Diving Can Be Dangerous For Marine Mammals
The Weddell seal has the ability to dive up to 2,000 feet below the surface of the sea. This is astonishing because at that depth, even most submarines would be crushed. Even more impressive is these seals can hold their breaths for up to 45 minutes in the icy waters of Antarctica, but the big question is whether there is any cost in doing so. A new study suggests that the mechanism which allows marine mammals to dive so deeply may in fact not be good for their hearts.
Heart rate slows down and then speeds up
When either a terrestrial or marine mammal dives, they experience a reduction in the heart rate which is known as bradycardia. The slowdown in the heart rate allows the diver to conserve oxygen whilst they are holding their breath. There are some marine mammals such as seals and dolphins which go on extended dives in search of food. This means they must find and chase their prey, all on a single breath. They must do all of that whilst enduring hydrostatic pressure which can be intense. The sheer physical exertion of this kind of activity produces what is known as tachycardia or a sharp increase in heart rate and until very recently it was not known how these marine mammals were able to deal with such dramatic cardiovascular disparity. The results of the study suggest they do not cope well with it at all.
Marine mammals suffer from irregular heartbeats
The researchers led by Dr. Terrie Williams assessed the diving behaviours of both Weddell seals and bottlenose dolphins. The team fitted these animals with specialised equipment which enabled them to measure both depth and heart rates on over 165 separate dives. In 73 per cent of all dives the animals suffered from an irregular heartbeat known as cardiac arrhythmia.
The two conditions work against each other
The results of the study suggest that the alteration between a slow and fast heart rate produces the erratic cardiac condition. Bradycardia and tachycardia work against one another it would seem though it is not clear what the effects of arrhythmia on the hearts of marine mammals are. Dr. Williams is hoping her work will inspire more research to that end.
“This paper is introducing people to the fact that we have some sort of ancestral baggage that seems to exist in these mammals. That dive response that we thought was such a great safety mechanism for marine mammals isn’t perfect.” Dr. Williams said.
Evolution is not perfect
Cetaceans such as dolphins and whales share a land dwelling ancestor that is common to both. Evolution however is far from being a perfect system in the sense that it does not create attributes from scratch but instead compromises between existing traits. For marine mammals this has resulted in two contradictory impulses which cannot be reconciled – to slow the heart when diving and to speed it up when exercising.
“The more we got into this, the more we realized that there are so many signals going to the mammalian heart, and as we started to look at this in detail for marine mammals, we noticed that the heart looks like the same heart you would see in a mountain lion. These animals are working with the same cardiovascular system, just packaged in marine mammal form. So maybe living in the ocean is asking an awful lot from that system.” Dr. Williams adds.
