Choloepus didactylus & C. hoffmanni
1) General Zoological Data
This family contains the species: Choloepus didactylus and C. hoffmanni. They differ from the three-toed sloths by having only two toes on their forearms, but having three on their legs. The designation "Choloepus" indicates this. It refers to being "maimed", because of the lack of a third front digit, when compared with three-toed sloths, according to Gotch (1979). They are South American animals, with a distribution of Hoffmann's sloth over the central Americas, and the two-toed sloth being confined to northern Brazil and Colombia (Wetzel, 1982). This author, and others (Nowak, 1999) now assign this new and perhaps unusual family name to these two animals; they were formerly listed as Choloepidae. The reasons are their relationships to extinct families of sloths. These relations and their putative evolution from larger ancestors have been reviewed in the molecular studies of Greenwood et al. (2001), and also by Delsuc et al. (2001). Several subspecies have been nominated for both species. They vary mostly in pelage and distribution but have not necessarily been fully defined by the various authors. The animals are herbivorous and, while Hoffmann's sloth has a mostly nocturnal activity, this is in contrast to his mostly diurnal three-toed cousins. These sloths are also somewhat larger, weighing up to 8.5 kg. Many of the references that I have cited in the chapter on the three-toed sloth, Bradypus, also apply to this chapter. They should be consulted in that chapter. There are, however, also a number of significant differences, especially chromosomal differences that also set the two toed sloths apart from Bradypus.
The longevity of two-toed sloths is 29 years, according to Moeller (1975, quoted by Wetzel, 1982) but it may even be longer in Hoffmann's sloth (up to 32 years, according to Nowak's sources). Pregnancy in C. hoffmanni lasts 11.5 months (309 days) according to Eisenberg & Maliniak (1978; 1985) who used special techniques to verify the gestations. It produces normally one offspring (Meritt, 1975). There is virtually no external sexual dimorphism in the two-toed sloths. Therefore, gender assignment has been difficult and is now aided by the DNA study of hair samples (Murata & Masuda, 1996). Main predators of sloths are the large carnivores and eagles. The animals are not currently endangered. While they have been relatively easily adapted to zoo maintenance, breeding in captivity has occurred only in a few zoos (McCrane, 1966). The Hoffmann's sloth placenta that is discussed in this chapter was kindly supplied by Dr. D.A. Meritt who, at Lincoln Park Zoo, Chicago, has bred the animals successfully. He has also written a detailed review of the animal's physiology, diet, immobilization and provided access to other physiologic studies (Meritt, 1985). This placenta was also depicted by Soma (1976), and it was described in a previous contribution (Benirschke & Powell, 1985).
|Choloepus didactylus at San Diego Zoo. The two forefoot digits and three leg digits are well visible.|
|Another photograph to show two and three digits.|
General Gestational Data
One young is produced after a pregnancy that lasts up to 11.5 months in Hoffmann's sloth (Eisenberg & Maliniak, 1978). These authors were forceful in their interpretation of the gestational length and cited all previous records. Some of those pregnancies cited were also very long. But, they dismissed suggestions of sperm storage and delayed implantation to account for the long gestation. Neonates weigh 350-454 g. Other observers have suggested that pregnancy is shorter for C. didactylus. Veselovsky (1966), who recorded birth weights of 376, 408 and 309 g, observed a birth 5 months and 20 days after mating in C. didactylus. Thus, there may be marked differences in reproduction of these two species that need further attention.
General Characterization of the Placenta
|Placenta of Choloepus hoffmanni with the typically bulging cotyledons. (Courtesy Dr. Dennis Meritt, Chicago). Some atrophy of lobules is seen at right.|
|Closer detail of the term placenta from a Hoffmann's sloth (courtesy Dr. D.A. Meritt, Chicago).|
|Choloepus didactylus gestation from the San Diego Zoo. It is within the uterus (left).|
|Choloepus didactylus gestation from the San Diego Zoo.|
(1921) considered the earlier publication on the placenta of Hoffmann's
sloth by Turner. The findings were essentially identical to those that he
found in a specimen of Bradypus. For that reason, and in order not
to be too repetitive, the reader is referred to the chapter on the three-toed
sloth. In my observations, no significant differences were ascertained when
I compared the gross or microscopic features. This is a multicotyledonary,
but fused placenta, in which the cotyledons (lobes) are separated by thin
maternal (decidual) septa that contain large maternal blood vessels. It
is more lamellar than villous, and the placenta is essentially endothelio-chorial
in nature. There is no vitelline tissue, and the amnionic sac directly apposes
the chorion. The chorionic membrane has many small blood vessels. The placental
location is mostly fundal but extends on the posterior uterine surface distally.
Details of fetal/maternal barrier
|Site of implantation with septum between two cotyledons.|
|During gestation, many lobules undergo degenerative changes, such as this cotyledon.|
|Cross section through placental labyrinth. E=endothelium; M= maternal vessel.|
|Electron micrograph of "barrier" in C. didactylus. BM=basement membrane, C.T.= connective tissue. The latter is the reason why this placenta has been considered to be syndesmochorial.|
|At arrows are the so-called epithelioid cells of the villous stroma = Hofbauer cells = macrophages. They are pigmented in H&E preparations.|
The umbilical cord has two arteries and one vein. It is about 10 cm long at term and is not twisted. No remnants of ducts were identified. The surface is smooth and clad with thin amnionic epithelium. Few spirals were detected in our specimens.
|Membranes apposed to the uterine wall. Note the numerous small blood vessels in the chorion.|
Trophoblast external to barrier
There is no extravillous trophoblast, and the invasion of maternal spaces is limited to the decidua. Here, the trophoblast destroys decidual cells, which accounts for much of the debris that is found at the base. It then remodels the decidua and maternal blood vessel walls so as to form the trabeculae, or lamellae, which are apposed by the fetal components. Maternal blood vessel invasion does not occur.
|Pregnant uterus of C. didactylus before opening.|
No subplacenta exists, but decidua basalis is left after delivery, whence the endometrium regenerates.
|Adrenal glands (0.311 g each), kidneys (0.55 & 0.85 g), and testes (0.065 & 0.058 g) of a fetal C. tridactylus that weighed 110 g.|
The karyotype of these two species has been confusing and it is very different from, and more complex than that of three-toed sloths. Especially the sex-determining chromosomes have been a problem in their clear delineation. Because it is difficult to determine the gender of two-toed sloths externally, Murata & Masuda (1996) developed a technique that employs DNA from hair for sex assignment. They amplified the SRY region and were able to determine maleness in newborns.
Several authors have studied the chromosomes of Hoffmann's sloths and found initially 2n=49, with the Y-chromosome translocated to a small autosome. Females, however, also have only 49 chromosomes, with a single X-chromosome. Two animals (perhaps representing subspecific hybrids) were supplied to us for study by Meritt (1977). Both, one male and one female, had 50 chromosomes. Jorge et al. (1985) found further variations, and identified animals with chromosome numbers between 2n=49 and 51 for Hoffmann's sloths. The animals that they studied were from specified regions in South America.
The situation is different in C. didactylus. Sonta et al. (1980) found the two sexes to have 53 chromosome and a somewhat similar XO female sex determination. They found a translocated Y/A as well. The most comprehensive study undertaken on this species is that of Jorge et al. (1985). They found six different karyotypes in animals from specified, different regions in South America. Their chromosome numbers varied from 53 to 64 elements. It is thus certain that there may well be subspecies/species differences in different regions of South America that require much more intensive study.
No interspecific hybrids have been produced, but Meritt (1977) referred to probable intersubspecific crosses in his study. Furthermore, the very unusual chromosomal distribution found in these sloths suggests that the nominated subspecies may actually be good species. Much further work is needed before the situation is resolved.
I know of no studies.
16) Physiologic data
Other remarks - What additional Information is needed?
Benirschke, K. and Powell, H.C.: On the placentation of sloths. Pp. 237-241, In, Montgomery, G.G., ed.: The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Smithsonian Institution, Washington, 1985.
Bush, M. and Gilroy, B.A.: A bleeding technique from nonpalpable vessels in anesthetized two-toed sloths (Choloepus didactylus) - plus hematologic data. J. Zoo Anim. Med. 10:26-27, 1979.
Delsuc, F., Catzeflis, F.M., Stanhope, M.J. and Douzery, E.J.: The evolution of armadillos, anteaters and sloths depicted by nuclear and mitochondrial phylogenies: implications for the status of the enigmatic fossil Eurotamandua. Proc. Roy. Soc. Lond. B Biol. 268:1605-1615, 2001.
Diniz, L.S. and Oliveira, P.M.: Clinical problems of sloths (Bradypus sp. and Choloepus sp.) in captivity. J. Zoo Wildl. Med. 30:76-80, 1999.
Eisenberg, J.F. and Maliniak, E.: Reproduction of the two-toed sloth Choloepus hoffmanni in captivity. Amer. Soc. Mamm. Abstr. Techn. Pap. 58th Ann. Meet. 41-42, 1978 (cited by Nowak, 1999).
Eisenberg, J.F. and Maliniak, E.: Maintenance and reproduction of the two-toed sloth Choloepus didactylus in captivity. Pp. 327-331, in, The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. G.G. Montgomery, ed. Smithsonian Institution, Washington, 1985.
Gilmore, D.P., da-Costa, C.P. and Duarte, D.P.: An update on the physiology of two- and three-toed sloths. Brazil. J. Med. Biol. Res. 33:129-146, 2000.
Gilmore, D.P., da Costa, C.P. and Duarte, D.P.: Sloth biology: an update on their physiological ecology, behavior and role as vectors of arthropods and arboviruses. Braz. J. Med. Biol. Res. 34:9-25, 2001.
Gotch, A.F.: Mammals - Their Latin Names Explained. Blandford Press, Poole, Dorset, 1979.
Greenwood, A.D., Castresana, J., Feldmaier-Fuchs, G. and Paabö, S.: A molecular phylogeny of two extinct sloths. Mol. Phylogenet. Evol. 18:94-103, 2001.
Griner, L.A.: Pathology of Zoo Animals. Zoological Society of San Diego, San Diego, California, 1983.
Heuser, C.H. and Wislocki, G.B.: Early development of the sloth (Bradypus griseus) and its similarity to that of man. Contrib. Embryol. Carnegie Inst. Washington 25:1-13, 1935.
Jorge, W., Meritt, D.A. and Benirschke, K.: Chromosome studies in edentates. Cytobios 18:157-172, 1978.
Jorge, W., Orsi-Souza, A.T. and Best, R.: The somatic chromosomes of Xenarthra. Pp.121 in, The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. G.G. Montgomery, ed. Smithsonian Institution, Washington, 1985.
M.P.: Birth, behavior and development of a hand-reared two-toed sloth.
Int. Zoo Ybk. 6:153-163, 1966.
D.A.: Edentate diets. II. Two-toed sloths. Lab. An. Sci. 23:543-543, 1973.
D.A.: Edentates. The Ark (Lincoln Park Chicago) 2:2-14, 1975.
D.A.: The natural history, behavior, nutrition, physiology, reproduction,
development and management of Hoffmann's sloth, Choloepus hoffmanni
(Peters). M.A. Thesis, Northeastern Illinois University, 124 pp., 1977.
(Kindly supplied by author).
Moser, H.G. and Benirschke, K.: The fetal zone of the adrenal gland in the nine?banded armadillo, Dasypus novemcinctus. Anat. Rec. 143:47?60, 1962.
Murata, K. and Masuda, R.: Gender determination of the Linné's two-toed sloth (Choloepus didactylus) using SRY amplified from hair. J. Vet. Med. Sci. 58:1157-1159, 1996.
Nowak, R.M.: Walker's Mammals of the World. 6th ed. The Johns Hopkins Press, Baltimore, 1999.
Seymour, C., Peralta, P.H. and Montgomery, G.G.: Viruses isolated from Panamanian sloths. Amer. J. Trop. Med. Hyg. 32:1435-1444, 1983.
Soma, H.: In chase of trophoblast: A comparison between human and other mammalian placentas. J. Tokyo Med. Coll. 34:873-883, 1976 (in Japanese).
Sonta S.-i., Hayata, I., Sasaki, M. and Kondo, N.: The karyotypes and sex-determining mechanism in the two-toed sloth, Choloepus didactylus. Chromos. Inf. Serv. 28:15-17, 1980.
Veselovsky, Z.: A contribution to the knowledge of the reproduction and growth of the two-toed sloth Choloepus didactylus at Prague Zoo. Int. Zoo Ybk. 6:147-153, 1966.
Wetzel, R.M.: Systematics, distribution, ecology, and conservation of South American edentates. Pp. 345-375, in "Mammalian Biology in South America. Special Publication Series of the Pymatuning Laboratory of Ecology, University of Pittsburg, Vol. 6, 1982.
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