Molossus rufus (E. Geoffroy)

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 : 160-165

Molossus rufus (E. Geoffroy)

Figures 64 , 65 , 67

VOUCHER MATERIAL: 10 females (AMNH *267266, *267267, *267273, *267539, *268597, *268600, *269101; MNHN *1995.976, *1995.977, *1995.978) and 13 males (AMNH *267263, *267264, *267268, *267269, *267270, *267546, *268595, *268596, *268598; MNHN *1995.979, *1995.980, 1995.981, 1995.982); see table 65 for measurements.

IDENTIFICATION: The history of nomenclature applied to the largest species of Molossus is complex, with different authors alternatively recognizing either M. ater or M. rufus as the correct name. Whereas the epithet ater was used by Goodwin (1960), Husson (1962, 1978), Handley (1976), Freeman (1981), Hill (1981), Brosset and Charles­ Dominique (1990), and Koopman (1993, 1994) among others, the name rufus was applied by Cabrera (1958), Dolan (1989), and Brosset et al. (1996). Both names originate from the same publication ( Geoffroy, 1805a). We follow Dolan (1989) in using rufus for the largest species of Molossus based on her lucid discussion of the historical confusion that led to the rufus / ater controversy.

The type locality of Molossus rufus was restricted by Miller (1913) to Cayenne, French Guiana. Measurements of the lectotype and paralectotype (both adult males) provided by Carter and Dolan (1978) fall slightly outside the range of variation among our male specimens from Paracou (tables 65, 66) in some dimensions. Compared with the Paracou males, the lectotype of rufus has a slightly narrower skull (cf. braincase breadth, mastoid breadth, zygomatic breadth, breadth across molars), and its toothrow is slightly longer. However, this degree of variation is comparable to that found by Dolan (1989) within large series of rufus from Central America. Measurements of both sexes of rufus from Paracou (table 65) fall within the range of variation of Central American rufus measured by Dolan (1989), confirming her observation that this species appears to be morphologically homogeneous across a wide geographic area.

A species similar to and sometimes confused with Molossus rufus is M. pretiosus . Both are large bats with dark, unicolored pelage that may be brown, red, or black; both have spatulate upper incisors; and both have skulls of similar shape with equivalently developed sagittal crests (Miller, 1902; Dolan, 1989). However, recent authors agree that consistent size differences indicate that rufus and pretiosus are distinct species ( Jones et al., 1971; Freeman, 1981; Dolan, 1989; Koopman, 1994). In South America, pretiosus has been reported from Colombia, Venezuela, and Guyana, but not from localities farther east or south ( Dolan, 1989; Koopman, 1994). Even the smallest of our specimens of rufus from Paracou are too large to be referred to pretiosus (for comparative measurements, see Miller, 1902; Jones et al., 1971; Freeman, 1981; Dolan, 1989).

Instead, our collections from Paracou include a different large Molossus species, M. sinaloae . Although rufus and sinaloae exhibit overlapping forearm measurements, they can be immediately and unambiguously identified in the field by pelage characters. Whereas the dorsal fur of rufus is unicolored brown, black, or red, sinaloae has dorsal fur that is bicolored dark brown or reddish brown with a white base comprising approximately one­half of the length of each hair (the base of the fur may appear gray in subadult sinaloae , but the dorsal pelage it is still clearly bicolored). Also, whereas adult male rufus are either black or red (the two color morphs occurring with roughly equal frequency), male sinaloae are brown or reddish brown (never black in our experience, and probably never the rich red color of many male rufus ). The fur over the shoulders is the same length (2–4 mm) as the rest of the dorsal fur in rufus , but a distinct ruff of longer fur (6–7 mm) is present over the neck and shoulders in sinaloae . The ventral fur is the same color as the dorsal fur, and the throat never appears pale in rufus , whereas the ventral fur is slightly paler than the dorsal fur and the white hair bases show through the fur of the throat in sinaloae . These species additionally exhibit nonoverlapping ranges of variation in body weight and many craniodental measurements (greatest length of skull, condyloincisive length, postorbital breadth, braincase breadth, mastoid breadth, zygomatic breadth, and breadth across molars), with rufus consistently larger than sinaloae (tables 65, 66).

