Iguana Ovaries and Testes
©2000 Melissa Kaplan
Germ cells, formed while the iguana is still an embryo in the egg, ultimately become either ova (eggs) or spermatozoa (sperm), depending on the sex of the iguana. During this time, the testes of the male iguana, already larger in relation to body mass than those of other animals, begin to enlarge as sperm is produced and stored.
Iguana ovaries and testes are tucked up well inside the torso of the iguana. The annotated drawing below is a little misleading as the testes and ovaries lay behind the liver, as one sees it when going into the coelomic cavity through an abdominal incision, as one does when spaying or neutering an iguana. The ovaries and testes are paired, attached by the mesovarium and mesorchium, respectively, to the left and right sides of the dorsal wall. The adrenal glands are embedded in the mesovarium and mesorchium mesentery as well.
Click on photo to see enlargement
Drawing from: Laboratory Anatomy of the Iguana, by Jonathan C. Oldham and Hobart M. Smith. 1975. Wm. C. Brown Company Publishers, Dubuque, IA
Each oocyte (individual single-cell) is enclosed in its own membrane; each ovary consists of a group of oocytes contained in an overall membrane. Fertilization occurs while the early developing eggs (secondary oocytes) are still in the ovaries. After fertilization, yolk development continues (vitellogenesis), with the ova continuing to grow, until the mature ova are 10-100 times their pre-vitellogenic size (depending on species). "Ovulation" is the rupture of the individual and overall ovarian membranes as the eggs are freed from the enclosing membranes. As the egg passes through the nearby oviduct (one for each ovary), layers of fibers and proteins are applied to each egg (with each layer being applied at specific points within the oviduct), with the final layer being calcium carbonate (calcite crystals in crocodilians and squamates; aragonite crystals in turtles). The oviducts empty into the cloaca, thus eggs are shelled just before they are actually laid.
Male iguanas have the largest testes for their body size of all reptiles. Some keepers of sexually mature adult males don't doubt this for a second, given how their sweet Dr. Jekyll lizard becomes the raging Mr. Hyde once breeding season hits.
Iguana testes comprise 1% the male iguana's body mass. By comparison, the great apes have testes ranging in size from 0.017% to 0.269%. The great apes, then, have one-fourth as much testicular tissue, relatively speaking, as do the green iguanas. (Human males come in somewhere in the great apes range, with a 6'0" 180 pound human male having testes weighing in at about 0.041% of body weight.) Iguana keepers who have had occasion to clean up iguana ejaculate are not surprised by this datum.
Germ cells are the cells that are stored in the testes and ovaries which, when triggered to start growing, become either the sperm or eggs. Follicles are the enlarging ova in ovaries, prior to fertilization. Vitellogenesis is the development of the egg yolk in the follicles. Fertilization occurs when the sperm meets the ova as they pass through the oviduct, a passage which also layers the outside of the egg with calcium to form a shell. Intromission is the act of copulation, during which the male transfers his sperm into the female. Oviposition is the laying of eggs.
When a male iguana is neutered, the surgery is the same as a spay, as the testes are in the same position inside the body as are the female's ovaries. Since there is little evidence to support the premise that neutering seasonally aggressive sexually mature males reduces or eliminates the breeding season aggression, those thinking about neutering their male iguanas should think again...
Sources for this article include:
Burghardt, Gordon, A. Stanley Rand. 1982. Iguanas of the World: Their Behavior, Ecology and Conservation. Noyes Publishing, Park Ridge NJ.\
Mader, Douglas R. (ed.). 1996. Reptile Medicine and Surgery. WB Saunders, Philadelphia PA.
Oldham, Jonathan C. and Smith, Hobart M. 1975. Laboratory Anatomy of the Iguana. Wm. C. Crown Company Publishers, Dubuque IA.
Zug, George R. 1993. Herpetology: An introductory biology of amphibians and reptiles. Academic Press, Division of Harcourt, Brace & Company, San Diego CA.
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