Biology subjects

Endler, J. A.

Publications and source records attributed to Endler, J. A..

10 recordsLinked to original sources

bioRxiv2023

Colour patterns are less variable in highly unpalatable nudibranchs

The selective factors that shape phenotypic diversity in prey communities with aposematic animals are diverse and coincide with similar diversity in the strength of underlying secondary defences. However, quantitative assessments of colour pattern variation and the strength of chemical defences in assemblages of aposematic species are lacking. We quantified colour pattern diversity using Quantitative Colour Pattern Analysis (QCPA) in 13 Dorid nudibranch species (Infraorder: Doridoidei) that varied in the strength of their chemical defences. We accounted for the physiological properties of a potential predators visual system (a triggerfish, Rhinecanthus aculeatus) and modelled the appearance of nudibranchs from multiple viewing distances (2cm and 10cm). We identified distinct colour pattern properties associated with the presence and strength of chemical defences. Colour patterns were also less variable among species with chemical defences when compared to undefended species. This confirms correlations between secondary defences and diverse, bold colouration while showing that chemical defences coincide with decreased colour pattern variability among species. Our study suggests that complex spatiochromatic properties of colour patterns perceived by potential predators can be used to make inferences on the presence and strength of chemical defences.

animal behavior and cognition↗

bioRxiv2022

Signal detectability and boldness are not the same: the function of defensive colouration in nudibranchs is distant-dependent

Aposematic signals advertise underlying defences in many species. They should be detectable (highly contrasting against the background) and bold (high internal pattern contrast) to enhance predator recognition, learning and memorisation. However, the signalling function of aposematic colour patterns may be distance-dependent: signals may be undetectable from a distance to reduce costs of increased attacks from naive predators but bold when viewed up close. To test this hypothesis, we quantified the chromatic and achromatic detectability and boldness of colour patterns in 13 nudibranch species that varied in the strength of their chemical defences, in terms of unpalatability and toxicity, using Quantitative Colour Pattern Analysis (QCPA) and data on the visual perception of a triggerfish (Rhinecanthus aculeatus). When viewed from larger distances, there were no differences in detectability and boldness between well-defended and undefended species. However, when viewed at close distances, well-defended species were more detectable and bolder than undefended species. The detectability of defended species decreased more significantly with increased viewing distances compared to boldness but remained relatively consistent over viewing distances for undefended species. We provide evidence for distance-dependent signalling in aposematic nudibranchs and highlight the importance of distinguishing between signal detectability and boldness in studies of aposematism.

ecology↗

bioRxiv2022

Highly defended nudibranchs do not 'escape' to visually diverse background habitats

The escape and radiate hypothesis predicts that once species have evolved aposematism, defended species can utilise more visually diverse visual backgrounds as they escape the need to be well camouflaged. This enables species to explore new ecological niches, resulting in increased diversification rates. To test this hypothesis escape component, we examined whether the background habitats of 12 nudibranch mollusc species differed among species depending on the presence and strength of chemical defences. We obtained a rich array of colour pattern statistics using Quantitative Colour Pattern Analysis (QCPA) to analyse backgrounds viewed through the eyes of a potential predator (triggerfish, Rhinecanthus aculeatus). Colour pattern analysis was done at viewing distances simulating an escalating predation sequence. We identified four latent factors comprising 17 non-correlated colour pattern parameters, which captured the among-species variability associated with differences in chemical defences. We found that chemically defended species, indeed, were found on visually distinct backgrounds with increased colour and luminance contrast, independent of viewing distance. However, we found no evidence for increased among-species background diversity coinciding with the presence and strength of chemical defences. Our results agree with the escape and radiate hypothesis, suggesting that potent chemical defences in Dorid nudibranchs coincide with spatiochromatic differences of visual background habitats perceived by potential predators. Lay SummaryAccording to the escape and radiate hypothesis, the ability to store potent chemical defences as protection from predators enables animals to be minimally dependent on matching visual backgrounds for camouflage. We found evidence supporting this hypothesis in 12 species of nudibranch molluscs. Aposematic species were found on visual backgrounds that differed consistently in appearance from the visual background of undefended cryptic species. However, we found no difference in the variability of visual backgrounds between species with or without chemical defences. This suggests that warning colouration in eastern Australian Dorid nudibranchs coincides with broadly generalisable spatiochromatic properties of visual backgrounds.

