-2,
5114, 5121, 5127, 5886, 6454, 6456; USNM
117384, 286831, 71368; AMNH 91146; NCSM
301-2, 472, 484-3, 491.

O. p. texensis. MCZ 2701-4, 2712-3, 2715-7,
2874, ANSP 14439; USNM 43299-300; AMNH
136499-500.

O. p. coloratus. MCZ 4454-59, 4461-2, 4465-9;
USNM 71354-5, 73747-9, 228418, 228420-22,
228425, 228427.

O. p. natator. MCZ 3056-60, 7047, 7049, 7051,
7128, 7133-4, 7241-2; USNM 1090, 2250, 2253,
3872, 6172-4, 6176, 6183, 64061-7, 64069,
64071, 78705, 142693, 142697, 142748,
142811, 163993, 163995.

O. p. sanibeli. YPM 4670-2; USNM 301534.

O. p. planirostris. USNM 301533.

Methods

Nine O. argentatus, 16 O. p. texensis, 25 O. p.
palustris, 24 O. p. coloratus, 39 O. p. natator,
four O. p. sanibeli, and one O. p. planirostris
skulls were used in the analysis. Variables
used were condylobasal length, zygomatic
breadth, nasal length, and nasal width. In the
original description of the species the ratios of
nasal length/nasal width and condylobasal
length/zygomatic breadth were used to
discriminate O. argentatus from all subspecies
of O. palustris. Pelage color was not used as
a variable because this character is so
definitive that it would have unfairly weighted
the data. In a canonical discriminant
analysis—Statistical Analysis System (SAS)
CANDISC procedure—we used only the
metric data and no ratios to generate the
models. In Model | we determined the
Mahalanobis distance between all seven
groups—the six races of O. palustris and O.
argentatus. In Model Il O. argentatus was
lumped with the insular rice rat O. p. sanibel.
In Model Ill O. p. sanibeli, O. p. planirostris and
O. argentatus, the three insular taxa, were
lumped. Roy's Greatest Root, a summary
statistic not affected by the number of
classification groups, was used to determine
which model best fit the data.

Postilla 198

The skull measurements were also
examined using the SAS DISCRIM procedure
with and without the ratios. Last we
performed a one-way ANOVA and a Duncan's
Multiple Range test on the ratio of nasal
length/nasal width between the seven
taxonomic groups.

Results

The Roy's Greatest Root, for which larger
values mean better fit, was largest for Model |
(0.82872), smaller for Model Il (0.814489), and
smallest for Model Ill (0.749208). This indicated
that it was better to view O. argentatus as a
separate taxon, distinct from the other insular
forms.

The Mahalanobis distances between O.
argentatus and the races of O. palustris (from
Model |) are shown in Table 1. The values
show that the O. argentatus centroid is farther
from all other groups than they are from each
other with the exception of one pair, O. p.
sanibeli and O. p. texensis which seem quite
different from each other. However, the
distance between O. argentatus and O. p.
texensis is the greatest of all. Canonical
variables 1 and 2 are plotted for Model | in
Figure 2. Though the program attempted to
maximally separate all seven groups, only the
O. argentatus lie distant from the main mixed
cloud of O. palustris subspecies. One
individual O. argentatus lies within the O.
palustris cloud. This was a captive female
runt, which died at three months, weighing
only 31 g (a normal female weighs 60 to 70 g).
It had badly recurved teeth and did not grow
at the rate of its litter mates. It died retaining
the juvenile skull proportions which caused
the misclassification. In the five discriminant
analyses performed it was consistently
grouped with O. palustris. No O. palustris was
ever grouped with O. argentatus.

The case of overlap should be
disregarded, as in Vogt and McCoy (1980), as
the result of one teratogenic individual.
However, cranial characters in combination
with coloration do render every O. argentatus
unequivocally distinct from every O. palustris.

4 Silver Rice Rat Postilla 198

CANONICAL VARIABLE 2

-4.5 -2.5 m0) 5S 1:5
CANONICAL VARIABLE |!

Fig. 2

Canonical variables one and two (Model |: see text)
for seven North American rice rats (Oryzomys) using
four cranial characters. Solid circles = O.
argentatus; open circles = O. palustris palustris;

X = O. p. texensis; + = O. p. coloratus; open
triangles = O. p. natator, solid triangles = O. p.
sanibeli, * = O. p. planirostris.

