When you think of colour-changing animals, what comes to mind? A slow-moving chameleon barely distinguishable from its spot in a tree? Or maybe an octopus camouflaging itself on the ocean floor? What about hundreds of bright yellow male Asian common toads (Duttaphrynus melanostictus) gathering to fight each other for the company of their somewhat drab, brown-coloured mates? During monsoon rains, the toads gather in groups of over 200 individuals at breeding sites throughout South and Southeast Asia. Before congregating, the male toads swiftly change colour to a conspicuous yellow, signalling to other males that they are ready to fight – and to distinguish themselves to potential female mates. Unsurprisingly, hormones associated with the ‘fight or flight’ response are high during these ‘explosive breeding’ events thanks to the stressful monsoon rains that coincide with the mating season and fighting for partners. Could these hormones have something to do with the toads’ striking colour change? Susanne Stückler and Doris Preininger from the University of Vienna, Austria, and their colleague Matthew Fuxjager from Brown University, USA, decided to investigate.

The trio turned to a group of toads housed in Vienna Zoo, Austria, to see which hormones were responsible for the colour change. First, they measured the colour of the toads’ backs – specifically, the hue (what we normally think of when we describe colour), brightness (lightness or darkness) and chroma (colour saturation). Having established the toads’ baseline colour, the researchers tested whether a dose of the reproductive hormone human chorionic gonadotropin (hCG) could trick the toads into thinking that it was breeding season and trigger a rapid colour change. Surprisingly, the toads did not turn bright yellow when they experienced an increase in their reproductive hormone levels. If reproductive hormones such as hCG aren't triggering a change in colour, could the hormones associated with a stressful experience be driving the colourful transformation instead?

To test this, the scientists gave the toads one of the stress hormones, either adrenaline (epinephrine) or noradrenaline (norepinephrine), and again recorded the toads’ hue, brightness and colour saturation. With either stress hormone, the toads became brighter and their shade of yellow became more saturated. Interestingly, for these toads, only a grape-sized patch of skin changed colour (about half the size of the body), and a few did not change colour at all.

To make sure any colour changes the scientists saw in these treatments could actually be seen by potential mates (or sparring partners), Stückler and colleagues also calculated how noticeable the colour changes would be to the eyes of a toad. Using the colour perception data of a close relative, the common toad (Bufo bufo), the scientists were able to estimate just how different this stress-induced yellow would be from the typical brown colour of females or males – and indeed, the yellow stood out! The scientists suggest that to trigger the whole-body colour change, perhaps there are other hormones at play that they haven't tested yet. Or perhaps physical signals such as monsoon rains and social cues such as the company of other toads work together with stress hormones during explosive breeding events in the wild.

This is the first time that stress hormones alone have been shown to cause animals to change colour rapidly without the help of other hormones that signal breeding. For Asian common toads, being a brighter, flashier yellow might make them more successful during these explosive breeding events, so it would make sense that they have embraced another means to quickly change colour. So, next time you think of colour-changing animals, perhaps you'll picture the bright yellow Asian common toad – and how their ‘fight or flight’ hormones have now been adopted for flashiness.

M. J.
Evidence that catecholaminergic systems mediate dynamic colour change during explosive breeding events in toads
Biol. Lett.