The reproductive biology of scorpions is unusual among arthropods in many respects. There is no direct contact between male and female copulatory organs; sperm is transferred by means of a packet deposited on the ground. Fertilized eggs develop inside the female's body and the young are born alive. The fact that the young spend their first days on the mother's back after birth is one of the best-known scorpion behaviours.
Courtship and the mating dance
Reproduction begins when the male finds a suitable female. During the breeding season, males are often observed to be more active and to move away from their shelters in search of females. The male is thought to detect the female through chemical traces and vibrations; the pectines, which extend close to the ground, play an important role in this detection. After the encounter, in some species the male approaches the female by jerking his body or moving his tail.
Telling males and females apart is not easy in every species. In many species, males have larger pectines with more pectinal teeth, while females have a broader body; in some species, the pincers or metasoma of males are distinctly different in shape. The structure of the genital operculum is also among the criteria used to distinguish the sexes. In closed-cycle production, correct pairing depends on making this distinction reliably.
Next comes the stage known as the mating dance. The male grasps the female's pincers with his own and steers her back and forth in search of a suitable surface. Depending on the species, this dance can last from minutes to hours. In some species the male has been observed to insert his sting into the female's body during the dance, often at a joint; this behaviour is thought to calm the female, but its function has not been fully clarified.
Spermatophore: indirect transfer
When the male finds a suitable surface, he attaches to it the structure that carries the sperm packet, called a spermatophore. The spermatophore resembles a capsule standing on a stiff stalk. The male then pulls the female forward so that her genital opening lies directly over the packet. Pressure from the female triggers a mechanism that opens the packet, and the sperm is transferred into her genital opening. This is why the nature of the surface matters: a surface that is too loose or too rough makes it difficult to anchor the packet.
Once the transfer is complete, the pair usually separate quickly. Females eating males is a behaviour that has also been reported in scorpions, but it should not be assumed that every mating ends this way. The position of the pectines and the genital opening on the body was described in the article on body structure.
Gestation and live birth
In scorpions, the embryos develop inside the female's body; the young do not hatch from eggs outside but are born alive. The mode of development differs between families. In some groups the embryo develops inside a yolk-rich egg; in others it is nourished directly by the mother within special sacs attached to the ovarian tube. The Buthidae family is cited as an example of the first group.
Gestation is long and, depending on the species, temperature and nutritional status, can range from a few months to more than a year. In the later stages of gestation, the female's mesosoma becomes noticeably wider. In some species females have been reported to give birth more than once after a single mating. In a small number of species, reproduction without fertilization, that is parthenogenesis, has also been described.
During birth, the female folds her front legs to form a basket-like structure beneath her body. The young emerge one by one from the genital opening, drop into this basket and from there climb onto the mother's back, holding on to her legs. Birth usually takes place inside the shelter and out of sight, so even in captivity it is often noticed only when the young are seen on the back. Depending on the species, the number of young in a single brood can vary widely, from a few individuals to nearly a hundred.
Why are the young carried on the mother's back?
A newborn is not a miniature copy of an adult. Its cuticle is thin and soft, its sting and pincers are not yet functional and it cannot hunt on its own. Instead of claws, the tips of its legs bear adhesive structures for gripping, which allow the young to hold firmly to the mother's back. The young get through this period on the nutrient reserve left over from the egg. The mother, too, generally moves less during this time and prefers to stay in her shelter.
During this fragile period, the mother's back serves several functions at once:
| Function | Explanation |
|---|---|
| Protection | Young with a thin cuticle are defenceless against predators; the mother's presence acts as a deterrent |
| Limiting water loss | The young do not come into direct contact with dry ground and stay in the humid environment of the mother's shelter |
| Staying together | Siblings are kept in the same shelter without dispersing |
| First moult | The young usually complete their first moult on the mother's back |
After the first moult, the cuticle of the young becomes stronger, claws appear at the tips of the legs, and within a short time the individuals leave the mother and begin to live independently. The time spent on the back varies with species and temperature but is generally between a few days and a few weeks. They moult several more times before reaching adulthood; this process is covered in the article on moulting.
The breeding period in closed-cycle production
In a closed-cycle facility, reproduction is the foundation of the colony's continuity and calls for a care regime of its own. The main elements of the general approach are as follows:
- Females thought to be gravid are moved to separate enclosures and left undisturbed.
- Females carrying young are not handled until the young have come down from their back; procedures such as venom extraction are not scheduled during this period.
- The date of birth and the number of young are recorded; the record helps to trace the history of the individual and the batch.
- When the young reach the dispersal stage, they are moved to separate enclosures; otherwise cannibalism may occur among siblings or between mother and young.
- Temperature and humidity are monitored with particular care throughout gestation and the first moult.
The place of these principles in colony management is described in the article on closed-cycle breeding, and the control of environmental conditions in the article on temperature and humidity. For species-specific reproductive data, the field and laboratory literature should be used as the basis; this article offers a general framework for scorpions.
