The passage from water to lands emerged is a chapter that never fails in stories about the history of life on Earth, having been a fundamental step for the subsequent evolutionary radiation of many animal and plant species. But even more interesting, from an evolutionary point of view, is to rethink the history of those vertebrate descendants of progenitors who, after adapting to the earthly life, have, so to speak, started to go back from mainland to water. Some, like turtles, even seem to have embarked on this trip several times in both directions, from land to water and back to earth.
Some mammals, such as whales and dolphins, have come to the end of this path in the opposite direction (in the sense that it is highly unlikely that their descendants will return to colonize the emerged lands), returning to a completely aquatic life like theirs ancient progenitors. Others stopped a bit earlier along this path, such as seals that, despite having developed anatomy suitable for swimming and diving, still spend a good deal of time on the earth, especially to lighten and nurture the little ones. The return to the aquatic life of these mammals has been made possible by numerous adaptations not only of their anatomical structure but also of their physiology. However, studying some of these species can be seen how the evolutionary process of refinement of these adaptations is still ongoing. Life in the water still proves the physiology of these organisms, especially during some high-energy activities.
This is a study published in Nature Communications on the Cardiovascular Dolphin Physiology (Tursiops truncatus) and the Weddell Seals (Leptonychotes weddellii). Marine predators such as dolphins and seals have to locate and pursue their diving prey while holding their breath and resisting the enormous hydrostatic pressures that lie deep. An extreme case, quoted by the authors, is that of the zihio, or a whalebone (Ziphius cavirostris), able to immerse itself up to 3000 meters deep and to hold the breath for more than two hours. For a long time, one wondered how the physiology of these animals could be adapted to respond efficiently to the effort required by such exercise in such extreme conditions. It is believed that the immediate response to diving by these cardiovascular system of these mammals is a slowing down of the heartbeat (bradycardia) and peripheral vasoconstriction, which limits the consumption of the oxygen reserve available during apnea. But during a physical effort, such as that required to track the prey, other mechanisms that produce a counter effect, ie an increase in heart rate (tachycardia), are activated. What is the combined effect of these two responses on the heart of these animals?
To find the answer to this question were some tools for detecting heartbeat, peak frequency (physical effort intensity measurement), depth and dive time of several Weddell and dolphin seal specimens. The results of these measurements show that the level of depth reached and physical effort altogether alter the intensity of bradycardia during diving, which in turn affects the occurrence of some heart abnormalities. The authors have reported an increase in the frequency of cardiac arrhythmias as physical and depth stress increases. The change in heart rate may be considered as the effect of two opposite sign stimuli, due to the action of the sympathetic and parasympathetic nervous system. The sympathetic system, during muscular effort, increases the heart rate, while the parasympathetic system is responsible fcor its decrease in response to diving. Thus, the physiological heart rhythm may be altered by rapidly alternating tachycardia and bradycardia, especially during the beginning of the ascent to the surface.
It’s like a human being when it comes to adrenaline, for example, playing poker or other games when don‘t know the rules. Sure, here you can find it, but the heart rhythm still can be high and you feel stressed. In order not to exceed the physiological limits imposed by their cardiovascular system, these mammals, during diving, have adopted a behavioral response that consists in controlling the intensity of the effort. For example, at the beginning of the deep descent, Weddell’s seals let it slide to the bottom and then begin swim by accelerating or decelerating more or less intensely and alternating the descent at short rises during the pursuit of the different prey.
It is interesting to note that such a change in heart rhythm occurs also in humans during diving underwater apnea. As the authors write, “50 million years of physiological evolution during sea-to-sea passage of marine mammals may not have completely solved the problem of balancing heart response to diving exercise.”
Tens of millions of years after the beginning of the journey that led the various progenitors of these mammals from mainland to water, their descendants still survive ancestral “terrestrial” traits, which testify that this evolutionary journey has not yet come to an end.