Oligoryzomys fulvescens (Saussure)

VOSS, ROBERT S., LUNDE, DARRIN P. & SIMMONS, NANCY B., 2001, The Mammals Of Paracou, French Guiana: A Neotropical Lowland Rainforest Fauna Part 2. Nonvolant Species, Bulletin of the American Museum of Natural History 2001 (263), pp. 1-236 : 116-120

Oligoryzomys fulvescens (Saussure)

VOUCHER MATERIAL: AMNH 267022 , 267023 ; MNHN 1998.673 . Total = 3 specimens. IDENTIFICATION: Although the genus Oli­

TABLE 31 Comparison of Character­State Frequencies for Occurrence of the Alisphenoid Strut in Oecomys bicolor and O. rutilus a

goryzomys (formerly a subgenus of Oryzomys ; see Carleton and Musser, 1989) has never been comprehensively revised, several publications have at least partially clarified the species­level systematics of Oligoryzomys in certain regions, notably Paraguay (Myers and Carleton, 1981), Bolivia (Olds and Anderson, 1987), and Central America (Carleton and Musser, 1995). Unfortunately, the Oligoryzomys of northern South America have received no revisionary attention to date. In a preliminary review of the genus, however, Carleton and Musser (1989) hypothesized that a single widespread polytypic species— O. fulvescens —extends from Mexico throughout most of Central America, thence southward into Colombia and northern Ecuador and eastward throughout Venezuela, Guyana, and Surinam. Included as subjective synonyms in a subsequent synopsis of O. fulvescens (see Carleton and Musser, 1995) were costaricensis J. A. Allen, delicatus J. A. Allen and Chapman , navus Bangs , messorius Thomas, tenuipes J. A. Allen , munchiquensis J. A. Allen, lenis Goldman, mayensis Goldman, engraciae Osgood, and pacificus Hooper. As defined geographically by Carleton and Musser (1989), O. fulvescens was not known to occur east of Surinam or south of the Amazon. Instead, populations of Oligoryzomys extending along the entire south bank of the Amazon from near the headwaters of that river to its mouth (near Belém) were identified as comprising a distinct species, O. microtis (J. A. Allen) .

The taxonomic status of Oligoryzomys populations in French Guiana (first reported as O. delicatus by Charles­Dominique, 1993) and in the Brazilian state of Amapá (tentatively identified as O. navus by Carvalho, 1962) has yet to be critically evaluated. Because these regions constitute a geographic hiatus between the known ranges of O. fulvescens and O. microtis (as delimited by Carleton and Musser, 1989; see above), we compared our Paracou vouchers with typical material of both species.

Craniodental measurements of our three Paracou specimens (table 32) fall almost entirely within the range of morphometric variation observed by Carleton and Musser (1995) for a large sample of typical Oligoryzomys fulvescens (topotypes and other specimens from Veracruz, Mexico); the few exceptions are Paracou values (of BM1, BIF, BZP, and BB) that only exceed the observed range in homologous dimensions of typical fulvescens by 0.1 mm. Our vouchers are larg­ er than average fulvescens from Mexico, but Carleton and Musser (1995) documented a southward cline of increasing size among their Central American samples; in craniodental measurements, our vouchers more closely resemble Costa Rican and Panamanian populations that Carleton and Musser referred to O. f. costaricensis than they do typical (Mexican) material.

The only noteworthy morphometric contrast between our Paracou vouchers and typical Oligoryzomys fulvescens appears to be the ratio of tail length to head­and­body length. That ratio is about 1.16 in our two specimens with intact tails, whereas the ratio of mean tail length to mean head­and­body length calculated from Carleton and Musser’s (1995) data for Mexican fulvescens is 1.31. The same ratio for Costa Rican and Panamanian samples of fulvescens measured by those authors ranges from 1.27 to 1.44. In side­by­side comparisons, Mexican and Central American skins of fulvescens appear visibly longer­tailed than our Paracou vouchers.