Using the DISCRIM procedure with no sanibeli and O. p. planirostris 0%. Predictably,
ratios, the program correctly classified with the ratios used instead of the raw data,
individuals as follows: O. argentatus, 79%; O. the program correctly classified more O.

p. natator, 77%; O. p. texensis, 63%; O. p. argentatus, 89% (i.e., all but the runt noted

palustris, 40%; O. p. coloratus, 25%; O. p. above); O. p. natator, 85%; O. p. palustris,

5 Silver Rice Rat

Table 1

Postilla 198

Mahalanobis distance values between seven North American Oryzomys. The distance values are in the up-
per triangle, and probability values for greater distance are in the lower triangle. O.p.p. = Oryzomys palus-
tris palustris, O.p.t. = O. p. texensis, O.p.c. = O. p. coloratus, O.p.n. = O. p. natator, O.p.s. = O. p. sanibe-

li, O.p.pl. = O. p. planirostris, O.a. = O. argentatus.

SPECIES O.P.P. O.P.T. O.P.C.
O.p.p. = 1.09 12
O.p.t. 4.37 = 2.28
O.p.c. 6.61 18.26 —

O.p.n. 8.03 29.24 2.69
O.p.s. 0.66 1.58 0.23
O.p.pl. 0.01 0.03 0.04
O.a. 5.80 8.92 4.28

32%; O. p. coloratus, 21%; O. p. texensis, O.
p. sanibeli and O. p. planirostris 0%. The only
O. argentatus misclassified was the female
runt, YPM 4664.

In the one-way ANOVA on the ratio of
nasal length divided by nasal width, there was
a highy significant difference between O.
argentatus and O. palustris (pb < 0.01). A
Duncan's Multiple Range test showed that
there were two significantly different groups:
one consisted of O. argentatus alone; the
other contained all the subspecies of O.
palustris (p < 0.05).

Discussion

Since it was originally described (Spitzer and
Lazell 1978) the validity of O. argentatus has
been repeatedly challenged (Humphrey and
Barbour 1979; Barbour and Humphrey 1982).
Because the species is so rare (Goodyear, in
press) collection of information has been slow.
Research on aspects of the natural history
and the results of this taxonomic re-evaluation
show that the animal is indeed as distinctive
as originally thought. Radiotelemetry of
individuals (Spitzer 1983) showed that, unlike
their closest geographic relative, O. p.
coloratus, they are primarily salt marsh
inhabitants that can have home ranges 10 to
100 times larger than one might expect for an
animal of their size and guild. Their pelage is
distinctive: always the silver-grey of the limey
Florida Keys’ mud regardless of condition of

O.P.N. O.P.S. O.P.PL. O.A.
1.18 Zier. 1.10 3.16
2.09 2.85 Ho) 3.37
0.68 1.28 1.98 2.68
— 1.47 1.46 255
0.20 = 2.71 2.25
0.01 0.36 = 2.86
2.68 7.68 16.15 —

molt or diet. The coats of rice rats from
temperate Florida tend to be grey-brown in
juveniles (because of shorter guard hairs) and
develop red hues when animals mature.
Humphrey et al. (1986) raise the point that
pelage is affected by ‘‘chemicals and light in
the environment and by fumigants in museum
cabinets.’’ This may cause some of the
variation they observed in specimens of the
Sanibel and Pine Island rice rats. We find that
silver rice rats raised in the laboratory on
Purina Dog Chow are identical in all respects
to those captured wild, and that our museum
specimens appear unchanged.

The silver rice rat's distinctive features may
be due to an early separation from the
mainland stock of Oryzomys and a
subsequent period of sympatry when
character divergence occurred. Oryzomys is
known to be a successful colonizer of oceanic
islands: the only rodent to have naturally
reached the Galapagos Archipelago over
more than a thousand kilometers of ocean
(Heller 1904). We suspect that Oryzomys
arrived in the Florida Keys as waters of the
Sangamon Interglacial receded and exposed
the islands (Lazell 1984; Goodyear, in press).
We calculate the probable earliest exposure
of land in what are now the Keys to be about
75 000 years BP from the curves provided by
Morris et al. (1977). This date is corroborated
by the climatic curves of Brunner (1982). If
they arrived over water, differentiation may
have begun before the Wurm reconnected the
islands with the mainland. During the transition

6 Silver Rice Rat

from interglacial to glacial, and in the more
recent reversal of that transition, Florida Bay
was largely freshwater swamp, continuous
with the Everglades; the oolitic Lower Keys
were connected by mangrove swamp
(Hoffmeister 1974). At these times, flanking
the 65 000 year glacial maximum, the
mainland and the Florida Keys rice rats may
have been sympatric. This could explain the
degree of character divergence seen in
osteology, pelage, behavior, and habitat
preferences that persist today after their
re-isolation.

