EMERGENCY PETITION TO LIST THE PYGMY THREE-TOED SLOTH

1056 R. P. ANDERSON AND C. O. HANDLEY, JR.changing environmental conditions such as those experiencedby the sloths of Bocas del Toro (Lande 1980, 1986; changein the ‘‘individual fitness function’’ of Kirkpatrick 1982).Furthermore, environmental change that fosters a change inselection pressures can promote a morphological shift of severalstandard deviations in a small, isolated population within‘‘a few thousand generations’’ (Lande 1980, p. 476). Theshift of seven standard deviations in body size in the Bradypusof Isla Escudo probably occurred in approximately 1500 generations,falling within the predictions of theory.However, the linearity of the response does not fulfill theoreticalexpectations. If a particular smaller body size is optimalon islands, then selection should be strongest immediatelyafter island formation, when the population is farthestfrom the new adaptive peak. Body size would then asymptoticallyapproach the equilibrium (optimal body size) overtime, as selection intensity decreased. This is clearly not thecase in the Bradypus from Bocas del Toro, where sloths fromthe youngest islands (Isla Popa and Isla Cristóbal) are notsignificantly different in body size from mainland samples.Skulls of sloths from those islands do appear slightly moregracile than those of mainland individuals, however (Andersonand Handley 2001). One possible explanation is thatthe basis for changes in optimal body size lies with the disappearanceof other species in the community, which doesnot occur immediately upon island formation. Alternatively,the adaptive peak for optimal body size might shift as afunction of the population’s average body size (presumablyby some frequency-dependent mechanism; see Bürger andLynch 1995). Because of these discrepancies, future work isnecessary to reconcile the present results with theory or perhapsdevelop theoretical models regarding conditions underwhich morphological divergence would be linear for thousandsof generations in new selective environments.Possible Selective ForcesChanges in diet, resource limitation, and absence of mainlandpredators may all have contributed to selection for smallersize. Many small sloths on the islands, and all from IslaEscudo, were collected in mangroves, a habitat that Bradypususually does not inhabit on the mainland. Thus, some slothson the outer islands, and certainly those on Isla Escudo, probablyeat primarily R. mangle leaves, which are coarse andleathery (Everett 1981). Mainland sloths eat the leaves ofmany terra firma forest trees, especially those of Cecropia,a short-lived successional tree with tender leaves protectedby ants rather than by chemicals or lignin (Carroll 1979;Everett 1981). Therefore, a poorer food source may haveinteracted with evolutionary processes in these sloths.Based on what is known of folivore biology, however, onewould not necessarily predict smaller body size given a poorerfood source. Principles of energetics and allometry of bodyproportions (Grand 1978; McNab 1978) demonstrate that inprimates, for example, the largest species must acquire proteinfrom leaves because they cannot gather enough insects(which are high in protein) and fruits are too low in protein(Hladik 1978). Furthermore, the relatively large storage capacityof herbivore intestines is related to their need to retainlarge volumes of plant matter for long periods of time, toaccommodate the fermentation process (Parra 1978). Smallerherbivores, in fact, ‘‘are faced with a paradox, that is as theirsize is decreased their energy requirements per unit weightare increased and their fermentation contents per unit weightare decreased’’ (Parra 1978, p. 208).Even though an energetically poorer food source mightfavor larger body size, a smaller sloth’s absolutely lowerenergy requirements could allow survival and successful reproductionwith less total resources (Roth 1992). Large mammalsare often resource limited on islands, favoring smallersize (Heaney 1978; Lomolino 1985; Roth 1992). Conversely,larger size may be advantageous for greater digestive efficiencyand in antipredator defense on the mainland (see alsodiscussion in Alder and Levins 1994). Several species offelids that prey on Bradypus are present on the mainland buthave not been observed on the outer islands of Bocas delToro and are assumed to have been extirpated subsequent toisolation (Handley 1993, unpubl. data). Additional studiesare clearly called for, investigating possible relationships betweenoptimal body size and metabolism, diet, and communitycomposition.ACKNOWLEDGMENTSWe thank M. D. Carleton, A. L. Gardner, D. Handley, A.P. Hendry, R. D. Holt, E. K. V. Kalko, C. M. McCain, V.L. Naples, L. E. Olson, S. L. Olson, A. T. Peterson, N. A.Slade, R. M. Timm, J. J. Wiens, and an anonymous reviewerfor constructive comments on earlier drafts of the manuscript.N. A. Slade provided lucid statistical advice, and M. Varnshared data on island age and formation sequence. COH’sresearch in Bocas del Toro was supported by the NationalMuseum of Natural History (Nelson Fund and Research OpportunitiesFund); Soundprints of Norwalk, Connecticut; theNational Science Foundation; and J. and P. Nelson of Hillsboro;Oregon. RPA received funding from the National Museumof Natural History Research Training Program, a KansasState University Putnam Scholarship, a National ScienceFoundation Graduate Research Fellowship, the E. RaymondHall Fund (University of Kansas Natural History Museum),and the American Museum of Natural History (RooseveltPostdoctoral Research Fellowship).LITERATURE CITEDAdler, G. H., and R. Levins. 1994. The island syndrome in rodentpopulations. Q. Rev. Biol. 69:473–490.Ahlborn, G., and L. Dempfle. 1992. Genetic parameters for milkproduction and body size in New Zealand Holstein-Friesian andJersey. Livest. Prod. Sci. 31:205–219.Anderson, R. P., and C. O. Handley, Jr. 2001. A new species ofthree-toed sloth (Mammalia: Xenarthra) from Panamá, with areview of the genus Bradypus. Proc. Biol. Soc. Wash. 114:1–33.Arnold, S. J. 1988. Quantitative genetics and selection in naturalpopulations: microevolution of vertebral numbers in the gartersnake Thamnophis elegans. Pp. 619–636 in B. S. Weir, E. J.Eisen, M. M. Goodman, and G. Namkoong, eds. Proceedings ofthe second international conference on quantitative genetics.Sinauer, Sunderland, MA.Arnold, S. J., and M. J. Wade. 1984. On the measurement of naturaland sexual selection: theory. Evolution 38:709–719.Avise, J. C. 1994. Molecular markers, natural history and evolution.Chapman and Hall, New York.Bartlett, A. S., and E. S. Barghoorn. 1973. Phytogeographic history