Subtle qualitative cranial differences are present between our three specimens and the Mexican and Costa Rican exemplars of fulvescens with which we compared them. For

TABLE 32 Sex, Measurements (mm), and Weights (g) of Oligoryzomys Vouchers from Paracou with Comparative Data from Samples of Typical O. fulvescens and O. microtis (All measurements from adult specimens.)

example, the frontal sinuses appear slightly more inflated in the Mexican and Central American samples, producing a noticeably larger swelling behind the lacrimal bone in the front of the orbit than that seen in our French Guianan material. The rostrum also appears relatively longer and more slender, and the upper incisors perhaps less strongly opisthodont in Mexican and Central American specimens than in our vouchers. These are not conspicuous contrasts, however, and their possible taxonomic significance is difficult to assess without a careful study of the many hundreds of museum specimens that are now available from dozens of geographically intermediate localities. In other qualitative characters, including those that have previously been used to diagnose species of Oligoryzomys (e.g., the dorsal projection of the capsular process of the lower incisor al­ veolus, position of the incisive foramina relative to the toothrows, pelage color and texture), our vouchers appear to be indistinguishable from Mexican and Central American examples of fulvescens .

Confusingly, the Paracou material is also morphometrically similar to typical Oligoryzomys microtis as represented by Allen’s (1916a) original series (and several topotypes, table 32) collected by Leo E. Miller on the ‘‘Lower Rio Solimoens (fifty miles above mouth)’’, a locality that can now be restricted with some confidence to a specific site on the north bank of the river. 17 Apparent

17 According to Allen’s (1916b) gazetteer of the Roosevelt Brazilian Expedition, Miller collected at this locality from 16 to 30 April 1914. A letter from Miller to F. M. Chapman (in the AMNH Ornithology archives) written in Manaus on 24 April 1914, however, states that he had just ‘‘... spent a week up on the Solimoens,

mensural differences between these very small samples (3 and 10 specimens, respectively) are unimpressive, especially when the considerable distance between collection localities (roughly 1300 km) is taken into account. Length of the molar toothrow is the only nonoverlapping measurement, but the mean sample difference (0.2 mm) is trivial. The few specimens at hand from immediately south of the Amazon (e.g., AMNH 95983, 188964) have toothrow measurements that do overlap those of our vouchers, so very large samples will probably be required to demonstate any significant divergence in this dimension between Oligoryzomys populations on opposite banks of the river, if any such difference in fact exists.

Allen’s (1916a) original description of Oligoryzomys microtis emphasized the diagnostic value of pale coloration, relatively small ears, and short tail for distinguishing this species from other congeners. In coloration, however, Allen’s material of microtis appears to be indistinguihable from typical fulvescens

with a [G]erman friend, and got some nice things in the mammal line... but I hope to go to another fazenda within a day or two.’’ Miller therefore visited (or intended to visit) two localities near Manaus between 16 and 30 April. Perplexingly, Miller’s skin tags give only ‘‘Solimoens’’ as the locality where he worked in this interval, and his field catalog (AMNH Mammalogy archives) contains no additional geographic information. Fortunately, this problem did not escape the notice of mammalogists who were still in time to obtain at least a partial first­hand clarification: in a letter dated 4 June 1945, G. H. H. Tate wrote Miller (then retired and living in Connecticut) asking him to provide additional geographic details about the types of Proechimys kermiti (collected 20 April 1914) and Oligoryzomys microtis (collected 29 April 1914). Miller replied in a letter dated 6 June 1945 that ‘‘After a short time in Manaos, I went down the Rio Negro and up the Solimoes about 50 or 60 miles to make the collections on the North bank of the river. The location is approximately between 3° and 4° S Lat., and Long. 61°. I think this will locate the spot closely enough for your purposes.’’ (italics ours; correspondence in AMNH Mammalogy archives). Because Miller did not mention collecting at separate localities on the 20th and the 29th, the second sojourn from Manaus mentioned in his letter to Chapman of 24 April 1914 may have been a return trip to his previously productive site. Based on Miller’s reply to Tate, Moojen (1948) published the restricted type locality of P. kermiti as Manacaparú, a settlement located 70 km WSW of Manaus on the north bank of the lower Solimões at 3°08̍S, 60°01̍W (Paynter and Traylor, 1991). We assume that the type of O. microtis was collected at the same place.

