The biomechanical basis of evolutionary change in a territorial display

Interspecific hybridization explains rapid gorget colour divergence in Heliodoxa hummingbirds (Aves: Trochilidae)

Hybridization is a known source of morphological, functional and communicative signal novelty in many organisms. Although diverse mechanisms of established novel ornamentation have been identified in natural populations, we lack an understanding of hybridization effects across levels of biological scales and upon phylogenies. Hummingbirds display diverse structural colours resulting from coherent light scattering by feather nanostructures. Given the complex relationship between feather nanostructures and the colours they produce, intermediate coloration does not necessarily imply intermediate nanostructures. Here, we characterize nanostructural, ecological and genetic inputs in a distinctive Heliodoxa hummingbird from the foothills of eastern Peru. Genetically, this individual is closely allied with Heliodoxa branickii and Heliodoxa gularis, but it is not identical to either when nuclear data are assessed. Elevated interspecific heterozygosity further suggests it is a hybrid backcross to H. branickii. Electron microscopy and spectrophotometry of this unique individual reveal key nanostructural differences underlying its distinct gorget colour, confirmed by optical modelling. Phylogenetic comparative analysis suggests that the observed gorget coloration divergence from both parentals to this individual would take 6.6-10 My to evolve at the current rate within a single hummingbird lineage. These results emphasize the mosaic nature of hybridization and suggest that hybridization may contribute to the structural colour diversity found across hummingbirds.

Studies of the Behavioral Sequences: The Neuroethological Morphology Concept Crossing Ethology and Functional Morphology

Simple Summary

Behavioral sequences analysis is a relevant method for quantifying the behavioral repertoire of animals to respond to the classical Tinbergen’s four questions. Research in ethology and functional morphology intercepts at the level of analysis of behaviors through the recording and interpretation of data from of movement sequence studies with various types of imaging and sensor systems. We propose the concept of Neuroethological morphology to build a holistic framework for understanding animal behavior. This concept integrates ethology (including behavioral ecology and neuroethology) with functional morphology (including biomechanics and physics) to provide a heuristic approach in behavioral biology.

Abstract

Postures and movements have been one of the major modes of human expression for understanding and depicting organisms in their environment. In ethology, behavioral sequence analysis is a relevant method to describe animal behavior and to answer Tinbergen’s four questions testing the causes of development, mechanism, adaptation, and evolution of behaviors. In functional morphology (and in biomechanics), the analysis of behavioral sequences establishes the motor pattern and opens the discussion on the links between “form” and “function”. We propose here the concept of neuroethological morphology in order to build a holistic framework for understanding animal behavior. This concept integrates ethology with functional morphology, and physics. Over the past hundred years, parallel developments in both disciplines have been rooted in the study of the sequential organization of animal behavior. This concept allows for testing genetic, epigenetic, and evo-devo predictions of phenotypic traits between structures, performances, behavior, and fitness in response to environmental constraints. Based on a review of the literature, we illustrate this concept with two behavioral cases: (i) capture behavior in squamates, and (ii) the ritualistic throat display in lizards.

Deep-time convergent evolution in animal communication presented by shared adaptations for coping with noise in lizards and other animals

Convergence in communication appears rare compared with other forms of adaptation. This is puzzling, given communication is acutely dependent on the environment and expected to converge in form when animals communicate in similar habitats. We uncover deep-time convergence in territorial communication between two groups of tropical lizards separated by over 140 million years of evolution: the Southeast Asian Draco and Caribbean Anolis. These groups have repeatedly converged in multiple aspects of display along common environmental gradients. Robot playbacks to free-ranging lizards confirmed that the most prominent convergence in display is adaptive, as it improves signal detection. We then provide evidence from a sample of the literature to further show that convergent adaptation among highly divergent animal groups is almost certainly widespread in nature. Signal evolution is therefore curbed towards the same set of adaptive solutions, especially when animals are challenged with the problem of communicating effectively in noisy environments.

Pygmy mouse songs reveal anatomical innovations underlying acoustic signal elaboration in rodents

Elaborate animal communication displays are often accompanied by morphological and physiological innovations. In rodents, acoustic signals used in reproductive contexts are produced by two distinct mechanisms, but the underlying anatomy that facilitates such divergence is poorly understood. 'Audible' vocalizations with spectral properties between 500 Hz and 16 kHz are thought to be produced by flow-induced vocal fold vibrations, whereas 'ultrasonic' vocalizations with fundamental frequencies above 19 kHz are produced by an aerodynamic whistle mechanism. Baiomyine mice (genus Baiomys and Scotinomys) produce complex frequency-modulated songs that span these traditional distinctions and represent important models to understand the evolution of signal elaboration. We combined acoustic analyses of spontaneously vocalizing northern pygmy mice (Baiomystaylori) in air and light gas atmosphere with morphometric analyses of their vocal apparatus to infer the mechanism of vocal production. Increased fundamental frequencies in heliox indicated that pygmy mouse songs are produced by an aerodynamic whistle mechanism supported by the presence of a ventral pouch and alar cartilage. Comparative analyses of the larynx and ventral pouch size among four additional ultrasonic whistle-producing rodents indicated that the unusually low 'ultrasonic' frequencies (relative to body size) of pygmy mice songs are associated with an enlarged ventral pouch. Additionally, mice produced shorter syllables while maintaining intersyllable interval duration, thereby increasing syllable repetition rates. We conclude that while laryngeal anatomy sets the foundation for vocal frequency range, variation and adjustment of central vocal motor control programs fine tunes spectral and temporal characters to promote acoustic diversity within and between species.

How do complex animal signals evolve?

Animal signals-involving combinations of acoustic, chemical, visual, and behavioral cues-are among the most diverse traits in nature. Testing hypotheses about signal evolution has been hampered by difficulties with comparing highly divergent signals among species. In this Primer, I describe recent advances in capturing signals and studying their evolution. I highlight new findings using an information theory-based approach to quantifying signal variation in the diverse birds-of-paradise. Growing access to signal databases in tandem with development of new analytical tools will open up new avenues for studying the proximate mechanisms and ultimate evolutionary causes of signal variation, both within and among species.