Sloths: the classic slow moving animal adored by many. Thoughts of sloths probably don’t evoke images of deep sea diving. However, a new study, led by Eli Amson from the Sorbonne in Paris, published in the journal Proceedings of the Royal Society B shows that sloths and the sea have an evolutionarily significant relationship. It is hypothesized that ancient sloths began to venture into the ocean to find food as their normal sustenance become harder to find on land. As land leaves grew scarce, sloths headed into open waters to search for nutrition.
Land animals tend to have low density bones to keep our body weight lower to aid in movement. However, in the water, this can be a problem because it causes high buoyancy. Because of this buoyancy, most water dwelling animals, especially those that perform dives, have higher density bones to help them dive deeper in the water column. The researchers looked at the bone densities of five species of sloths and measured the changing bone density over time among them.
The study revealed that earlier sloths had a lower bone density and as time passed the density increased. In addition, parts of the bone structure, like the ribs, became more compact. The researchers hypothesize that this was an adaptation to allow deeper dives. Higher density and more compact bones are able to sink further down in the water column, which means that the sloths were able to reach and eat deeper growing plants. This adaptation is significant because it happened over 4 million years – a relatively short time span in evolutionary terms.
Sloths did not make the full transition back to the water like their mammalian relatives dolphins and seals did today. However, sloths exhibit an interesting case that shows us how quickly evolution can work and how unlikely behaviors can spring up in surprising ways in response to a changing environment.
Amson, E., de Muizon, C., Laurin, M., Argot, C., & de Buffrénil, V. (2014). Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru. Proceedings of the Royal Society B: Biological Sciences, 281(1782), 20140192.
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