Dammit Jim!

When I was writing up the last post on the Great Turtle Race, I came across this Wikipedia page that has details on every Colbert report ever (what doesn’t Wikipedia have?). It included this quote on Stephen Colbert’s leatherback:

Stephen is unhappy at the fact that Stephanie Colburtle The Turtle did not win The Great Turtle Race, after being bested by another turtle named Billy. He claims Billy is a male, and demands a re-race. (After explaining that one can tell the sex of a turtle by the concavity of its plastron, Stephen says that he checks the plastron on “all [his] dates, and if it’s not concave, [he is] outta there.” However, a concave plastron denotes a male turtle.)

Now I’m not totally sure the concave plastron bit works with leatherbacks since they’re more barrel-shaped than turtle-shaped but I guess it’s possible. But on the topic, I just thought I’d share a couple tips for determining leatherback sex.

First, is it on a beach? If so, it’s female. Healthy male leatherbacks never return to land after their initial crawl from the nest to the ocean. That makes research programs that catch turtles at sea the only way to look at male leatherbacks.

Second, does it have a long tail that trails well behind the shell? Then it’s a male. Leatherbacks (and other sea turtles) store their penis in their tail. The tails of female turtles barely extend past their shell. The tails of male turtles, shall we say, hang low and wobble to and fro. I couldn’t dig up a picture of a male leatherback but here’s a picture of a male green sea turtle tail (from seaturtle.org courtesy of Daha Diew and Alain Gibudi) that should give an idea (that’s its rear flippers in the left edge of the picture).

And now you know.

I’m a bit late on this one (I don’t know how I managed to miss it since I had to put the data together) but National Geographic and Conservation International had a Great Turtle Race with a bunch of leatherback turtles tagged by my old adviser. They took data from turtle tracked from Nova Scotia to South America and had a big two-week event watching which turtle reached the Caribbean first. They have a pretty cool animation of the satellite tracking (although of course not quite as good as mine) and some cute leatherback artwork (complete with leathery back instead of shell, although why are they green?).

That site also has the first leatherback game I’ve ever seen. The artist did a really good job since the view is pretty much identical to the view we get from a shoulder mounted turtleCam. Unfortunately, the game turtle handles like a tank which really doesn’t do justice to the maneuvering ability of leatherbacks. They’re huge animals but in the water they’re really quite graceful and they can turn on a dime (as I quickly found out when we were trying to catch them).

Anyway, it looks like the turtle named Backspacer (it’s weird to see all the interesting names since we always call the turtles by their tag ID number), sponsored by Pearl Jam, yes that Pearl Jam, won the race. Turtle Cali won the diving portion of the race and received an Iron Turtle Award. Here’s a nice post-race summary and also Olympic swimmer (and turtle coach) Jason Lezak’s take on it. It’s great to see so much public interest in leatherback turtle tracking and National Geographic and Conservation International did a great job promoting and running the event.

There’s a new stories about a “rare giant sea turtle” being caught and eaten in the Gaza Strip going around now. I dug up a video for it and it’s definitely a leatherback so I figured I’d throw in my two cents since I just posted about leatherback turtles.

Here’s the video. Sorry if there’s an advertisement in front of it. I’m not making anything from it but it was the only source I could find. They do show the turtle being killed so it’s not really fun to watch.

A few corrections to the video first, it’s not “thought” to be a leatherback. There’s no mistaking a leatherback for any other turtle. That’s a leatherback. The fisherman says it’s 600-700 kilograms. I’m never good at estimating weights but the biggest one weighed in Canada so far was just a bit above 600 kilograms and that one looks on the small side so probably less than 400 kilograms (still a big animal though).

I thought it was odd they were talking about eating the meat since I’d often heard that leatherback flesh is poisonous but I can’t find a good citation for that and there do seem to be substinence fisheries for them so I guess leatherbacks are either edible or only occasionally poisonous. Also, I’d heard of people eating the eggs for ‘viagra’ effects but never the blood or the whole ailing children thing. I’m not sure what conditions are like in Gaza but if they’re not killing the turtle for necessary food, it really seem like a shame to kill an endangered species for bogus penis enhancement.

Also the fisherman says the turtle ruined a bunch of fishing gear. This is actually a common problem with leatherbacks. They don’t seem to have a way to reverse directions. So if they run into a net or even a loose rope in the water, they can easily become entangled. This can often end badly when the loops get caught around their neck and choke them or the tide rises while the ropes hold them underwater (or people decide to drag the turtle ashore and drink its blood to improve their sex lives).

In more encouraging news, the Reuters article ends with this:

A smaller Leatherback was caught off the Gaza coast last month but the turtle was released after fishermen discovered it carried a tag classifying it as an endangered species.

Thanks to William F. Landell for pointing this story out in the comments.