While the ecological significance of the
elongate nasal bones of O. argentatus is not
apparent to us at present, the other three
characters seem the very sorts which
selection for character divergence might
produce. While O. argentatus and the distant
forms of O. palustris utilize salt marsh (Spitzer
1973), the proximate form O. p. coloratus
seems confined to freshwater areas. We
have trapped extensively for coloratus in the
Everglades and Upper Keys and have never
taken it in brackish or salt habitats. The vast
home ranges documented for O. argentatus
indicate a very different foraging strategy for
this species compared to any subspecies of
O. palustris (Goodyear, in press).

Finally, pelage color is far more different
between O. argentatus and its nearest
neighbor O. p. coloratus than between
argentatus and geographically remote forms
like nominate palustris and texensis. The
Everglades O. p. coloratus is richly patterned
in russet and ochraceous tones; O. argentatus
is overall chinchilla grey to ash white. The
grey-brown nominate palustris and texensis,

Literature Cited

Postilla 198

geographically remote from O. argentatus,
most closely resemble it in color. The insular
form O. p. sanibeli has a dark grey-brown
dorsum shading to warm fawn-brown on the
sides (Hamilton 1955; YPM specimens); the
ventral hairs of sanibeli are cream-white
tipped with plumbeus bases. The ventral hairs
of O. argentatus are ashy to the bases.

Pelage color differences are often strong
evidence of character divergence in small
mammals. For example, two species of mice,
Peromyscus leucopus and P. gossypinus, can
scarcely be separated on any consistent
characters where far removed from each
other, but sharp color (and hind foot)
distinctions are seen where their ranges
overlap (Webster et al. 1985). A similar case is
described by these authors involving the
shrews Blarina brevicauda and B. carolinensis.

Character divergence between presently
allopatric, isolated species resulting from past
contact was discussed theoretically by
Williams (1969). Lazell (1972, p. 103-104)
provided further discussion and an Antillean
example.

Acknowledgments

Our field work was supported in part by The
Conservation Agency, Friends of the
Everglades, U.S. Fish and Wildlife Service,
Earthwatch, The Explorers Club, The Nature
Conservancy, and State of Florida Department
of Parks and Recreation. The University of
Rhode Island provided computer time. George
Garrett and numerous others assisted us in
the field.

Barbour, D. B. and S. R. Humphrey. 1982. Status of the silver rice rat (Oryzomys argentatus). Florida

Sci. 45, 2:106-112.

Brunner, C. A. 1982. Paleoceanography of surface waters in the Gulf of Mexico during the late

Quaternary. Quat. Res. 17:105-119.

Goodyear, N. C. In press (1987). Distribution and habitat of the silver rice rat, Oryzomys argentatus. J.

Mammal. 68.

Hamilton, W. J. 1955. Two new rice rats (genus Oryzomys) from Florida. Proc. Biol. Soc. Washington

68:83-86.

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Heller, E. 1904. Mammals of the Galapagos Archipelago, Exclusive of the Cetacea. Proc. Calif. Acad. Sci.
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Hoffmeister, J. E. 1974. Land from the Sea: the Geologic Story of South Florida. U. Miami Press, Coral
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threatened rodents in Florida. Spec. Sci. Rep. No. 2. Office of Ecological Services, Fla. State Mus., 17 p.
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1972. The anoles (Sauria, Iguanidae) of the Lesser Antilles. Bull. Mus. Comp. Zool. (Harvard) 143, 1:

ao:

Morris, B., J. Barnes, F. Brown and J. Markham. 1977. The Bermuda marine environment. Bermuda
Biol. Stn. Spec. Publ. 15. iv + 120 p.

Spitzer, N. C. 1973. Rice rat's world. Man and nature, Mass. Audubon Soc., March:24-26.

1978. Cudjoe Key rice rat, p. 7-8. In J. Layne (ed.), Rare and Endangered Biota of South Florida.
Vol. |, Mammals. Gainesville, Univ. Presses Florida, 52 p.

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Rhode Island, Kingston, 100 p.

Spitzer, N. C. and J. D. Lazell, Jr. 1978. A new rice rat (genus Oryzomys) from Florida’s Lower Keys. J.
Mammal. 59, 4:787-92.

Vogt, R. C. and C. J. McCoy. 1980. Status of the emydine turtle genera Chrysemys and Pseudemys.
Ann. Carnegie Mus. 49, 5:93-102.

Webster, W., J. Parnell and W. Biggs. 1985. Mammals of the Carolinas, Virginia, and Maryland. Univ.
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Williams, E. E. 1969. The ecology of colonization as seen in the zoogeography of anoline lizards on small
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The Authors

Numi C. Goodyear. Doctoral candidate,
Department of Zoology, University of Rhode
Island, Kingston, RI 02881.

James D. Lazell, Jr. Curatorial Affiliate for
Recent Vertebrates, Peabody Museum of
Natural History, Yale University, and President,
The Conservation Agency, 6 Swinburne Street,
Jamestown, RI 02835.

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