Adaptations to light contribute to the ecological niches and evolution of the terrestrial avifauna

Eye morphology contributes to the ecology and evolution of the aquatic avifauna

Aquatic birds are notable among the global avifauna for living in environments exposed to large amounts of light. Despite growing evidence that visual adaptations to light underly the ecology and evolution of the avian tree of life, no comprehensive comparative analysis of visual acuity as approximated by eyes size exists for the global aquatic avifauna. Here, I use Stanley Ritland's unpublished dataset of measurements for axial length collected from museum specimens to explore the ecology and evolution of eye size variation for half of the aquatic avifauna (N = 464 species). After correcting for body mass allometry and incorporating phylogenetic relationships, aquatic species had significantly smaller eyes compared to terrestrial species. Furthermore, species using hyperopic foraging manoeuvres, exhibiting carnivorous and insectivorous diets, and displaying nocturnal behaviour had larger eyes. Plunge-divers (e.g. boobies and tropic birds) and stalkers (e.g. herons) had the largest relative eye sizes, especially species identifying prey at higher altitudes or longer distances. Underwater pursuit-divers foraging at greater depths had larger eyes, likely due to the dramatic attenuation of light in the deep ocean. Overall, residual eye size was phylogenetically conserved (l = 0.94), with phylogeny alone explaining 62% of residual eye size variation. Collectively, these results suggest that the relatively bright environments found in aquatic ecosystems negate the adaptive benefits of costly metabolic investments associated with developing and maintaining larger eyes, while also reducing the potential occurrence of disability glare. Strong correlations between eye size and foraging ecology in different aquatic environments corroborate similar comparative studies of terrestrial birds and underscore the central role that vision has played in driving the ecology and evolution of the global avifauna.

Comparative study of the ciliary body and iris morphology in the anterior eye chamber of five different vertebrate classes

One crucial component of the optical system is the ciliary body (CB). This body secretes the aqueous humour, which is essential to maintain the internal eye pressure as well as the clearness of the lens and cornea. The histological study was designed to provide the morphological differences of CB and iris in the anterior eye chambers of the following vertebrate classes: fish (grass carp), amphibians (Arabian toad), reptiles (semiaquatic turtle, fan-footed gecko, ocellated skink, Egyptian spiny-tailed lizard, Arabian horned viper), birds (common pigeon, common quail, common kestrel), and mammals (BALB/c mouse, rabbit, golden hamster, desert hedgehog, lesser Egyptian jerboa, Egyptian fruit bat). The results showed distinct morphological appearances of the CB and iris in each species, ranging from fish to mammals. The present comparative study concluded that the morphological structure of the CB and iris is the adaptation of species to either their lifestyle or survival in specific habitats.

Ecological and morphological correlates of visual acuity in birds

Birds use their visual systems for important tasks, such as foraging and predator detection, that require them to resolve an image. However, visual acuity (the ability to perceive spatial detail) varies by two orders of magnitude across birds. Prior studies indicate that eye size and aspects of a species' ecology may drive variation in acuity, but these studies have been restricted to small numbers of species. We used a literature review to gather data on acuity measured either behaviorally or anatomically for 94 species from 38 families. We then examined how acuity varies in relation to (1) eye size, (2) habitat spatial complexity, (3) habitat light level, (4) diet composition, (5) prey mobility and (6) foraging mode. A phylogenetically controlled model including all of the above factors as predictors indicated that eye size and foraging mode are significant predictors of acuity. Examining each ecological variable in turn revealed that acuity is higher in species whose diet comprises vertebrates or scavenged food and whose foraging modes require resolving prey from farther away. Additionally, species that live in spatially complex, vegetative habitats have lower acuity than expected for their eye sizes. Together, our results suggest that the need to detect important objects from far away - such as predators for species that live in open habitats, and food items for species that forage on vertebrate and scavenged prey - has likely been a key driver of higher acuity in some species, helping us to elucidate how visual capabilities may be adapted to an animal's visual needs.

