A sweat bee (Augochlorella sp.)
Bryan Reynolds / Alamy
Many bees have a brain cell density greater than that of small birds – but most ant brains contain a far lower density of neurons. The difference may be down to the insects’ lifestyles: because bees fly, they may need more brain cells than ants do in order to process visual information.
Scientists have already compared the size and weight of various insects’ brains – which contain independent specialised regions to process visual information, sounds, smells, and even memories.
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But brain size, whether in insects or vertebrate animals like birds and mammals, doesn’t always give a realistic idea of brain power. This is because some animals – especially flying animals like birds that would be weighed down by a large and heavy brain – have many neurons compacted into a smaller space, making the cell density higher.
So, Rebekah Keating Godfrey at the University of Arizona and her colleagues studied insect brains using a recently developed technique for counting brain cells.
They removed the brains of 450 insects, belonging to 32 different species including bees, wasps, ants, and a species of fly.
They ground up each brain and soaked it in a solution that frees the nucleus of each brain cell. Then they added a dye that makes those nuclei fluoresce, allowing the researchers to count the number of nuclei in a small sample under a special ‘epifluorescence’ microscope using ultraviolet light. From that number, they could estimate the number of neurons in the animal’s entire brain.
They found that some of the bees – in particular, the metallic green sweat bee (in the genus Augochlorella) – had very high numbers of neurons for their brain sizes: about 2 million per milligram. This is higher even than those seen in the neuron-rich cerebellums of many birds: the goldcrest, for example, has 490,000 cells per milligram.
But compared to bees and wasps, ants had small brains and relatively few brain cells. One ant species in the study (Novomessor cockerelli) had just 400,000 cells per milligram.
However, the difference in the insects’ brain cell counts probably has little to do with intelligence, says team member Wulfila Gronenberg, also at the University of Arizona. Most likely, the researchers think, flying insects need more neurons to power the enhanced vision they need for flight – an idea they will test in future.
“Brain tissue is expensive, in terms of energy, to keep – so if you don’t need it, you don’t have it,” says Gronenberg.
“We see some pretty dense brain structures in birds, and some of our small bees have that kind of cellular density,” Godfrey says. “Ant brain cell numbers, though, are surprisingly low.”
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2021.0199
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