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Larger Grass Snakes Took Larger Prey, While Fish Were Proportionally Larger Than Frogs

Grass snake holding a common toad as prey
Research · Ecology

Larger grass snakes (Natrix natrix) contained larger prey in a small historical dataset from Plovdiv, Bulgaria, but the relationship differed between fish and frogs. The study, published in 2026, analysed previously unpublished measurements from snakes collected on 21–28 August 1974.

Forty snakes had originally been examined, but one had an empty stomach and was excluded. Of the 39 snakes with prey, 24 had eaten fish and 15 had eaten frogs. In both groups prey length increased with snake length, showing size-dependent scaling between predator and prey. The relationship was much tighter for frogs: snake length explained considerably more of the variation in frog size than in fish size.

Fish were proportionally larger prey than frogs

The researchers also calculated relative prey size by dividing prey length by snake length. This asks not simply whether a large snake ate a long prey item, but how large that prey was in relation to the predator. By this measure, fish were proportionally larger prey than frogs.

The two findings therefore describe different aspects of the same feeding pattern. Frog size tracked snake size more closely, whereas fish tended to occupy a larger fraction of the snake’s body length. The author interprets this as evidence that prey type changes the size constraints on feeding.

One possible explanation is that fish body shape, swimming ability and capture in water loosen the relationship between predator length and prey length, while robust amphibian bodies may impose different swallowing constraints. These mechanisms were not measured directly in the 39 snakes, so they remain interpretations of the observed size relationships rather than tested causes.

Sources and image credit

  1. Mollov, I. (2026). Prey type influences predator–prey size relationship in Natrix natrix (Linnaeus, 1758) (Reptilia: Colubridae). ZooNotes, 299, 1–4. https://doi.org/10.69085/zn20260299Primary study published 28 May 2026. The analysis used previously unpublished measurements from grass snakes collected in Plovdiv on 21–28 August 1974; 39 snakes with prey measurements showed a positive predator–prey size relationship, stronger for frogs, while fish were proportionally larger prey.

Image: Aleksandr Abrosimov / Wikimedia Commons — grass snake consuming a common toad in Estonia, photographed 19 May 2016; representative feeding image, not a snake from the Plovdiv dataset · CC BY-SA 4.0 · Source

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