Sperm whales dive to great depths (to more than 2 km or 1.4 mi deep) to catch one of their favorite foods: giant squid. But how did the first sperm whale know it would find giant squids in the ocean depths? The following story is speculation that makes sense but has no facts to support it. Scientists often refer to such stories as just-so stories, named after the stories by author Rudyard Kipling. But while Kipling’s just-so stories are fanciful, my story is plausible.
Before whales evolved, it could be that giant squids lived near the ocean surface. After whales evolved and discovered that squids are tasty, the giant squids might have started living in deeper water, to escape the whale predators. Some whales might have started diving more deeply (and developed specialized physiology allowing them to hold their breath up to 90 minutes and to resist the great pressure at depth) so they could feast on the squid, so the squid might have responded by living deeper still. Cycles like this, between predators and prey, are examples of co-evolution. This cycle could have continued until the squid lived in some of the deepest parts of the ocean, and the sperm whales dove to those great depths to eat the tasty squids. That just-so story might explain why giant squid live at depth, and how sperm whales are able to dive that deep to find them.
Humans hunted sperm whales heavily from the 1700s to the middle 1900s and reduced their numbers possibly to a third what they were historically. Fewer whales would mean less predation pressure on giant squids. With reduced predation pressure, giant squids might venture into shallower water.
It is possible that such a change in squid behavior could lead to more sightings of giant squids over the last few decades (squids caught in fishing nets and caught on cameras). Or it is possible that improvements in technology explains the increased sightings. You might be thinking that humans have interacted with giant squids for centuries – consider the myths of giant squids attacking ships. I agree that humans have known about giant squids for centuries, but I doubt anyone had previously ever seen one alive. Humans are likely to have known about giant squids from examining the gut contents of sperm whales, or possibly from a squid carcass that floated to the ocean surface after it died.
We might never know the real answer to why giant squid live at depth, and why sperm whales are able to dive to such great depths (and how they know squids are there). This co-evolutionary just-so story is a plausible explanation.
Timothy A. Pearce, PhD, is the head of the mollusks section at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.