A new Perspective out today in Nature Reviews Biodiversity, by Matt Symonds, Sara Ryding, and Marcel Klaassen (Deakin University), with Josh Tabh and Andreas Nord (Lund University) and myself, tackles a problem that has been nagging at climate-change biologists for years: why do studies of a single trait, like body size, or metabolic rate, so often give inconsistent answers about how animals are responding to a warming world? Textbook “rules” like Bergmann’s (smaller bodies in warm climates) and Allen’s (bigger appendages in warm climates) might predict tidy, single-trait shifts through time as the planet heats up.

But real animals don’t have the luxury of changing one thing at a time. Morphology, physiology and behaviour are tangled together, and a change in one can mask, amplify, or completely offset a change in another. A bird that isn’t shrinking might instead be shifting its foraging schedule to the cooler part of the day; a bird with no obvious behavioural change might be quietly ramping up its evaporative cooling capacity instead. Look at any one trait in isolation and you risk concluding “no effect” when there’s really just a hidden trade-off.

Our Perspective lays out what’s currently known about how each of these three trait categories from form to function responds to geographic and temporal temperature gradients, then digs into the ways they interact with and constrain each other. Rather than just cataloguing the problem, we sketch out a path forward: comparative multi-species studies, wide-ranging single-species studies across many populations, and controlled mesocosm experiments that track morphology, physiology and behaviour simultaneously in the same individuals, so trade-offs actually become visible instead of disappearing into separate, disconnected data sets. The hope is that this kind of integrative, multi-trait thinking will let us better identify which species are truly vulnerable to climatic warming, rather than mistaking a “silent” trait for a species that isn’t responding at all.

Citation

Symonds, M.R.E., Tabh, J.K.R., Nord, A. Klaassen, M, Tattersall, G.K., and Ryding, S. Moving beyond single traits to predict endotherms’ responses to climatic warming. Nature Reviews Biodiversity (2026). https://doi.org/10.1038/s44358-026-00195-5

For access to the full paper, this is a shareable link: https://rdcu.be/ypK7CBgbw1va