The term “fitness” congers images of six-pack abs, yoga poses, and 90’s spandex leotards (for me, anyway). But those images can be misleading when it comes to understanding evolutionary fitness.
Good “fitness” in evolution means “a good fit.” Organisms that fit well in their environmental niche spend less energy just trying to survive; they have more energy left over for reproduction. So fitness is often measured by the number of offspring produced.
A sloth is well camouflaged with its slo-mo movement. Many predators detect prey by movement (if we stick with the 90’s imagery, remember the T.rex in Jurassic Park “can’t see us if we don’t move”). Low metabolism in sloths means very low food requirements. One study measured sloth metabolic rate at 174 kJ (kilajoules) per day. (1) Just for comparison, an average human requires 6000-7000 kJ per day.
- Nagy, KA and GG Montgomery. 1980. Field metabolic rate, water flux, and food consumption in three-toed sloths (Bradypus variegatus). Journal of Mammalogy 61(3)465-472
RedNewtGallery 
A very charming illustration to go with an interesting text – thank you!
Thank you, blue! Dripping watercolor may be my new favorite. 🙂