Phenotypic signatures of urbanization? Resident, but not migratory, songbird eye size varies with urban-associated light pollution levels

Urbanization now exposes large portions of the earth to sources of anthropogenic disturbance, driving rapid environmental change and producing novel environments. Changes in selective pressures as a result of urbanization are often associated with phenotypic divergence; however, the generality of phenotypic change remains unclear. In this study, we examined whether morphological phenotypes in two residential species (Carolina Wren [Thryothorus ludovicianus] and Northern Cardinal [Cardinalis cardinalis]) and two migratory species (Painted Bunting [Passerina ciris], and White-eyed Vireo [Vireo griseus]), differed between urban core and edge habitats in San Antonio, Texas, USA. More specifically, we examined whether urbanization, associated sensory pollution (light and noise) and brightness (open, bright areas cause by anthropogenic land use) influenced measures of avian body (mass and frame size) and lateral eye size. We found no differences in body size between urban core and edge habitats for all species except the Painted Bunting, in which core-urban individuals were smaller. Rather than a direct effect of urbanization, this was due to differences in age structure between habitats, with urban-core areas consisting of higher proportions of younger buntings which are, on average, smaller than older birds. Residential birds inhabiting urban-core areas had smaller eyes compared to their urban-edge counterparts, resulting from a negative association between eye size and light pollution and brightness across study sites; notably, we found no such association in the two migratory species. Our findings demonstrate how urbanization may indirectly influence phenotypes by altering population demographics and highlight the importance of accounting for age when assessing factors driving phenotypic change. We also provide some of the first evidence that birds may adapt to urban environments through changes in their eye morphology, demonstrating the need for future research into relationships among eye size, ambient light microenvironment use, and disassembly of avian communities as a result of urbanization.

Plasticity and genetic effects contribute to different axes of neural divergence in a community of mimetic Heliconius butterflies

Changes in ecological preference, often driven by spatial and temporal variation in resource distribution, can expose populations to environments with divergent information content. This can lead to adaptive changes in the degree to which individuals invest in sensory systems and downstream processes, to optimize behavioural performance in different contexts. At the same time, environmental conditions can produce plastic responses in nervous system development and maturation, providing an alternative route to integrating neural and ecological variation. Here, we explore how these two processes play out across a community of Heliconius butterflies. Heliconius communities exhibit multiple Mullerian mimicry rings, associated with habitat partitioning across environmental gradients. These environmental differences have previously been linked to heritable divergence in brain morphology in parapatric species pairs. They also exhibit a unique dietary adaptation, known as pollen feeding, that relies heavily on learning foraging routes, or trap-lines, between resources, which implies an important environmental influence on behavioural development. By comparing brain morphology across 133 wild-caught and insectary-reared individuals from seven Heliconius species, we find strong evidence for interspecific variation in patterns of neural investment. These largely fall into two distinct patterns of variation; first, we find consistent patterns of divergence in the size of visual brain components across both wild and insectary-reared individuals, suggesting genetically encoded divergence in the visual pathway. Second, we find interspecific differences in mushroom body size, a central component of learning and memory systems, but only among wild caught individuals. The lack of this effect in common-garden individuals suggests an extensive role for developmental plasticity in interspecific variation in the wild. Finally, we illustrate the impact of relatively small-scale spatial effects on mushroom body plasticity by performing experiments altering the cage size and structure experienced by individual H. hecale. Our data provide a comprehensive survey of community level variation in brain structure, and demonstrate that genetic effects and developmental plasticity contribute to different axes of interspecific neural variation.

Ecological predictors of interspecific variation in bird bill and leg lengths on a global scale

Bills and legs are two vital appendages for birds, and they exhibit huge interspecific variation in form and function, yet no study has examined the global predictors of this variation. This study examined global gradients in the relative lengths of bird bills and tarsi (i.e. exposed leg parts) to body size across non-migratory birds, while accounting for phylogeny. We found that relative bill length and tarsus length were related to diet, habitat density, latitude, annual mean temperature, temperature variability and hand-wing index (HWI), a proxy for birds' flight efficiency. Among these factors, diet played a primary role in predicting bill length, with nectar-feeding pollinators, vertivores, invertivores and omnivores having longer bills; HWI emerged as the predominant predictor of tarsus length, wherein species with higher HWI had shorter tarsi. However, the effects of these factors differed between passerines and non-passerines, with some temperature-related effects exhibiting opposite trends between these two groups. Our findings highlight the compromise in adaptations for feeding, thermoregulation and flight performance between the two distinct appendages.

