AboutZoos, Since 2008


Evo­lu­tion in facial appear­ance pre­vents inter­breed­ing between related mon­key species

pub­lished 28 June 2014 | mod­i­fied 28 June 2014

Old World mon­keys have under­gone a remark­able evo­lu­tion in facial appear­ance as a way of avoid­ing inter­breed­ing with closely related and geo­graph­i­cally prox­i­mate species, researchers from New York Uni­ver­sity (NYU) and the Uni­ver­sity of Exeter have found. Their research pro­vides the best evi­dence to date for the role of visual cues as a bar­rier to breed­ing across species. The find­ings have been pub­lished on 26 June in Nature Communications.

Guenon-monkeyfacesEvo­lu­tion pro­duces adap­ta­tions that help ani­mals thrive in a par­tic­u­lar envi­ron­ment, and over time these adap­ta­tions lead to the evo­lu­tion of new species,” explains James Higham, an assis­tant pro­fes­sor in NYU’s Depart­ment of Anthro­pol­ogy and the senior author of the study, which appears in the jour­nal Nature Com­mu­ni­ca­tions. “A key ques­tion is what mech­a­nisms keep closely related species that over­lap geo­graph­i­cally from inter-​breeding, so that they are main­tained as sep­a­rate species.”

Our find­ings offer evi­dence for the use of visual sig­nals to help ensure species recog­ni­tion: species may evolve to look dis­tinct specif­i­cally from the other species they are at risk of inter-​breeding with. In other words, how you end up look­ing is a func­tion of how those around you look. With the pri­mates we stud­ied, this has a pur­pose: to strengthen repro­duc­tive iso­la­tion between populations.”

These results strongly sug­gest that the extra­or­di­nary appear­ance of these mon­keys has been due to selec­tion for visual sig­nals that dis­cour­age hybridisation
William Allen, lead author, School of Bio­log­i­cal, Bio­med­ical and Envi­ron­men­tal Sci­ences, Uni­ver­sity of Hull »

Lead author Allen, under­took the work while a post-​doctoral researcher in NYU’s Depart­ment of Anthro­pol­ogy. The researchers stud­ied guenons, a group of more than two dozen species of mon­keys indige­nous to the forests of Cen­tral and West Africa. Many dif­fer­ent species of guenons are often sym­patric — they live in close prox­im­ity to each other, with mul­ti­ple species often trav­el­ling, feed­ing, and sleep­ing side-​by-​side. There­fore inter­breed­ing, which could result in afflicted infer­tile off­spring, remains an unwel­come possibility.

In the 1980s, Oxford zool­o­gist Jonathan King­don tried to explain the diver­sity in facial appear­ance of guenons, which show mark­ings such as dif­fer­ently coloured eye­brow patches, ear tufts, nose spots, and mouth patches. He hypoth­e­sized that sym­patric guenon species had under­gone facial changes that visu­ally rein­forced dif­fer­ences among their species in order to avoid the risks of hybridising.

How­ever, Kingdon’s ideas were pri­mar­ily based on obser­va­tions with the naked eye, and he failed to find evi­dence for his hypothe­ses. The NYU and Uni­ver­sity of Exeter sci­en­tists sought to test Kingdon’s con­clu­sions quan­ti­ta­tively using sophis­ti­cated meth­ods, such as facial recog­ni­tion algo­rithms that can iden­tify and quan­tify detailed fea­tures in faces.

To do this, they pho­tographed nearly two dozen species of guenons in var­i­ous set­tings, over an 18-​month period: in zoos in the United States and the United King­dom and in a wildlife sanc­tu­ary in Nige­ria. Armed with more than 1,400 stan­dard­ised pho­tographs, the researchers employed what is known as the eigen­face tech­nique, which has been used in the field of com­puter vision for machine recog­ni­tion of faces, in order to dis­tin­guish pri­mate fea­tures and then to deter­mine whether the appear­ance of each guenon species was related to the appear­ance of other species.

Their results showed that, as pre­dicted, the face pat­terns of guenon species have evolved to become more visu­ally dis­tinc­tive — specif­i­cally from those guenon species they over­lap with geo­graph­i­cally — and hence those that they are risk of hybri­dis­ing with.

These results strongly sug­gest that the extra­or­di­nary appear­ance of these mon­keys has been due to selec­tion for visual sig­nals that dis­cour­age hybridi­s­a­tion,” observes lead author Allen, now at the Uni­ver­sity of Hull. “This is per­haps the strongest evi­dence to date for a role for visual sig­nals in the key evo­lu­tion­ary processes by which species are formed and main­tained, and it is par­tic­u­larly excit­ing that we have found it in part of our own lineage.”

(Source: New York Uni­ver­sity press release, 26.06.2014)

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