Can Sharks Smell Blood?

Sharks are famous for their keen sense of smell, particularly their ability to detect blood in the water. But how strong is a shark's smell? We will explore this by looking at the paper “The Role of Olfaction in Shark Predation” [1]. In this experiment, they studied blacktip, grey, hammerhead, and tiger sharks in both laboratory and large outdoor pools [1].

Ethical Concerns

It’s important to note that this research took place in the 1950s and 60s, and many methods used, such as keeping sharks in cramped tanks, surgically blinding them for tests, and starving them [1], would be considered unethical today. Some sharks were injured or died during the experiments [1], raising serious concerns about animal welfare in the past.

Sensitive Nose

To test their sense of smell, researchers exposed the sharks to various scents, including tuna, eel, human blood and sweat, amino acids (molecules that combine to form proteins), and even water from tanks containing stressed fish [1]. Some sharks were temporarily blinded so they could focus on smell alone [1]. Remarkably, even without sight, many sharks can still find food with impressive accuracy, demonstrating just how finely tuned their sense of smell is [1].

The results showed that sharks mostly reacted strongly to fresh scents [1]. Fresh fish or blood drew their attention, while older, decomposing matter was less appealing or even repulsive [1]. That said, some species, like grey sharks, were still interested in rotting blood or flesh, suggesting that responses may vary across species [1]. However, it still suggests that their senses are strongest when food is fresh. Surprisingly, even rotting shark flesh, once thought to repel, sometimes triggered interest [1].

Human blood, even in tiny amounts (as little as 0.01 parts per million), was a powerful attractant, with a pool of blood exciting the sharks [1]. On the other hand, human sweat usually repelled blacktip sharks, especially when they were blinded [1]. Interestingly, decomposing blood had a less consistent effect, with blacktips being repulsed; however, grey sharks were still interested [1].

The responses to other substances, like certain amino acids or chemicals, showed species-specific results [1]. For instance, a lachrymator, a chemical that causes tearing, repelled grey sharks but didn’t bother tiger sharks [1].

Sharks can Smell Fear!!

One of the most intriguing findings was that sharks could detect stress signals from live, uninjured fish [1]. Calm fish were ignored, but as soon as fish became agitated or panicked, sharks took notice and quickly homed in with remarkable speed and accuracy [1]. The result suggests sharks can detect chemical signals released by distressed prey [1]. Guess sharks can sense fear.

This supports the idea that olfactory cues, particularly those linked to fear or stress, play a critical role in shark predation [1].

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These findings challenge the Hollywood portrayal of sharks as mindless eating machines. In reality, they are highly selective hunters who rely on a complex mix of senses, especially smell, to locate and select prey.

The study gives us a rare glimpse into how sharks perceive the world and just how sensitive their sense of smell really is. While the science is fascinating, it also serves as a reminder of how far we have come in how we treat animals in research.

 References

Reference paper “The Role of Olfaction in Shark Predation”

1.       Tester, A.L., 1963. The role of olfaction in shark predation.