Here is some info I recently became privy too. It's one of those "so simple things."
This info was copied from V a r a n u s . n l
Varanus.nl is a monitor forum whose main focus is the breeding of monitors.
The author is Dr. Danny Brown
Again, it was NOT written by anyone here. It is posted on another website!
Hemipenal transillumination as a sexing technique in Varanids
By Dr Danny Brown
In 2007, Davis and Leavitt adapted a means of checking for gravidity in the very small skink species Xantusia vigilis as a means of sexing these very small skinks. I have adopted this technique (which I have called Hemipenal Transillumination technique or HTI technique) and have subsequently utilised it for sexing over 60 species of Australian lizards at various ages including dragons, geckos, monitors and skinks. The technique can be used on adults and juveniles and does not require that the individuals are sexually mature before the characteristics are visible. Like any technique, practise makes perfect and regular examination of individuals of known gender can allow you to develop a “feel” for what to expect and for which instrumentation works best for which species.
The technique is as follows :
The animal is positioned on its back so that the tail is directed towards the handler. A small, focussed, very bright (but non heat producing) light source is positioned behind the dorsal side of the tail to direct a beam of light through the tail base. I have found that visualisation is improved if the light beam is directed only behind the tail i.e. stray light around the edges of the tail are limited. This light allows the internal anatomy of the tail base to be visualised. In male lizards, the hemipenes are visualised both by their increased blood supply compared to adjacent tissue and the increased density of tissue in this area. They will be seen as either red dots, red ovals or as a “dull redness”. The latter refers to an overall red glow in the tail base. This is caused by a combination of an increase in blood supply and tissue density in the tail base (dorso-ventral thickening to accommodate the hemipenes) and degrees of “shadowing” (limiting light penetration) and “luminance” (as the light penetrates the hemipene and shines through what essentially becomes a red light filter) that creates an appearance of an overall red glow. In a female, an absence of red structures and a general yellowish glow is observed (as the light is only penetrating pure tail tissue). In some adults, the appearance will vary with mood, body temperature and breeding season depending on factors such as seasonal hemipenal size, voluntary extrusion of hemipenes , tail position and hemipenal blood supply changes (often one will appear more “engorged” than the other).
The primary limitations of this technique are light intensity and tissue penetration of light. These two go hand in hand to some degree in that specimens with a dorso-ventral tail diameter of 8-10 mm or smaller are the most ideal candidates for this technique. Specific features such as heavy dorsal pigmentation, heavy dorsal scalation , tail thickness >8-10 mm and handling difficulties may also limit this technique although technique modifications (such as side on viewing) can be used to work around this. The “side on” technique involves (as the name suggests) placing the light source against the side of the tail. In those species where this technique is warranted due to dorsal visualisation issues, male exhibit the “dull redness” as described above whilst females exhibit a clean yellow glow. In species with lateral flattening of the tail and its base, such as water monitors, side on viewing through this significantly narrower area of tissue may allow hemipenes to be observed in the same detail as if the light is dorsally directed (as in other species).
I maintain two torch types, an incandescent or halogen bulb producing a yellowish light and an LED torch producing bright white light. In some species, tail density is too thin for the use of an LED light as the light “blasts straight through” whereas an incandescent bulb produces a less harsh light that enhances the hemipenes more appropriately. In larger or spiny species, an incandescent bulb may be too subtle and a LED light is require to “blast past” the impediments to visualisation. The reduction of light scatter around the edges of the tail can also improve visualisation. This may be easily overcome using thick tape over the torch lens (with a viewing holes cut centrally) or by placing a cap over the torch end (plastic chair leg rubbers are ideal) with a viewing hole cut centrally. Oval shaped viewing holes are preferred over round ones. Technically, a purpose built sexing table or box could also be built and may allow a much larger torch to be used (or alternatively a dimmable electric globe). Be very careful with excessively intense light sources as these may also produce significant heat and may cause heat damage to the tail if the animal is left in position for an excessive period. The darker the room that the technique is used in, the better the visualisation. In the field, I have used this technique inside a dark backpack with good results.
In summary, using this technique in monitors has produced the following results :
• Very easy with small arboreal species (e.g. V. bushi, V. gilleni, V. caudolineatus).
• Small terrestrial species (e.g. V. storri, V. brevicauda, V. primordius) are quite easy up to 10 mm tail thickness.
• Juveniles of many smaller species sexable from 5 cm snout to vent length.
• Juveniles of larger species may be sexed as long as tail thickness is > 10 mm. I have only had the opportunity to trial a single larger species being a 3 month old V. spenceri but in this specimen hemipenes were clearly visible (using a side on technique).
• Some species may partly evert hemipenes when handled (e.g. V. primordius and V. brevicauda are notorious) making them difficult to visualise in this state (they are essentially tucked up under the cloacal rim but not visible externally) . This can be overcome by sexing whilst cooled or by placing light pressure at the hemipenal base with a finger to stop the hemipenes being extruded.
• Hemipenes in adults will appear as long red ovals except if partly extruded (when they will be seen as red dots very close to the cloacal edge). In juveniles they may appear as thin ovals but are more commonly represented by red "dots".
• Larger species can be sexed “side on” and males will appear as a “dull redness” rather than a yellow clear glow.
Sexing of an adult male V. storri using HTI
Sexing of an adult female V. storri using HTI
Sexing of an juvenile male V. storri using HTI
Sexing of an juvenile female V. storri using HTI
I would welcome feedback from anybody with regards to the use of this technique in any other species(that I have not had access to for trials), particularly V. prasinus, glauerti, kingorum, pilbarensis and the larger monitor species.
Davis, A.R. and Leavitt, D. H., 2007. “Candlelight vigilis: A noninvasive method for sexing small, sexually monomorphic lizards”, Herpetological Review. 38 (4): 402-404.