The research, led by Julieta Nattero of the Universidad de Buenos Aires and CONICET together with colleagues including Delfina Trezza-Neumayer, Federico G. Fiad, Ricardo E. Gürtler, and María S. Gaspe, set out to answer four linked questions. Does the frequency of the melanic form change along a gradient running from city to countryside? Do melanic bugs cluster in space? Do the two color forms differ in traits linked to flight and dispersal? And which features of the surrounding landscape best predict where melanic individuals turn up in and around human dwellings? The answers carry practical weight, because understanding what drives house reinfestation is critical for the final push against a disease that still burdens millions of people in Latin America.

To do this, the team assembled an unusually detailed dataset. They collected 901 adult Triatoma infestans from 67 infested houses spanning urban, peri-urban, and rural environments in Avia Terai. Rather than relying on subjective visual judgments of color, the researchers quantified dorsal coloration using red, green, and blue pixel metrics taken from two body regions: the forewing and the connexivum, the flattened lateral margin of the abdomen. They then used K-means clustering, an unsupervised machine learning method, to sort individuals into chromatic groups objectively. This quantitative approach matters because color variation in insects can be subtle, and consistent classification is the foundation for every downstream analysis.

The first striking result is rarity combined with strict environmental restriction. Melanic individuals made up just 16.98 percent of the bugs analyzed, and they were found only in rural environments, never in peri-urban or urban houses. That pattern suggests the dark morphotype is tied to particular ecological conditions that exist in the countryside but not in more built-up settings. Whether those conditions involve temperature, humidity, background substrates, or something else remains an open question, but the urban-to-rural gradient clearly acts as a filter on this phenotypic variation.

Color, however, is only one part of the story. The team also examined whether melanic and non-melanic bugs differ in traits related to dispersal, a key question because flying adults are thought to be important agents of reinfestation after insecticide campaigns. Using geometric morphometrics on the forewings, a technique that captures size and shape variation independent of overall body size, the researchers found that wing size and shape were broadly similar between the two morphotypes. The single exception was male wing shape, which differed between color forms. Flight muscle development, assessed directly, did not differ between morphotypes or between sexes, indicating that darker bugs are not inherently better or worse fliers by this measure.

One dispersal-related trait did stand out. The researchers calculated a dispersal index based on the ratio between the membranous and stiff portions of the forewing, a morphological feature that in other insects correlates with flight capacity. Non-melanic individuals had higher values of this index than melanic ones. Taken together with the similar flight muscle development, the result paints a nuanced picture: the color morphotypes are not simply fast and slow dispersers, but subtle differences in wing architecture hint that the two forms may move through the landscape in different ways, with consequences for how quickly they can colonize newly sprayed houses.

The spatial analysis delivered perhaps the most consequential finding. Using Moran’s I, a standard statistic for spatial autocorrelation, and Local Indicators of Spatial Association, or LISA, which pinpoint where clusters of similar values occur, the team detected significant positive spatial autocorrelation in melanic frequency at distances of 2,500 meters, a pattern that persisted out to 5,000 meters. In practical terms, houses with melanic bugs are not distributed at random across the map. Instead, they form localized hotspots where the dark morphotype occurs at unusually high frequency, and these hotspots are large enough to span several kilometers. Such spatial structure implies that the processes producing melanic bugs, whether local population persistence, restricted gene flow, or shared environmental conditions, operate at landscape scales rather than house by house.

What landscape features explain those hotspots? To find out, the researchers calculated landscape metrics within eight concentric buffers around each dwelling, ranging from 200 to 3,200 meters in radius. This multi-scale design allowed them to identify the scale of effect, the spatial extent at which the surrounding environment most strongly influences the phenomenon of interest. They then modeled melanic frequency using negative binomial generalized linear models combined with multi-model inference based on Akaike’s Information Criterion corrected for small samples, a rigorous framework for comparing competing explanations while accounting for overdispersed count data.

The models converged on a clear answer. Landscape metrics measured at buffers of 800 to 1,000 meters best predicted the frequency of melanic individuals, and higher values of those metrics, associated with increased landscape connectivity, corresponded to more melanic bugs. In other words, dark-colored Triatoma infestans are most frequent around houses embedded in well-connected rural landscapes, where patches of suitable habitat are linked in ways that may facilitate bug movement and population persistence. The identification of this specific scale of effect gives vector control programs a concrete spatial radius to consider when assessing reinfestation risk around treated houses.

