Evolution and diversity in avian vocal system: An Evo-Devo model from the morphological and behavioral perspectives

Cellular computation and cognition

Contemporary neural network models often overlook a central biological fact about neural processing: that single neurons are themselves complex, semi-autonomous computing systems. Both the information processing and information storage abilities of actual biological neurons vastly exceed the simple weighted sum of synaptic inputs computed by the "units" in standard neural network models. Neurons are eukaryotic cells that store information not only in synapses, but also in their dendritic structure and connectivity, as well as genetic "marking" in the epigenome of each individual cell. Each neuron computes a complex nonlinear function of its inputs, roughly equivalent in processing capacity to an entire 1990s-era neural network model. Furthermore, individual cells provide the biological interface between gene expression, ongoing neural processing, and stored long-term memory traces. Neurons in all organisms have these properties, which are thus relevant to all of neuroscience and cognitive biology. Single-cell computation may also play a particular role in explaining some unusual features of human cognition. The recognition of the centrality of cellular computation to "natural computation" in brains, and of the constraints it imposes upon brain evolution, thus has important implications for the evolution of cognition, and how we study it.

Roars, groans and moans: Anatomical correlates of vocal diversity in polygynous deer

Eurasian deer are characterized by the extraordinary diversity of their vocal repertoires. Male sexual calls range from roars with relatively low fundamental frequency (hereafter fo ) in red deer Cervus elaphus, to moans with extremely high fo in sika deer Cervus nippon, and almost infrasonic groans with exceptionally low fo in fallow deer Dama dama. Moreover, while both red and fallow males are capable of lowering their formant frequencies during their calls, sika males appear to lack this ability. Female contact calls are also characterized by relatively less pronounced, yet strong interspecific differences. The aim of this study is to examine the anatomical bases of these inter-specific and inter-sexual differences by identifying if the acoustic variation is reflected in corresponding anatomical variation. To do this, we investigated the vocal anatomy of male and female specimens of each of these three species. Across species and sexes, we find that the observed acoustic variability is indeed related to expected corresponding anatomical differences, based on the source-filter theory of vocal production. At the source level, low fo is associated with larger vocal folds, whereas high fo is associated with smaller vocal folds: sika deer have the smallest vocal folds and male fallow deer the largest. Red and sika deer vocal folds do not appear to be sexually dimorphic, while fallow deer exhibit strong sexual dimorphism (after correcting for body size differences). At the filter level, the variability in formants is related to the configuration of the vocal tract: in fallow and red deer, both sexes have evolved a permanently descended larynx (with a resting position of the larynx much lower in males than in females). Both sexes also have the potential for momentary, call-synchronous vocal tract elongation, again more pronounced in males than in females. In contrast, the resting position of the larynx is high in both sexes of sika deer and the potential for further active vocal tract elongation is virtually absent in both sexes. Anatomical evidence suggests an evolutionary reversal in larynx position within sika deer, that is, a secondary larynx ascent. Together, our observations confirm that the observed diversity of vocal behaviour in polygynous deer is supported by strong anatomical differences, highlighting the importance of anatomical specializations in shaping mammalian vocal repertoires. Sexual selection is discussed as a potential evolutionary driver of the observed vocal diversity and sexual dimorphisms.

A Call to Expand Avian Vocal Development Research

Birds are our best models to understand vocal learning – a vocal production ability guided by auditory feedback, which includes human language. Among all vocal learners, songbirds have the most diverse life histories, and some aspects of their vocal learning ability are well-known, such as the neural substrates and vocal control centers, through vocal development studies. Currently, species are classified as either vocal learners or non-learners, and a key difference between the two is the development period, extended in learners, but short in non-learners. But this clear dichotomy has been challenged by the vocal learning continuum hypothesis. One way to address this challenge is to examine both learners and canonical non-learners and determine whether their vocal development is dichotomous or falls along a continuum. However, when we examined the existing empirical data we found that surprisingly few species have their vocal development periods documented. Furthermore, we identified multiple biases within previous vocal development studies in birds, including an extremely narrow focus on (1) a few model species, (2) oscines, (3) males, and (4) songs. Consequently, these biases may have led to an incomplete and possibly erroneous conclusions regarding the nature of the relationships between vocal development patterns and vocal learning ability. Diversifying vocal development studies to include a broader range of taxa is urgently needed to advance the field of vocal learning and examine how vocal development patterns might inform our understanding of vocal learning.

