Data from: Evolution of heterophil/lymphocyte ratios in response to ecological and life-history traits: a comparative analysis across the avian tree of life

1. Lymphocytes and heterophils are the two most abundant leukocyte 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. 2. 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). 3. I compiled published data on
leukocyte 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. 4. 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. 5. 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 intra-specific variation in physiological stress, but also as an
important evolutionary trait which probably have adaptive significance
visible in a wider phylogenetic perspective.