Anteaters and sloths are members of the same ancient biological lineage, making them relatives despite their differences in appearance and behavior. Both are unique South American mammals that share a deep evolutionary history. They represent two distinct branches of a group that originated on the continent millions of years ago.
Defining the Relationship: The Superorder Xenarthra
The relationship between anteaters and sloths is based on their classification under the Superorder Xenarthra. This group, which also includes armadillos, represents one of the four major clades of placental mammals. The name Xenarthra, meaning “strange joints,” refers to a unique skeletal feature uniting all members.
Xenarthrans possess extra articulations, called xenarthrous processes, on the lumbar and posterior thoracic vertebrae. These contacts strengthen the lower back and hips, a modification that historically aided activities like digging. This feature confirms their shared ancestry.
The Superorder Xenarthra is divided into two living orders: Cingulata (armadillos) and Pilosa (sloths and anteaters). This classification confirms that sloths and anteaters are more closely related, sharing a more recent common ancestor within the Pilosa order. All xenarthrans also share traits like reduced or absent dentition and a notably low metabolic rate compared to other mammals of similar size.
Specialized Adaptations and Evolutionary Divergence
Sloths and anteaters diverged significantly through adaptive radiation, specializing in different ecological niches. This evolutionary split is estimated to have occurred around 60 million years ago in the early Paleocene era. This divergence led to the development of highly specialized body plans suited to their respective environments.
Sloths adapted to an arboreal, leaf-eating lifestyle, requiring a slow metabolism to process their low-calorie diet. This folivorous specialization resulted in a large, multichambered stomach for fermenting tough leaves. Anteaters, conversely, specialized in myrmecophagy, the consumption of ants and termites.
The anteater evolved as an efficient insectivore, developing an elongated snout and losing all teeth. Their powerful claws and robust forelimbs are adapted for tearing open termite mounds and ant nests to access prey. Sloths, in contrast, developed hook-like claws for suspensory locomotion, allowing them to hang and move slowly through the forest canopy.
Contrasting Lifestyles: Diet, Movement, and Habitat
The distinct evolutionary paths are reflected in their contrasting lifestyles. Anteaters are strictly insectivorous, relying on colonial insects, with some species consuming up to 30,000 ants and termites daily. Their feeding requires a long, sticky tongue, which can extend up to two feet in the Giant Anteater, to rapidly lap up prey.
Sloths are primarily folivores, feeding on leaves, buds, and twigs. This low-energy diet dictates their signature slow movement, as they spend most of their time resting or feeding in the forest canopy. Their movement is characterized by suspensorial locomotion, hanging upside down from branches.
Regarding habitat, the largest species, the Giant Anteater, is terrestrial, living on the ground in grasslands and savannas. Smaller species, like the Tamandua, are semi-arboreal, moving through both trees and the ground. Sloths are almost exclusively arboreal, inhabiting the trees of humid tropical forests.