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Rivas, J. A. and Ávila, T. M. 1996. Sex identification in juvenile green iguanas (Iguana iguana) by cloacal analysis. Copeia. 1996: 219-221. click here for a pdf
SEX IDENTIFICATION IN JUVENILE GREEN IGUANAS (Iguana iguana) BY CLOACAL ANALYSIS. Most adult lizards are sexually dimorphic, with males being larger and having wider femoral pores (Dellinger and von Hegel, 1990). Adult male green iguanas have larger dorsal crests, dewlaps, and skulls, and develop a darker, more reddish coloration than do females (Dugan, 1982; Fitch and Henderson, 1977). These sexually dimorphic features develop gradually as the animal grows (Dugan, 1982).
Rodda (1991) developed a method for sexing adult green iguanas. However, no sexing techniques for hatchling green iguanas have been published. The lack of such techniques has limited the potential of field studies on the population ecology and behavior of this species (Dugan, 1982; Van Devender, 1982).
In 1988, we studied green iguanas at the Fundo Pecuario Masaguaral, a cattle ranch in the central Venezuelan llanos (Guárico state, 8º 34' N. 67º 35' W.). We opportunistically collected 468 recent hatchlings and assessed the sex of all individuals by determining the presence or absence of hemipenes. Hemipenial eversion was accomplished by holding the animal upside-down and applying soft pressure on both sides of the base of the tail, pressing from the tail toward the head. In some cases it was necessary to open the cloaca slightly.
For a sub-sample of 84 individuals, the snout-vent length (SVL) was measured to the nearest mm and cloacal width and depth were measured with a caliper to 0.05 mm. Additionally, to measure the depth of the cloacal pouch in relative terms, a probe was inserted with the hind leg held adjacent to the tail. The depth of the pouch could then be recorded as equivalent to a given number of femoral pores (which extend distally from the cloaca along the ventral surface of the leg). This technique provided a relatively size independent measure of the depth of the cloacal pouch.
Eversion of the hemipenes proved to be an effective technique for sexing hatchling iguanas (239 females and 229 males), as hemipenes were easily everted. It was also possible to sex embryos in an advanced state of development (these came from five eggs accidentally broken while handling, approximately two weeks prior to hatching).
Sex determined through hemipenial eversion
was always consistent with the results achieved using the method of Dellinger and
von Hegel (1990) for Amblyrhynchus cristatus. Cloacal depth was significantly
greater in males (p< 0.01), both in absolute and relative terms (Table 1). Although in A. cristatus the differences are
greater (Dellinger and von Hegel, 1990), cloacal width was also significantly
greater in male
For 33 (14 females, 19 males) five month old iguanas, the sex determined through the depth of the cloacal pouch (both relative and absolute) was always consistent with that determined through cloacal width, but it was very difficult to evert the hemipenes. The sex determination of these individuals was 100% consistent with a later exam (14 month old) based on hemipenial eversion .
At 14 months sex determination in 92 iguanas (47 females and 45 males), ranging from 131 to 177 mm SVL, was always consistent based on all these measures: cloacal width, depth of the cloacal pouch and hemipenial eversion.
These results show that it is possible to identify the sex of iguanas before the appearance of secondary characters. Sex of neonates can be determined by eversion of the hemipenes or by measurement of the cloacal pouch depth or cloacal width. Larger juveniles (< one year old) are best sexed by measurement of the cloacal width or the cloacal pouch depth, because of the difficulty of everting the hemipenes in males of this age.
Acknowledgments: We thank T. Blohm for his collaboration and for his permission to work on his ranch. We also thank the New York Zoological Society (Wildlife Conservation International) for financial support of this work, and P. Andreadis, G. M. Burghardt, R. Lord, Z. Tárano, and J. Thorbjarnarson for editorial and technical suggestions in preparation of the manuscript.
REFERENCES
DELLINGER, T. AND G. VON HEGEL 1990. Sex identification through cloacal probing in juvenile marine iguanas (Amblyrhynchus cristatus). J. Herpetol. 24: 424-426.
DUGAN, B. 1982. The mating behavior of the
green iguana, Iguana iguana, p. 320-341. In Iguanas of the World: Their
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Noyes Publ.,
FITCH, H. AND R. HENDERSON 1977. Ages and sex
differences, reproduction and conservation of Iguana iguana. Contrib. Biol.
Geol. Milwaukee Pub. Mus. Vol 13.
RODDA, G.
1991. Sexing Iguana iguana. Bull.
VAN DEVENDER, R. 1982. Growth and ecology of
spiny-tailed and green iguanas in