Strict carnivores manage vitamin A differently than omnivores and herbivores. Carnivores are more tolerant of high intakes of retinol because those species have the ability to excrete retinol and retinyl esters in urine. Carnivores also have the ability to store more in the liver, due to a higher ratio of liver HSCs to hepatocytes compared to omnivores and herbivores. For humans, liver content can range from 20 to 30 μg/gram wet weight. Notoriously, polar bear liver is acutely toxic to humans because content has been reported in range of 2,215 to 10,400 μg/g wet weight. As noted, in humans, retinol circulates bound to RBP4. Carnivores maintain R-RBP4 within a tight range while also having retinyl esters in circulation. Bound retinol is delivered to cells while the esters are excreted in the urine. In general, carnivore species are poor converters of ionone-containing carotenoids, and pure carnivores such as felidae (cats) lack the cleaving enzyme entirely. They must have retinol or retinyl esters in their diet.