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

Abstract

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 defences 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 defences 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.

Genomic correlates for migratory direction in a free-ranging cervid

“…The ability of an animal to maintain its immune system or mount an immune response can depend on its nutritional health and energetic condition. Variations in immune function and condition have been linked to migration in bats (Rogers et al, 2022) and stopover behaviour along a migration route in birds (Hegemann et al, 2018). MAN2A2, RNF216, and IKZF3 are involved in metal- and zinc-binding while EMC9 is part of the endoplasmic reticulum membrane protein complex.…”

Genomic correlates for migratory direction in a free-ranging cervid

“…The ability of an animal to maintain its immune system or mount an immune response can depend on its nutritional health and energetic condition. Variations in immune function and condition have been linked to migration in bats [73] and stopover behaviour along a migration route in birds [74]. FITM1 is of note as it is related to the formation of lipid droplets and has been associated with fat storage in humans [75].…”

Migration strategy varies with novel environment response in common noctule bats

“…Furthermore, they observed a high degree of consistency in migratory behaviour within individuals of common noctule bats. While morphological and physiological trade-offs for partially and differentially migrating bats have been described (Rogers et al, 2022), so far, we still lack evidence whether behavioural variation, specifically in response toward novel environmental cues, are linked to intra-specific migratory strategies in bats.…”

“…Bats use energy-saving strategies during migration by travelling at optimal flight speeds (Troxell et al, 2019) and by entering torpor during stopover periods (McGuire et al, 2014). Wing morphology and immunological parameters of North American partial migratory bats differ between migratory and non-migratory conspecifics (Rogers et al, 2022). However, climate change poses new threats to migratory bats because they are exposed to novel human-made infrastructures, such as wind energy facilities, with cause high numbers of casualties in bats (Voigt et al, 2022).…”