The broader significance of the study lies in its integration of phenotypic, spatial, and landscape perspectives. Chromatic dimorphism in Triatoma infestans was first reported in sylvatic populations, bugs living in wild habitats away from houses, and later documented in domestic and peridomestic settings, but its ecological meaning has remained murky. By showing that the melanic morphotype is spatially aggregated, environmentally restricted, and landscape-dependent, the new work suggests that color variation is not mere noise. It appears to reflect genuine ecological differentiation tied to the settings where bug populations persist and from which they reinvade treated houses. If melanic bugs mark populations with particular spatial dynamics, then their presence could serve as a phenotypic signal of reinfestation risk, helping surveillance teams prioritize villages and households for follow-up inspection and spraying.

The study also underscores a methodological lesson for disease ecology more generally. Phenotypic variation in disease vectors is often studied at the level of individual insects or single houses, yet the patterns documented in Avia Terai emerge only when the surrounding landscape and spatial structure are taken into account. As the authors conclude, understanding phenotypic variation in Triatoma infestans requires integrating landscape context and spatial structure, because the occurrence of the melanic morphotype is associated with particular ecological settings and spatial dynamics that influence persistence and reinfestation. For a vector that continues to infest houses despite decades of control efforts, every clue about where and why certain bug populations thrive is a clue about how to stop them, and in the dark wings of these rural hotspots, researchers have found a marker written in color that points toward the landscapes where the battle against Chagas disease is hardest to win.

Subject of Research: Spatial distribution and landscape associations of melanic Triatoma infestans, the Chagas disease vector, along an urban-to-rural gradient in Argentina

Article Title: Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient

Article References: Nattero, J., Trezza-Neumayer, D., Fiad, F. G., Gürtler, R. E., & Gaspe, M. S. (2026). Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07712-7

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07712-7

Keywords: Triatoma infestans, Chagas disease, melanism, chromatic dimorphism, spatial autocorrelation, landscape connectivity, geometric morphometrics, vector control, Argentina, Parasites & Vectors, reinfestation, flight traits

Despite its name, the eastern grey squirrel, known scientifically as Sciurus carolinensis, does not always wear a grey coat. Individuals of the species can appear in a black, or melanic, form, and the relative abundance of these two colour morphs has long intrigued biologists. In many North American cities, black squirrels are a familiar sight, and previous work on urban wildlife has often treated cities as broadly uniform environments, comparing animals living in urban cores with those in surrounding rural areas. The Montreal study challenges that framing. Rather than asking whether urbanization alone explains coat colour, the researchers asked whether conditions differing from one neighbourhood to the next, and even from one side of a river to the other, leave a measurable signature in the squirrel populations that live there.

The data behind the study came from an unusually direct and low-tech source: bicycle wheels and patient observation. Francis Quang-Hai Dinh, the study’s first author, was an undergraduate student at McGill when he began the fieldwork. During his first summer at the university, he visited almost 200 parks around the city in his free time, cycling between them and recording what he saw. In total, he observed more than 800 squirrels, noting which individuals were grey and which were black. This effort was complemented by park surveys and citizen science observations, allowing the team to assemble a fine-scale map of squirrel colouration across a complex urban landscape. The project has since been included in a special feature published by five ecology journals highlighting how researchers are using large, open-access datasets and collaborative approaches to address major ecological questions.

The analysis revealed that grey coats predominate across Montreal, as they do in most of the species’ range. But the proportion of black squirrels was not distributed randomly. It was highest in parts of the city with cooler winter temperatures and greater forest canopy cover. This pattern is consistent with the idea that darker coats may confer a biological advantage in colder conditions, because dark fur can retain heat more effectively, an advantage that becomes meaningful during long northern winters. In shaded, leafy neighbourhoods where winter temperatures drop, black squirrels appear to fare relatively better, and their colouration becomes more common in the population.

