Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Temperature of egg incubation determines sex in Alligator mississippiensis


The factors controlling sexual differentiation in crocodilians are unknown, but heteromorphic sex chromosomes are absent from all species1. Nichols and Chabreck2 speculated that the sex of Alligator mississippiensis was not rigidly determined at the time of hatching but could be influenced by the post-hatching environment. They presented little evidence to support their hypothesis3 (no histological sections of hatchling gonads, no indication of the sex ratio at hatching), and their study failed to take account of habitat preferences of adult male and female alligators4. Here we demonstrate by laboratory and field experiments, that in A. mississippiensis: (1) Sex is fully determined at the time of hatching and naturally irreversible thereafter, and depends on the temperature of egg incubation, temperatures 30 °C producing all females, 34 °C yielding all males. (2) The temperature-sensitive period is between 7 and 21 days of incubation. (3) Natural nests constructed on levees are hotter (34 °C) than those constructed on wet marsh (30 °C), thus the former hatch males and the latter females. (4) The natural sex ratio at hatching is five females to 1 male. (5) Females hatched from eggs incubated at 30 °C weigh significantly more than males hatched from eggs incubated at 34 °C. This weight difference constitutes a possible selective evolutionary advantage of temperature-dependent sex determination (TSD) in alligators in that females become large and sexually mature as early as possible. The occurrence of TSD in alligators has wide-ranging implications for embryological, teratological, molecular, evolutionary, conservation and farming studies as well as for theories relating to the extinction of other Archosaurs.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Similar content being viewed by others


  1. Cohen, M. M. & Gans, C. Cytogenetics 9, 81–105 (1970).

    Article  CAS  Google Scholar 

  2. Nichols, J. D. & Chabreck, R. H. Am. Nat. 116, 125–137 (1980).

    Article  Google Scholar 

  3. Ferguson, M. W. J. Archs oral Biol. 26, 427–443 (1981); J. Craniofacial Genet. dev. Biol. 1, 123–144 (1981); Trans. zool. Soc. Lond. 36, 99–152 (1982); in Biology of the Reptilia Vol. 12 (eds Gans, C. & Billett, F. S.) (in the press); in The Reproductive Biology and Conservation of Crocodilians (ed. Tryon, B. W.) (in the press); Science 214, 1135–1137 (1981); Evolution (submitted).

    Google Scholar 

  4. Joanen, T. & McNease, L. Proc. A. Con. J. SE Assoc. Fish & Wildl. Agencies 23, 141–151 (1969), 24, 175–193 (1970), 25, 106–116 (1971), 26, 252–274 (1972), 28, 482–500 (1974), 29, 407–415 (1975), 32, 179–181 (1978); Int. Zoo Yb. 19, 61–66 (1979).

    Google Scholar 

  5. Bull, J. J. & Vogt, R. C. Science 206, 1186–1188 (1979).

    Article  ADS  CAS  Google Scholar 

  6. Bull, J. J. Q. Rev. Biol. 55, 3–21 (1980).

    Article  Google Scholar 

  7. Pieau, C. C.r. hebd. Séanc. Acad. Sci., Paris 272, 3071–3074 (1971); 274, 719–722 (1972); 277, 2789–2792 (1973).

    CAS  Google Scholar 

  8. Yntema, C. L. J. Morph. 125, 219–252 (1968); 159, 17–28 (1979).

    Article  CAS  Google Scholar 

  9. Yntema, C. L. & Mrosovsky, N. Herpetologica 36, 33–36 (1980).

    Google Scholar 

  10. Mrosovsky, N. Am. Zool. 20, 531–547 (1980).

    Article  Google Scholar 

  11. Charnov, E. L. & Bull, J. J. Nature 266, 828–830 (1977).

    Article  ADS  CAS  Google Scholar 

  12. Charnov, E. L., Los-den Hartogh, R. L., Jones, W. T. & Van der Assem, J. Nature 289, 27–33 (1981).

    Article  ADS  CAS  Google Scholar 

  13. Fisher, R. A. The Genetical Theory of Natural Selection (Oxford, 1930).

    Book  Google Scholar 

  14. Bull, J. J. Heredity 46, 9–26 (1981).

    Article  Google Scholar 

  15. Packard, G. C., Packard, M. J., Boardman, T. J. & Asher, M. D. Science 213, 471–473 (1981).

    Article  ADS  CAS  Google Scholar 

  16. Coulson, R. A. & Hernandez, T. Biochemistry of the Alligator (Louisiana State University Press, Baton Rouge, 1964).

    Google Scholar 

  17. Chabreck, R. H. & Joanen, T. Herpetologica 35, 51–57 (1979).

    Google Scholar 

  18. Fisher, R. A. & Yates, F. Statistical Tables for Biological, Agricultural and Medical Research 6th edn (Longman, London, 1963).

    MATH  Google Scholar 

Download references

Author information

Authors and Affiliations


Rights and permissions

Reprints and permissions

About this article

Cite this article

Ferguson, M., Joanen, T. Temperature of egg incubation determines sex in Alligator mississippiensis. Nature 296, 850–853 (1982).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing