Anteaters and sloths, despite their distinct appearances and behaviors, are related. They share a common evolutionary heritage, belonging to a unique and ancient group of mammals. This shared ancestry explains fundamental biological similarities, even as evolution has shaped them into highly specialized creatures adapted to distinct roles. Understanding their relationship involves exploring their shared taxonomic classification and unique traits of their lineage.
The Unifying Link: Superorder Xenarthra
Anteaters and sloths are grouped within the Superorder Xenarthra, an ancient lineage of placental mammals originating in South America around 60-65 million years ago. They diversified extensively during the continent’s long isolation. Xenarthra encompasses three main living branches: Cingulata, including armadillos, and Pilosa, comprising anteaters and sloths.
Within the Pilosa order, anteaters belong to the suborder Vermilingua, meaning “worm tongue,” and sloths are classified under the suborder Folivora, or “leaf-eaters.” While these suborders represent distinct evolutionary paths and specialized adaptations, their placement within the same superorder highlights their deep, shared evolutionary history. This classification underscores that armadillos, anteaters, and sloths are more closely related to each other than to any other group of mammals.
Defining Characteristics of Xenarthra
Xenarthra members possess several unique anatomical and physiological features. The most prominent is “xenarthrous vertebrae,” extra articulations on their lumbar and sometimes thoracic vertebrae. These joints provide increased skeletal rigidity and support to the lower back and hips, aiding activities like digging. While common to the superorder, tree sloths have lost some articulations, allowing greater flexibility for arboreal locomotion.
Xenarthrans also have remarkably low metabolic rates, among the lowest of all placental mammals. This adaptation allows them to subsist on specialized, often low-energy diets like leaves or insects. Correspondingly, xenarthrans typically maintain lower body temperatures compared to most other mammals, with some species, like the giant anteater, having an average body temperature around 33°C (91°F). Additionally, xenarthrans generally have relatively small and simple brains compared to other mammals of similar size.
A further distinctive feature is their dentition, or often, its absence. Anteaters are completely toothless, a characteristic that led to the obsolete grouping “Edentata.” Sloths and armadillos, while not entirely toothless, possess unique, simple, peg-like teeth lacking enamel. These teeth grow continuously, compensating for wear, and are suited for their specialized diets.
Divergent Paths: Specialized Adaptations
Despite shared ancestry and xenarthran traits, anteaters and sloths evolved highly specialized adaptations, occupying distinct ecological niches. Their dietary preferences represent a significant divergence. Anteaters are myrmecophagous, feeding almost exclusively on ants and termites. Their bodies are adapted for this diet, featuring elongated snouts, powerful foreclaws for breaking into insect nests, and long, sticky tongues that can extend up to 60 centimeters (24 inches) and flick rapidly to collect prey.
Sloths are primarily folivorous, consuming mainly leaves, buds, and twigs. Their digestive systems reflect this, with large, multi-chambered stomachs hosting symbiotic bacteria to break down tough plant material, resulting in an exceptionally slow digestion process that can take days or even weeks. Two-toed sloths can have a more varied diet, occasionally including insects, carrion, or small lizards.
Differences in primary habitats and locomotion also illustrate their divergent evolutionary paths. Giant anteaters are largely terrestrial, while other anteater species, like tamanduas and silky anteaters, are arboreal or semi-arboreal. Terrestrial anteaters walk on their front knuckles to protect their long, sharp claws, used for digging and defense. They can move with a shuffling gait but also gallop and swim. Sloths are exclusively arboreal, spending nearly their entire lives hanging upside down in trees. Their long, curved claws are adapted for gripping branches, making them clumsy and vulnerable on the ground, though surprisingly capable swimmers. Their slow movement, a consequence of their low-energy diet and metabolic rate, also serves as camouflage against predators that hunt by sight.