Assessing visual requirements for social context-dependent activation of the songbird song system

Social context has been shown to have a profound influence on brain activation in a wide range of vertebrate species. Best studied in songbirds, when males sing undirected song, the level of neural activity and expression of immediate early genes (IEGs) in several song nuclei is dramatically higher or lower than when they sing directed song to other birds, particularly females. This differential social context-dependent activation is independent of auditory input and is not simply dependent on the motor act of singing. These findings suggested that the critical sensory modality driving social context-dependent differences in the brain could be visual cues. Here, we tested this hypothesis by examining IEG activation in song nuclei in hemispheres to which visual input was normal or blocked. We found that covering one eye blocked visually induced IEG expression throughout both contralateral visual pathways of the brain, and reduced activation of the contralateral ventral tegmental area, a non-visual midbrain motivation-related area affected by social context. However, blocking visual input had no effect on the social context-dependent activation of the contralateral song nuclei during female-directed singing. Our findings suggest that individual sensory modalities are not direct driving forces for the social context differences in song nuclei during singing. Rather, these social context differences in brain activation appear to depend more on the general sense that another individual is present.     

Articulatory capacity of Neanderthals, a very recent and human-like fossil hominin

Scientists seek to use fossil and archaeological evidence to constrain models of the coevolution of human language and tool use. We focus on Neanderthals, for whom indirect evidence from tool use and ancient DNA appears consistent with an adaptation to complex vocal-auditory communication. We summarize existing arguments that the articulatory apparatus for speech had not yet come under intense positive selection pressure in Neanderthals, and we outline some recent evidence and analyses that challenge such arguments. We then provide new anatomical results from our own attempt to reconstruct vocal tract (VT) morphology in Neanderthals, and document our simulations of the acoustic and articulatory potential of this reconstructed Neanderthal VT. Our purpose in this paper is not to polarize debate about whether or not Neanderthals were human-like in all relevant respects, but to contribute to the development of methods that can be used to make further incremental advances in our understanding of the evolution of speech based on fossil and archaeological evidence.     

The effects of the false vocal fold gaps on intralaryngeal pressure distributions and their effects on phonation

Human phonation does not merely depend on the vibration of the vocal folds. Research by clinical and computer simulations has demonstrated that the false vocal fold (FVF) is an important laryngeal con-striction that plays a vital role during human voice production. This study explored the effects of the FVF gaps using both the three-dimensional Plexiglas model and the numerical computation methods. Twelve FVF gaps (ranging from 0.02 to 2.06 cm) were used in this study at three glottal angles (uniform and convergent/divergent 40°), two minimal glottal diameters (Dg) (0.04 cm and 0.06 cm) separately, and the constant subglottal pressure (8 cm H2O). The results suggested that (1) the intralaryngeal pressure was the lowest and the flow was the highest (least flow resistance) when the FVF gap was 1.5-2 times greater than Dg; (2) the divergent glottal angle gave lower pressure and greater flow than the conver-gent and uniform glottal angle as there were no FVF conditions; (3) the presence of the FVF decreased the effects of the glottal angle to a certain extent; and more importantly, (4) the presence of the FVF also moved the separation points downstream, straightened the glottal jet for a longer distance, decreased the overall laryngeal resistance, and reduced the energy dissipation, suggesting the significance of FVF in efficient voice production. These results may be incorporated in the phonatory models (physical or computational) for better understanding of vocal mechanics. The results might also be helpful in exploring the surgical and rehabilitative intervention of related voice problems.     

Beyond illness: Variation in haemosporidian load explains differences in vocal performance in a songbird

In animal communication, signals are expected to evolve to be honest, so that receivers avoid being manipulated by signalers. One way that signals can evolve to be honest is for them to be costly, with only high‐quality individuals being able to bear the costs of signal expression. It has been proposed that parasites can introduce costs that affect the expression of sexually selected traits, and there is evidence to support the role of parasitism in modulating animal behavior. If host infection status or intensity is found to relate to differences in signal expression, it may indicate a fitness cost that mediates honesty of signals. Birdsong is a good model for testing this, and physically challenging songs representing complex motor patterns provide a good example of sexually selected traits indicating individual condition. We performed a field study to evaluate the relationship between song performance and avian malaria infection in a common songbird. Previous work on this subject has almost always evaluated avian malaria in terms of binary infection status; however, parasitemia—infection intensity—is rarely assessed, even though differences in parasite load may have profound physiological consequences. We estimated parasitemia levels by using real‐time PCR. We found that birds with higher parasitemia displayed lower vocal performance, providing evidence that this song trait is an honest signal of parasitic load of haemosporidian parasites. To our knowledge, this study links parasite load and the expression of a sexually selected trait in a way that has not been addressed in the past. Studies using song performance traits and parasitemia offer an important perspective for understanding evolution of characters via sexual selection.