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Poi­so­nous frogs devel­oped warn­ing col­oration first, loud mat­ing calls later

pub­lished 27 Octo­ber 2014 | mod­i­fied 27 Octo­ber 2014

Warn­ing col­oration paved the way for louder, more com­plex calls in cer­tain species of poi­so­nous frogs

Frog A bilinguisFrogs are well-​known for being among the loud­est amphib­ians, but new research indi­cates that the devel­op­ment of this trait fol­lowed another: bright col­oration. Sci­en­tists have found that the tell­tale colours of some poi­so­nous frog species estab­lished them as an unap­pe­tiz­ing option for would-​be preda­tors before the frogs evolved their elab­o­rate songs. As a result, these ini­tial warn­ing sig­nals allowed dif­fer­ent species to diver­sify their calls over time.

Zool­o­gists at the National Evo­lu­tion­ary Syn­the­sis Cen­ter (NES­Cent), the Uni­ver­sity of British Colum­bia, and other research uni­ver­si­ties assem­bled an acoustic data­base to analyse more than 16,000 calls from 172 species within the poi­son frog fam­ily, Den­dro­bati­dae. The paper, which will appear in the Decem­ber issue of the Pro­ceed­ings of the Royal Soci­ety B, is now avail­able online.

The study included both frogs that dis­play bright colours and oth­ers that rely on cam­ou­flage for pro­tec­tion. Each call was exam­ined in terms of pitch and dura­tion, and researchers also fac­tored in the size of the frogs and their vis­i­bil­ity to preda­tors. They found that because warn­ing col­oration pro­tected them from preda­tors, they were bet­ter able to attract a mate with low-​pitch, puls­ing vocal­iza­tions in plain sight than their qui­eter, darker-​hued relatives.

This allows the frog to have a unique type of call — a noisy call. These noisy kinds of calls, in gen­eral, are what the females really like.
Juan C. San­tos, lead author, Uni­ver­sity of British Columbia »

Sci­en­tists already under­stood that preda­tors shied away from brightly coloured frogs because of visual cues, but San­tos (who worked for­merly at NES­Cent) and his col­leagues hypoth­e­sized that some species evolved to include audio sig­nals, as well. Such a warn­ing sys­tem is not unprece­dented: Tiger moths emit ultra­sonic chirps to com­mu­ni­cate their unsavoury taste to bats. With­out a sim­i­lar abil­ity, frogs nav­i­gate a pre­car­i­ous dilemma in which they must either risk detec­tion by preda­tors or forgo pos­si­ble courtship.

Frog chronogramIni­tially the researchers expected that audio warn­ings pre­ceded col­oration, but the results indi­cate the oppo­site. Using mol­e­c­u­lar data and sta­tis­ti­cal analy­ses, they were able to infer a phy­lo­ge­netic tree and pin­point which trait came first. Their find­ings indi­cate that visual traits estab­lished the frogs as poi­so­nous and cleared the way for louder, more elab­o­rate calls.

Species rely­ing on cam­ou­flage for defense will not invite atten­tion with bois­ter­ous calls, while their pro­tected rel­a­tives — includ­ing non-​poisonous frogs that mimic the appear­ance of their toxic coun­ter­parts — can be loud and more nuanced.

The type of colour they have is in the range of the noisy ones,” San­tos said. “When you’re mim­ic­k­ing some­body that’s already pro­tected, you have some free­dom to be found by poten­tial mates.”

These calls require high energy expen­di­tures, but the boon of attract­ing females with­out preda­tory threats makes it a reward­ing behav­iour for males. Less is known about the rea­sons females are attracted to the nois­ier males and how they appraise the var­i­ous calls. San­tos explained that if the females are being espe­cially picky, it will drive male diver­sity by push­ing them to cre­ate even more com­plex songs.

“What can the females get from this infor­ma­tion? Maybe females — by being very picky — increase male diver­sity,” San­tos said. A more diverse pool of poten­tial mates increases the like­li­hood that their off­spring will have more advan­ta­geous genes over time.

(Source: NES­Cent press release, 24.10.2014)

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