Most animals that carry chemical weapons make their own. Venomous spiders produce their toxins in dedicated glands. Poison dart frogs synthesize compounds in their skin. But a group of snakes in Asia has taken a strikingly different approach: they simply steal their defenses from the animals they eat.
The rednecked keelback (Rhabdophis subminiatus), a slender, modestly-sized snake found across Indonesia, does not produce a single molecule of its own toxin. Instead, when it swallows a toad, it extracts cardiotonic steroids — compounds called bufadienolides — from the toad’s skin and shuttles them into a row of specialized sacs embedded just beneath the skin of its neck. These organs, called nuchal glands, can rupture under pressure, releasing a yellowish, foul-smelling fluid that can deter or disorient a predator.
It is a remarkable strategy, but it raises an immediate question: does the snake know how much toxin it has stored? Can it tell whether its chemical arsenal is full or empty?
A new study published in the journal Ethology may have an answer to that question. Despite being experimentally drained of their toxin reserves, the snakes showed no meaningful change in defensive behavior. They continued to perform the same threat displays, at roughly the same frequency, as if their glands were still full. The finding, the researchers say, suggests that rednecked keelbacks do not directly monitor the contents of their own nuchal glands.
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It helps to know what these glands actually do in a confrontation
When a predator grabs or pins the snake, the pressure ruptures the glands, releasing the stored bufadienolides directly onto the attacker. But the snakes also perform a set of elaborate physical displays designed to make the most of their chemical weaponry. They flatten their necks, arch their heads sharply downward so the snout almost touches the ground, and sometimes swing the back of the neck forcefully against a threat — a behavior called neck butting. These movements are thought to help position and expose the glands for maximum effect. The snakes can even release the fluid voluntarily.
Tomonori Kodama — a biologist at the National Research and Innovation Agency of Indonesia — and colleagues collected 23 juvenile and adult rednecked keelbacks from West and Central Java. To eliminate the effects of any recently consumed toxins, snakes that had been fed toads were kept for at least a month before the experiments began. All snakes were then fed only frogs — not toads — during the study period, ensuring that no new toxins would be sequestered.
Each snake was tested twice. In the first trial, its nuchal glands were intact and full. Researchers placed the snake in an arena and used a padded metal hook to gently pin it — simulating the kind of contact a predator might make — 18 times over the course of a session, recording the snake’s behavioral response to each prod. Then they extracted the contents of the nuchal glands by pressing forceps against each sac while placing a tissue over the neck to absorb the fluid. After waiting four to seven days, just long enough for the handling stress to subside but not long enough for any toxin replenishment (which requires eating another toad), they ran the test again.
That the extraction actually worked was confirmed in two ways. First, the fluid collected during the second extraction was colorless or red rather than the characteristic yellowish color of toxin-laden fluid — the yellow pigment comes from the bufadienolides themselves. Second, among the 17 snakes for which measurements were possible, the mass of extracted components dropped sharply between the first and second sessions, from an average of about 8.6 milligrams down to just over 1 milligram.
This means that the snakes were genuinely depleted. What they did not do was act like it.
Across all seven behaviors recorded — biting, fleeing, freezing, body flattening, neck flattening, neck arch, and neck butting — no statistically significant differences emerged between the full-toxin and toxin-depleted conditions. One behavior, the neck arch, showed a slight (but not statistically meaningful) tendency to increase after depletion, which was actually the opposite of what would be expected if the snakes were modulating their behavior in response to lower toxin levels. The researchers say this finding was consistent even when they reanalyzed the data using only the snakes that had carried the most toxin to begin with, ruling out the possibility that a few low-toxin individuals were skewing the result.
This places the rednecked keelback in an interesting contrast to a close relative. An earlier study on the tiger keelback (Rhabdophis tigrinus) — a species with nearly identical nuchal gland anatomy — found that individuals raised on toads were far more likely to use their nuchal gland displays, while those fed on non-toxic prey like fish and frogs tended simply to flee. That research suggested the tiger keelback has some awareness of its own chemical status. But crucially, that awareness appeared to be tied to the snake’s feeding history — whether it had recently eaten toads — rather than to any direct internal check on what was actually sitting in the glands.
The researchers suggest this distinction matters. The rednecked keelback, their data imply, may work the same way: it infers something about its toxicity from whether it has recently eaten toads, not from any real-time monitoring of gland contents. They say that “Rhabdophis species might indirectly recognize toxin reserves in nuchal glands through factors related to actual toad-feeding, such as feeding experience and physiological processes, rather than by directly monitoring gland contents.”
Why would a snake not evolve the ability to check its own glands? Each rednecked keelback carries between 12 and 17 pairs of nuchal glands. Completely emptying all of them in a single predator encounter would be very difficult — an attacker would have to rupture every one. The researchers note that “complete depletion would be unlikely in natural conditions, even after several predator encounters,” and that “recognition of feeding history on toads may provide sufficient information for adaptive decision-making, and precise toxin metering may not be required.”
There is also another possibility that could relax the need for precise monitoring. The nuchal gland fluid carries a powerful, disagreeable odor — during the study, the smell was detectable from about 1.5 meters away when snakes voluntarily expelled it. The researchers raise the possibility that “if the deterrent effect of the nuchal gland secretion partly depends on its strong odor or on the display itself, rather than solely on toxin concentration, natural selection for precise regulation of gland use may be further relaxed.” Even a mostly depleted set of glands might still make a predator hesitate if the residual scent is strong enough.
“Toxin recognition ability may have evolved only in some lineages,” they write, suggesting that the capacity for chemical self-monitoring may be a derived feature — one that appeared in certain branches of the family tree and not others.
