Young lizards of many species often have a vibrantly blue tail that’s also detachable. That way, when would-be predators attack, their attention is diverted towards the expendable body part that can be regenerated – and not the vital ones that can’t. But why blue? According to new findings published in the Journal of Zoology, bright blue evolved as an anti-predator strategy against snakes and weasels.

Blue-colored tails have independently evolved in several lizard families. These conspicuous tails, combined with contrasting stripe patterns, distract the predator’s attention away from the head and the trunk. Sometimes, the tail might even wiggle after the lizard drops it.

The blue tails of juvenile far eastern skinks (Plestiodon latiscutatus) in Japan vary across populations, especially those living on islands. Lizards living in the northern Izu Islands and Izu Peninsula – where diurnal snakes and carnivorous weasels are the major predators – have a yellowish, white-striped body and a blue tail of various shades. Meanwhile, juveniles from the southern Izu Islands – where birds are the main predators – have an almost uniformly brown hue, except for a blue tail tip. 

A team led by Takeo Kuriyama of Toho University wanted to see if variation within the species has to do with differences in the color vision capabilities of their different predators. So they examined lizards from three island populations: Kozu Island with snake predators only, Hachijokojima Island with just birds, and the mainland Izu Peninsula where weasels live.

Lizard tails with vivid blue reflectance, they found, evolved in areas with either weasels or snakes – two animal groups that have the ability to detect blue wavelengths. However, UV reflectance was much higher in the populations that have only snake predators to contend with: Snakes can detect UV, and weasels can’t. High UV reflectance increases the tail’s conspicuousness to snakes. Additionally, brown tails evolved in places where birds are the primary predators. The team thinks that, since birds have keen visual acuity, camouflage would be more helpful than diversion.

Previous work revealed that the skink’s blue color is created with the help of light-reflecting platelets in iridophore pigment cells. "Nano-particle pigments in skin produce the anti-predator tail coloration," Kuriyama explained in a statement. With transmission electron microscopy, the team found that the wavelength and intensity of light that’s reflected by the tail can be predicted by the thickness of the reflecting platelets as well as iridophore density.