I’ve seen mother sea turtles laying their eggs on the beach (in Costa Rica), but I’ve never seen them hatch. Here’s a great short video of 100 baby loggerhead turtles (Caretta caretta) hatching in the Florida Keys. The explanation is below; I always wondered how they knew where the water was:

Using infrared lighting, a live-streaming, high-definition “turtle webcam” positioned on a beach in the Florida Keys recorded the hatch of about 100 baby loggerhead sea turtles on Friday, July 25, just before 9 p.m.

Friday evening, the 3-inch-long babies erupted from a hole, came out en masse and headed to the Atlantic Ocean under dim moonlight.

The camera uses infrared lighting so hatchlings won’t be confused by artificial light and will go to sea — guided by moonlight reflecting on the water — instead of pushing further onto land.

Wikipedia reports that the females reach sexual maturity between 17 and 33 years (that’s a long time!), and have a total lifespan of 47-87 years. They are also reported to produce four clutches in a year and then go quiescent for 2-3 years before reproducing again. Let’s do a calculation. Assume that they become sexually mature at 25 years (the average of 17 and 33), and live an additional 42 years (assuming an average lifespan of 67 years).  Let’s say they reproduce once every 2.5 years (the average of 2 and 3). That means they have 16.8 reproductive bouts. If they produce 400 eggs per bout, that’s a total lifetime output of 6,720 hatchlings.  If the population is stable, each female produces two surviving adults per lifetime. The pre-reproductive mortality of newly-hatched turtles is therefore about 6718/6720, or 99.97% (I hope I’ve calculated correctly). That means that there is a huge juvenile mortality.

Remember, the turtles don’t have a huge juvenile mortality simply because they produce so many offspring; rather, they produce so many offspring partly because they face such a high mortality rate; evolution is compensating for the environmentally determined survival rate. It was the population geneticist Ronald Fisher who realized this—not for sea turtles, but for anything that produces a large number of offspring.

1. GBJames
Posted July 27, 2014 at 11:43 am | Permalink

Amazing.

2. Posted July 27, 2014 at 12:15 pm | Permalink

I love turtles. John Fahey loved turtles. Darwin loved turtles. What more does one need to justify preservation?

3. Posted July 27, 2014 at 12:21 pm | Permalink

The other thing that means is that the turtles are an important part of the food chain; lots of other animals are going to depend on the turtles for food.

Given their fecundity, they might also make for an interesting species for aquaculture. They’re currently endangered. Could commercial viability help save them?

b&

• Moarscienceplz
Posted July 27, 2014 at 1:23 pm | Permalink

Only if you consider the beef industry as having “saved” the wild aurochs.

• Lars
Posted July 27, 2014 at 4:45 pm | Permalink

Archie Carr tried this with the Green Turtle (Chelonia mydas) in the Caribbean, starting in the 60s; I’m not sure how the experiment panned out. However his priority was to re-establish turtle populations across the Caribbean – they’d been practically wiped out by egg-collecting and the harvest of adults. Taking the pressure of wild, recovering populations by establishing a commercial turtle-farming industry was worth a try, but ran the risk of serving as a mask for continued wild-turtle harvesting.

4. Diana MacPherson
Posted July 27, 2014 at 12:21 pm | Permalink

Yay baby turtles! I wonder what happens if they happen to hatch during new moon or if they never hatch during a new moon and the moon phases have something to do with hatching.

• Posted July 27, 2014 at 12:31 pm | Permalink

…or if they hatch when it’s overcast….

The moon reflections theory sounds fishy to me. On the other hand, they’ve spent their entire egg existence in a fixed orientation with the sound and vibrations of the surf. Seems more likely that that would explain it, and they just head towards the noise.

I just re-watched the video. The nest is in a depression, with no line of sight (at baby turtle height) to anywhere else on land. The turtles that emerge on the side closest to the camera are all veering off to the sides, and the ones farthest from the camera are all heading straight away. They can’t have seen the ocean yet, but they’re already heading in the correct direction.

b&

• Diana MacPherson
Posted July 27, 2014 at 12:32 pm | Permalink

The surf sound makes more sense to me too.

• Lars
Posted July 27, 2014 at 4:52 pm | Permalink

Check this out (http://www.jstor.org/stable/4535039); there’s been a lot of work on orientation in sea turtle hatchlings, and the data suggest that it’s visual cues they respond to.

