EMERGENCY PETITION TO LIST THE PYGMY THREE-TOED SLOTH

1048 R. P. ANDERSON AND C. O. HANDLEY, JR.TABLE 1. Estimated age (since separation from the adjacent mainland),area, and distance from the mainland for the islands of Bocasdel Toro, Panama. Data are from the Anderson and Handley (2001).IslandIsla EscudoIsla ColónIsla BastimentosCayo NancyCayo AguaIsla PopaIsla CristóbalAge(years ago)8900520047004700340010001000Area(km 2 )4.359.051.56.814.553.036.8Distance fromthe mainland(km)17.61.56.39.56.61.80.3(three-toed sloths), are extant (Webb 1985; Patterson et al.1992). Here we consider only the three-toed sloths, Bradypus.We recently described a new species of Bradypus from IslaEscudo (Anderson and Handley 2001), bringing the numberof species known in the genus to four (Wetzel and Avila-Pires 1980; Wetzel 1985; Anderson and Handley 2001). Thisnew species, B. pygmaeus, is closely related to B. variegatus,which is widespread in both Central and South America. Theother two species in the genus, B. tridactylus and B. torquatus,are restricted to South America.Three-toed sloths are arboreal folivores with low dispersalability (Carvalho 1960; Montgomery and Sunquist 1975; Chiarello1998b). Concomitant with their energy-poor diet, theyhave extremely low metabolic rates and are not fully homeothermic(Britton and Atkinson 1938). Sloths avoid predationlargely by avoiding detection, moving very slowly intrees (Brattstrom 1966) throughout small home ranges, whichaverage 1.6 ha (Montgomery and Sunquist 1975; see alsoChiarello 1998a). Sloths move even more slowly on theground than in trees, traveling on average 0.4 km per hour(Britton and Kline 1939; see also Mendel 1985), althoughthey are known to swim well in rivers (Beebe 1926, pp. 7–9; Carvalho 1960). We have found no reference to their swimmingin salt water, however; perhaps they have a behavioralaversion to salt water or to rough water and wave action.Their relatively large size, restricted diet, and low dispersalpotential make sloths a model system for investigating theeffects of isolation on body size in large insular mammals.TABLE 2. Descriptive statistics for greatest length of skull (mm) andscores on the first principal component (PC I) for adult Bradypus fromBocas del Toro, Panama. Measurements and scores are given as themean two standard errors and the range of minimum to maximum.Sample size is given in parentheses. Data are from Anderson andHandley (2001). Islands are arranged from outermost to innermost;mainland localities from west to east.LocalityIsla EscudoIsla BastimentosCayo AguaIsla ColónCayo NancyIsla CristóbalIsla PopaAlmiranteTierra OscuraValienteÑuriGreatest length of skull69.0 1.3467.5–72.2(6)70.4 1.5368.3–73.4(6)72.5 1.1868.7–74.9(12)73.4 2.9670.5–76.6(4)76.0 2.6073.2–80.0(5)79.3 2.0076.5–82.1(5)80.0 1.2977.7–82.5(6)79.7 2.8875.9–82.5(4)80.5 2.5076.1–86.0(7)80.3 1.9977.7–83.5(6)78.4 1.1377.0–80.6(7)First principal component(PC I)0.28 0.050.35 to 0.22(5)0.14 0.070.18 to 0.07(3)0.04 0.030.14–0.02(11)0.08 0.120.20–0.00(3)0.05 0.070.15–0.05(5)0.06 0.080.06–0.13(4)0.11 0.030.09–0.16(5)0.09 0.070.04–0.15(3)0.12 0.050.02–0.19(7)0.09 0.040.04–0.17(6)0.06 0.030.01–0.10(5)Body Size in Sloths from Bocas del ToroIn a morphological and morphometric study of the slothsof Bocas del Toro (Anderson and Handley 2001), we documenteda repeated pattern of dwarfism in sloths from severalof the islands. The populations of three-toed sloths of theouter (and older, Fig. 1, Table 1) islands of Bocas del Toro—Isla Colón, Isla Bastimentos, Cayo Nancy, Cayo Agua, andIsla Escudo—are significantly smaller in body size than theBradypus from most or all of the four localities on the adjacentmainland (by Tukey’s tests of multiple pairwise comparisonswith a conservative familywide error rate of 0.05; from Anderson and Handley 2001; see also Table 2).Furthermore, some of the populations on those five islandsthemselves vary in mean body size, with that from Isla Escudoclearly being the smallest (Anderson and Handley 2001;Fig. 2; Table 2; see Fig. 3; in addition to the statistical significanceof the magnitudes of these differences in mean relativeto within-group variation, the magnitudes are clearlybiologically significant as well). In contrast, the samples fromIsla Popa and Isla Cristóbal, young islands close to shore,are not significantly different in body size from any populationon the adjacent mainland (Anderson and Handley 2001;Table 2; see Fig. 3).Although sloths from the five outer islands share smallbody size, pelage characters indicate similarities between insularand mainland populations that were once contiguous(Anderson and Handley 2001). Several pelage charactersevaluated throughout the range of B. variegatus in Centraland South America show that sloths from Bocas del Torogroup most closely with those from nearby samples in centralPanama and extreme northwestern Colombia (Anderson andHandley 2001). Furthermore, a few discrete pelage charactersvary within Bocas del Toro, with geographically proximatepopulations there sharing the same character states. For example,there is a west-to-east cline from uniformly coloredto blotchy dorsal pelage. Similarly, the only populations inBocas del Toro with individuals lacking a dorsal stripe arefound at Tierra Oscura and the adjacent islands of Isla Cris-