That's because stress releases cortisol, the «fight or flight» hormone, resulting in faster heart rate, increased blood pressure, and release of sugar into the blood stream. (mindbodygreen.com)
The normal heart rate can vary widely between species and it most cases faster heart rate is related to smaller size. (themeowingvet.com)
The husband's disappointment in the marriage was also correlated to his wife's faster heart rate during the marital interaction (increasing the likelihood of flooding). (gottman.com)