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Sexes of Heloderma

Sexes of Heloderma

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Two male heloderma

Sex determination of Heloderma suspectum proves to be difficult. Even experienced field biologists and breeders lead to false sex determinations (Ref. 19).

Obese specimen (a common tendency !) are not to be differentiated by external body shape criteria (Ref. 24).

All sexes of the depicted adult Heloderma are verified by copulation and/or depositing of eggs.

Methods for sex determination in adult Heloderma:

1. Body shape

  1. Head

    In comparison to females, the head of a male lizard is much more massive, broader and "higher". A look from the top to a females head shows softer contours and their cheeks are flattened.

    Head of male Gila monster Head of male Gila monster Head of female Gila monster Head of female Gila monster
    Head of male Gila monster Head of male Gila monster Head of female Gila monster Head of female Gila monster
    Male, adult
     
    Female, adult

  2. Body

    The body shape of a male resembles an elongated oval, whereas the body of a female shows a pear shape toward the rear legs.

    Body shape of male adult Gila monster Body shape of male adult Gila monster Body shape of female adult Gila monster Body shape of female adult Gila monster
    Male, adult
    Female, adult

  3. Tail

    The tail of a male tapers evenly to its tip, whereas the female´s tail appears "dented" near the cloaca and does not taper until the last third of the tip.

    Tail shape of adult male Gila monster Sexes: Tail shape of adult male Gila monster Sexes: Tail shape of adult female Gila monster Tail shape of adult female Gila monster
    male, adult
    female, adult

  4. Total impression of male and female

    Body impression of male Body impression of male Body impression of female Body impression of female
    male, adult
    female, adult

2. Ultrasound

With the help of an experienced physician, inner sexual organs such as gonads and follicles are unequivocally detectable and documented.

The sexual organs are dominated by a yearly cycle. The gonads and follicles enlarge from October to March/April which is the mating season. At the end of May, these have reduced their size and functionality. Taking this into consideration, the best time to determine the sex of the specimens using the ultra-sound method would be shortly before the actual mating season (ref. 46).

Prints of ultrasound examinations of doubtful sexes. You can observe in text on top of the pictures that two sexes had been falsely determinated by only using body shape characteristics of the individuals:
Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma
Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma Sex-determination: Gonades in adult Heloderma
>> video sequence <<

Ultrasound examinations of sexually mature females in March, representing developing and mature follicles.
Sex-determination: developing follicles Sex-determination: developing follicles Sex-determination: developing follicles Sex-determination: developing follicles
>> video sequence <<      
Sex-determination: developing follicles Sex-determination: developing follicles Sex-determination: developing follicles Sex-determination: developing follicles
>> video sequence <<      

Ultrasound examinations of adult male Heloderma suspectum in the ventral area of the tail spanning 2 cm in distance to the vent.
You can observe large and variable paired soft tissue structures representing the tips of the hemipenisses with their hemipenis pockets (Ref. 35).
Sex-determination in males with ultrasound: hemipenis pockets Sex-determination in males with ultrasound: paired structures Sex-determination in males with ultrasound: hemipenis pockets Sex-determination in males with ultrasound: paired structures
Sex-determination in males with ultrasound: hemipenis pockets Sex-determination in males with ultrasound: paired structures Sex-determination in males with ultrasound: hemipenis pockets Sex-determination in males with ultrasound: hemipenis pockets


Ultrasound examinations of adult female Heloderma suspectum in the ventral area of the tail spanning 2 cm in distance to the vent.
In comparison to the male tail there is no similar structure. The tail in its full length exposes a much more consistent structure using ultrasound imaging (Ref. 35).
Sex-determination in females: no paired structures Sex-determination in females: no paired structures Sex-determination in females: no paired structures


Sex determination in young Heloderma:

After several consecutive breedings and observing many hatchlings, I am now of the opinion that it is possible to determine the sex in young Heloderma. Sex determination can be successful in the hatchlings when they are still filled with absorbed yolk. This makes their body shape quite distinct. After hatching, both sexes show characteristic body and head shapes. The length and shape of the tail is less useful for sex determination.

1. Body

The body of a new born male is oval shaped and appears to look bloated. The body of a female hatchling looks bloated only to her rear legs and appears more "pear-shaped". The hatchling female nevertheless has an elongated body shape.

2. Head

In comparison the head of a young male Gila Monster is more triangular in shape. This gives the impression of a larger head than that of a female. The head of a hatchling female is similar in appearance to that of an arrowhead as it softly tapers down into the neck.


Examples of male and female hatchlings from 2005:


Body shape of male hatchling Body shape of male hatchling Body shape of female hatchling Body shape of female hatchling
male
female

After about one month of growth, these characteristics become difficult and sex determination may not be certain. With the age of about one year young Gila Monsters show more significant body shapes for sex determination.


Examples of the body shape criteria for sex determination of the same individuals after 12 month´s growth.

Body shape of male after 12 months Body shape of male after 12 months Body shape of female after 12 months Body shape of female after 12 months
male
female


Comparison of head shape
from the same individuals in pairs at about 12 months after hatching.

Comparison of head shape Comparison of head shape
left male, right female
left female, right male


There is a much greater need of experience in observation for sex determination in young Heloderma than for adult individuals. Once experienced in the propagation of Heloderma suspectum, a herpetoculturist can have successful accuracy. It is especially true with having additional specimens of the same age for comparison. Photo documentation of specimens immediately after hatching supports this, especially from the side and from the dorsal view.

2. Ultrasound

We tried to determin the sexes of heloderma susp. from 16 months of age by ultrasound examination. The individuals came out of their first hibernation after three months and then tested.

Ultrasound examination of the sexual organs and of the ventral areas of the tail of two 16 month old twins of Heloderma suspectum.

Single gonade of MO-2 gonade (?) of MO-2 Hemipenis pockets (?) of MO-2 Longitudinal structures
Mo-2-06 (hatched 10/30/06)
Abdominal cavity of MO-3-06 Gonade (?) of MO-3-06 Gonade (?) of MO-3-06 Gonade (?) of MO-3-06
MO-3-06 (hatched 10/30/06)

Both specimens have a snout-vent length (SVL) of more than 230 mm and should therefore have sexual maturity (Ref. 1). In both individuals (twins) we could detect singular roundish structures (gonade ?) , each. They are located deep in the abdominal caverty. The future will show if there will be a developement of "secondary gonades" . 

The two individuals came out as females. Since 2010 eggs were laid and healthy offsprings hatched.

Ultrasound was performed at the Department for radiology (Prof. Dr. G. Alzen) Universitatsklinikum Giessen.
Equipment: Elegra Advanced Ultrasound. Siemens Co. with a 7,5 MHz linear ultrasound head.

Ultrasounds of the tails were performed with a 7,2 - 12,0 MHz linear ultrasound head.


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