A Medical Hypothesis: Excessive Adaptation Results in Relative Hypothermia and Lymphocytopenia, As Seen in Carnivorous Aquatic Mammals

Body temperature is an important clinical indicator of health and illness. Many studies of human body temperature have been conducted; however, there are no studies that compare the body temperatures of humans and other homeothermic animals. Twenty-six homeothermic animal species, including humans, were selected and characteristics of their internal environment were studied based on previous reports. The studied species were divided into two groups based on habitat (eight aquatic and eighteen terrestrial species) and three groups based on diet (carnivores, herbivores, and omnivores). Body temperatures, erythrocyte counts, and lymphocyte percentages were compared between species and between groups. Our results showed that carnivores had lower body temperatures and erythrocyte numbers than herbivores, and lower lymphocyte ratios than both herbivores and omnivores. Aquatic mammals that experienced a second adaptation event during their evolutionary process had lower body temperatures and lymphocyte ratios than terrestrial animals. These results suggest that excessive adaptation induced by stress or a change in environment may result in relative hypothermia and lymphocytopenia, features that aquatic mammals with stressful evolutionary backgrounds share with human cancer patients. However, our study is based on analysis of previous observations and reports, and further research (e.g., larger-scale studies) is needed to support this hypothesis.