Sand and Stars

The star-gazing beetle with a tiny brain

A dung beetle rolling his ball of dung 

21 Nov 2018

Who would ever imagine that one would sit gazing at the brilliant Milky Way arching across the sky from horizon to horizon and think of dung beetles?

Admittedly, these little beetles are fascinating insects! They are nature’s clean-up crew of the veld, converging by the hundreds on the piles of brown stuff almost as soon as it is dumped and cleaning up the mess. Each beetle sculpts a dung ball, which they roll away. Far from the pile, they bury the dung ball and use it as a food source and a nursery for their young.

Their quick burial of dung hastens its decomposition, prevents the loss of nutrients, aerates the soil, changes the texture of soil particles, increases porosity and percolation, and increases the depth of soil containing organic material. This is extremely important for other soil animals and microbes and provides a boost of nutrients to plant roots. I reckon that dung beetles are serious competitors for ants and bees in the Hard Worker of the Year stakes!  

Dung beetles have a tiny brain – it is several times smaller than a match head and contains fewer than a million neurons. Yet within this tiny brain lies a complex sky compass that employs sophisticated celestial navigation skills. I am dumbfounded when I think of the ancient Polynesians (1,500–800 B.C.) using the stars to navigate the vast Pacific Ocean in their outrigger canoes and the ancient Tuareg and Bedouin tribes using the stars for navigating the immense expanse filled with shifting sand dunes that is the Sahara. But a dung beetle navigating by the stars? They really are underrated.

Diurnal dung beetles use our friendly neighbourhood star. Nocturnal dung beetles use the moon and one species – the African dung beetle Scarabaeus satyrus – navigates using the pattern of the Milky Way across the night sky. 

A dung beetle’s survival depends upon rolling his ball of dung in a straight line and like a blindfolded human he will walk in circles because he can’t see where he is going while rolling. Dung beetles propel their balls with their hind legs, moving backwards with their heads pointed at the ground. 

The problem for the little beetles is that they are thieving, combative beasts so they need to get the hell out of Dung Dodge as fast as possible. They will happily smash each other up to swipe a dung ball. And if two beetles leaving the pile bump into one other, they can get into a brutal wrestling match. Wandering around in circles boosts the odds of a theft and fight exponentially. Thus, dung beetles have therefore evolved the ability to navigate to safety in quick straight lines.

During the day diurnal dung beetles steer by the sun. They can see polarization patterns in the daytime sky, and use these patterns to hold course. A single patch of blue sky is sufficient. It works at night, too. A number of animals, including bees and butterflies, use sunlight’s polarization in their navigations, but the dung beetle is the only known creature who can see the polarization of moonlight, which is 100 million times weaker than daylight polarization. Studies show that dung beetles can roll their dung balls straight even when the Moon is a faint crescent.

But what happens when there’s no moon?

The nocturnal African dung beetles, Scarabaeus satyrus, navigates by the glow of the Milky Way. Image Credit: Current Biology, Dacke et al.

Researchers discovered that when the nocturnal African dung beetles, Scarabaeus satyrus, roll their tiny balls of dung in straight lines across the sands of South Africa on a moonless night, they use the glow of our Milky Way galaxy for navigation.

Their brain mechanism to perform their celestial navigation is dumbfounding. They are able to look up into the sky and take a “celestial snapshot,” which they store as a map in their brains showing them how the sun or the moon or the stars of the Milky Way are positioned. Before rolling its ball of dung away, the dung beetle climbs to the top of its ball of dung and performs a “dance” whereby it rotates about its vertical axis. It’s during this dance that a beetle takes its snapshot, which it stores in its brain for reference. When the beetle starts to roll its ball of dung, it’s able to move in a straight line by matching the internally stored image of the sky with its current environment. 

According to researchers, their celestial compass cues dominate straight-line orientation in dung beetles so strongly that, as far as scientists know, this is the only animal with a visual compass system that ignores the extra orientation precision that landmarks can offer.

What excellent little astronomers they are! 

Copyright © Susan Young 2018