Size Matters: Zoo Data Analysis Shows that the White Blood Cell Ratio Differs between Large and Small Felids

SIMPLE SUMMARY: The white blood cells (WBCs) are some of the components of vertebrates’ immune systems. In some orders of mammals, the body mass of the animal correlates positively with the number of WBCs at the interspecific level. However, the different types of WBC play different roles in mammalian immunity, and we suggested that their number may vary between species as well. We estimated the number and ratio values of WBC types in 26 felid species and compared them with their body masses. We found that large cats had more neutrophils and monocytes and fewer lymphocytes than smaller ones. These differences may be explained by their diets. Large cats evolved as the hunters of medium and large-sized ungulates. They utilize the kills for long time intervals (days), resulting in the growth of fungi, protozoa and bacteria in the kills. That may explain the high number of neutrophils and monocytes in large cats to prevent infection by these organisms. The roles of different hunting styles, such as different times of kill utilization and the potential for a greater neutrophils/lymphocytes ratio in larger felids needs further investigation. This comparative study is helpful for zoo and wildlife veterinarians by allowing them to apply these results to endangered and poorly studied felid species. ABSTRACT: The total number of white blood cells (WBCs) is related the immune system. In mammals, it is affected by the body mass, but it is unclear how the numbers of different WBC types correlate with this parameter. We analyzed the effect of body mass on WBC number and ratio in felids, where species are similar in diet (warm-blood vertebrates) and reproductive strategy (promiscuity). Based on zoo veterinary data (ZIMS database) we analyzed the effect of body mass on WBC number and neutrophils/lymphocytes ratio in 26 species of felids. The number of WBCs correlated with the body masses of animals: large cats had more WBC, which may be due to greater risks of infection associated with larger body surface, lifespan and home range size. For the first time we found obvious differences in the number of WBC types. Large cats also had more neutrophils and monocytes but fewer lymphocytes than smaller cats. The ratio of neutrophils to lymphocytes is greater in large felids. This phenomenon may be related to diet (relative prey size and kill utilization time), which suggests regular contact of large cats with bacterial and protozoal pathogens in contrast to the small cats.