I’ve had this blog a year and a half now and although I’ve had a good time, I’ve never really wrote about anything I’m actually (somewhat) knowledgeable. So I think I’ll do a few posts on leatherback turtles. I’m no turtle expert but I did spend a couple years working on their migration and movement. My first first-author paper just got officially published so I’ll start with that. <ego>It made the cover of Behavioral Ecology</ego> which isn’t really a big deal but I thought it was kind of cool. The paper was about leatherback turtles migrating from their summer feeding grounds near Nova Scotia to head down to wintering and nesting areas in the south. (If you don’t feel like reading all this, you could just skip to the pretty cool {at least to me} movie down at the bottom).

First a little background. If you don’t know much about Nova Scotian geography, don’t feel too bad a lot of my relatives in Michigan were surprised when I told them I was in Canada but still south of them. Here’s a map for orientation. Leatherback turtles come up to Nova Scotian waters to eat jellyfish in the summer and then head back south to nest, mate and presumably avoid the cold waters. Leatherback turtles are pretty cool animals and I’ll probably do another post with more details on them later. For now, you just need to know they’re big animals reaching more than half a ton and their size, blubber and counter-current heat exchange allow them to maintain a high body temperature in cold waters almost like a warm-blooded animal.

So back to the paper, my coauthor Mike James has tagged a whole bunch of turtles now and we were curious if we could see any pattern in when they begin their migration. One problem was that unlike migratory birds it’s pretty hard to define when a turtle begins migrating. Luckily the state space models of Ian Jonsen provide a method to estimate turtle behavior as either foraging or transiting. By finding the last foraging in a season, we could have a pretty good idea of when the turtles began their migration.

Once we figured out when the turtles began their migration, we tried to figure out what might be triggering it. We guessed it would have something to do with colder temperatures or declining prey abundance. I could get temperature from satellite data but there isn’t an easy way to measure jellyfish (leatherback food) abundance. As a rough proxy, we used chlorophyll estimates from satellite images. As a side note, it’s really cool that NASA provides their data for free. Just to cover as many bases as possible, we threw position, day length, the North Atlantic Oscillation index, water depth and the sex and size of the turtle into the mix and stuck it all into a stats model. After a bit of calculating, the model came back predicting that the position of the turtle and temperature and chlorophyll of the water appear to correlate with departures.

The biggest factor for predicting departure was the position of the turtle. Northern turtles appear to leave earlier than southern turtles and turtles around the longitude of Georges Bank (abundant shelf ecosystem) and Nova Scotia (the study area and lots of jellyfish) stay longer. The contour plot to the right shows the relative probability of departure from low (red) to high (yellow). The contour lines and dates show when we would expect 50% of the turtles in a region to depart. Knowing these dates could be pretty useful for conservation since regulations could be lifted after their departure.

Northern turtles leaving earlier was pretty much what we expected since northern waters get colder sooner. So it was pretty surprising when the model also predicted that turtles are actually more likely to leave when the water is warmer and greener. Now I’m not sure exactly why this is but one possible guess is that warmer chlorophyll-rich waters provide more nourishment sooner and allow turtles to head south earlier. Another possible explanation is that jellyfish population might decrease earlier since many jellyfish die off after reproducing and warmer waters and increased prey allow jellyfish to reproduce sooner. Of course, a final possibility is that the correlation is just a funny coincidence in the data but hopefully this is unlikely.

Since turtles didn’t appear to be leaving when the water cooled off, why were they still leaving early from the north? One guess might be that turtles leave earlier from the north if they have farther to swim to their southern nesting grounds but the difference from the southern and northern foraging areas is only about 500 km. Since leatherbacks can easily swim 2 km/hr, they could cover this distance in 10 days or less but we observed differences of more than a month. Without temperature or distance to explain it, the disparity could possibly be related to some difference in habitat quality. Unfortunately this will remain just a hypothesis until we get a lot more data on jellyfish distribution and feeding rates. It is interesting to note that although many leatherbacks are still foraging in southern waters (blue dots in the figure to the right), no northern foraging turtle (red dots) has been observed moving into southern foraging grounds late in the season (red tracks).

Now that you made it through all that text, here’s a video I made of the turtles foraging, transiting and migrating (higher quality here). By the way, it costs quite a bit of work and money to get a transmitter on a leatherback so each one of those points is a good bit of work done by my coauthor Dr. James.

A couple extra interesting things to watch for are turtles hitting the Gulf Stream east of the continental shelf and being swept to the northeast and turtles getting stuck in the Bay of St. Lawrence (there used to be a channel between Cape Breton Island and mainland Nova Scotia but it was recently filled in with a causeway). See the map above if you need help finding those. Anyway, that movie’s pretty much the highlight of my thesis. I must have seen it several dozen times already but I still get a kick out of watching the turtles swim around.

Reference

Sherrill-Mix, S.A., James, M.C., Myers, R.A. (2007). Migration cues and timing in leatherback sea turtles. Behavioral Ecology, 19(2), 231-236. DOI: 10.1093/beheco/arm104