It is one of the most unusual primates on the planet — famed for its large eyes, big ears and thin, bony finger used for probing. Often persecuted as a harbinger of evil, the aye-aye has fascinated scientists, in particular how and why it evolved such unusual features.
Aye aye in the wild.
Image by Nomis-Simon. Creative Commons license (CC BY 2.0)
When two aye-ayes were first brought back to Europe from their native Madagascar by French explorers in 1780, they were “ranked with the rodents” and believed to be “more closely allied to the genus of squirrel than any other”.
High bite force
By the mid-19th Century the aye-aye had been correctly identified as a primate, but its squirrel-like appearance is often cited as a striking example of “evolutionary convergence”, or how unrelated species can independently evolve the same traits.
Now, using techniques developed in collaboration by researchers at the University of York, a new study has used high-resolution microCT scanning to image the skulls of the two species, mapping and modelling the level of convergence in their physical features.
The findings, published on 1 August in the journal Biology Letters, 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.
Iconic
Senior author of the study, Dr Philip Cox, said: “Examples of convergent evolution can be seen throughout nature — for example, despite belonging to separate biological groups, dolphins and sharks have converged in body shape due to their shared need to move efficiently through the water.
Dr Philip Cox, co-author, Department of Archaeology and Hull York Medical School, University of York, United Kingdom
“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,” Cox added.
3D reconstructions
Using skeletons borrowed from the collections of natural history museums, the research team 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.
Crania and mandibles of the aye-aye (Daubentonia madagascariensis) and the grey squirrel (Sciurus carolinensis). Not to scale.
Image credit: Dr. Philip Cox, University of York; Creative Commons license (CC BY 4.0)
Plotting the evolutionary trees of the two biological groups allowed the team to visualise 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.
Dr Cox added: “Our study shows the extent to which functional pressures, such as having to eat mechanically demanding food, can significantly alter an animal’s skeleton and result in distantly-related species evolving to resemble one another very closely”.
The aye-aye (Daubentonia madagascarienis), Gerald Durrell’s favourite mammal species, is classified as Endangered by the IUCN Red List of Threatened Species. The species’ distribution is restricted to Madagascar, where it occurs in fragmented pockets (though in very low population densities) across almost the whole of the coastal area. Besides habitat destruction throughout their range the aye-aye is hunted for food and it is killed in some areas as a harbinger of evil and as a crop-pest. Another threat may be export of live animals on a small scale.
(Source: University of York news release, 01.08.2018; IUCN Red List)
