Testosterone Levels In Carcharhinus leucas: Is It All Bull?

ResearchBlogging.orgSo, Shark Diver set out a challenge for Ocean of Pseudoscience Week that I simply had to look into. He wanted to know whether there's any science backing the notion that Bull Sharks, Carcharhinus leucas, have extraordinarily high testosterone levels and might thus be usable as a source for legally obtainable steroids - an idea, apparently, started by some video game.

The rumor, as I'd heard it, is that the fierce attitudes of these large and aggressive sharks is due to unfathomably high circulating levels of testosterone. Specifically, these menacing monsters supposedly have higher serum testosterone levels than any species on the planet, land or sea, and that even a female bull shark has higher levels than a testosterone-raged male elephant in musth.

I know I'm as much to blame as anyone, as I've repeated that line myself. But suddenly, I realized, I don't know if it's true. How do the circulating testosterone levels compare between bull sharks and other species? Could you procure enough testosterone by catching and eating bull sharks to beef up your body? I simply couldn't let Shark Diver's challenge go unanswered.

Let's start with that elephant. In the red corner, standing up to 11.5 feet tall and weighing in at up to 20,000 lbs, we have the African Bush Elephant (Loxodonta africana). What's the testosterone level in this whomping beast? During much of the year, not much. Male elephants, on average, have less than 2 ng/ml ciculating in their plasma. But wait! Big boy wants to get his groove on, and he is getting ready for a season of fighting and fornicating. So what's the male elephant in musth's testosterone level? As high as 64.4 ng/ml! About a 60 fold increase in average circulating testosterone1. Ai! That's a lot of anger-pumping hormone.

How about his opponent, the every day female bull shark? In the blue corner, weighing in at a maximum 505 lbs and stretching almost 12 feet long, is our large and in charge girl. She's bigger than her man, no doubt, but she still has to keep her femininity about her. After all, as a girl, if her testosterone levels are too high, she might have reproductive issues. Surely her circulating levels are lower than the male elephant's?!

According to the only, extremely obscure published reference* with testosterone levels in a female bull shark, actually, yes, they are2. Her circulating testosterone level is right around 0.1 ng/ml, a whole lot lower than that angry elephant. Sorry to burst your (and my) bubble, but she ain't gonna give anyone roid rage.

Her man, though... My oh my. One of the two male adult bull sharks in that study had a circulating testosterone level of 358 ng/ml. Yeah, that's one roided out shark.

Problem is, the other male bull shark in that study only had 2.7 ng/ml of testosterone in his serum - which is probably less than you male readers out there have pumping in your blood right now. So it's not like all male bull sharks are running around roided out of their minds. Of course, these were just single sharks, caught once and tested once. Without a more complete study of the average hormone levels in bull sharks, by size, season, etc, we can't really say that bull sharks have abnormally high or low testosterone levels.

Of course, there is a more complete study. Not a super detailed one, but a study none the less. Rasmussen & Murru3 studied androgen levels in a number of captive sharks over time. They found testosterone levels of 10 ng/ml to 20 ng/ml in two captive bull sharks when they measured every June for three years, just after the sharks' normal breeding season. Not too impressive, boys - not too impressive, but of course, that is in captivity, and it's unknown how captivity may affect their hormone levels. In wild bull sharks caught right before the breeding season, the serum levels were much higher: 185 ng/ml on average (n=3). This was 4 to 10 times higher than the levels they found in two other shark species, and is pretty impressive compared to the elephant.

But, it turns out, it's not that hard to find high testosterone levels in fish. Other sharks have high levels, too - like the bonnethead, Sphyrna tiburo, whose highest levels have been recorded at 303 ng/ml4. And in that species, even the girls have higher levels than our elephant - a whopping 74 ng/ml at max5. Believe me, I'd rather go hunting bonnethead than bull sharks any day.

Other fish, too, have been found to have high testosterone. Male rainbow trout have levels around the same as those of the bonnetheads6, and heck, they sound a whole lot tastier to me than the other options. Just sayin'.

Sorry folks, but according to the best, albeit limited scientific information we have, the idea that bull sharks are super juiced-up compared to other animals just isn't true.

Rasmussen & Murru.pngEven still, it is the scientific community that is to blame for the impression that bull sharks are testosterone-pumped. Every other paper I read about shark hormones since the two with bull sharks cites them, specifically mentioning 358 ng/ml and that bull sharks have much higher levels than other sharks. Where did that come from? Now that I read the papers, I see. It's the original authors that claim that bull sharks have higher testosterone than other sharks, even without presenting evidence to back it up. It started with how Rasmussen & Gruber were quick to point out how high that 358 ng/ml value is, saying it's "among the highest recorded in vertebrate serum," but didn't talk at all about why the other mature male bull shark (by their own identification) was more than one hundred fold lower. But it's really Rasmussen & Murru (hmm... that first name sounds familiar), in their discussion, who seem to overinflate their own data. They state that "a species differences in absolute concentrations appears to exist because concentrations of testosterone in both wild and captive bull sharks were about two times higher than those in mature sandbar and lemon sharks" (emphasis mine). Yet their included figure showing the yearly serum concentrations for the two captive bull sharks studied clearly shows the levels between 5 ng/ml and 20 ng/ml, while the levels for the two captive sandbar sharks sampled at the same and different times of year range from 0 ng/ml to over 40 ng/ml! Two to four times higher in bull sharks? Where? When?!