ecology↗

bioRxiv2022

Constructed Synchronized Multimodal Signalling in Great Bowerbirds

Great bowerbird males build bowers for attracting females for mating. Bowers consist of a thatched twig tunnel (avenue) which opens onto two flat courts covered with objects. Male displays on a court are seen by a female from within the avenue. She sees and hears displays through the avenue entrance but can only see the males head and objects in his bill as it passes repeatedly across the entrance. We investigated bower acoustic properties by playing standard sounds from multiple court positions and recorded the resulting sounds at the females typical avenue head position within the avenue. Bower geometry significantly affects both his acoustic and visual display components and physically synchronizes them as he repeatedly moves in and out of the females view. Consequently, complex neural circuitry is unnecessary for linking sound to vision. Experimentally removing bower objects shows that objects significantly increase higher frequencies, hence bandwidth and loudness received inside the avenue. Great Bowerbird bowers produce a synchronized multimodal signal to females which may increase male attractiveness more than if a single sensory mode were used. This multimodal signal is unusual in that it is constructed rather than being part of the body and synchronized as a result of physics rather than neurons. Summary StatementBowerbird bower geometry jointly effects both visual and auditory signal components and synchronizes them without the need for additional neural circuitry.

animal behavior and cognition↗

bioRxiv2022

Colour pattern edge contrast statistics can predict detection speed and success at ecologically relevant viewing distances in triggerfish (Rhinecanthus aculeatus)

Edge detection is important for object detection and recognition. However, we do not know whether edge statistics predict the detection of prey by non-human predators. Understanding the link between image statistics and animal behaviour is crucial and of increasing importance given the growing availability of image analyses and their application across non-human visual systems. Here, we investigated whether Boundary Strength Analysis (BSA), Local Edge Intensity Analysis (LEIA) and the Gabor Ratio (GabRat) could predict the speed and success with which triggerfish (Rhinecanthus aculeatus) detected patterned circular stimuli against a noisy visual background, in both chromatic and achromatic presentations. We found that individual pattern statistics could only explain up to 2% of the variation in detection time, whereas PCA regression analysis considering all edge statistics simultaneously explained up to 6% of the variation. This suggests that other factors explained more behavioural variation than individual edge statistics. Furthermore, different statistics significantly correlated with detection speed depending on treatment, viewing distance, and changes in fish response over time, while highlighting the importance of considering spatial acuity and relevant viewing distances in the study of visual signals. Our results demonstrate the need for broad and unbiased approaches for identifying task-specific predictive relationships between pattern statistics and animal behaviour using image statistics capturing different aspects of colour patterns. We require robust statistical approaches to investigate correlations between ecological effect and the ever-increasing dimensionality and size of datasets in the field of visual ecology, rather than pre-emptively narrowing down the choice of image statistics unless warranted by specific hypotheses. Summary statementCorrelations between edge detecting colour pattern statistics and animal behaviour are complex. Specifically, correlations are unlikely to be explained by single image statistics and depend upon observer distance.

animal behavior and cognition↗

bioRxiv2021

The price of defence: toxins, visual signals and oxidative state in an aposematic butterfly

