Speciation by Sensory Drive in the Paragalago zanzibaricus Species Complex

Abstract

Cryptic animal species are often distinguishable by nonvisual communication signals used in specific-mate recognition. The three cryptic taxa comprising the Paragalago zanzibaricus species complex use distinct loud calls that do not obviously reflect molecular phylogenetic relationships. Paragalago granti and P. cocos both have incremental calls but are not sister taxa; P. cocos is the sister taxon to P. zanzibaricus, which has a rolling (trilling) call like that of P. rondoensis, one of the outliers to the complex. To test current hypotheses of species delimitation, I conducted 378 playback trials, using conspecific and heterospecific loud calls and recording both vocal responses and movement toward the speaker. To investigate the sensory drive hypothesis (acoustic adaptation), I measured reverberation duration of 215 short “yap” mobbing calls and examined 282 sonotopes (local soundscapes). I tested the hypothesis that different species’ ear lengths allow the animals to filter out interfering frequencies in local background noise. On Zanzibar Island, P. zanzibaricus responded solely to conspecific calls (23/32). In South Africa, P. granti individuals responded to conspecific calls (12/22) but also sometimes to P. cocos calls (9/22). In Kenya, P. cocos individuals responded to conspecific calls (34/40) but also sometimes to P. zanzibaricus calls (8/40). The three habitats showed differences in reverberation duration, and soundscapes differed in background orthopteran frequencies. Incremental calls probably evolved in the ancestor of the complex, in response to high levels of reverberation and a continuous backdrop of katydid stridulation in dry forest. Secondary evolution of trilling and decreased ear size probably evolved as result of decreased reverberation and higher frequency katydid interference in the P. cocos–P. zanzibaricus ancestor. A loss of short units in P. zanzibaricus probably occurred as result of decreased reverberation in thicket. The study validates the three species of the complex and provides information supporting speciation by sensory drive.