A Col­lec­tion of News by Moos


Rea­son dis­cov­ered why elu­sive aye-​aye devel­oped such unusual features

pub­lished 04 August 2018 | mod­i­fied 04 August 2018

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 wildAye aye in the wild.
Image by Nomis-​Simon. Cre­ative Com­mons license (CC BY 2.0)

But now a new study has, for the first time, mea­sured the extent to which the endan­gered aye-​aye has evolved sim­i­lar fea­tures to squir­rels, despite being more closely related to mon­keys, chimps, and humans.

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.

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.

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 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.

3 Dmodels of aye aye and squirrel skullsCra­nia 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”.

Aye-​aye con­ser­va­tion sta­tus
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)

Goal: 7000 tigers in the wild

Tiger range countries map

Tiger map” (CC BY 2.5) by Sander­son et al., 2006.

about zoos and their mis­sion regard­ing breed­ing endan­gered species, nature con­ser­va­tion, bio­di­ver­sity and edu­ca­tion, which of course relates to the evo­lu­tion of species.
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