Bergmann’s rule is driven not only by evolution, but also early-life stress and short-term environmental shifts.

Burrowing owls across North America follow a classic ecological understanding: the higher the latitude where they live, the bigger they are. But new research shows that this pattern isn’t the result of long-term evolution alone. It also reflects the lasting impacts of early-life conditions, as well as rapid responses to shifting food supplies.

In 1847, the German biologist Carl Bergmann reported that animals in colder climes were seemingly larger than those inhabiting warmer regions. Known as Bergmann’s rule, the trend has been attributed to climatic factors, but there is a limited, sometimes conflicting, understanding of what developmental or evolutionary mechanisms may be at play. While the rule largely deals with differences between species, differences between members of the same species living across diverse habitats has also been analyzed and tells us how species may react to changing climates. Temperature and precipitation can influence animal metabolism and reproduction along with vegetation and resource availability.

For the last few decades, the Conway Lab at the University of Idaho, led by researcher Courtney Conway, has explored a wide variety of behaviors of burrowing owls living in western North America. With such a large dataset available from studying these owls that inhabit a wide range of climes across the Americas, the research team could test Bergmann’s Rule and understand what mechanisms may lie underneath.

“Burrowing owls were initially selected for this investigation of Bergmann’s rule largely because of the sheer geographic extent of our dataset, which sparked a seemingly simple question: does this arid grassland predator follow this commonly observed pattern of body size variation along a latitudinal gradient?” says first author Kurt Ongman in an email to Advanced Science News. 

The researchers had measurements on mass, wing and leg bone lengths for 5,597 owls across 54 sites in the western US, recorded from 2000 to 2020. The 54 breeding sites stretched across 1600 km latitudinally, 1500 km longitudinally, and ranged in elevation from 70 m below sea level to 2285 m. The team also used drought data, which affects burrowing owl reproduction, from the National Centers for Environmental Information (2025) nClimDiv dataset and sourced additional climate data from the PRISM Group (2026) at Oregon State University as well as vegetation and productivity data from the USDA’s VegScape Condition Explorer and NASA’s Global Vegetation Greenness.

Indeed, burrowing owls did follow Bergmann’s Rule, with owls in cooler northern areas tending to be larger, with the heaviest and longest-winged representatives clustering in the northwest. With this geographic pattern validated, the team wanted to understand how long-term evolutionary adaptations and short-term responses to the environment played into the established size gradient.

“By separating environmental conditions across these time scales, we were able to evaluate whether geographic differences in body size were more consistent with genetic adaptation to thermal extremes, developmental stunting from early-life heat and resource stress, or reversible weight fluctuations driven by recent resource availability,” adds Ongman. They focused on testing three theories put forth in earlier research: heritable adaptations to local conditions, developmental adaptations during early life, and reversible changes in adulthood.

“Overall, while we found general support for Bergmann’s rule, the underlying mechanisms differed depending on whether a trait was plastic, such as adult body mass and, to some extent, wing length, or more fixed, such as tarsus length,” writes Ongman.

Both adult mass and wing length were closely linked to 21-year average temperatures rather than more recent weather conditions, suggesting local, heritable adaptations were responsible. Body mass in adults dropped by 0.41% while wing lengths grew shorter by 0.16% for every degree Celsius rise in temperature.

But juvenile body mass was strongly linked to more immediate, radical changes in temperature and precipitation. Extreme heat and drought in the prior breeding season, which is the year before hatching, affect prey availability and influence reproduction in female birds, likely affecting juvenile birds. Such extreme weather events in the previous year also affected adult owls, resulting in shorter leg bones, implying that thermal stress and resource limitations may shape structural growth in young adults.

Short-term environmental conditions also played a role. Sudden, strong rain showers within six months of measurements influenced both wing growth and body mass in adults, reflecting rapid changes in resource availability.

Looking ahead, Ongman notes that “Future work could apply a similar framework to a species with reliable adult aging to better separate developmental plasticity from local adaptation, allowing us to predict how body size might respond under future climate scenarios. Adding detailed information on migratory behavior would allow us to examine how morphology and movement strategies interact, which is key for predicting range shifts and population responses.”

“Together, these approaches could help forecast which populations are most vulnerable to warming, drought, or other climate-driven changes, and identify the mechanisms driving phenotypic variation in rapidly changing environments,” adds Ongman.

Reference: Kurt M. Ongman et al., Bergmann’s rule: Why does body size increase with latitude?, Functional Ecology (2026). DOI: https://doi.org/10.1111/1365-2435.70281