Assistant Professor SAWADA Kiyoto, Institute of Life and Environmental Sciences

Many people feel afraid when they see snakes. However, Professor SAWADA says, "If you see a snake, please be happy that the area has a rich environment with diverse living organisms." As top predators in the food chain, snakes cannot survive without a natural habitat that provides sufficient and varied prey.

How do different snake species coexist? Professor SAWADA has revealed the coexistence mechanism through field research on Sado Island in Niigata Prefecture. For snakes, avoiding overlap in prey targets is considered essential to species coexistence. However, he demonstrated that differences in activity location, timing, and season are also crucial factors.

Eight snake species are found in Honshu, Japan. Seven of these species, namely Elaphe climacophora, Elaphe quadrivirgata, Euprepiophis conspicillatus, Lycodon orientalis, Gloydius blomhoffii, Hebius vibakari, and Rhabdophis tigrinus, are resident on Sado Island. This is the highest number of snake species of any Japanese island outside the subtropical Ryukyu Archipelago. Sado Island spans 855 km² (approximately 1.5 times the size of Tokyo's 23 wards). The S-shaped island hosts rich biodiversity, with the Osado Mountains in the north, the Kosado Mountains in the south, and the Kuninaka Plain (a key granary) stretching between them.

From 2019 to 2023, while a graduate student at the University of Tsukuba, Professor SAWADA visited the site from March to July and September to November annually. He searched for snakes across diverse environments, from mountains to lowlands, and forests to rice paddies. Because snakes hibernate, the winter months were excluded from the survey period. When snakes were found, they were captured using nets or other tools, and the date, time, and location were recorded. Their stomach contents were also examined by gently pressing their abdomens to induce regurgitation.

Each survey lasted 4-6 hours, totaling 185 daytime surveys and 80 nighttime surveys. In total, 564 snakes were captured. Professor SAWADA says with a smile, "I can now recognize the sound of snakes sliding on the ground, and I can even distinguish seven species by their smell. The eyes of R. tigrinus are so cute."

The survey results grouped the seven snake species into three dietary categories. Those that fed mainly on rodents, such as rats, included E. climacophora, E. conspicillatus, and G. blomhoffii. Hebius vibakari primarily consumed earthworms, whereas E. quadrivirgata and R. tigrinus mainly ate frogs. The diet of L. orientalis remained unknown, although it is believed to feed on reptiles.

Activity locations fell into two groups: species mainly in lowlands (E. climacophora, E. quadrivirgata, E. conspicillatus, H. vibakari, and R. tigrinus) and those found even in mountainous areas (G. blomhoffii and L. orientalis). Activity time was divided into three categories: strictly diurnal (E. climacophora, E. quadrivirgata, and R. tigrinus), nocturnal (L. orientalis), and active during both day and night (E. conspicillatus, G. blomhoffii, and H. vibakari). Seasonal activity was grouped into three: active from spring to autumn (E. climacophora, E. quadrivirgata, E. conspicillatus, and R. tigrinus), mainly active in summer (G. blomhoffii), and mostly active in autumn (H. vibakari and L. orientalis).

These findings show that species with a large dietary overlap tend to show less overlap in where and when they are active. For example, among the three rodent-eating species, only G. blomhoffii mainly inhabited mountainous areas. The habitats of E. climacophora and E. conspicillatus overlapped in the lowlands. However, E. climacophora was fully diurnal, whereas E. conspicillatus was active both day and night and more active in autumn compared with E. climacophora. Notably, E. quadrivirgata and R. tigrinus shared similar diets, locations, seasonal timing, and activity periods, but R. tigrinus showed increased activity in autumn.

The range of environmental conditions and resources that support a species' population is called its ecological niche. Professor SAWADA's research revealed that snakes on Sado Island coexist through "multidimensional niche partitioning," where species differ in multiple factors, e.g., diet, time and place of activity, and season. He explains, "Although the results may seem obvious, no comprehensive study has ever examined niche partitioning across the three major resources (food, time, and habitat) in terrestrial snakes. Demonstrating niche partitioning with empirical data is highly valuable."

Snake populations worldwide are reportedly declining in many regions due to habitat loss and other pressures. Based on this survey's findings, conserving snakes requires protecting diverse environments that support niche partitioning.

Professor SAWADA began his research on Sado Island because of his interest in the poisonous snake R. tigrinus. On mainland Japan, this species feeds on poisonous toads and stores the toad's toxins ("nuchal gland toxin") in the back of its head (neck). When threatened by predators, such as weasels, R. tigrinus, displays its neck as a warning and can release the toxin to defend itself. However, there were originally no toads on Sado Island, and R. tigrinus on the island did not exhibit these defensive behaviors. In the 1960s, the Eastern-Japanese common toad was introduced to the island and became established in the southwestern region.

Professor SAWADA hypothesized that R. tigrinus feeding on toads on Sado Island might acquire nuchal gland toxin and use it to deter predators. This led him to conduct field research on the diet and behavior of R. tigrinus, which later expanded into a study of all seven snake species on the island and resulted in the current findings.

The survey also showed that R. tigrinus living alongside introduced toads had acquired nuchal gland toxin and displayed defensive behaviors. In contrast, those living apart from these toads lacked the toxin and often fled from predators. Professor SAWADA notes, "If predators avoid R. tigrinus because of its toxin and begin preying on other snake species, it could destabilize the ecosystem.

Professor SAWADA now plans to explore how niche partitioning among snakes supports ecosystem stability, and new discoveries are anticipated.

Article by Science Communicator at the Bureau of Public Relations