with the other. Gum color Pale pink: the healthy color of gums, indicates good circulation. May brighten slightly after strenuous work due to an increase in circulation. Very pale pink: due to contracted capillaries. May indicate anemia, fever, or blood loss. Pale blue, gray, or whitish: indicative of anemia (low red blood cell count). May indicate severe shock or illness. A serious sign; often the best option is to call the veterinarian. Yellow with a tinge of brown: indicates jaundice and liver failure. Very serious and the veterinarian should see the animal immediately. Yellowish: a bright yellow tinge may occur if the horse has high levels of beta-carotene in its diet, such as horses that eat a good deal of alfalfa hay. Does not indicate any serious problem. Dark red: indicates enlarged capillaries, due to poisoning or severe dehydration. The veterinarian should be contacted immediately. Capillary refill time The capillary refill time is determined by pressing a finger against the horse's gums for about 2 seconds, so that a white "thumbprint" is left. After releasing, it should take no longer than 2 seconds for the gum color to return to normal. If it takes longer for the gum color to return, the horse may be experiencing shock. Cardiovascular capacity Measurements of heart size do not appear to correlate directly with racing speed, stride length, or stride frequency. However, the ability of the body to pump blood can help identify athletic potential in an unproven horse. There is a hypothesis that measurements of a horse's heart at rest are directly related to the same horse's cardiac function during exercise. Therefore, attempts have been made to take resting measurements of horses using an electrocardiograph (ECG). This has led to the development of the "heart score", which measures the QRS interval. However, no work has correlated this to a horse's oxygen uptake (VO2Max) and the test has not been a good predictor of future athletic ability. On the other hand, the Pearson correlation coefficient has been found to provide a link between oxygen uptake and echocardiographic measures. There is also evidence that maximal oxygen consumption and heart size are more important predictors of performance for horses that run longer distances because their energy consumption is mainly aerobic. "X factor" The X factor theory proposes that a mutation within a gene located on the X chromosome of horses causes a larger-than-average heart. A larger-than