Introduction
Since the Upper Paleolithic age more than 20,000 years ago, humans have been fascinated by animal movement, as evidenced by early rock art depicting animal migration (Bacon et al., 2023). From this long-standing interest in animal movement has come a rich history in studying animal migration. Most studies of animal migration have primarily focused on latitudinal migration, although altitudinal migration—the seasonal movement of populations across elevational or bathymetric gradients (Hsiung et al., 2018; Milligan et al., 2020)—has garnered increasing attention across taxonomic groups. Altitudinal migrants often pass through multiple habitats with different environmental conditions and experience a similar or even greater breadth of ecological interactions (e.g., predation, interspecific competition, interactions with parasites, etc.) compared to strict latitudinal migrants (Williamson & Witt, 2021). As the taxonomic representation of studies on altitudinal migration has grown, so too have inconsistencies in the language used to describe this phenomenon. Here we propose a functional definition of altitudinal migration and provide guidelines towards a unified conceptual framework of altitudinal migration that highlights its biological importance and prevalence across taxa (Figure 1). We also discuss emerging opportunities and challenges, outstanding questions in the field, and future directions to advance its study. Collective thought towards an improved conceptual framework will enable researchers to better compare and contrast emergent patterns and identify idiosyncrasies of altitudinal migration behavior among taxa and biogeographic regions.