Myotis nigricans (Schinz)

Simmons, Nancy B. & Voss, Robert S., 1998, The mammals of Paracou, French Guiana, a Neotropical lowland rainforest fauna. Part 1, Bats, Bulletin of the American Museum of Natural History 237, pp. 1-219 : 142-145

Myotis nigricans (Schinz)

Figures 57 , 58

VOUCHER MATERIAL: 27 females (AMNH *267223, *267225, *267227, *267228, *267229, 267230, *267231, *267509, *267510, *267511, *267512, *267514, *267516, *267517, *267519, *267520, *267522, *268539; MNHN *1995.937, *1995.938, *1995.939, *1995.940, *1995.941, *1995.942, *1995.943, *1995.944, *1995.945) and 2 males (AMNH *267221, *267513); see table 57 for measurements.

IDENTIFICATION: LaVal’s (1973) revision is still the standard reference for Neotropical Myotis , but descriptions and measurements useful for identifying M. nigricans can also be found in Goodwin and Greenhall (1961), Husson (1962, 1978), Wilson and LaVal (1974), and Brosset and Charles­Dominique (1990). Myotis nigricans as currently recognized is a highly variable species. As noted by LaVal (1973: 6), ‘‘Any specimen [of Neotropical Myotis ] that does not seem to fit the diagnosis of another species is probably nigricans .’’ Four or five subspecies of Myotis nigricans are currently recognized, of which the nominate form M. n. nigricans occurs throughout Central America and most of tropical South America (LaVal, 1973; Wilson and LaVal, 1974; Koopman, 1994).

Despite confusion concerning the limits and diagnosis of Myotis nigricans , specimens from the Guianan region (presumably referable to the nominate subspecies) share a common craniodental morphology (described by Husson, 1962, 1978). Given appropriate specimen preparations (see below), this species can be easily distinguished from M. riparius and M. albescens , both of which also occur in the region (see below and appendix 1).

We found that specimens of Myotis from Paracou could not be identified unambigously without extracting and cleaning skulls. Although pelage characters (e.g., color, banding and/or frosting, degree of ‘‘silkiness’’ or ‘‘woolliness’’) and measurements (e.g., forearm length) have been cited previously as morphological criteria for identifying Myotis species (e.g., by LaVal, 1973; Brosset and Charles­Dominique, 1990), we found considerable overlap between M. nigricans and M. riparius with respect to all of these features (see below). By contrast, M. nigricans and M. riparius are easily distinguished by craniodental morphology (figures 57, 58), and we found no species overlap in five of seven craniodental measurements (table 57).

Myotis nigricans can be distinguished from M. albescens throughout South Amer­ ica based on postorbital breadth and braincase breadth, both of which are greater in M. albescens where these taxa occur in sympatry or near­sympatry ( Husson, 1962, 1978; LaVal, 1973; Baud and Menu, 1993). Husson (1962, 1978) noted that Surinamese specimens of M. albescens have a braincase breadth> 6.8 mm and a postorbital breadth> 3.6 mm, whereas these dimensions are smaller in M. nigricans from Surinam. None of our specimens from Paracou has a braincase breadth> 6.75 mm, and only one specimen has a postorbital breadth>3.50 mm (table 57). In our sample of 29 individuals, the specimen with the largest postorbital breadth (MNHN 1995.943, postorbital breadth 3.65 mm) has a braincase breadth identical to the sample mean (6.41 mm). Based on these data and comparisons of external morphology, we are confident that none of the Myotis collected at Paracou represents M. albescens . Myotis albescens has yet to be reported in French Guiana despite its occurrence in Surinam and northeastern Brazil. However, we note that several specimens identified as M. nigricans by Brosset and Charles­Dominique were reported to have postorbital breadth ≥ 3.7 mm, and it is possible that one or more of these specimens may represent M. albescens . Although M. albescens generally has a distinctively ‘‘frosted’’ pelage, LaVal (1973) noted that some specimens lack this feature and externally resemble M. nigricans .

Our voucher material of Myotis nigricans falls within the lower range of size variation previously reported for this species (e.g., by LaVal, 1973; Wilson and LaVal, 1974). With the exception of postorbital breadth (noted above), the Paracou specimens are similar in all dimensions to specimens reported from other localities in French Guiana and Surinam ( Husson, 1962, 1978; Brosset and Charles­Dominique, 1990). Most of our specimens have long, silky, brownish dorsal fur. However, several have fur that is brownish but woollier in texture, and one individual (a male) has almost black fur. The ventral fur is always slightly lighter in color than the dorsal fur, appearing frosted in many specimens.

FIELD OBSERVATIONS: We made 29 vouchered captures of Myotis nigricans at Paracou, 25 of which were in ground­level mistnets and 4 of which were in elevated nets. Of the 25 ground­level captures, 3 were in creekside primary forest, 9 were in manmade clearings, and 13 were over roadside puddles. The elevated net captures were made between 4 and 13 m above a narrow dirt road.

See the following species account for habitat comparisons with Myotis riparius and for information about captures of unidentified Myotis .

Figures

Fig. 57. Dorsal (A) and ventral (B) views of the skull of Myotis nigricans (AMNH 267228; female) from Paracou. Dorsal (C) and ventral (D) views of the skull of M. riparius (AMNH 268591; female) from Paracou. Note differences in structure of the anterior braincase, palate, and auditory region. Scale bars = 5 mm.

Fig. 58. Lateral views of the skull and lower jaw of (A) Myotis nigricans (AMNH 267228; female) and (B) M. riparius (AMNH 268591; female). Scale bars = 5 mm.