Evolution of Avian Eye Size Is Associated with Habitat Openness, Food Type and Brain Size

The eye is the primary sensory organ that obtains information from the ecological environments and specifically bridges the brain with the extra environment. However, the coevolutionary relationships between eye size and ecological factors, behaviours and brain size in birds remain poorly understood. Here, we investigate whether eye size evolution is associated with ecological factors (e.g., habitat openness, food type and foraging habitat), behaviours (e.g., migration and activity pattern) and brain size among 1274 avian species using phylogenetically controlled comparative analyses. Our results indicate that avian eye size is significantly associated with habitat openness, food type and brain size. Species living in dense habitats and consuming animals exhibit larger eye sizes compared to species living in open habitats and consuming plants, respectively. Large-brained birds tend to possess larger eyes. However, migration, foraging habitat and activity pattern were not found to be significantly associated with eye size in birds, except for nocturnal birds having longer axial lengths than diurnal ones. Collectively, our results suggest that avian eye size is primarily influenced by light availability, food need and cognitive ability.

Neuroanatomical shifts mirror patterns of ecological divergence in three diverse clades of mimetic butterflies

Microhabitat partitioning in heterogenous environments can support more diverse communities but may expose partitioned species to distinct perceptual challenges. Divergence across microhabitats could therefore lead to local adaptation to contrasting sensory conditions across small spatial scales, but this aspect of community structuring is rarely explored. Diverse communities of ithomiine butterflies provide an example where closely related species partition tropical forests, where shifts in mimetic coloration are tightly associated with shifts in habitat preference. We test the hypothesis that these mimetic and ecological shifts are associated with distinct patterns of sensory neural investment by comparing brain structure across 164 individuals of 16 species from three ithomiine clades. We find distinct brain morphologies between Oleriina and Hypothyris, which are mimetically homogenous and occupy a single microhabitat. Oleriina, which occurs in low-light microhabitats, invests less in visual brain regions than Hypothyris, with one notable exception, Hyposcada anchiala, the only Oleriina sampled to have converged on mimicry rings found in Hypothyris. We also find that Napeogenes, which has diversified into a range of mimicry rings, shows intermediate patterns of sensory investment. We identify flight height as a critical factor shaping neuroanatomical diversity, with species that fly higher in the canopy investing more in visual structures. Our work suggests that the sensory ecology of species may be impacted by, and interact with, the ways in which communities of closely related organisms are adaptively assembled.

Toward Multiplexed Optogenetic Circuits

Owing to its ubiquity and easy availability in nature, light has been widely employed to control complex cellular behaviors. Light-sensitive proteins are the foundation to such diverse and multilevel adaptive regulations in a large range of organisms. Due to their remarkable properties and potential applications in engineered systems, exploration and engineering of natural light-sensitive proteins have significantly contributed to expand optogenetic toolboxes with tailor-made performances in synthetic genetic circuits. Progressively, more complex systems have been designed in which multiple photoreceptors, each sensing its dedicated wavelength, are combined to simultaneously coordinate cellular responses in a single cell. In this review, we highlight recent works and challenges on multiplexed optogenetic circuits in natural and engineered systems for a dynamic regulation breakthrough in biotechnological applications.

Host and brood parasite coevolutionary interactions covary with comparative patterns of the avian visual system

In coevolutionary arms-races, reciprocal ecological interactions and their fitness impacts shape the course of phenotypic evolution. The classic example of avian host–brood parasite interactions selects for host recognition and rejection of increasingly mimetic foreign eggs. An essential component of perceptual mimicry is that parasitic eggs escape detection by host sensory systems, yet there is no direct evidence that the avian visual system covaries with parasitic egg recognition or mimicry. Here, we used eye size measurements collected from preserved museum specimens as a metric of the avian visual system for species involved in host–brood parasite interactions. We discovered that (i) hosts had smaller eyes compared with non-hosts, (ii) parasites had larger eyes compared with hosts before but not after phylogenetic corrections, perhaps owing to the limited number of independent evolutionary origins of obligate brood parasitism, (iii) egg rejection in hosts with non-mimetic parasitic eggs positively correlated with eye size, and (iv) eye size was positively associated with increased avian-perceived host–parasite eggshell similarity. These results imply that both host-use by parasites and anti-parasitic responses by hosts covary with a metric of the visual system across relevant bird species, providing comparative evidence for coevolutionary patterns of host and brood parasite sensory systems.