A confusing case is presented by the measurement data and pelage variation reported for Molossus ‘‘ ater ’’ by Husson (1962). Several specimens referred to M. ater by Husson exhibit measurements that fall in the range of M. sinaloae , not M. rufus (i.e., the two smallest males and the two smallest females in his table XXXI: RMNH 12998, 12999, 13001; ZMH 1632). Husson (1962: 262) also described another specimen (RMNH 13010, for which no measurements were provided) as having ‘‘dorsal fur... dark brown, the ventral surface being light brown with the exception of the chin and the area surrounding the gular sac, which are whitish as in Molossus molossus .’’ In our experience, this description fits M. sinaloae but not M. rufus . The pelage patterns described by Husson for other specimens in his sample correspond closely with our observations of M. rufus . Unfortunately, Husson did not discuss banding (or lack of banding) of the fur in his 1962 discussion of these bats.

Husson (1962) was apparently unaware of Goodwin’s (1959) description of Molossus trinitatus (which we recognize as a subspecies of M. sinaloae ; see below) from Trinidad. Husson regularly mentioned extralimital species that might eventually be found in Surinam, and had he known of M. trinitatus he would likely have done so. We suspect that Husson, unaware of the potential existence of another large species of Molossus in northern South America, actually described a mixed collection of M. rufus and M. sinaloae trinitatus in his 1962 account of M. ater . This hypothesis is supported by close examination of Husson (1978), which was compiled by L. B. Holthuis and M. Boeseman (op. cit.: xiii–xv) from Husson’s 1962 monograph and later notes. The 1978 volume reported two specimens of M. trinitatus from Surinam (RMNH 13010 and 13014), both of which had been included in Husson’s 1962 treatment of M. ater . In what must have been an editorial oversight, however, the description of the pelage of one of these specimens (RMNH 13010) was still included in the 1978 account of M. ater .

One effect of the mistakes in Husson’s 1962 and 1978 accounts is that the range of size and pelage variation of Molossus rufus in the Guiana region has been overestimated, possibly leading to misidentifications in the subsequent literature. For example, Brosset and Charles­Dominique (1990: 546) referred three French Guianan specimens from Piste St.­Élie to this species, a large black male and two much smaller ‘‘light brown’’ females; citing Husson (1962), they attributed the disparities in color and size among these specimens to sexual dimorphism. Comparisons of the published measurements of these individuals with our Paracou data, however, suggest that Brosset and Charles­Dominique’s collection of Molossus ‘‘ ater ’’ is a mixed series. Whereas the black male from Piste St.­Élie clearly represents M. rufus (measurements of this individual falling in every case within 0.1 mm of the range of variation among male rufus from Paracou), the ‘‘light brown’’ females from the same locality probably represent M. sinaloae (being much smaller than female rufus from Paracou in most dimensions).

In our experience, correct identification of females is often more difficult than identification of males since female Molossus rufus lack the distinctive red or black pelage of adult males; specimens that we misidentified in the field were invariably female M. sinaloae that we initially attributed to M. rufus . In contrast to other molossid species that can be distinguished on the basis of forearm measurements, we found that the best way to quickly identify large Molossus species was to look for the white hair bases characteristic of both sexes in M. sinaloae .

In the event that a trinomial nomenclature for Molossus rufus seems warranted, our Paracou material would be unambiguously referable to the nominate form.

FIELD OBSERVATIONS: We recorded 47 captures (possibly including some recaptures) of Molossus rufus at Paracou, of which 19 were in ground­level mistnets over roadside puddles and 28 were in nets suspended between 13 and 23 m over a narrow dirt road.

Figures

Fig. 64. Dorsal (A) and ventral (B) views of the skull of Molossus rufus (AMNH 269101; female) from Paracou. Dorsal (C) and ventral (D) views of the skull of M. sinaloae (AMNH 269107; female) from Paracou. Scale bars = 10 mm.

Fig. 65. Lateral views of the skull and lower jaw of (A) Molossus rufus (AMNH 269101; female) and (B) M. sinaloae (AMNH 269107; female). Scale bars = 10 mm.

Fig. 67. Comparative palatal cross sections of (A) Molossus rufus (AMNH 269101; female), (B) M. sinaloae (AMNH 269107; female), and (C) Promops centralis (AMNH 269114; female).