Snow leopards, Panthera uncia, prowl way up in the snowy Himalayas at elevations above 6,000 meters. Researchers long assumed that the secret must be in their blood. It turns out that when it comes to low oxygen adaptations, their blood is no different than that of house cats, according to new work published in the Journal of Experimental Biology this week.
Hemoglobin is the oxygen-carrying protein in blood. Other animals that have adapted to oxygen-deficient conditions typically have genetically-based modifications of hemoglobin function that allow them to transport more oxygen. Cats, on the other hand, are notoriously bad at coping when oxygen is scarce. “Members of the cat family have hemoglobin with unusually low oxygen affinities,” Jay Storz from the University of Nebraska, Lincoln, says in a statement. This makes it tough for cats of all shapes and sizes to scavenge oxygen from the thin air of high altitudes to keep their tissues oxygenated.
To see if snow leopards have (cat-defying) modified hemoglobin, Storz and colleagues gathered up blood samples from big cats in zoos across the U.S. These included two African lions, one tiger, a leopard, a panther, and four snow leopards. Then they sequenced the genes for hemoglobin.
Purified hemoglobin from snow leopards and African lions alike, they discovered, exhibited equally low oxygen affinities and DPG sensitivities. (DPG, short for 2,3-diphosphoglycerate, helps hemoglobin to offload oxygen whenever it’s necessary.) So not only was their hemoglobin only weakly binding to oxygen, they weren’t even taking advantage of DPG. In other words, their hemoglobin is structurally and functionally more or less identical to other cats.
Both low oxygen affinities and unresponsiveness to DPG are the result of a single amino acid swap that occurred in the common ancestor of the family Felidae. This single substitution compromised the ability of hemoglobin in all cats to carry oxygen efficiently, keeping them in a narrow physiological niche.
For now, snow leopards remain a mystery, though the researchers think it’s possible that the cats are compensating for the poor oxygen capacity of their blood by simply breathing harder.