Perhaps the most surprising finding, however, was what the researchers did not see. Contrary to patterns reported in many other cities, the prevalence of black squirrels in Montreal was generally lower in more heavily urbanized areas. In several urban studies elsewhere, melanic squirrels have been found at elevated frequencies in dense city centres, and explanations have ranged from thermoregulation to the behaviour of predators and people. The Montreal result runs against that expectation, and the researchers themselves note that this intriguing outcome will require future examination. The finding serves as a reminder that urban patterns observed in one city cannot simply be assumed to hold in another, and that the drivers of adaptation may interact with local geography and climate in ways that vary from place to place.

Coat colour was not the only factor at work in shaping the city’s squirrel populations. The study found that squirrel colouration across the Montreal region is influenced not only by urbanization but also by features of the physical landscape, including forest canopy cover and the presence of the St. Lawrence River and its tributaries. Rivers can act as barriers to movement, and for an animal that travels on foot and by branch, a wide waterway can effectively divide a population into distinct groups. Over time, such separation can allow different populations to diverge, with local conditions favouring different traits on either side of the barrier. In this way, the geography of a city, its rivers, its parks, and its tree cover, can create a mosaic of subtly different selective environments within a single metropolitan area.

The implications extend well beyond squirrels. The findings highlight how wildlife can respond to environmental conditions at a very local scale, showing that cities are not uniform habitats but complex landscapes in which differences between neighbourhoods, as well as geographic features such as rivers, can influence how animals adapt and evolve over time. For evolutionary biologists, this is a significant reframing. Classic studies of urban adaptation often rely on broad urban-to-rural gradients, treating the city as one end of a line and the countryside as the other. The Montreal work suggests that such coarse comparisons may miss much of the story. Two parks a few kilometres apart, separated by a river or differing in canopy cover, may host populations experiencing meaningfully different selective pressures.

Allison Roth, the study’s senior author, who was a GRIL postdoctoral fellow in McGill’s Department of Biology during the data collection and is now an Assistant Professor at the University of Missouri, emphasized this point. People often think of cities as a single type of environment, she noted, but the findings show that the conditions animals experience can vary dramatically from one neighbourhood to another. These local differences, along with barriers such as rivers, can shape how wildlife populations evolve, which means, she argued, that researchers need to look beyond simple urban-rural comparisons to understand adaptation in cities. The statement captures a growing recognition in urban ecology that the city is best understood not as a single habitat type but as a patchwork of micro-environments, each with its own temperature profile, vegetation structure, and connectivity.

The study also offers a case study in how modern ecological research can be conducted. Dinh’s fieldwork was carried out on a bicycle, in his free time, during a single summer, supplemented by observations contributed by members of the public through citizen science platforms. The project demonstrates how discoveries can be accessible, as Dinh put it, and that science can be done in one’s own backyard. The integration of personal field observation with open-access citizen science data reflects a broader trend in ecology, in which large collaborative datasets allow researchers to detect patterns that would be invisible to any single observer working alone. The special feature of five ecology journals in which the study appears celebrates exactly this kind of approach, showcasing how open data and teamwork are being used to tackle big ecological questions.

For city dwellers, the research adds a layer of meaning to an everyday encounter. The black squirrel crossing a path in a leafy park and the grey squirrel foraging in a sparser, warmer neighbourhood are not merely variants of the same animal; they are, in a sense, records of local evolutionary history, shaped by winter temperatures, tree cover, and the rivers that carve the city into distinct territories. As urban areas continue to expand and climates continue to change, understanding how animals adapt at these fine scales will become increasingly important for conservation and city planning alike. The Montreal squirrels suggest that the answers may lie not in sweeping urban-rural contrasts but in the details: the shade of a particular park, the chill of a particular valley, and the width of the water that separates one population from the next.

Subject of Research: Fine-scale spatial variation in melanism of eastern grey squirrels across an urban landscape

Article Title: Montreal’s squirrels provide insight into evolutionary change in urban species

Article References: Montreal’s squirrels provide insight into evolutionary change in urban species. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: eastern grey squirrel, melanism, urban evolution, Montreal, coat colour, forest canopy, urban ecology, citizen science, Journal of Animal Ecology, St. Lawrence River, thermoregulation, local adaptation