Sloths and the Apparent Imperfections of Natural Selection

Jasmine Gunton

Natural selection is a concept often misinterpreted as being a result of some divine destiny; a process ensuring the best adaptations for survival in a given environment. Some animals, however, defy this perception and display how evolution more often functions as a mechanism that just ensures the longevity of a few individuals, with seemingly random consistency. Sloths are perceived as slow, non-threatening, and generally quite useless at survival. When compared to other organisms that inhabit the sloth’s environment, this impression is not too far from the truth. This is not to say that sloths are unimportant - they are essential to the functioning of the ecosystems which they inhabit, and are objectively adorable. Impractical animals such as the sloth are in fact my favourite group of all, as their populations should not be able to thrive, yet they continue to persist for millions of years.

Image by Selina Bubendorfer from Unsplash

Evolutionary History

One thing should first be made clear when discussing the sloth’s biology. There are two distinct genera of sloth, the two-toed sloth (Choloepus), and the three-toed sloth (Bradypus). This may seem insignificant to mention, but the two and three-toed sloths are in fact likely less related to each other than humans are to chimpanzees [1, 2]. The two-toed sloth is more closely related to a family of extinct ground sloths (Scelidotheriidae), and is mainly nocturnal. Three-toed sloths are mostly diurnal, and are more closely related to the giant extinct Megatheriidae sloth family [3]. Additionally, the three-toed sloth has nine vertebrae, while the two-toed sloth only has six [4]. However, the fact that the two extant species of sloth evolved to appear similar to each other suggests that the sloth body plan is beneficial to survival in a tropical rainforest biome. For the purpose of simplicity, I will narrow the majority of my discussion of sloth biology to the two-toed sloth.

Heat Regulation

Although naturally abundant, sloths are only typically found in the tropical rainforests in Central and South America [5]. This is because although sloths are endotherms [6], they cannot afford to spend too much metabolic energy on heat generation, so require a relatively hot environment. Instead of regulating their body heat like other mammals through mechanisms such as shivering, the sloth prefers to rely on the absorption of radiant heat energy from the sun. When the sloth gets too cold, it shuts down its metabolism and enters a state of torpor [7]. Torpor describes a metabolic state in which the sloth decreases its physiological activity and energy expenditure, a process similar to hibernation in other mammals. However, having a very slow metabolism has significant costs.

Photo by Alexander Schimmeck on Unsplash

Diet

The primary diet of the two-toed sloth consists of leaves, buds, and twigs. This plant material is often quite low in nutrients, as the sloth requires little energy due to its low metabolism. The sloth would certainly not be able to catch any substantial animal prey due to its extremely slow movements. Additionally, the sloth is almost blind and has very little hearing, mostly relying on its sense of smell and touch [8]. However, because the sloth has such a slow metabolism, it can die from starvation because it is unable to extract sufficient nutrients from the food it has eaten [9]. To me, an animal that can die from starvation even with a full stomach is one that should not be able to exist. Yet, the sloth perseveres.

Locomotion

So just how slow is the sloth? Sloths tend to move only when necessary and move at speeds around 4-4.5 metres per minute on land [10]. They cannot walk and instead move by dragging themselves across the ground. In comparison, the herbivorous capybara in which the sloth shares its habitat can run at speeds up to 35 miles per hour, or 938 metres per minute [11]. However, the sloth does have one locomotive adaptation that puts the animal at somewhat of an advantage. Surprisingly, the sloth has a substantial swimming capability, in the water reaching speeds of up to 13.5 metres a minute [12]. This adaptation could be useful for escaping predators, but is useless for finding food, as the majority of the sloth’s diet is derived from arboreal browsing.

Behaviour

The swimming adaptation of sloths appears even more confusing considering that sloths are often quite reluctant to leave their trees. In fact, in some cases, infant sloths will die from falling because the mother is unwilling to leave the tree canopy to retrieve their young [13]. One of the main reasons sloths do not want to go on the ground is that they will be vulnerable to predation from animals, such as jaguars and ocelots [14]. Typically, the only time that a sloth will leave its tree is once a week to defecate, which is apparently the most dangerous thing that a sloth can do [15]. Afterwards, the sloth will (very slowly) bury its waste, and then climb back up the tree. Sloths are thought to bury their waste underneath the tree which they occupy as part of a complex symbiotic relationship with native moths. Moths lay eggs in the sloth’s faeces, which then mature and fly up to the sloth. The moths will live in the sloth’s fur, promoting the growth of algae, which the sloth then feeds on [15].

Reproduction

Partially due to their reluctance to move, sloths live mostly solitary lives, only meeting to mate. Although able to reproduce once a year, a female sloth may take longer than one year to find a fertile male, despite their abundance in equatorial rainforests [16]. The sloth is an example of a k-selected species, because it has a relatively long gestation period, and only produces one offspring at a time [17]. This seems ineffective as opposed to having multiple offspring where at least one of them should be more likely to survive. However, having such a slow metabolism, the sloth would not be able to provide sufficient care and nutrients for more than one baby. With a low reproductive rate, it is difficult to imagine how the sloth could have survived for around 60 million years [18].

Conclusion

The contradicting ecology and tenacity of the sloth shows us that natural selection can work in ways that are not often clear, and we still have limited understandings as to what allows a population to survive. Sloths just like to hang out for most of their lives, and it obviously works very well for them. Only two of the six species of sloth are categorised as threatened by the IUCN (mostly as a result of habitat loss), with the other species being classified as of ‘least concern’ [19]. So who are we to judge their puzzling adaptive strategies?

References

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