HORMONES IN THE WEDDELL SEAL

Tavishi

The weddell seal, or Leptonychotes weddellii, is one of my favorite pinniped species. I routinely follow the work of the Montana State University Weddell Seal Lab in Antarctica, and so I get a very good supply of seal pup pictures and videos, including:

Antarctica has months of endless dark and months of endless sunlight, rather than the day and night cycle we equator-dwellers are accustomed to. Because of this, circadian variation exists within the organisms that inhabit the frozen paradise. A prime example is the Weddell seal. Instead of melatonin within the pineal gland being used to regulate the circadian rhythms, melatonin has other functions within pinnipeds. The pineal gland is actually pretty large in a lot of the lobodontine seals (See previous post!), contrary to their issues with circadian rhythms. In many lobodontine seals, neonatal pups are born with a large blood concentration of melatonin, with upwards of 1000 pg/mL. Why is there so much damn melatonin? Good question. One more prominent idea is that this may stimulate non-shivering thermogenesis. Melatonin promotes the production of T3, a thyroid hormone, which also results in NST. Neonate pups in the Antarctic would uh, likely be very cold to say the least. Almost as cold as Arizona in the winter....

Another hypothesis is that the high levels of melatonin may prevent hypoxia in utero during deep dives (of the mother, not the pup, despite the pup being there for the ride.) Weddell seals are also known for their adrenal hormones, most notably cortisol and adrenaline. Cortisol is a hormone released by the adrenal glands, which are regulatory glands sitting just above the kidneys. The adrenal glands release hormones from both the adrenal cortex and medulla. The adrenal cortex, or the outer bit of the glands, produces aldosterone (RAAS and raas post coming maybe...?!), cortisol, and androgens like DHEA and androstenedione. Cortisol plays an important role as a primary stress hormone, and the release of cortisol results in vasoconstriction and an increase of blood pressure, in addition to an increase in blood sugar.

Cortisol is elevated in newborn Weddell seals. Which, I suppose, birth is a terrifying experience. Being shot out of the only warm darkness within which I have existed into a very bright, very very cold, world would certainly stress me outI s.

However, that's likely not why cortisol is elevated in Weddell seal neonates. A potential hypothesis may be linked to the shedding of lanugo, or this sort-of premature fur that develops in utero. Cortisol can trigger hair loss, and elevated cortisol has been detected along with periods of molting. Nonetheless, this remains a mystery for the most part. One of the most beautiful things about this sort of field is how much uncertainty there is. Trying to link hormones to functions based off of ecology is fascinating. And somehow, you still end up looking at the biochemistry. It often feels like a scavenger hunt in the best way possible. Plus, seal pups are adorable!

Finally, adrenaline and noradrenaline: two other adrenal gland products. Adrenaline and noradrenaline are a fun breed of hormones called catecholamines. The catecholamines are a group of hormones derived from my favorite amino acid of them all: tyrosine! Noradrenaline and adrenaline are also key players in the stress response, often released alongside cortisol. Rather than being produced within the adrenal cortex, however, these catecholamines are produced within the adrenal medulla.

Noradrenaline and adrenaline actually increase in concentration during deep dives of Weddell seals. Why? Well, this is something we have a lot more idea of. This increase in blood catecholamine concentration actually results in splenic contraction, consequently increasing RBC count in the blood. During long dives, oxygen is definitely lacking to the body. Since we don't want to induce hypoxia in our seals, there are so many creative ways to keep the body going. We humans suck at diving. Not so much pinnipeds, though. Weddell seals actually often serve as a model for diving physiology on thousands of accounts- see the last page of Campbell Biology circulation!!!!

Campbell, my beloved <3

I will leave you all with one last realization which makes a lot of sense. Pinnipeds have a low ratio of insulin to glucagon. Now, this naturally makes sense; as carnivores, they're not consuming much sugar in their diet. But it does mess with my head a little to see pinnipeds so lacking in such a cool hormone. I will say, this is perhaps one of the more fundamental realizations I've made. Weddell seals are very cool, mostly because they live in the Antarctic, but also because of so many of their physiological adaptations!