How on Earth did you get like that?

A little mathematics has shown how animals get their extraordinary patterns.

Previous studies have revealed how animal patterns assist animal survival – colours and shapes help them hide, scare away predators and regulate their body temperature, for example – but little is known about how they evolved over time.

In a paper in the journal Science Advances, Seita Miyazawa from Japan’s Osaka University suggests that it happened through a relatively simple mechanism where colours and patterns blended together as animals – in this case in fish – reproduced.

A major difficulty in animal pattern studies is not knowing where a pattern motif originated. From a logical point of view, it may seem like they start simple and become more complex over time.

Building on a previous hypothesis that came from a mathematical model, Miyazawa analysed 18 thousand fish species using more modelling and genomics.

He found that fish with intricate patterns were more closely related to fish with simple patterns. The maze-like shapes on fish actually came from simple, different coloured spots that blended together as species hybridised.

Hybridisation occurs when two genetically distinct animals breed and produce offspring with a mixture of both parental traits.

“I found that several fish species with maze patterns have actually been derived from hybridisation between light- and dark-spotted species,” Miyazawa says. “Although expected, this was amazing.”

This meant that, instead of viewing the fish pattern as a maze made of wobbly lines, they are, in truth, different coloured dots that they inherited from spotty ancestors, superimposed on each other.

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Mathematically predicted blended pattern compared to real pufferfish. Credit: Seita Miyazawa, Hiroshi Senou, Kagoshima University Museum, Keiichi Matsuura.

Miyazawa mathematically modelled this pattern blending and found a striking similarity between computer-generated patterns of fused dots with the patterns found on maze-like patterned pufferfish.

Even more remarkably, these were consistent with the fish’s lineage, where motifs could be mathematically predicted from an ancestor’s spots and colours.

This might also explain why some fish were historically categorised as unrelated but were later found to be related when genomic data became available.

“I anticipate that some of them may be just hybrids and have been deceiving taxonomists with their camouflaged colouration to be given unworthy taxonomic positions as novel/distinct species,” Miyazawa writes in his paper.

Colours and patterns are an important visual identifier for animals, and they are more likely to breed with similar looking animals. This means that the hybridised patterns would likely breed together, instead of with other spotty animals. With the extra advantages that come with complex patterns, the fish had the opportunity to evolve and speciate very fast.

“This indicates that the pattern blending mechanism may be extensively involved in the enrichment of colour pattern diversity,” says Miyazawa, “possibly in other animal groups as well.”

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