More of the same: allopatric humpback whale populations share acoustic repertoire

BACKGROUND

Humpback whales (Megaptera novaeangliae) are a widespread, vocal baleen whale best known for producing song, a complex, repetitive, geographically distinct acoustic signal sung by males, predominantly in a breeding context. Humpback whales worldwide also produce non-song vocalizations ("calls") throughout their migratory range, some of which are stable across generations.

METHODS

We looked for evidence that temporally stable call types are shared by two allopatric humpback whale populations while on their northern hemisphere foraging grounds in order to test the hypothesis that some calls, in strong contrast to song, are innate within the humpback whale acoustic repertoire.

RESULTS

Despite being geographically and genetically distinct populations, humpback whales in Southeast Alaska (North Pacific Ocean) share at least five call types with conspecifics in Massachusetts Bay (North Atlantic Ocean).

DISCUSSION

This study is the first to identify call types shared by allopatric populations, and provides evidence that some call types may be innate.

Toward a computational framework for cognitive biology: unifying approaches from cognitive neuroscience and comparative cognition

Progress in understanding cognition requires a quantitative, theoretical framework, grounded in the other natural sciences and able to bridge between implementational, algorithmic and computational levels of explanation. I review recent results in neuroscience and cognitive biology that, when combined, provide key components of such an improved conceptual framework for contemporary cognitive science. Starting at the neuronal level, I first discuss the contemporary realization that single neurons are powerful tree-shaped computers, which implies a reorientation of computational models of learning and plasticity to a lower, cellular, level. I then turn to predictive systems theory (predictive coding and prediction-based learning) which provides a powerful formal framework for understanding brain function at a more global level. Although most formal models concerning predictive coding are framed in associationist terms, I argue that modern data necessitate a reinterpretation of such models in cognitive terms: as model-based predictive systems. Finally, I review the role of the theory of computation and formal language theory in the recent explosion of comparative biological research attempting to isolate and explore how different species differ in their cognitive capacities. Experiments to date strongly suggest that there is an important difference between humans and most other species, best characterized cognitively as a propensity by our species to infer tree structures from sequential data. Computationally, this capacity entails generative capacities above the regular (finite-state) level; implementationally, it requires some neural equivalent of a push-down stack. I dub this unusual human propensity "dendrophilia", and make a number of concrete suggestions about how such a system may be implemented in the human brain, about how and why it evolved, and what this implies for models of language acquisition. I conclude that, although much remains to be done, a neurally-grounded framework for theoretical cognitive science is within reach that can move beyond polarized debates and provide a more adequate theoretical future for cognitive biology.

Development of the trigeminal motor neurons in parrots: implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest-derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology.

Evolutionary Developmental Biology and Human Language Evolution: Constraints on Adaptation

A tension has long existed between those biologists who emphasize the importance of adaptation by natural selection and those who highlight the role of phylogenetic and developmental constraints on organismal form and function. This contrast has been particularly noticeable in recent debates concerning the evolution of human language. Darwin himself acknowledged the existence and importance of both of these, and a long line of biologists have followed him in seeing, in the concept of "descent with modification", a framework naturally able to incorporate both adaptation and constraint. Today, the integrated perspective of modern evolutionary developmental biology ("evo-devo") allows a more subtle and pluralistic approach to these traditional questions, and has provided several examples where the traditional notion of "constraint" can be cashed out in specific, mechanistic terms. This integrated viewpoint is particularly relevant to the evolution of the multiple mechanisms underlying human language, because of the short time available for novel aspects of these mechanisms to evolve and be optimized. Comparative data indicate that many cognitive aspects of human language predate humans, suggesting that pre-adaptation and exaptation have played important roles in language evolution. Thus, substantial components of what many linguists call "Universal Grammar" predate language itself. However, at least some of these older mechanisms have been combined in ways that generate true novelty. I suggest that we can insightfully exploit major steps forward in our understanding of evolution and development, to gain a richer understanding of the principles that underlie human language evolution.

