A sweat bee - credit Jeremiah Bender
Some bees really do change colour with the weather, according to new research that shows humidity can temporarily alter the shimmering hues of certain species.
In a study published today in Biology Letters, scientists led by Dr Madeleine Ostwald of Queen Mary University of London found that moisture in the air can cause sweat bees to change colour - and then change back again when conditions dry out.
Sweat bees are known for their bright, metallic greens and blues. Until now, reports that their colours could shift have been anecdotal. This new research provides the first experimental proof.
The team studied museum specimens of a North American sweat bee, Agapostemon subtilior. When the bees were placed in dry air, they appeared deep blue. But when humidity increased, they took on a warmer, copper‑green colour. Once dried again, the bees returned to blue.
Unlike most animals, whose colours come from pigments, these bees get their colour from microscopic structures on their bodies that reflect and scatter light at particular wavelengths. The same effect creates the iridescent feathers of hummingbirds and the shifting skin colours of cuttlefish.
In some animals, these tiny structures swell slightly when exposed to moisture, causing them to reflect redder colours. The researchers believe a similar process may be happening in bees, although more work is needed to fully understand the mechanism.
The scientists also looked at colour changes in the wild. By analysing hundreds of public photos from the citizen science app iNaturalist, they compared bee colour with local humidity levels. While many factors influence a bee's appearance, the team found that bees in drier areas tended to look bluer - matching the lab results.
Interestingly, older museum specimens showed the strongest colour changes. The researchers think this may be because bees' outer shells slowly degrade over time, allowing moisture to enter more easily.
The findings suggest this colour‑changing effect could be common among bees, which display a wide range of shimmering colours and live in environments ranging from deserts to rainforests.
Insects use colour for many reasons, including temperature control, communication, and camouflage. Whether these subtle colour shifts affect how bees behave or survive is still unknown.
Dr Madeleine Ostwald, Lecturer in Ecology, Conservation & Biodiversity at Queen Mary said: "When people think of bees, they often picture drab, brown honey bees. In reality, bees are incredibly diverse and colourful - and we're only just starting to understand how their appearance reflects the climate they live in."
She added: "Most people associate colour‑change with animals like chameleons that actively control it. These bees aren't choosing to change colour - it's happening passively, simply in response to the humidity around them. That adds a whole new layer of mystery to why these colours evolved in the first place."
The study was carried out with researchers Leslie Cervantes Rivera, Jorge De La Cruz and Katja Seltmann from the Cheadle Center for Biodiversity and Ecological Restoration at the University of California, Santa Barbara.
"Humidity induces structural colour change and contributes to biogeographic colour variation in sweat bees" was published on 00.05 BST April 22, 2026 in Biology Letters and can be accessed here:
