Relating wing morphology and immune function to patterns of partial and differential bat migration using stable isotopes

Migration is energetically expensive and is predicted to drive similar
morphological adaptations and physiological trade-offs in migratory bats
and birds. Previous studies suggest that fixed traits like wing morphology
vary among species and individuals according to selective pressures on
flight, while immune defenses can vary flexibly within individuals as
energy is variably reallocated throughout the year. We assessed
intraspecific variation in wing morphology and immune function in
silver-haired bats (Lasionycteris noctivagans), a species that follows
both partial and differential migration patterns. We hypothesized that if
bats experience energy constraints associated with migration, then wing
morphology and immune function should vary based on migratory tendency
(sedentary or migratory) and migration distance. We predicted that
long-distance migrants would have reduced immune function and more
migration-adapted wing shapes compared to resident or short-distance
migrating bats. We estimated breeding latitude of spring migrants using
stable hydrogen isotope techniques. Our sample consisted primarily of male
bats, which we categorized as residents, long-distance northern migrants,
short-distance northern migrants, and southern migrants (apparent breeding
location south of capture site). Controlling for individual condition and
capture date, we related wing characteristics and immune indices among
groups. Some, but not all, aspects of wing form and immune function varied
between migrants and residents. Long-distance northern migrants had larger
wings than short-distance northern migrants and lower wing loading than
southern migrants. Compared with resident bats, short-distance northern
migrants had reduced IgG while southern migrants had heightened
neutrophils and neutrophil-to-lymphocyte ratios. Body fat, aspect ratio,
wing tip shape, and bacteria killing ability did not vary with migration
status or distance. In general, male silver-haired bats do not appear to
mediate migration costs by substantially downregulating immune defenses or
to be under stronger selection for wing forms adapted for fast,
energy-efficient flight. Such phenotypic changes may be more adaptive for
female silver-haired bats, which migrate farther and are more constrained
by time in spring than males. Adaptations for aerial hawking and the use
of heterothermy by migrating bats may also reduce the energetic cost of
migration and the need for more substantial morphological and
physiological trade-offs.