In a variety of aposematic species, the conspicuousness of an individuals warning signal and the quantity of its chemical defence are positively correlated. This apparent honest signalling is predicted by resource competition models which assume that the production and maintenance of aposematic defences compete for access to antioxidant molecules that have dual functions as pigments and in protecting against oxidative damage. We raised monarch butterflies (Danaus plexippus) on their milkweed host-plants (Apocynaceae) with increasing quantities of cardenolides to test whether (1) the sequestration of secondary defences is associated with costs in the form of oxidative lipid damage and reduced antioxidant defences; and (2) that reduced oxidative state can decrease the capacity of individuals to produce aposematic displays. In male monarchs conspicuousness was explained by an interaction between oxidative damage and sequestration: males with high levels of oxidative damage become less conspicuous with increased sequestration of cardenolides, whereas those with low oxidative damage become more conspicuous with increased levels of cardenolides. There was no significant effect of oxidative damage or concentration of sequestered cardenolides on female conspicuousness. Our results demonstrate a physiological linkage between the production of coloration and protection from autotoxicity, and differential costs of signalling in monarch butterflies.

evolutionary biology↗

bioRxiv2019

Quantitative Colour Pattern Analysis (QCPA): A Comprehensive Framework for the Analysis of Colour Patterns in Nature

O_LITo understand the function of colour signals in nature, we require robust quantitative analytical frameworks to enable us to estimate how animal and plant colour patterns appear against their natural background as viewed by ecologically relevant species. Due to the quantitative limitations of existing methods, colour and pattern are rarely analysed in conjunction with one another, despite a large body of literature and decades of research on the importance of spatiochromatic colour pattern analyses. Furthermore, key physiological limitations of animal visual systems such as spatial acuity, spectral sensitivities, photoreceptor abundances and receptor noise levels are rarely considered together in colour pattern analyses.\nC_LIO_LIHere, we present a novel analytical framework, called the Quantitative Colour Pattern Analysis (QCPA). We have overcome many quantitative and qualitative limitations of existing colour pattern analyses by combining calibrated digital photography and visual modelling. We have integrated and updated existing spatiochromatic colour pattern analyses, including adjacency, visual contrast and boundary strength analysis, to be implemented using calibrated digital photography through the Multispectral Image Analysis and Calibration (MICA) Toolbox.\nC_LIO_LIThis combination of calibrated photography and spatiochromatic colour pattern analyses is enabled by the inclusion of psychophysical colour and luminance discrimination thresholds for image segmentation, which we call Receptor Noise Limited Clustering, used here for the first time. Furthermore, QCPA provides a novel psycho-physiological approach to the modelling of spatial acuity using convolution in the spatial or frequency domains, followed by Receptor Noise Limited Ranked Filtering to eliminate intermediate edge artefacts and recover sharp boundaries following smoothing. We also present a new type of colour pattern analysis, the Local Edge Intensity Analysis (LEIA) as well as a range of novel psycho-physiological approaches to the visualisation of spatiochromatic data.\nC_LIO_LIQCPA combines novel and existing pattern analysis frameworks into what we hope is a unified, user-friendly, free and open source toolbox and introduce a range of novel analytical and data-visualisation approaches. These analyses and tools have been seamlessly integrated into the MICA toolbox providing a dynamic and user-friendly workflow.\nC_LIO_LIQCPA is a framework for the empirical investigation of key theories underlying the design, function and evolution of colour patterns in nature. We believe that it is compatible with, but more thorough than, other existing colour pattern analyses.\nC_LI

