By GEORGE BALAZS & RODERIC MAST

Of the seven species of sea turtles, only the green turtle is known to emerge from the sea to bask, or, as defined by Merriam-Webster, “to lie or relax in a pleasant warmth or atmosphere.” Greens of both sexes and all sizes and life stages exhibit this fascinating behavior, but it only occurs at a few well-documented locations in the Pacific, including the Galápagos Islands, Mexico’s Socorro Island, Australia’s Wellesley Islands, and—most notably—the Hawaiian archipelago.

In Hawaii, basking occurs by day or night on shorelines where nesting occurs or adjacent to algal foraging pastures; turtles have also been observed basking on floating objects. Turtles may crawl ashore on their own or passively surface to bask as a result of falling tides in shallow bays. Basking has even been documented in captive animals at Hawaii’s Sea Life Park, where turtles emerge onto artificial nesting beaches. Basking turtles often cluster together, suggesting that the behavior may have a social function; there may also be a genetic component involved. Substrates on which turtles bask include black to light-colored sand of varying particle sizes, rocks and old lava flows, limestone benches and the tops of offshore coral heads, shipwrecks, and even beach lounge chairs!

In the Galápagos and northwestern Hawaiian Islands, green turtle basking has been known for centuries from the logbooks of early European voyagers. But in the main Hawaiian Islands, basking didn’t exist before the 1990s; then the behavior began to spread rapidly in both scope and magnitude, concomitant with sharp increases in turtle populations tied to the 1970s ban on commercial harvest. Now a normal and iconic feature of Hawaiian beach landscapes, basking greens (locally called honu) have grown accustomed to people being close to them, and honu has facilitated an array of life history research projects about the phenomenon. Not surprisingly, human conflicts have erupted over the need to manage touristic “turtle viewing” on beaches often shared with bathers, surfers, fishers, and others. (See “Trapped in the Crossroads of Honu Conservation” from SWOT Report, vol. XII, pp. 38–39)

Not unlike other ectotherms, from freshwater turtles to snakes, lizards, and more, green turtles bask to optimize body temperature; a major thermal ecology study has gone deeper to suggest why. For instance, elevated body temperatures can mobilize stored fat and theoretically accelerate egg maturation in nesting females; warmer body temperatures can also speed and promote digestion in all sizes of turtles. Beyond warmth, basking may also be a means for females to avoid unwanted copulation attempts and for both sexes to stay out of harm’s way from predators like tiger sharks. And, intuitively, basking serves to conserve energy, since a turtle out of the water doesn’t need to periodically rise to the surface to breathe as it would when resting in underwater refugia. Another hypothesis, now supported by research, is that carrying capacities of certain foraging pastures in Hawaii are being exceeded as a result of the increased turtle population in recent decades, so basking may result from suboptimal nutrition. Further research will help shed light on this unique behavior of Pacific green turtles.

The first published photo of “a green turtle asleep on a sandy beach” in the northwestern Hawaiian Islands appeared in a 1925 issue of National Geographic Magazine. The caption provided what is possibly the most concise answer as to why green turtles bask: “These grotesque creatures browse in submarine fields of algae until hunger is satisfied, and then crawl heavily out to sprawl in the sand, safe from enemies in the sea.”

Sea turtles can elicit tricky questions from those curious about their mysterious lives and natural histories. And even sea turtle specialists can struggle to answer some of the most seemingly basic questions about sea turtle biology and conservation. If you are among the many specialists who have stumbled to concisely answer things such as “How many sea turtles are there?,” “How old do turtles get?,” or “Where do baby turtles go?,” then this feature is for you. Our hope is to set the record straight about often-asked questions with answers written by top experts who will prepare you to respond like an expert yourself. Moreover, we hope that for those questions about sea turtles that may still have no firm answers, this series can pique SWOT readers’ curiosity and drive them to conduct the research needed to solve the mysteries.

This article originally appeared in SWOT Report, vol. 16 (2021). Click here to download the complete article as a PDF.