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The Nile crocodile is one of the most dangerous animals in the world. They are very efficient hunters, relying on ambush tactics to indiscriminately capture any prey in their path. National Geographic estimates that up to 200 people are killed by these deadly reptiles every year, but what is the secret to their success?
It has long been known that crocodilians—crocodiles and alligators—are able to hold their breath underwater for an extraordinarily long time. This adaptation allows them to kill large mammal prey by dragging them into the water and drowning them. Their remarkable breath-holding capacity is believed to have evolved due to adaptive alterations in the protein that carries oxygen around their blood: hemoglobin.
Hemoglobin is found in red blood cells: it carries oxygen around the body via the blood. The strength with which hemoglobin binds to oxygen varies at different oxygen concentrations: in the lungs, where the blood is rich in oxygen, hemoglobin binds strongly to these oxygen molecules; however, when the blood reaches the muscles, oxygen concentrations plummet, and the hemoglobin lets go of the bound oxygen.
Other factors, such as temperature and pH, can affect this binding too, but in crocodilians, the binding of oxygen is also regulated by bicarbonate molecules, which are produced when carbon dioxide dissolves in the blood. When bicarbonate ions bind to the hemoglobin, the protein loosens its grip on the oxygen molecules even more, thus unloading its oxygen cargo in areas where bicarbonate is particularly high.
The cells that are respiring the most—those that need the most oxygen—produce the most bicarbonate, which causes hemoglobin to release its oxygen. By binding to this bicarbonate, the hemoglobin is also able to efficiently remove this waste product from the respiring cells. In this way, the crocodile's hemoglobin is able to efficiently deliver oxygen to the cells that need it the most, while preventing the build up of carbon dioxide in the blood.
The mechanisms behind the evolution of crocodilians' adaptive advantage have so far remained a mystery. But research from the University of Nebraska-Lincoln, published in the journal Current Biology, has now identified a new piece in this evolutionary puzzle.
"It's a super-efficient system that provides a kind of slow-release mechanism that allows crocodilians to efficiently exploit their onboard oxygen stores," one of the study's authors, Jay Storz, said in a statement. "It's part of the reason they're able to stay underwater for so long."
By comparing crocodile hemoglobin to that of its ancient ancestor, the researchers were able to conclude that the crocodilian's unique hemoglobin was born from a sequence of 21 interconnected mutations, which not only introduced this bicarbonate-mediated oxygen-carrying capacity, but also relieved the protein's sensitivity to the molecules that regulate hemoglobin's affinity with oxygen in our own cells.
The complexity of these mutations has meant that no other creatures have evolved this unique skill, despite tens of millions of years of evolution, precluding any competition with crocodiles for using this hunting strategy.
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References
Natarajan C, et al., Evolution and molecular basis of a novel allosteric property of crocodilian hemoglobin, Current Biol, December 21 2022, https://doi.org/10.1016/j.cub.2022.11.049
About the writer
Pandora Dewan is a Senior Science Reporter at Newsweek based in London, UK. Her focus is reporting on science, health ... Read more