animal behavior and cognition↗

bioRxiv2018

pavo 2.0: new tools for the spectral and spatial analysis of colour in R

O_LIBiological colouration presents a canvas for the study of ecological and evolutionary processes. Enduring interest in colour-based phenotypes has driven, and been driven by, improved techniques for quantifying colour patterns in ever-more relevant ways, yet the need for flexible, open frameworks for data processing and analysis persists.\nC_LIO_LIHere we introduce pavo 2, the latest iteration of the R package pavo. This release represents the extensive refinement and expansion of existing methods, as well as a suite of new tools for the cohesive analysis of the spectral and (now) spatial structure of colour patterns and perception. At its core, the package retains a broad focus on (a) the organisation and processing of spectral and spatial data, and tools for the alternating (b) visualisation, and (c) analysis of data. Significantly, pavo 2 introduces image-analysis capabilities, providing a cohesive workflow for the comprehensive analysis of colour patterns.\nC_LIO_LIWe demonstrate the utility of pavo with a brief example centred on mimicry in Heliconius butterflies. Drawing on visual modelling, adjacency, and boundary strength analyses, we show that the combined spectral (colour and luminance) and spatial (pattern element distribution and boundary salience) features of putative models and mimics are closely aligned.\nC_LIO_LIpavo 2 offers a flexible and reproducible environment for the analysis of colour, with renewed potential to assist researchers in answering fundamental questions in sensory ecology and evolution.\nC_LI

animal behavior and cognition↗

bioRxiv2018

An Ishihara-style test of animal colour vision

O_LIColour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, even though animal visual systems differ from our own. Behavioural tests of non-human animals are required to understand how colour signals are perceived by them.\nC_LIO_LIHere we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. These charts consist of dots that vary in colour, brightness and size, and are designed so that a numeral or letter is distinguishable from distractor dots for humans with normal colour vision. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide Matlab code for creating these stimuli, which can be modified for use with different animals.\nC_LIO_LIWe demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, and highlight behavioural parameters that can be measured, including success of finding the target dot, time to detect dot and error rate. Triggerfish quickly learnt to select target dots that differed from distractors dots regardless of the particular hue or saturation, and proved to use acute colour vision. We measured discrimination thresholds by testing the detection of target colours that were of increasing colour distances ({Delta}S) from distractor dots in different directions of colour space. At least for some colours, thresholds indicated better discrimination than expected from the Receptor Noise Limited (RNL) model assuming 5% Weber fraction for the long-wavelength cone.\nC_LIO_LIThis methodology seems to be highly effective because it resembles natural foraging behavior for the triggerfish and may well be adaptable to a range of other animals, including mammals, birds, bees and freshwater fish. Other questions may be addressed using this methodology, including luminance thresholds, sensory bias, effects of sensory noise in detection tasks, colour categorization and saliency.\nC_LI

animal behavior and cognition↗

bioRxiv2018

Boundary Strength Analysis: Combining colour pattern geometry and coloured patch visual properties for use in predicting behaviour and fitness

Colour patterns are used by many species to make decisions that ultimately affect their Darwinian fitness. Colour patterns consist of a mosaic of patches that differ in geometry and visual properties. Although traditionally pattern geometry and colour patch visual properties are analysed separately, these components are likely to work together as a functional unit. Despite this, the combined effect of patch visual properties, patch geometry, and the effects of the patch boundaries on animal visual systems, behaviour and fitness are relatively unexplored. Here we describe Boundary Strength Analysis (BSA), a novel way to combine the geometry of the edges (boundaries among the patch classes) with the receptor noise estimate ({Delta}S) of the intensity of the edges. The method is based upon known properties of vertebrate and invertebrate retinas. The mean and SD of{Delta} S (m{Delta}S, s{Delta}S) of a colour pattern can be obtained by weighting each edge class{Delta} S by its length, separately for chromatic and achromatic{Delta} S. This assumes those colour patterns, or parts of the patterns used in signalling, with larger m{Delta}S and s{Delta}S are more stimulating and hence more salient to the viewers. BSA can be used to examine both colour patterns and visual backgrounds. BSA was successful in assessing the estimated conspicuousness of colour pattern variants in two species, guppies (Poecilia reticulata) and Gouldian finches (Erythrura gouldiae), both polymorphic for patch colour, luminance and geometry. The pattern difference between chromatic and achromatic edges in both species reveals the possibility that chromatic and achromatic edges could function differently. BSA can be applied to any colour pattern used in intraspecific and interspecific behaviour. Seven predictions and four questions about colour patterns are presented.

animal behavior and cognition↗