based on our side­by side comparisons of skins. Small differences in ear size are hard to evaluate without measurements taken in the flesh, which Leo E. Miller (the collector of Allen’s specimens) did not record, and our visual comparisons of dried ears between typical material of microtis and fulvescens revealed no obvious size contrast. The reputedly diagnostic short tail of microtis is also hard to assess. Although Miller’s field measurements of the type (AMNH 37091) indicate that its tail was slightly shorter than the head­and­body, 18 three paratypes (AMNH 37088, 37089, 37097) have an average ratio of tail to head­and­body of 1.20. Because Miller did not indicate whether his measurements were of complete or bobbed tails (AMNH 37096 has an obviously bobbed­tail measurement of 20 mm with no accompanying notation to that effect), and because most of his skins no longer have intact tailtips (the result of bending and compression in museum trays), it is now impossible to evaluate the ratio of tail to head­and­body length in Allen’s series. Other specimens of microtis (sensu Carleton and Musser, 1989) , however, are not very short­tailed: one specimen from the south bank of the lower Amazon (AMNH 203400) has a tail:body ratio of 1.17, whereas another (AMNH 188964) has an improbably large ratio of 1.57. Given the usual methodological inaccuracies associated with muroid tail measurements (Howell, 1924), any taxonomic inferences based on this dimension in the small samples at hand would be premature.

We found no obvious qualitative characters to distinguish Oligoryzomys fulvescens (as represented by our Mexican and Central American exemplars) from typical O. microtis , and in view of the negligible morphometric differences indicated by table 32 we are unable to confidently assign our Paracou material to one or the other taxon. Although

18 Miller’s original measurements, recorded in pencil on a small field label tied to the right hindfoot of the type skin, were correctly reported by Allen (1916a) as 183 mm (total length), 90 mm (tail), and 20 mm (hindfoot). However, Olds and Anderson (1987: 271) stated that ‘‘... the tail of the holotype is more than half the total length, a discrepancy noted by Goodwin (1953).’’ In fact, Goodwin (p. 299) did not note any discrepancy and reported the same external measurements as Allen.

an obvious implication of the preceding discussion is that fulvescens and microtis are conspecific, it is also possible that multivariate analyses of larger samples could detect morphological discontinuities that are not now apparent; karyological and biochemical comparisons among key geographic populations could likewise contribute to an assessment of these and other phenotypically similar nominal taxa of Oligoryzomys as valid species. Lacking the time to undertake such a critical revisionary study for this faunal report, we simply apply the older name to our Paracou material.

OTHER SPECIMENS EXAMINED: Brazil — Amazonas, near Manacaparú on north bank of lower Rio Solimões ( AMNH 37088 , 37089 , 37091 [holotype of microtis ], 37092– 37097, 37157) ; Para´, Capim ( AMNH 188964 , 203400 ), Pôrto de Moz ( AMNH 95983 ), Vilarinho do Monte ( AMNH 95984– 95986 , 95997 ) . Costa Rica — Puntarenas, Cañas Gordas ( AMNH 142440–142458 , 142490–142495 , 142500 ) . Mexico — Veracruz, Jalapa ( AMNH 12536 /10846–12541/ 10851, 12543/10853–12549/10959, 12583– 12585) .

FIELD OBSERVATIONS: All of our definite records of Oligoryzomys fulvescens at Paracou are based on collected specimens. All three of our vouchers were caught by hand at night in roadside secondary growth (fig. 13), where two were encountered running on the ground through sparse weeds and the third was climbing among the dried leaves of a felled tree.