Convergent differential regulation of parvalbumin in the brains of vocal learners

Spoken language and learned song are complex communication behaviors found in only a few species, including humans and three groups of distantly related birds – songbirds, parrots, and hummingbirds. Despite their large phylogenetic distances, these vocal learners show convergent behaviors and associated brain pathways for vocal communication. However, it is not clear whether this behavioral and anatomical convergence is associated with molecular convergence. Here we used oligo microarrays to screen for genes differentially regulated in brain nuclei necessary for producing learned vocalizations relative to adjacent brain areas that control other behaviors in avian vocal learners versus vocal non-learners. A top candidate gene in our screen was a calcium-binding protein, parvalbumin (PV). In situ hybridization verification revealed that PV was expressed significantly higher throughout the song motor pathway, including brainstem vocal motor neurons relative to the surrounding brain regions of all distantly related avian vocal learners. This differential expression was specific to PV and vocal learners, as it was not found in avian vocal non-learners nor for control genes in learners and non-learners. Similar to the vocal learning birds, higher PV up-regulation was found in the brainstem tongue motor neurons used for speech production in humans relative to a non-human primate, macaques. These results suggest repeated convergent evolution of differential PV up-regulation in the brains of vocal learners separated by more than 65–300 million years from a common ancestor and that the specialized behaviors of learned song and speech may require extra calcium buffering and signaling.

The evolution of syntax: an exaptationist perspective

The evolution of language required elaboration of a number of independent mechanisms in the hominin lineage, including systems involved in signaling, semantics, and syntax. Two perspectives on the evolution of syntax can be contrasted. The “continuist” perspective seeks the evolutionary roots of complex human syntax in simpler combinatory systems used in animal communication systems, such as iteration and sequencing. The “exaptationist” perspective posits evolutionary change of function, so that systems today used for linguistic communication might previously have served quite different functions in earlier hominids. I argue that abundant biological evidence supports an exaptationist perspective, in general, and that it must be taken seriously when considering language evolution. When applied to syntax, this suggests that core computational components used today in language could have originally served non-linguistic functions such as motor control, non-verbal thought, or spatial reasoning. I outline three specific exaptationist hypotheses for spoken language. These three hypotheses each posit a change of functionality in a precursor circuit, and its transformation into a neural circuit or region specifically involved in language today. Hypothesis 1 suggests that the precursor mechanism for intentional vocal control, specifically direct cortical control over the larynx, was manual motor control subserved by the cortico-spinal tract. The second is that the arcuate fasciculus, which today connects syntactic and lexical regions, had its origin in intracortical connections subserving vocal imitation. The third is that the specialized components of Broca’s area, specifically BA 45, had their origins in non-linguistic motor control, and specifically hierarchical planning of action. I conclude by illustrating the importance of both homology (studied via primates) and convergence (typically analyzed in birds) for testing such evolutionary hypotheses.

Comparative analysis of mineralocorticoid receptor expression among vocal learners (Bengalese finch and budgerigar) and non-vocal learners (quail and ring dove) has implications for the evolution of avian vocal learning

Mineralocorticoid receptor is the receptor for corticosteroids such as corticosterone or aldosterone. Previously, we found that mineralocorticoid receptor was highly expressed in song nuclei of a songbird, Bengalese finch (Lonchura striata var. domestica). Here, to examine the relationship between mineralocorticoid receptor expression and avian vocal learning, we analyzed mineralocorticoid receptor expression in the developing brain of another vocal learner, budgerigar (Melopsittacus undulatus) and non-vocal learners, quail (Coturnix japonica) and ring dove (Streptopelia capicola). Mineralocorticoid receptor showed vocal control area-related expressions in budgerigars as Bengalese finches, whereas no such mineralocorticoid receptor expressions were seen in the telencephalon of non-vocal learners. Thus, these results suggest the possibility that mineralocorticoid receptor plays a role in vocal development of parrots as songbirds and that the acquisition of mineralocorticoid receptor expression is involved in the evolution of avian vocal learning.

Comparative gene expression analysis among vocal learners (bengalese finch and budgerigar) and non-learners (quail and ring dove) reveals variable cadherin expressions in the vocal system

Birds use various vocalizations to communicate with one another, and some are acquired through learning. So far, three families of birds (songbirds, parrots, and hummingbirds) have been identified as having vocal learning ability. Previously, we found that cadherins, a large family of cell-adhesion molecules, show vocal control-area-related expression in a songbird, the Bengalese finch. To investigate the molecular basis of evolution in avian species, we conducted comparative analysis of cadherin expressions in the vocal and other neural systems among vocal learners (Bengalese finch and budgerigar) and a non-learner (quail and ring dove). The gene expression analysis revealed that cadherin expressions were more variable in vocal and auditory areas compared to vocally unrelated areas such as the visual areas among these species. Thus, it appears that such diverse cadherin expressions might have been related to generating species diversity in vocal behavior during the evolution of avian vocal learning.