3 Things About Seals

3 Things About Seals

Adaptations and senses

One thing that is instantly discernible about seals is their chunkiness. They have a blubber layer under the dark skin colour for temperature insulation and absorption, buoyancy in water, also energy reserves during food scarcity. Some can reach up to 50% body fat.

The roundness, coupled with no ear flaps, grants a streamlined shape to swim over 20mph, using their front/hindlimb flipper surface and webbing to propel themselves. Unluckily, their land mobility is not good. They bobble along, looking like a banana on their abdominals, with the front flippers to the sides for stability, although the claws help a little with grip.

For protection, they have a transparent nictating membrane and lacrimal gland mucous secretions countering saltwater and freezing dry air on the surface. For some reason, they have no tear ducts for drainage, which is why they appear teary-eyed. In addition, nostrils are closed airtight when relaxed, and ear openings get shut underwater to cover the drum from pressure.

Their vision is better underwater, with a large, round eye lens helping to disperse light and focus more of it onto the retina, mostly comprised of rod cells for darker conditions. Their tapetum lucidum structure also reflects and amplifies visible light back through the retina to help with low-light conditions. Otherwise, this comes at the expenses of cone cells, so they are likely colourblind, as well as being unable to take in as much high-light above water - where the pupils constrict and the lens cannot focus as much.

Yet, blind seals can navigate and forage just fine using their hearing and whiskers. At surface level, their hearing range is similar to a human's, whereas, as particles are closer together in liquid than gas (sound waves bounce off more often), they hear better underwater with a range of potentially over 180 kilohertz - 9x our maximum.

Their whiskers have hundreds of nerve endings, being sensitive to water current velocity and vibrations to follow hydrodynamic trails of prey. Wavy shaping lets them hold still while moving through water, so there is minimal interference. The velocity and vibration strength gets dictated by factors such as animal size and speed - for example, an average-sized fish could get detected 10 metres away, but a smaller, slower fish may need to be closer to trigger the nerve action potential.

Dive Response/Reflex

Seals have evolved sophisticated cardiovascular adjustments, conserving intrinsic oxygen stores to remain underwater longer. Diving mammals like seals have particularly high abundances of red blood cell haemoglobin and muscle cell myoglobin proteins, meaning more availability for oxygen.

The dive response/reflex is defined generally by reactions to immersion of the body in water with a notable temperature drop, where breathing ends temporarily (apnea) in air-breathing animals. Typically, bradycardia (slowing heart rate), vasoconstriction, and blood pressure rise will all occur to combat hypoxia (oxygen deficit).

Diving instantly induces increased blood circulation, favouring flow to the brain and heart as the most vital organs - peripheral blood vessels serving more unnecessary or hypoxia-tolerant tissues, become intensely constricted to reduce flow and preserve cellular oxygen. Still, even the brain can drop a few degrees in temperature after ~15 minutes because superficial forelimb veins supply cold blood, so oxygen consumption lowers.

For prolonged hypoxia, extreme bradycardia can be employed, with heart rates dropping to 0.25 bpm to forage. Spending most of their lives holding their breaths may contribute to their longevity, which is a good couple or few decades via 'saving' the heartbeats. Above water, they recover with a physiological 'reset'.

Elephant seals could somehow reach up to 2 hours of no breathing and plausibly dive almost 2km deep. Some weigh over 4 tonnes and are significantly over 4 metres in length. It implies an insane lung capacity and a body robust enough to resist a crushing ~5500 psi/366x surface pressure under all that water. A human would die pretty much instantly, equating to the weight of roughly four polar bears standing on a tiny ten-pence coin.

Threats

As a prevalent marine mammal, seals are afflicted considerably by anthropogenic activity, including but not limited to:

-heavy vessel traffic in vulnerable areas resulting in propellor lacerations, crush injuries and conceivable skin abrasions

-fishing gear, nets and marine debris - severe injuries, embedment, infection and fatigue are plausible; entanglements can be dragged to the surface if manageable, except compromising feeding ability

-violence, trauma and environmental intrusion eventuating in invoked distress, supposable injury alongside degrading health

-chemical oil spills and waste discharge, leading to direct mortality; if not, long-term health impacts involving topical, respiratory and gastrointestinal damage; nutritional deficiencies plus immunosuppression/disease due to contaminant accumulation in fat stores, not to mention its amass along the lower trophic levels in their food chain

Juvenile pups under one year of age are the most vulnerable and regular casualties. Since they have not attained their adult characteristics of fully developed physical parameters - mainly their overall body mass, length and internal bone and organ structures.

Pups of all seal species are highly precocious; hence, individuals abandoned or separated from their mother will likely be conscious of distress and hunger. Newborns will die eventually if not rescued and rehabilitated, likewise concerning most stranded older pups.

Because pup survival is emphatically dependent on the importance of mass, the visually lowest body condition scores - presumably a 1/5 or even 1/9, could be one of the most extreme states, residing in emaciation and hypoglycaemia. Depictions are a lack of rounded shaping and underlying fat layer reserves, with rolls of skin and bones being significantly more noticeable - regarding the neck, pelvis and ribs.