Final answer:
Species with higher heart rates tend to have shorter life expectancies, correlating with Type I survivorship curves where mortality is low until old age, and this trend is generally influenced by natural selection.
Step-by-step explanation:
The best description of the association between the average heart rates of mammals and their life expectancy is that species with higher heart rates tend to have shorter life expectancies. This relationship is often observed in mammals and is supported by comparative studies involving life tables and survivorship curves.
Survivorship curves categorize organisms based on their patterns of mortality over the course of their lifespan. Mammals usually follow a Type I curve, where mortality is low until old age. This type of curve often correlates with a lower heart rate and longer life. Conversely, species with high heart rates often have shorter life spans, which can be associated with factors such as higher metabolic rates and faster living processes.
It is essential to understand that these patterns are influenced by natural selection and can vary among different species or populations. Nevertheless, the general trend indicates that a higher heart rate correlates with a shorter lifespan among mammals.