Dung beetles are able to roll their precious balls of manure along a straight line using celestial cues. According to a new study, nocturnal beetles orient using different cues than beetles who are active in the daytime. The findings, published in Proceedings of the National Academy of Sciences, also show how cue preference is reflected in the activity of compass neurons in the brain.
In order to avoid competition for food at the dung pile, South African ball-rolling dung beetles detach a piece of the dung, fashion it into a ball, and roll it away and across the savanna along a straight-line path. The nocturnal Scarabaeus satyrus and the diurnal (or day-active) Scarabaeus lamarcki both use dung for sustenance and as breeding chambers. And despite extremely dim lighting – moonlight is polarized, or scattered, making it at least a million-fold dimmer than sunlight – the nocturnal species is able to move in the same way through the same environment as its diurnal relatives.
To see if diurnal and nocturnal species use similar cues for straight-line orientation, researchers led by Lund University’s Basil el Jundi conducted field experiments where they coerced the beetles into rolling their dung balls at times of the day when they’re not typically active. They found that both species use the Sun as a compass during the day, but things are different at night: While the diurnal beetles use the Moon as a compass, the nocturnal beetles rely on the polarization pattern of moonlight as their main orientation cue, not the Moon itself.
In other words, diurnal beetles use the celestial body (the Sun or the Moon) as their compass, day or night. Nocturnal beetles use the celestial body (the Sun) during the daytime, but they switch to polarized skylight at night.
Then, to see what the neuronal basis of cue preference is, the team studied electrophysiological recordings of the dung beetles’ brains as they respond to simulated lighting cues. They focused on a network of cells in a brain region called the central complex, which houses the internal compass for celestial orientation in both species. The patterns of neural coding matched the beetles’ behaviors in the field: In the diurnal beetle, the central complex neurons are only tuned to the Sun, but these same neurons switch between the Sun and polarized moonlight in the nocturnal beetle.
The flexibility of the nocturnal species helps them use the more available cue under varying light intensities. After all, as with every creature of the night, Scarabaeus satyrus must overcome the challenge of maintaining high orientation precision even in extremely dim light.