The aye-aye (Daubentonia madagascariensis) is one of the most unusual primates on Earth famed for its large eyes, big ears and bony finger used for probing. This species has fascinated biologists for centuries, in particular how and why it evolved such unusual features. A new study, published in the journal Biology Letters, measured the extent to which the aye-aye has evolved similar features to squirrels, despite being more closely related to monkeys, chimps, and humans.
The aye-aye (Daubentonia madagascariensis). Image credit: Simon / CC BY 2.0.
When two aye-ayes were first brought back to Europe from Madagascar by French explorers in 1780, they were ‘ranked with the rodents’ and believed to be more closely allied to a genus of squirrels than any other.
By the mid-19th century the species had been correctly identified as a primate, but its squirrel-like appearance is often cited as a striking example of ‘evolutionary convergence.’
Now, Dr. Philip Cox from the University of York and the Hull York Medical School and co-authors have used high-resolution microCT scanning to image the skulls of the two species, mapping and modeling the level of convergence in their physical features.
“The aye-aye is a highly unusual lemuriform primate that has evolved a dentition similar to that of rodents: it possesses large, ever-growing incisors which it uses to strip the bark from trees in order to feed on wood-boring beetle larvae,” the researchers said.
“Indeed, such is the similarity that some of the earliest classifications of the aye-aye placed it in the squirrel genus Sciurus.”
“We aimed to test the degree of convergence between the skulls and lower jaws of squirrels and the aye-aye.”
The scientists made 3D reconstructions of the skulls and mandibles of the aye-aye and squirrel, plus a variety of other primates and rodents.
They then took 3D co-ordinates from these reconstructions and put this data into statistical software.
Plotting the evolutionary trees of the two biological groups allowed the team to visualize how the evolutionary paths of the aye-aye and squirrel incline towards each other – showing the high degree of convergence in the skull and jaw, despite the completely different ancestry of the two species.
The team’s findings suggest that the demands of needing to produce a high bite force with the two front teeth — in the squirrel for cracking nuts and in the aye-aye for biting into tree bark to feed on wood-boring beetle larvae — have not only led to the aye-aye evolving the ever-growing incisors characteristic of rodents, but has also given it a squirrel-like skull and jaw.
The study shows how lifestyle and ecology can have such a strong influence on the way a species looks that they can almost override ancestry.
“Examples of convergent evolution can be seen throughout nature. Aye-ayes and squirrels have become an iconic example of convergence because of their similar teeth, but our study has shown for the first time that the evolution of their skulls and jaws has also converged,” Dr. Cox said.
“Our analysis suggests that the skulls of both species have not evolved simply to house their teeth, but that the distinctive shape may be what allows them to exact a high bite force.”
“The shape of the skull is what makes the aye-aye look so similar to squirrels in particular.”
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Philip J.R. Morris et al. Convergent evolution in the Euarchontoglires. Biol. Lett 14: 20180366; doi: 10.1098/rsbl.2018.0366
