Scientists have explored how crocodile hemoglobin works, revealing why these reptiles can stay submerged longer than most animals and still surface ready to strike. This study, reported in a recent biology-focused publication, sheds new light on how blood chemistry supports extended underwater endurance in crocodiles.
Crocodiles are famed ambush predators. When they lie in wait beneath the water, they hold their breath and stay nearly motionless until prey comes within reach. Then they surge upward, skim the surface, and seize with their powerful jaws. Breathing air is essential for these reptiles, so their ability to dive deeply relies on a remarkable adaptation: hemoglobin that enables efficient oxygen management during extended submersion. The research shows that a special kind of hemoglobin in crocodile blood helps distribute oxygen to tissues over long periods without constant breathing, facilitating extended dives when needed for hunting or evading threats.
Lead investigator Jay Stortz and his team from the University of Nebraska–Lincoln, along with colleagues, pursued how this unique trait evolved across jawed vertebrates. Hemoglobin binds oxygen in the lungs and releases it to tissues as needed. In most vertebrates, organic phosphates bound to hemoglobin influence the release of oxygen. Yet in crocodiles, bicarbonate produced during carbon dioxide breakdown takes the place of phosphate and tunes oxygen delivery. The high levels of tissue carbon dioxide drive bicarbonate formation, which in turn signals hemoglobin to release oxygen where it is most required, enabling efficient extended underwater performance.
To trace how this system appeared through evolution, the researchers reconstructed three ancestral hemoglobin variants: a distant crocodile-like ancestor dating back about 240 million years (archosaurus), the last common ancestor of all birds, and an 80 million year old common ancestor of modern crocodiles. Their findings showed that only the crocodile’s direct ancestral hemoglobin lost phosphate binding and became sensitive to bicarbonate, marking a key turning point in the evolution of this trait.
Subsequently, scientists introduced crocodile-specific mutations into the archosaurus hemoglobin to observe how the protein’s properties shifted toward those found in modern crocodiles. This approach helped identify which mutations nudged the molecule toward bicarbonate sensitivity and away from phosphate-driven regulation, revealing the genetic steps that refined oxygen transport during diving.
The results demonstrated that changes affecting bicarbonate responsiveness and phosphate responsiveness arose from distinct sets of mutations. Turning on bicarbonate sensitivity did not require the loss of phosphate sensitivity, and vice versa. This indicates that the evolution of crocodile hemoglobin involved modular changes, with multiple pathways contributing to enhanced oxygen delivery in submerged conditions.
Ultimately, the study supports a broader view of evolution: combinations of mutations can produce functional effects greater than the sum of their parts. A mutation that seems neutral on its own may pave the way for other changes that unlock clear, immediate advantages in the organism’s physiology and behavior. The work adds a piece to the puzzle of how highly specific blood properties can adapt in response to ecological demands, enabling a predator to excel in a niche that requires long submergence and powerful bursts of action when prey appears. and colleagues.