Simple SummaryTiti monkeys—a diversified group of pair-bonded, territorial neotropical primates exhibiting biparental care—produce elaborate, powerful vocal duets used for long-range communication. While the callicebine taxonomy has been centered mainly on the biogeography, morphology, anatomy, and genetics of titi populations, vocal attributes have received little attention as potentially informative markers of phylogenetic relationships. We conducted acoustic analysis of callicebine loud calls recorded from ten species of titis at sites in Bolivia, Peru, and Ecuador and found four distinct patterns of duetting that only partially match three major clades identified in recent molecular genetic studies. In particular, we found that the loud calls of the San Martin titi monkey, P. oenanthe, and the Urubamba brown titi, P. urubambensis, strikingly differ from putative relatives within the donacophilus lineage. Our findings highlight interplay between genes and environment on the expression of vocal behavior and suggest that closer interaction between taxonomists, ethologists, and molecular biologists should be rewarding in resolving the callicebine phylogeny. Such concerted efforts, in turn, will most likely generate valuable recommendations for the conservation of some endangered populations of titi monkeys, such as the vocally distinctive San Martin titi.AbstractLong-range vocal communication in socially monogamous titi monkeys is mediated by the production of loud, advertising calls in the form of solos, duets, and choruses. We conducted a power spectral analysis of duets and choruses (simply “duets” hereafter) followed by linear discriminant analysis using three acoustic parameters—dominant frequency of the combined signal, duet sequence duration, and pant call rate—comparing the coordinated vocalizations recorded from 36 family groups at 18 sites in Bolivia, Peru and Ecuador. Our analysis identified four distinct duetting patterns: (1) a donacophilus pattern, sensu stricto, characteristic of P. donacophilus, P. pallescens, P. olallae, and P. modestus; (2) a moloch pattern comprising P. discolor, P. toppini, P. aureipalatii, and P. urubambensis; (3) a torquatus pattern exemplified by the duet of Cheracebus lucifer; and (4) the distinctive duet of P. oenanthe, a putative member of the donacophilus group, which is characterized by a mix of broadband and narrowband syllables, many of which are unique to this species. We also document a sex-related difference in the bellow-pant phrase combination among the three taxa sampled from the moloch lineage. Our data reveal a presumptive taxonomic incoherence illustrated by the distinctive loud calls of both P. urubambensis and P. oenanthe within the donacophilus lineage, sensu largo. The results are discussed in light of recent reassessments of the callicebine phylogeny, based on a suite of genetic studies, and the potential contribution of environmental influences, including habitat acoustics and social learning. A better knowledge of callicebine loud calls may also impac...

Worldwide urban expansion and deforestation have caused a rapid decline of non-human primates in recent decades. Yet, little is known to what extent these animals can tolerate anthropogenic noise arising from roadway traffic and human presence in their habitat. We studied six family groups of titis residing at increasing distances from a busy highway, in a park promoting ecotourism near Santa Cruz de la Sierra, Bolivia. We mapped group movements, sampled the titis’ behavior, collected fecal samples from each study group and conducted experiments in which we used a mannequin simulating a human intrusion in their home range. We hypothesized that groups of titi monkeys exposed to higher levels of anthropogenic noise and human presence would react weakly to the mannequin and show higher concentrations of fecal cortisol compared with groups in least perturbed areas. Sound pressure measurements and systematic monitoring of soundscape inside the titis’ home ranges confirmed the presence of a noise gradient, best characterized by the root-mean-square (RMS) and median amplitude (M) acoustic indices; importantly, both anthropogenic noise and human presence co-varied. Study groups resided in small, overlapping home ranges and they spent most of their time resting and preferentially used the lower forest stratum for traveling and the higher levels for foraging. Focal sampling analysis revealed that the time spent moving by adult pairs was inversely correlated with noise, the behavioral change occurring within a gradient of minimum sound pressures ranging from 44 dB(A) to 52 dB(A). Validated enzyme-immunoassays of fecal samples however detected surprisingly low cortisol concentrations, unrelated to the changes observed in the RMS and M indices. Finally, titis’ response to the mannequin varied according to our expectation, with alarm calling being greater in distant groups relative to highway. Our study thus indicates reduced alarm calling through habituation to human presence and suggests a titis’ resilience to anthropogenic noise with little evidence of physiological stress.

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