Pit vipers may be able to sense how much venom they have in reserve to deal with a threat, a study suggests.
Previous research indicates that venomous animals, including spiders, scorpions and snakes, use their precious venom frugally, essentially metering it out when they are hunting prey. However, little was known about the possibility of doing so in other contexts, such as self-defence.
and colleagues at the Chinese Academy of Sciences in Chengdu have now shown that sharp-snouted pit vipers (Deinagkistrodon acutus), which are notoriously aggressive, appear to behave differently when threatened depending on their venom levels.
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鈥淭his conclusion makes considerable intuitive sense since many other venomous organisms are understood to 鈥榤eter鈥� their venom,鈥� says at the University of Melbourne, Australia. 鈥淚 applaud the authors 鈥� it is one thing to predict a phenomenon and quite another to demonstrate its existence in nature.鈥�
The team housed 23 juvenile snakes in separate boxes, dividing them into three groups defined by venom levels: low, replenishing and normal.
The low and normal venom groups followed a three-day testing cycle over 24 days. For the first two days of each cycle, the team enticed individuals in the low venom group to bite into a cloth once each day to ensure they had little venom. The normal venom group experienced a blank test, during which they were gently poked instead. Every third day, all snakes underwent a behavioural trial.
The replenishing venom group initially followed the same pattern as the low venom group. But for the final three testing cycles, the researchers gave these snakes four days of respite after each behavioural trial to allow them to replenish their venom.
During the behavioural trials, each snake was provoked for a minute using a piece of medical silicone skin at the end of a snake hook. Snakes responded by sounding a warning, fleeing or striking and injecting the silicone skin with venom.
Snakes with normal venom reserves performed more strikes overall. Conversely, low-venom snakes showed more fleeing behaviour. The venom-replenishing vipers became more willing to strike in the second half of the experiment, when they were given more time to build up venom reserves.
The researchers argue this suggests that the snakes can sense their venom levels and tailor their response to a threat accordingly.
鈥淭his carefully designed study certainly provides a lot of thought-provoking evidence,鈥� says at the University of Porto, Portugal. However, he cautions that fatigue, habituation or hunger could also be factors in explaining the results.
The researchers are confident the experiments ruled these out. 鈥淪nakes were tested only once a day, so they got nearly 24 hours to relax, and snakes鈥� responses showed little sign of habituation,鈥� says Piao, adding that there were enough 鈥渄ry鈥� bites and other defensive moves to suggest strikes were non-predatory.
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