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Turtles Seek Heat While Still in the Egg…Do They Also Choose Their Sex?!

Pelodiscus sinensisTurtles and other reptiles are full of surprises when it comes to reproduction.  In the past few decades we’ve learned that incubation temperature, not genetics, determines the sex of many species, that some have dispensed entirely with males (i.e. the Brahminy Blind Snake) and that the massive Komodo Dragon is capable of reproducing without fertilization.  Recently (May, 2011), biologists have determined that turtle embryos move within the egg and actively seek heat.  This finding may cause us to re-examine conservation techniques, and raises an array of important questions – i.e., can turtle embryos actually determine what sex they will be?

Active in the Egg

Researchers from the Chinese Academy of Sciences found that basking behavior may be as important to turtle embryos as it is to fully-formed animals.  In laboratory experiments, embryos of the Chinese Soft-shelled Turtle, Pelodiscus sinensis, were able to sense slight temperature differences (1.8 Degrees F) and could move towards a heat source located outside of the egg.

The researchers repeated their findings in semi-natural conditions.  They oriented 540 eggs so that they could later trace the embryos’ movements by shining a strong light at the shell (a process known as “candling”).  Some of the eggs were buried in “nests” located on sloping ground, where sun exposure would vary, while others were located on flat ground, where the sun would strike the eggs evenly.  All of the embryos in the sloped nests migrated towards that portion of the egg that received the most sun; the embryos in the other nests did not move.

Conservation Implications

Being able to predict the sex of turtle hatchlings has important implications for captive breeding and “head-starting” programs.  In most turtles that have been studied, warmer incubation temperatures produce male hatchlings.  In Chinese Softshells, warmer temperatures also speed up hatching by approximately 4.5 days for each rise of 1.8 F.

I’ve assisted in field research which revealed that certain female Green and Leatherback Turtles usually choose to nest in sunny locations, while others often nest beneath beach vegetation (cooler sites, in theory).  An egg’s location within the nest – top or bottom – will also affect its incubation temperature.  Perhaps these are ways of assuring a beneficial sex ratio.  But if the embryos themselves are actually regulating their own temperatures, we have yet another variable to consider.

We do not yet know how this new information will affect our understanding of turtle biology and conservation…until the Chinese Academy of Sciences study, we were not aware that turtle embryos could sense heat and move within the egg.  Just goes to show the importance of paying attention to details, no matter how common the species…even very basic information is still coming to light.

Chinese Softshells in the Food Trade

Turtles in Korean MarketThe Chinese Softshell is bred commercially, as a food item, in several countries.  While farming does take pressure off wild populations, the turtles are, unfortunately, subjected to horrendous treatment in many food markets.

Years ago I tried, unsuccessfully, to induce the FDA to regulate the handling of Chinese and Florida Softshells in NYC.  Sadly, I still see them in food markets here, and they continue to be held under terrible conditions.  Recent efforts to stem the sale of live turtles and frogs by Wal-Mart (in China) have fallen upon deaf ears (please see this article for details).



Further Reading

Study abstract

Video of Captive Chinese Soft Shell

Turtles of the world

Pelodiscus sinensis image referenced from wikipedia and originally posted by Bastet78

Turtles in Korean Market image referenced from wikipedia and originally posted by Vmenkov


  1. avatar

    Strange how asian food markets, even in western countries, are so unregulated, but the sale of the same animals for the pet trade causes much trouble as concerns legalities, public health, etc. Probably if we herpers would subscribe as a separate cultural group free to exercise our rights, we would have been free of all these problems.

    • avatar


      Enforcement depends upon the country. I’ve been involved in this here in NYC, where laws are strict and Asian food markets checked carefully…have had many hundreds of frogs and turtles turned over to the zoo via confiscation over the years. Of course, national security and other crimes take precedence, but species, sizes, licenses, and time of harvest is checked. Regs vary by state, and of course enforcement depends upon budgets. In NY, diamondback terrapins may only be taken in winter; no regs on Florida or Chinese softshells as far as I know. However, where human health is involved a hard line is taken. For example, despite tight city budgets some years back I was able to get funding to check Mercury levels in food market softshells. Actaually, at least here in the USA, food animals are generally scrutinized more closely than pet trade species…human health factor, etc., plus animal rights folks are always on alert for mishandled animals in markets..pet stores, however poorly run, keep animals in better conditions. But pet laws becoming stricter each year as well..in NY State, it is now illegal to keep or sell any species native to the state, or a subspecies even if that subspecies is not native (to avoid ID problems) best, frank

About Frank Indiviglio

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Being born with a deep interest in animals might seem unfortunate for a native Bronxite , but my family encouraged my interest and the menagerie that sprung from it. Jobs with pet stores and importers had me caring for a fantastic assortment of reptiles and amphibians. After a detour as a lawyer, I was hired as a Bronx Zoo animal keeper and was soon caring for gharials, goliath frogs, king cobras and everything in-between. Research has taken me in pursuit of anacondas, Orinoco crocodiles and other animals in locales ranging from Venezuela’s llanos to Tortuguero’s beaches. Now, after 20+ years with the Bronx Zoo, I am a consultant for several zoos and museums. I have spent time in Japan, and often exchange ideas with zoologists there. I have written books on salamanders, geckos and other “herps”, discussed reptile-keeping on television and presented papers at conferences. A Master’s Degree in biology has led to teaching opportunities. My work puts me in contact with thousands of hobbyists keeping an array of pets. Without fail, I have learned much from them and hope, dear readers, that you will be generous in sharing your thoughts on this blog and web site. For a complete biography of my experience click here.
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