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.