Cooperative breeding

Sociality

Phylogenetic comparative methods

Phylogenetic tree

Lymphocyte

Evolutionary biology

Biology

Phylogenetics

Zoology

Gene

Trait

Host (biology)

Evolutionary ecology

Trade-off

Ecology

Programming language

Neutrophil to lymphocyte ratio

Computer science

Life history

Genetics

Reproduction

Avian clutch size

Life history theory

Journal of Animal Ecology

Lymphocytes and heterophils are the two most abundant leucocyte types, which play a major role in adaptive and innate immune defence, respectively. The ratio of heterophils to lymphocytes (H/L ratio) may reflect a readiness to cope with infection through injury (via heterophils) rather than with a communicable disease (via lymphocytes). Since elevated H/L ratio constitutes a corticosterone-mediated response to external stressors, this trait is generally acknowledged as the robust measure of physiological stress in vertebrates. Here, I hypothesized that baseline H/L ratios of birds (as measured under normal physiological function) could be an important evolutionary trait shaped by species' ecology and life history. I predicted that H/L ratios should be determined by traits related to pathogen exposure (migratoriness, sociality, breeding latitude) and trade-offs between investment in reproduction (clutch size) and self-maintenance (longevity). I compiled published data on leucocyte profiles for nearly 250 bird species representing over half of all extant avian orders and nearly 30% of all extant avian families. Phylogenetically informed comparative methods were used to assess evolutionary associations of H/L ratios with ecological and life-history traits across the avian tree of life. Relatively strong phylogenetic signal and phylogenetic autocorrelation in avian H/L ratios indicated that most diversification in this trait occurred relatively early in bird radiation. Fluctuating selection and drift were identified as the major forces responsible for the evolution of H/L ratios in birds, while low H/L ratios were identified as an ancestral state in birds. Finally, phylogenetically informed Bayesian models showed that H/L ratios correlated negatively with longevity and positively with sociality and breeding latitude. This study was the first to describe the patterns of avian H/L ratio evolution in a broad phylogenetic framework. The results indicate that H/L ratios should not be merely recognized as a proxy for the intraspecific variation in physiological stress, but also as an important evolutionary trait, which probably have adaptive significance visible in a wider phylogenetic perspective.

Evolution of heterophil/lymphocyte ratios in response to ecological and life‐history traits: A comparative analysis across the avian tree of life