Migration in turtles and tortoises
Hey y’all, it’s Ben from the Herpetology Team, and I hope y’all have had an excellent holiday. Today, I’d like to discuss the migratory patterns of turtles. When many folks think of migratory animals, turtles are usually not the type of animals that come to mind. When I think of migratory animals, the rufous hummingbird, Selasphorus rufus, or the Arctic tern, Sterna paradisaea, are the first examples that come to mind. While it is true that many species of turtles have a relatively small range throughout their lifetimes, some species demonstrate the exact opposite throughout their lifecycle.
The behaviors of turtles and tortoises, like most animals, vary seasonally, annually, and throughout their lifecycle. Generally, migratory behavior is cyclical and may take place on a daily, seasonal, annual or even longer basis. Species often migrate in search of more favorable environmental conditions. Those favorable environmental conditions can include finding habitats with more food, along with increased breeding and/or nesting opportunities. I’ll proceed by briefly going over two examples of chelonian migration.
Chelonoidis nigra, or the Galapagos tortoise, is the largest tortoise species on Earth. The largest individuals can reach weights of over 800 pounds and lengths around six feet. This species is so large that one may think that it doesn’t have the mobility to migrate long distances. While it is true that they aren’t true long-range migrants like some birds or ungulates, they are certainly migratory! This species displays a phenomenon known as altitudinal migration. This process involves animals migrating from one elevation to another and is typically a response to changes in climate and food availability at certain elevations. During the wet seasons on the Galapagos Islands, the giant tortoises spend their time at lower elevations. The increased rainfall during this season causes these habitats to turn into grassy plains, an excellent food source for this species. During the dry season, the Galapagos tortoise will make the gradual trek to meadows at higher elevations. While rainfall is uncommon at these higher elevations during the dry season, the increased humidity at these elevations allows grasses to thrive. So, this altitudinal migration of the Galapagos tortoise is likely a pursuit of food. Many Galapagos tortoises often utilize the same routes throughout their migration. Over generations this process has created tortoise highways of sorts. This example is fascinating to me, as it documents that even some of the slowest terrestrial vertebrates on Earth can still display migratory behavior!
The largest of all living turtles is Dermochelys coriacea, the leatherback sea turtle. This species can attain lengths up to seven feet and weights up to 1500 pounds. These animals are truly massive, yet for such large creatures, they are surprisingly mobile. Some individuals have been documented diving to depths of 3900 feet and are one of the most highly migratory animals on Earth. The leatherback sea turtle may migrate more than 10,000 miles each year as they transition between foraging, breeding, and nesting sites.
Sea turtles migrate thousands of miles each year from foraging grounds to very specific nesting areas. These incredible animals may nest within a few square miles from where they hatched. Huge numbers of sea turtles will often congregate in very small areas in order to nest. Due to their specific nesting requirements, the preservation of these nesting areas is of utmost importance for the conservation of imperiled sea turtle species. While the migratory behaviors and site fidelity of sea turtles is well documented, the mechanism of how they accomplish this feat is still mysterious. Scientists have hypothesized that sea turtles detecting changes in environment, climate, and photoperiod will trigger these migratory behaviors. Ocean currents that propel the sea turtles back towards their more equatorial nesting sites assist the movement of these populations. Once migrating, scientists have postulated that sea turtles will navigate these waters by using magnetic crystalline structures within their brains as an internal compass. This internal compass functions by using Earth’s magnetic field.
The migratory behaviors of turtles are fascinating. While many chelonian species have a relatively short-range migrations, some chelonians have annual migrations that transverse the world’s oceans. I am captivated by the fact that both of the world’s largest extant chelonian species both exhibit migratory behavior. I hope y’all learned something new about chelonians today and I hope y’all have a Happy New Year!
Keeper I, Herpetology