Innovation and Speciation: whale changes (Evolution)

by David Turell , Friday, September 23, 2022, 17:50 (790 days ago) @ David Turell
edited by David Turell, Friday, September 23, 2022, 18:31

Head circulation is modified:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/23_september_2022/4...

"More than 50 million years ago, terrestrial ancestors of dolphins and whales reinvaded the oceans in one of the most revolutionary events in mammalian history. The transition from land to sea required marked remodeling of the terrestrial mammalian form to withstand high hydrostatic pressures at depth, exponential increases in drag forces when moving locomotor appendages through water, and extreme breath-hold durations when diving (exceeding 3 hours in Cuvier’s beaked whales, Ziphius cavirostris). The changes were so radical that evolutionary selection pressures seem insurmountable. Yet, the transitions did occur, resulting in 47 extant cetacean (dolphin and whale) family lineages that radiated throughout the global oceans. How this evolutionary leap was accomplished has been the subject of much speculation. On page 1452 of this issue, Lillie et al continue this multimillennial investigation on aquatic adaptations in cetaceans, detailing how specialized vascular networks provide protection for their brains during submergence." (my bold)

Comment: land mammals dove into various waters fifty million years ago to make whales and dolphins. "Evolutionary insurmountable pressures" is Darwin-speak for the necessary scramble to find enough mutations to do the job. A designer fits the bill. It not just whales and dolphins, all aquatic mammals come from land.

The article summary

Retia mirabilia: Protecting the cetacean brain from locomotion-generated blood pressure pulses

"Cetaceans have massive vascular plexuses (retia mirabilia) whose function is unknown. All cerebral blood flow passes through these retia, and we hypothesize that they protect cetacean brains from locomotion-generated pulsatile blood pressures. We propose that cetaceans have evolved a pulse-transfer mechanism that minimizes pulsatility in cerebral arterial-to-venous pressure differentials without dampening the pressure pulses themselves. We tested this hypothesis using a computational model based on morphology from 11 species and found that the large arterial capacitance in the retia, coupled with the small extravascular capacitance in the cranium and vertebral canal, could protect the cerebral vasculature from 97% of systemic pulsatility. Evolution of the retial complex in cetaceans—likely linked to the development of dorsoventral fluking—offers a distinctive solution to adverse locomotion-generated vascular pulsatility.

"Numerous cardiovascular adaptations allow cetaceans (whales, dolphins, and porpoises) and pinnipeds (seals, sea lions, and walruses) to make extraordinary breath-hold dives, but some of the adaptations are group specific. The blood supply to the cetacean brain and spinal cord differs radically from that in pinnipeds, passing through a series of massive retia mira-bilia, or vascular networks located in the thorax, vertebral canal, and cranial cavity. Such differences indicate that diving cetaceans and pinnipeds face different vascular challenges". (my bold)

Comment: all of those aquatic mammals came from land animals. The article shows the complex morphological circulatory changes that had to occur/evolve for full (whales) or parttime (seals) aquatic activities to happen. The fact this all happened in a short period sure smells of design. A simple explanation of the physiological problem:

https://www.sciencealert.com/an-anatomical-quirk-could-explain-why-whale-brains-arent-p...

Humans have concocted all sorts of equipment to help us overcome the intense water pressures of the ocean's depths.

Yet our fellow mammalians, the cetaceans (dolphins, whales, and porpoises), can somehow go far deeper while completely naked – and stay down for hours without taking a breath.

And these animals are working against more than just external pressure – fluking, the powerful up-and-down movement of a whale's tail, can create internal pressure that builds up on their cardiovascular system. For land-dwelling animals, we'd simply exhale that pressure out. But cetaceans don't have that luxury.

When cetaceans dive holding their breath, each tail kick sends waves of increased pressure coursing through their abdomen and thorax, and into the bloodstream.

If these pressure pulses reached their brains it would pulverize the delicate capillaries that perfuse it. So where does all that extra pressure go?

A new study may have found the answer: a mysterious, massive network of blood vessels collectively called the retia mirabilia may act as a a literal safety net to buffer this pressure.

***

While most mammals have fairly direct blood flow to the brain, cetaceans' blood goes through the retia mirabilia, which is Latin for "wonderful net", a network of blood vessels (both veins and arteries).

While this structure has been studied for decades, its function has remained largely mysterious.

With their modeling the team found the retia mirabilia has the potential to protect cetaceans' brains from a whopping 97 percent of pressure pulses.

Comment: more extreme complexity to be an aquatic mammal.