• Posted July 28, 2014 at 7:25 am | Permalink

Thanks for that. Still doesn’t make intuitive sense…but if they’ve done the research….

b&

• Marlene Zuk
Posted July 28, 2014 at 7:20 am | Permalink

More recent than Lars’ suggestion is the work of Ken Lohmann at University of N Carolina; his website (http://www.unc.edu/depts/geomag/) has lots of cool information on navigation in sea turtles as well as other animals. I visited his lab a while back and it was great to see the turtles in their harnesses for the experiments. I explained the set-up to my students later and one referred to the “turtle bikinis” in an exam.

5. Gregory Kusnick
Posted July 27, 2014 at 12:56 pm | Permalink

Remember, the turtles don’t have a huge juvenile mortality simply because they produce so many offspring; rather, they produce so many offspring partly because they face such a high mortality rate; evolution is compensating for the environmentally determined survival rate.

On the other hand, if the turtles invested more in child-rearing instead of leaving them to fend for themselves, the juvenile survival rate would be higher, and they wouldn’t have to produce so many. So in some sense the fact that turtles have opted for low investment and large numbers does feed back into the survival rate.

• Lars
Posted July 27, 2014 at 4:58 pm | Permalink

Complex problem – you have long-lived adults that can breed repeatedly over decades, and furthermore are shaped to carry a large cargo of small eggs. Limited investment in individual offspring is a logical outcome.
Some workers think that turtles have worked themselves into a corner, although it’s not a bad corner – they don’t seem to have ever been live-bearers, and can’t do a lot for their offspring, but on the other hand they are well-suited to produce a lot of small cheap offspring because once they’ve reached a certain size, they don’t have a lot of worries and can keep reproducing over a long lifespan. It’s worked since the Triassic.

6. Jim Thomerson
Posted July 27, 2014 at 2:38 pm | Permalink

There are a lot of different tactics for, on average, replacing yourself and a partner in the population. If you can manage that, your lineage is at least hanging in there.

7. gravelinspector-Aidan
Posted July 27, 2014 at 2:40 pm | Permalink

If the population is stable, each female produces two surviving adults per lifetime. The pre-reproductive mortality of newly-hatched turtles is therefore about 6718/6720, or 99.97% (I hope I’ve calculated correctly). That means that there is a huge juvenile mortality.

The other side of that coin is that, IF conservationists can intervene to bring (for example) 10 eggs from each clutch to a degree of maturity where they’re at much lower risk of predation WITHOUT adverse effects on behaviours like homing instincts, then the population can be rapidly ramped back up with fairly low levels of intervention.
Potentially it would be sufficient to (for example) retrieve the test specimens on a hatching night, stock them locally in a controlled sea-water fed aquarium and fatten them up for a month, then release them at their original capture site one full moon later. But as we know, Nature is rarely so simple and accommodating.

8. Tess
Posted July 28, 2014 at 4:24 am | Permalink

Sometimes the hatching event is called a “boil,” because they all “boil” out of the sand. The deepest babies need the higher ones to get out of the way.

9. Posted July 28, 2014 at 5:45 am | Permalink

On a smaller scale, this is not unlike human reproduction. Until the beginning of last century, humans tended to have a relatively large number of offspring (and still do in some third-world countries) because only two or three would survive to adulthood and provide for their parents when their parents became old, weak and vulnerable. Modern medicine and especially vaccinations changed all that, especially in western industrial countries. This led to abortions and family planning/contraception and much smaller families. However, vaccines have led to large families in third world countries in which most and even all children survive, and this increases poverty as there are so many more mouths to feed in these families. Of course there are campaigns and NGOs that go around villages to teach family planning, but they have little effect in some cultures which are against it.

• Diana MacPherson
Posted July 28, 2014 at 9:19 am | Permalink

I’ve always wanted to lay eggs.

• Posted July 28, 2014 at 10:35 am | Permalink

Me too, until I realized that, to be viable, they would have to be as large as a newborn in its amniotic sac, whether they were fertilized or not. Still, they would make an omelet large enough to feed a large family…

• Diana MacPherson
Posted July 28, 2014 at 4:38 pm | Permalink

Ha! I’d rather lay lots of eggs like insects do or the female in the movie Species.

10. Carole
Posted July 28, 2014 at 7:00 am | Permalink

Amazing!!