In a new study, researchers compared the features of 30 different stingray spines to find out why some break off while others remain sturdy
Scientists have gained a fascinating insight into how stingrays’ iconic spines differ across species and the trade-off between different features.
“Stingray spines are highly modified scales,” says Emily Poulin, a PhD candidate at UC Irvine.
Found on the tail of the stingray, these spines are used for protection when the ray feels under threat. “When they use these spines, they puncture them really deeply into their attacker,” she says. “They can deliver venom into the wound.”
People often assume that stingrays might attack humans with their venomous tail barb but they are purely for self-defence. “They use their spines as a last resort to protect themselves,” says Poulin. “Usually, they would rather make an escape, if possible.”
Because the rays wield these weapons when being attacked, it’s vital that they work. “It's life or death if these tools are effective,” she says.
Poulin and colleagues had noticed that some stingrays have spines that seem to have evolved to break off – “you'll see them wedged in the skeleton of certain sharks or large predatory fishes,” she says – while others have more durable barbs that stay intact.
In a new study, published in Proceedings of the Royal Society B: Biological Sciences, they explored how different features affect performance across 30 different species of stingray.
To do this, they gathered a collection of different spines from various species. “It's hard to observe them actually striking,” says Poulin, so the team tracked down specimens from museum collections around the world and used micro-computed tomography (micro-CT) scanning to get a high-resolution picture of their key features.
“Our idea was to break down the spines into their mechanical components,” she says. “We measured properties of the spines that we thought would be important for their defensive function, and… we basically plotted all of the functional traits that we saw.”
Previous studies into how biological weapons such as stingray spines work test the strike at very slow speeds so they could take very precise measurements, Poulin says. They took a different approach because “predator-prey interactions don't actually happen at those slow speeds.”
Because of this, they wanted to recreate the strike as it would happen in nature, based on the speed reported in another study. “If you yank it out slowly, that's not completely representative of what that interaction would look like in real life,” she says.
To test the spines’ ability to puncture a surface, they dropped them from half a metre (53cm) high. Then, they had to find a way to test how easily the spine could be dislodged.
“We were tinkering, and we had to get creative,” says Poulin. “Basically, I made a giant pendulum.” As the pendulum accelerates, it “yanks the spine out,” she says.
Rather than replicate the species’ spines directly, they created models exploring how things like how angles, sharpness and serration impacted the spines’ function. “We wanted to get at what is the main force driving the trade off in spine function,” she says.
By doing this, the researchers started to see a pattern. “A lot of the stingrays have these really long narrow spines that break more easily, while others have shorter, sturdier spines that seem more like a slicing tool,” says Poulin.
Although stingrays can regrow their spines – in a similar way that some lizards can lose their tail and grow another – losing their defensive barb is costly. “You'll have a period where you don't have a fully mature spine to protect yourself with,” she says.
This means that there’s a trade-off to be made: “the cost of surviving that interaction versus protecting yourself in the future,” she explains.
The study showed that stingrays with long, slender spines were ideal for creating a deep puncture wound but were more likely to break off while some species (particularly freshwater stingrays) had much sturdier stingers. “They could probably use those spines for multiple defensive interactions,” she says.
The difference could be to do with the animals’ habitat. “Stingrays who are in more exposed and sandy habitats, maybe it would be better for them to sacrifice their spine,” she says, “but it could cause more pain to the predator if it breaks off inside of them.”
Meanwhile, freshwater stingrays likely have more places they can hide when they get away so they could benefit from hanging onto their weapon in case of another encounter.
Poulin still has more questions she wants answered about stingray’s self-defence. For example, she wonders: “do they change their behaviour based on what kind of predators are targeting them, or is it more they just have a strike that they do, and it's the same every time?”.
