Colour scales with climate in North American ratsnakes: a test of the thermal melanism hypothesis using community science images

Abstract

Animal colour is a complex trait shaped by multiple selection pressures that can vary across geography. The thermal melanism hypothesis predicts that darker coloration is beneficial to animals in colder regions because it allows for more rapid solar absorption. Here, we use community science images of three closely related species of North American ratsnakes (genus Pantherophis ) to examine if climate predicts colour variation across range-wide scales. We predicted that darker individuals are found in colder regions and higher elevations, in accordance with the thermal melanism hypothesis. Using an unprecedented dataset of over 8000 images, we found strong support for temperature as a key predictor of darker colour, supporting thermal melanism. We also found that elevation and precipitation are predictive of colour, but the direction and magnitude of these effects were more variable across species. Our study is the first to quantify colour variation in Pantherophis ratsnakes, highlighting the value of community science images for studying range-wide colour variation.

Temporal stability of the hybrid zone between Calocitta magpie‐jays revealed through comparison of museum specimens and iNaturalist photos

“…Finally, our study demonstrates how citizen science data associated with photo‐vouchers can be used to investigate evolutionary patterns such as temporal changes across zones of phenotypic transition. While discrete throat color differences offer a particularly amenable example, recent studies demonstrate that other more nuanced aspects of organismal coloration can also be quantified from citizen science photos (Hantak, Guralnick, Cameron, et al, 2022; Laitly et al, 2021), allowing for more varied study of hybrid zones and geographic variation in general. Photos are not replacements for museum specimens, as they capture only one axis of biological variation, whereas specimens allow study of internal anatomy, parasites, and genetics (Rocha et al, 2014), and open the door to future techniques that have not yet been developed.…”

Integrating Genomics, Collections, and Community Science to Reveal Speciation in a Variable Monitor Lizard (Varanus tristis)

“…The precise location of threatened taxa is often obscured, most observations are restricted to the Global North (Rosa et al 2022), and the processing of a large number of records can pose practical challenges. However, these limitations can be ameliorated by putting procedures in place to facilitate access to obscured metadata by researchers, using observations posted on social media (Chowdhury et al 2023a), and by the implementation of automated image processing workflows (de Solan 2020; Hantak et al 2022) (see below), respectively.…”

“…This kind of algorithms can also be used to extract phenotypic data from photographs obtained under different conditions, as is the case of community observations (de Solan 2020; Hantak et al 2022). Machine learning can then be used to analyze these data solely or jointly with molecular data to assign them to known species with great accuracy (Wäldchen and Mäder 2018; Valan et al 2019; Yang et al 2022).…”

“…Our analyses were thus aimed at testing whether variation in the other putative species is indicative of gene flow with the former or is instead reflecting local adaptation or plasticity within the latter. To test this, we fitted Bayesian generalized nonlinear regression models in the 'brms 2.19.0' R package (Bürkner 2017), implementing a modified version of the approach described by Hantak et al (2022). Color was indicated as a response variable under a Bernoulli distribution; i.e., we modelled the probability of being light-colored.…”

“…Community observations—with iNaturalist (https://www.inaturalist.org/) as their best known repository—have been mainly leveraged in the context of conservation (McKinley et al 2017; Chowdhury et al 2023b). However, these observations also convey important information about the geographic and temporal distribution of morphological variation (Drury et al 2019; Hantak et al 2022). In that way, community science can provide important information about evolutionary processes and inform taxonomy (Fritz and Ihlow 2022; Fritz et al 2023; Pizarro et al 2023).…”