Tiny frogs host an illusion on their backs

Would you recognize a stop sign if it was a different shape, though still red and white? Probably, though there might be a bit of a delay. After all, your brain has long been trained to expect a red-and-white octagon to mean “stop.”

The animal and plant world also uses colorful signals. And it would make sense if a species always used the same pattern to signal the same thing — like how we can identify western black widows by the distinctive red hourglass found on the adult spiders’ underbellies. But that doesn’t always happen. Even with really important signals, such as the ones that tell a predator, “Don’t eat me — I’m poisonous.”

Consider the dyeing dart frog (Dendrobates tinctorius), which is found in lowland forests of the Guianas and Brazil. The backs of the 5-centimeter-long frogs are covered with a yellow-and-black pattern, which warns of its poisonous nature. But that pattern isn’t the same from frog to frog. Some are decorated with an elongated pattern; others have more complex, sometimes interrupted patterns.

The difference in patterns should make it harder for predators to recognize the warning signal. So why is there such variety? Because the patterns aren’t always viewed on a static frog, and the different ways that the frogs move affects how predators see the amphibians, according to a study published June 18 in Biology Letters.

Bibiana Rojas of Deakin University in Geelong, Australia, and colleagues studied the frogs in a nature reserve in French Guiana from February to July 2011. They found 25 female and 14 male frogs, following each for two hours from about 2.5 meters away, where the frog wouldn’t notice a scientist. As a frog moved, a researcher would follow, recording how far it went and in what direction. Each frog was then photographed.

Sixty-four percent of the frogs appeared to move randomly. The remaining 36 percent, though, kept to a single direction. These frogs also moved around three times faster than their random-moving brethren.

The frogs’ color patterns split into the same two groups: Random movers had more interrupted patterns. Directional frogs tended to have more elongated coloring.

A frog’s movement affects what a potential predator sees. For the directional frogs, “this pattern–movement combination might create the illusion of a static pattern or a pattern with a greatly reduced speed that affects predators’ abilities to track the trajectory of moving individuals and predict their attack angle,” the researchers write. “This may be more pronounced when movements occur at a higher speed and over longer segments, as in these frogs.”

The random- and slow-moving frogs may get a different benefit from their patterning: “Interrupted patterns may be visually disruptive or cryptic at a distance, and the combination of disruptive patterns and slower movements, or alternating movement and freezing, might be advantageous for the avoidance of motion-oriented predators,” the researchers note.

Since both combinations of pattern and movement can be of benefit to the frogs, natural selection is unlikely to weed out one or the other.

Editor's note: This post was updated on September 28, 2014, to note that black widows' hourglass markings are on their undersides, not their backs.