How crocodiles became such ruthless, efficient apex predators

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 each year, but what is the secret of their success?

It has long been known that crocodiles—both crocodiles and alligators—can hold their breath underwater for exceptionally long periods of time. This adaptation allows them to kill large mammalian prey by dragging them into the water and drowning them. Their remarkable breath-holding capacity is thought to have evolved due to adaptive changes in the protein that carries oxygen through 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, the oxygen concentration drops and the hemoglobin releases the bound oxygen.

Other factors such as temperature and pH can also affect this binding, but in crocodilians, oxygen binding is also regulated by bicarbonate molecules, formed 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, unloading its oxygen load at sites where bicarbonate is particularly high.

The cells that breathe the most—the ones that need oxygen the most—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 most, while preventing the build-up of carbon dioxide in the blood.

The mechanisms behind the evolution of the adaptive advantage of crocodiles have so far remained a mystery. But research from the University of Nebraska-Lincoln, published in the journal Current Biologyhas 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 crocodiles to efficiently use their onboard oxygen supplies,” said one of the study’s authors, Jay Storz, in a statement. “That’s one of the reasons they can stay underwater for so long.”

By comparing crocodile hemoglobin to that of its ancient ancestor, the researchers were able to conclude that the crocodile’s unique hemoglobin arose from a sequence of 21 interconnected mutations that not only introduced this bicarbonate-mediated oxygen-carrying capacity, but also the protein’s relieved sensitivity to it the molecules that regulate hemoglobin’s affinity for oxygen in our own cells.

The complexity of these mutations has meant that despite tens of millions of years of evolution, no other creatures have developed this unique ability, ruling out any competition with crocodiles in using this hunting strategy.

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references

Natarajan C, et al., Evolution and Molecular Base of a Novel allosteric property of crocodilian hemoglobin, Current Biol, December 21, 2022,