The Sea Snake Trick with Going Deep
Wollongong, Australia, circa 2013: Me and a few dive buddies are scuba diving here to observe sea lions. The marine mammals do show up. We admire their fast and effortless freediving – of course these are air-breathing mammals with lungs, just like us, but they have just adapted through evolution to be able to hold their breaths much longer, and thrive in coastal ecosystems underwater. They feed on the ample (well, less ample now due to humans than throughout their evolutionary history) fish off the Australian east coast. It’s always great to be underwater, I have rarely had bland or boring scuba dives. But seeing marine mammals, so much closer to us than fishes, is always special.
We ended up observing the sea lions for a few minutes, and then they disengaged. In a situation like this, it’s the wildlife which decides how long the observation lasts. But soon we saw the group of sea lions again, and we swam over to observe them for a few more minutes. This cycle repeated a few times – some minutes with the sea lions, interrupted by them taking off, and us finsing them again, a few meters deeper.
Eventually we looked at our dive computers and we saw that we were almost 30 meters deep – by no means a dangerous depth for advanced scuba divers, and we had enough air as well, but a depth close to the recommended maximum depth while diving with one scuba tank. At depth you need more air per time, and the peculiarities of hyperbaric physiology allow you to stay for only shorter periods of time. Hence, we had to come up really fairly soon, sooner than planned, and not in the spot we had planned to. This was no diving accident and not even an incident, but the sea lions had clearly tricked us.
After coming back to Sydney, and sharing this episode in a pub with a non-diving friend, he had something like this to say: “Yes! They did that to my mate’s dog too! It was chasing a sea lion, and the sea lion would come close, and then retreat, a couple of times. Eventually the dog followed it too deep into the surf zone, and we had to rescue it out of the ocean!”.
Besides the fact that the mate is an irresponsible dog owner – don’t let your pet chase wildlife! – the parallels are curious: The marine mammal lured the land mammal so far into the ocean that it was in danger. Problem solved, from the point of the marine mammal, potential predator has gone away, and maybe learned a lesson.
In this video, also something similar happened, with a sea snake, which I followed during a dive. I ended up much deeper than I had planned to dive:.
Why do Snakes and Seals do this?
What is the common denominator here? Both the sea lions and the snakes are secondary aquatic animals. At one point their ancestors in evolution were air-breathing (they still are) land animals (which they are no more). In both instances, they lured actual land animals which they probably saw as a threat into deeper water where they weren’t so safe and comfortable anymore, and which we had to leave.
I am speculating here, of course, but is this a general strategy which secondary marine vertebrates, like marine reptiles and sea lions, have acquired to defend themselves from land predators? I have not found any literature in animal behavior science on this, though I haven’t looked very thoroughly yet. I’d be grateful for any pointers. In contrast to fish who have never had to come up to breathe in all of their evolution (with the exception of some unique Amazonian species), seals and sea snakes on some level probably understand that a terrestrial predator eventually needs to breathe, and that luring him where he can’t is a defensive strategy. It worked with us, as well as with my mate’s mate’s dog. It worked for the sea lions, and for the sea krait. Anecdotal evidence, I know, but hopefully a pointer for thought.
I hope this was interesting,
Best Fishes,
Klaus
and … more marine reptiles on Flickr for your viewing pleasure:
