Ackie monitors reach sexual maturity around the time they’re ~1 year old and/or measure 12” (30-31cm) from snout to vent.
Monitor lizards are monomorphic, which means that the sexes are difficult to differentiate. However, that doesn’t mean male and female ackie monitors look exactly the same. They do have some key differences if you look closely:
Male ackie monitors are generally larger and have blockier heads. Most notably, they have grippy spur-like scales on the underside of their tail which will hook onto a fingernail when run from the head to the tail. They tend to have bright red eyes (Kuhn & Julander, 1999).
Female ackie monitors are generally smaller with narrower, pointier heads. They also have spur-like scales under their tails, but these are smooth rather than grippy. They tend to have soft orange/brown eyes.
You can also use the hemipenal transillumination technique (aka flashlight technique) on individuals with tails less than 10mm (Brown, 2009). This technique can be used to look for hemipenes in a noninvasive manner, and can be used on juveniles 4” long from snout to vent and larger:
- Candle the base of the tail from above with a focused LED light, like a flashlight.
- Look at the glow on the underside of the tail.
- Hemipenes look like shadowy red dots, red ovals, or a general red glow.
- Females will have an absence of red structures and produce a generally yellowish glow.
Bear in mind that hemipenal appearance may vary with mood, body temperature, and breeding season.
If you need a visual aid, here’s a helpful video by Professor Herp on sexing ackie monitors:
“Popping” as a sexing technique is potentially harmful to the lizard and not recommended.
More ackie monitor health topics:
- Egg Binding
- Egg Cycling
- Metabolic Bone Disease (MBD)
- Respiratory Infection