Warning Signals Evolved From Other Functions

Some of the most strikingly colored animals are also quite deadly, their conspicuous colours advertising their toxicity in order to warn away potential predators. Warning colours or markings have to overcome a peculiar hurdle when they evolve. In order to work, the markings have to be common. When a marking first appears or is still uncommon in the population, predators won't be aware of its significance; the warning won't work. The warning is only effective if its widespread enough for predators to learn (or evolve) to avoid it, so how do the early adopters survive for long enough to establish its credibility?

In a paper appearing in PNAS, Paul Marek and Wendy Moore offer some insight into this conundrum based on their study of a group of millipedes. Their story starts with the discovery of a pair of small millipede males in California in 1967. Nearly 30 years later, they were classified as a new species, Xystocheir bistipita, but no other specimens of the species were known. The situation remained unchanged for the next two decades, two alcohol-preserved males representing the sum of our knowledge of this species. In 2013, the species was rediscovered in the wild. The duo captured several males and females and took them back to the lab to learn more about them.

One of the most surprising things Marek and Moore observed was that their X. bistipita specimens glowed a soft blue/green in the dark. Other Xystocheir species don't bioluminesce; coupled with phylogenetic data, this prompted the pair to reclassify the species in the genus Motyxia, dubbing it Motyxia bistipita. The other species of Motyxia millipedes are all bioluminescent, which scientists believe is a warning to nocturnal mammals that might hunt them. The millipedes themselves are blind, so their glow isn't for communication; it's to keep them safe by advertising the toxin they carry.

Marek and Moore next assembled a phylogeny of Motyxia and mapped the strength of each species' bioluminescence onto the resulting tree. They wanted to test whether there was a directional trend in the evolution of bioluminescence — that is, whether it had evolved gradually in the group. If so, then they expected bioluminescence would correlate with how far a species was from the base of the group. When they crunched the numbers, that's precisely what they found. A gradual, directional model of evolution fit their data best.

These results don't explain how a warning signal can evolve gradually; they just demonstrate that this particular signal did. Marek and Moore suggest that bioluminescence might have evolved gradually in Motyxia because it didn't always serve as a warning. The strength of the luminescence also correlates with altitude, and the duo think this may hold the key to the puzzle. They propose that bioluminescence originally evolved in Motyxia living at lower altitudes, like M. bistipita, and served as an anti-oxidant in their hot, dry environment. As Motyxia species moved higher up, the need for an anti-oxidant decreased while predation by smalls mammals increased, since they are more common at higher altitudes. In other words, bioluminescence was originally a metabolic process, so it was already common in the population when the time came to use it as a warning. Marek and Moore haven't conclusively shown that this is how the warning evolved, nor is it clear whether warning signals in other groups followed the same evolutionary course. Despite that, it's a helpful example of how things could happen, and a tidy little evolutionary story.

Ref
Marek, PE and Moore, W. Discovery of a glowing millipede in California and the gradual evolution of bioluminescence in Diplopoda. PNAS 112(20):6419-6424. (2015) doi: 10.1073/pnas.1500014112

Image credits
The image is from Figure 1 in the paper.