Hidden Social Networks in Butler's Gartersnakes: Insights from a 12-Year Field Study

When people think of social animals, they usually picture birds, mammals, or even insects that live in visible groups. These animals often gather in large numbers and may even possess structured societies with divisions related to age or sex.

Snakes, in contrast, have long been regarded as solitary creatures. Their social interactions are difficult to observe, and their behavior is often subtle and concealed. Because of these factors, together with longstanding stereotypes, many people have assumed that snakes are cold-blooded animals lacking meaningful social behavior.

Photograph of Thamnophis butleri （圖片來源：Caudatejake ，採用 CC BY-SA 4.0 授權。）

However, a study published in 2023 fundamentally challenged this assumption. The research followed Butler's gartersnake (Thamnophis butleri) populations for twelve years and revealed a previously unrecognized pattern of social organization.

Although studies of snake sociality remain relatively rare, several examples already hint at more complex behavior than previously assumed. For instance, the Southern African python (Python natalensis) shows parental care, the Indian python (Python molurus) can form dominance hierarchies, the timber rattlesnake (Crotalus horridus) uses shared information about the environment, and the Taiwanese kukri snake (Oligodon formosanus) displays territorial tendencies.

The Formosa kukri snake, also known as the Taiwanese kukri snake (Oligodon formosanus)（圖片來源：台灣生物多樣性網絡，採用 CC BY 4.0 授權。）

Against this background, Butler's gartersnake became an especially valuable study species because its home range is relatively small, making individuals easier to track in the wild.

Butler's gartersnake is a small to medium-sized diurnal snake distributed across parts of the northern United States and southern Ontario in Canada. The species exhibits sexual size dimorphism, with females typically larger than males. Its diet consists mainly of earthworms, although leeches are occasionally consumed as well.

During winter, these snakes enter a communal dormant state similar to hibernation, known as brumation. After emerging in spring, they mate in groups. Females later give birth to live young during summer or early autumn, but they do not provide parental care afterward.

The long-term research project that eventually revealed their social structure actually began as a conservation effort. Near Windsor, Ontario, roughly two kilometers from the Detroit River, a road construction project was planned that could threaten a local population of Butler's gartersnakes.

To monitor the potential impact, researchers began studying the population in 2009. From 2009 to 2020, they monitored snakes across an area of roughly 250 hectares.

Between 2011 and 2015, when road construction was underway, temporary fences were installed along the construction zone to prevent snakes from entering the work area. Any individuals captured near the site were relocated about fifty meters away. After construction was completed, permanent barriers replaced the temporary fences, and monitoring continued outside the road corridor.

Over the twelve-year study period, researchers recorded a total of 3,801 individual Butler's gartersnakes. Of these, 3,463 individuals met the criteria for complete analysis, including 53.3% females and 46.7% males.

Whenever a snake was captured, researchers recorded basic information such as body length, mass, age, and sex. Each snake was then marked with an identification tag so it could be recognized if captured again. After processing, the animals were released back into the environment.

To reconstruct potential social relationships, the researchers treated each individual snake as a node within a social network. If two snakes were captured near the same place and around the same time, they were considered likely to have interacted or shared space.

Two thresholds defined these potential connections: the snakes had to be within 50 meters of each other spatially and within 14 days temporally. If two individuals were found at exactly the same location on the same day, the probability of association was treated as 100%. As the spatial or temporal distance between captures increased, the probability of association decreased. Once the threshold limits were exceeded, the snakes were considered unconnected.

Using these rules, the researchers constructed a detailed social network for the population.

The analysis revealed a striking pattern. Butler's gartersnakes tended to interact preferentially with individuals of similar age and sex. Snakes of the same age group were more likely to appear together, and females in particular often associated with other females.

An especially interesting pattern emerged with age. As female snakes grew older, they became increasingly social, showing more frequent associations with other individuals. Males showed the opposite trend: older males became less socially connected and tended to avoid interactions.

These findings matched recent laboratory experiments suggesting that these sex-specific differences may arise from innate behavioral tendencies or learned social strategies during development.

Researchers also discovered that younger snakes often followed older individuals. Evidence suggested that juvenile snakes rely on chemical cues left behind by adults when selecting shelter sites.

For young snakes, following adults may reduce the risks associated with exploring unfamiliar environments and increase the likelihood of locating safe refuges. In this way, adult individuals effectively act as hubs within the social network, linking different generations even without direct cooperation.

Further analysis revealed a connection between health and social connectivity. Snakes that were integrated into the social network generally had better physical condition than individuals that remained isolated.

Group living may offer several advantages. These include improved thermoregulation, reduced risk of predation, decreased water loss, and access to environmental information by observing other snakes. At the same time, living near conspecifics may also impose costs, such as competition for resources or increased exposure to parasites and disease.

Ultimately, the study highlights that even animals traditionally regarded as solitary and difficult to observe can possess surprisingly complex social relationships.

From a conservation perspective, understanding these hidden social networks is important. Knowledge of social structure can improve assessments of population health, clarify patterns of disease transmission, and guide more effective conservation strategies.

Future research expanding on this approach may reshape how scientists understand snakes and other animals that have historically been labeled as non-social. As these hidden networks come to light, they may become an increasingly important consideration in wildlife management and conservation planning.

Author: Bai Leng

Reference:

Skinner, M., Hazell, M., Jameson, J., Lougheed, S. C. (2023). Social networks reveal sex- and age-patterned social structure in Butler's gartersnakes (Thamnophis butleri). Behav Ecol.