Starting around this time every year, thousands flock to Florida's beaches with only one thing on their minds. They come from all along the Atlantic and the Gulf in droves to spend a few days getting wild with others they've barely met. That's right it's time for Spring B....eginning of loggerhead sea turtle nesting season.
What did you think I was talking about?
Courtesy: Daytona Spring Break via Flickr
Well actually spring break and loggerhead (Caretta caretta) nesting season don't match up perfectly. Thank god, can you imagine what kind of stuff a bunch of drunk bros would get up to with hundreds of sea turtles? Only the very first nesters come in during April, and the breeding season continues through the summer. Around the world's sub-tropical regions loggerheads return to the beaches they were born at to pass on their genes. The Southeast coast of the US hosts more loggerhead sea turtles than most other breeding regions around the world. Florida is especially important to these turtles. Ninety percent of all loggerheads born in the US nest in Florida.
Spring breakers and sea turtles have other things in common besides an intense urge to mate. They both have many stepped journeys that take them to the beaches of the southeast. After birth, young turtles head out to the open ocean and usually spend their first 5-7 years out at sea. It may seem a little odd for baby turtles to head out to the open ocean since there aren't a lot of places to hide, but there are far fewer predators out there. Plus the Atlantic has a unique region called the Sargasso sea which is named for its abundant floating fields of Sargassum (pronounced: sarj-ass-um) algae. Sargassum is perfect for little turtles because it's about the same color as they are and has a lot of spatial complexity for them to hide in. In these floating prairies, young loggerheads can grow fat on a diet of just about anything. Unlike other toddlers they aren't particularly picky and chow down on jellies, squid, shrimp, crabs, clams (even Tridacna), and fish. As they grow loggerheads rely on increasingly hearty prey so, like a college student raiding your fridge when he/she comes home for the summer, the turtles return to more abundant coastal waters to feed.
Courtesy: rosepetal236 via Flickr
But how do the turtles find their way? Human college students have the benefit of Google maps and Expedia, but loggerheads' flippers make it hard to type. Instead sea turtles use the magnetic field of the earth to navigate. To understand how this works we need to know a little bit about the magnetic field itself.
Beneath the planet's mantle, but above the very center is a region called the outer core. The outer core is made of liquid iron mixtures and it moves around in loops thanks to the heat put out by the inner core. The conductive nature of the iron ensures that as the mixture moves around it generates electrical current, which in turn creates a magnetic field. The planet's magnetic field varies in several ways across the surface of the earth. It has an angle relative to the center of the planet that becomes steeper the closer you get to the poles. The magnetic field also varies in its strength changing along seemingly more random lines (It's weakest around South America and strongest around Siberia and the ocean south of Australia). If you can somehow detect these different lines of magnetism (called isolines) you can use them to figure out where you are on the planet, just like you would use lines of latitude and longitude.
Courtesy: Wendell Reed via Flickr
Amazingly scientists have enough behavioral evidence to confidently say that turtles navigate by detecting the earth's magnetic field, but don't have enough evidence to figure out exactly how they do it. The problem is it's easy to do experiments and scan data to see where turtles are going, but it's really hard to find possibly microscopic structures that sense magnetism. Perhaps the most compelling way animals might detect isolines is by having evenly spaced crystals of magnetite in their bodies. Magnetite is an iron mineral that has magnetic north and south poles, just like man made magnets. If a series of magnetite crystals is laid out in a line then they'll push away from each other when they're turned perpendicular to a magnetic field, but pull towards one another if they're lined up with it. Magnetite has been found in salmon, several species of birds, and sea turtles, all of which are highly migratory. Scientists think these animals might be able to gauge how much the crystals are pulling or pushing on one another to figure out how isolines are oriented.
Orienting themselves by magnetic fields is so important to loggerheads that they familiarize themselves with the unique magnetic coordinates of their home beach as they hatch. Then as the turtles return, if the isolines have shifted which they often do, the turtles will dig nests away from their birth place.
Despite their incredible abilities loggerheads are an endangered species. Their biggest threat is coastal development and negative interactions with humans since they prefer the same beaches we do. So for anyone out there planning a trip to Florida this spring break; remember that loggerhead turtles are there to have a good time too. If we treat each other with respect then humans and turtles can all have as much fun as this kid:
References:
Brothers, J. Roger, & Lohmann, Kenneth J., "Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles", Current Biology, Vol 25 Issue 3, 2015, DOI 10.1016/j.cub.2014.12.035. Accessed via:
Lohmann, Kenneth J. & Johnson, Sonke, "The Neurobiology of Magnetoreception in Vertebrate Animals", Trends in Neuroscience, No. 23, 2000. Accessed via:
The Earth's Magnetic Field, University of North Carolina's Oceanweb. Accessed via: http://www.unc.edu/depts/oceanweb/turtles/juvenilemap/EarthMF.html
"Caretta caretta, Loggerhead Sea Turtle"
The Encyclopedia of Life
Accessed via: http://eol.org/pages/1056566/details
"Caretta caretta, Loggerhead Sea Turtle"
The Encyclopedia of Life
Accessed via: http://eol.org/pages/1056566/details
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