But enough griping about inaccurate inflation of results. Let's say, for a hypothetical moment, that there is a time of year, size, or whatever where you could go out with a shark hook and some dead fish and guarantee getting a big boy bull shark with upwards of 300 ng/ml in his system. It's time to address the other part of the myth: Should serious users think about going fishing?

You'll have one big fish to fry if you're trying to get a nice dose of testosterone by ingesting sea creatures. First off, I hope you're feeling vampiric. You want the blood, not the tissues. We don't really know a lot about the doses of testosterone in tissues, and besides, that super high amount was in the blood, so yeah. Cheers.

Second off, unless you're planning on shooting up shark blood, you're not getting the dose you think. Orally ingested testosterone is rapidly absorbed by the gut, but it's also converted to inactive metabolites, leaving you with only 1/6th the dose you took remaining in active form. That's why pills and injections aren't actually of straight testosterone, they're of slightly modified chemicals that the body doesn't metabolize as easily. It also means that to get the same dose from shark as you would from a prescription (or black market) pill, you have to drink six times what you think you have to.

So let's say you want to replace that 40 mg pill you bought with bull shark blood. Even if you catch that one shark that had 350 ng/ml in his serum, that means you'll have to drink down three cups of shark plasma to equal one pill.

A shark tends to be about 12.3% blood by weight according to previous studies7 - that's 6.8% blood cells and 5.5% serum, which has a specific gravity (weight per volume) of around 1.03. So say you caught an average bull shark, weighing only 350 lbs instead of the max of 500. He'll have around 44 cups of blood in him, which is only 44% plasma, so you'll need to drink 6.8 cups of blood per pill. So at 2-5 pills a day, that shark will only last you one to three days.

Hey - I guess it's legal. Though somehow, I don't think athletes are going to get away with the old "I was just drinking shark blood" excuse just because of that.

2. Harold L. Pratt, Jr., Samuel H. Gruber, & Toru Taniuchi (editors) (1990). Elasmobranchs as Living Resources: Advances in the Biology, Ecology, Systematics, and the Status of the Fisheries NOAA Technical Report NMFS 90, 143-155
3. Rasmussen, L., & Murru, F. (1992). Long-term studies of Serum Concentrations of reproductively related Steriod Hormones in individual captive Carcharhinids Marine and Freshwater Research, 43 (1) DOI: 10.1071/MF9920273
4. Manire, C. (1997). Serum Concentrations of Steroid Hormones in the Mature Male Bonnethead Shark,Sphyrna tiburo General and Comparative Endocrinology, 107 (3), 414-420 DOI: 10.1006/gcen.1997.6937
5. Manire, C. (1995). Serum Steroid Hormones and the Reproductive Cycle of the Female Bonnethead Shark, Sphyrna tiburo General and Comparative Endocrinology, 97 (3), 366-376 DOI: 10.1006/gcen.1995.1036
6. Scott, A. P., & Baynes, S. M. (1982). Plasma levels of sex steroids in relation to ovulation and spermiation in rainbow trout (Salmo gairdneri) Proc. Int. Symp. Reprod. Physiol. Fish, 103-106
7. Thorson, T. (1962). Partitioning of Body Fluids in the Lake Nicaragua Shark and Three Marine Sharks Science, 138 (3541), 688-690 DOI: 10.1126/science.138.3541.688

*Just for your information, do you have any idea how hard it was to find those two papers with bull sharks in them? I mean seriously! I had to go to the library and everything. One was in a section of the library I'd never been in, and in the waaaaaaay back corner of this huge room of endless bookshelves, and I swear it was right out of a teen slasher flick. I expected that any second some masked man with a blade would pop out and say some corny line about how it'll do no good to scream and chase me around while I hopelessly search for a way out. I actually got lost back there. Lost. For about ten minutes. I got turned around and I couldn't find the way I'd come in. Just thought you should know. The things I do for you guys...

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Is there a clear association between testosterone and aggressioN?

Wouldn't testosterone be sequestered in fatty tissues, being derived (and not too far) cholesterol?

It just seems logical to me, but I don't actually know. Not wanting to take shark testosterone, I suppose I don't really need to know.

By that logic, shouldn't cholesterol be sequestered in fat tissue? But it's in the blood...

You'd also have to deal with the fact that sharks carry a lot of urea in their tissues and blood for osmoregulation. That's why shark meat is often bled out before it's sent to market.

There is some evidence in other animals about higher levels of testosterone in fat (eg this article), but there hasn't been much research on testosterone levels in different shark tissues, and there isn't any on concentrations in other bull shark tissues

Christie, don't feel bad, I've been running around repeating that line as well. I think we can pin it on a certain IMS professor we had who taught us that...