Many sharks hunt small, plentiful prey. Great hammerheads take a different approach: Eat other sharks.
It’s an energy-intensive strategy, but one that pays off for this critically endangered species, according to Institute of Environment research. As part of her Ph.D. research, Erin Spencer — working alongside world-renowned FIU shark ecologist Yannis Papastamatiou — tagged great hammerheads off Florida’s coast with special trackers equipped with video cameras, as well as speed and sonar sensors.
The data helped build computer models comparing the energetic costs and rewards of different prey. The findings, published in Oecologia, were striking: When food is scarce, catching a single 55-pound blacktip shark about every three weeks can sustain a 250-pound hammerhead for up to two months.
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Video transcript
Take a look at this video shot in the shallow waters off the east coast of Florida. Why is this great hammerhead shark chasing these smaller sharks? That's one of the questions researchers from Florida International University in Miami are seeking answers to as they work to save these critically endangered hammerheads.
Great hammerheads are one of the most recognizable sharks in the ocean, but there's actually quite a lot about their basic biology and ecology that we still don't know. South Florida is actually a great place to study great hammerheads because they love following prey that are along this Gulf Stream that comes really close off of South Florida. So, we're able to go right off of the FIU campus and find great hammerheads really close.
In order to explore the ecological role of these sharks, FIU researchers use data sensors called biologgers. Biologgers are like a Fitbit for sharks. Researchers attach them to live sharks in the wild to collect data about the animals' hunting behavior. This enables the scientists to study the shark predator-prey relationship and helps them understand the overall dynamics of the ocean ecology.
These biologging tags tell us about the foraging behavior of these animals. But we have to understand that within this context of optimal foraging theory. This is this idea that evolution should select for behaviors in animals that maximize their fitness. This becomes an important question when it comes down to what shall you eat? Should you just eat whatever fish you come across? Or should you be searching for a large fish which is going to provide more energy but might be harder to catch? And these are the decisions these animals have to make. And optimal foraging theory lets us predict how they should behave.
Great hammerheads are kind of unique among large sharks because they actually prefer most times to go after larger prey. A great hammerhead can easily consume prey that's up to 25% of their own size, which is pretty big.
One of the goals of this study was to essentially compare the benefits of two different prey types. What's the benefit of going after a blacktip shark compared to going after a smaller reef fish that might be easier to catch?
One of the perhaps lesser known secrets about predators is that the big ones, especially, fail a lot in terms of trying to catch their prey. The majority of the time they don't succeed, and this creates a problem for them because each time they chase their prey they can burn up a lot of energy doing so, and if they keep on failing then they can basically drive themselves to starvation.
When we look at success rate for these predators, for great hammerheads, we estimated they only succeed perhaps about 15% of the time. And this is true for a lot of big predators, whether you're talking about wolves or polar bears or great white sharks. The rate of success of predation is pretty low. So predators have to do the best they can and have to forage as optimally as possible. Otherwise, they run this risk of burning themselves out.
One of the findings of our model was that even when the blacktip shark densities are low, there's still an energetic advantage to go after them. And a great hammerhead shark would only need to eat about one blacktip shark every 3 weeks and then would even be able to go 2 months without eating before starvation. That same shark would have to eat a smaller bony fish about every day in order to keep up the same amount of energy.
This research helps explain why these sharks might like to go and target other sharks, because it's much more beneficial for them from a foraging standpoint. They only have to hunt once a month, or at least catch prey once a month, versus having to try and go out and catch something every single day.
In order to protect critically endangered species like great hammerheads, we need this data to help inform management. And so it's really important that we continue to study these animals and answer these big questions about their biology and ecology so that we can better protect them in a changing ocean.