Maternal care in scorpions

Issue 7, December 1996

Scorpions are found in the warmer regions of the world and this includes southern Europe. They are not restricted to desert regions, as is often supposed, but can be found in forests too. However, since they are largely nocturnal they spend most of the day under logs or deep in the leaf litter and so are not readily seen. They vary in length from 13 - 180 mm. Two of their most striking features are their pincers and their sting, see Figure 1. The pincers are used to grip and manipulate their prey, mainly insects and spiders, which are stung. The venom is inserted into the victim via a sting at the end of the 'tail'. Although most scorpion stings are only painful to humans a few species are more dangerous, and may be fatal, especially to children and elderly people.

Figure 1. A scorpion.

Figure 2. Mean sprint speed of female scorpions during three reproductive states (N= 1 7 for pregnant and post- dispersal groups, N=6 for carrying group).

Figure 3. The relationship between the sprint speed of female scorpions while carrying offspring and the number of offspring carried.

Figure 4. The relationship between the sprint speed of female scorpions while carrying offspring and the total mass of offspring carried.

Courtship between the male and female consists of a 'dance'. The male deposits a spermatophore on the ground and then he guides the female into position, through the interlocking of their pincers, so that the spermatophore enters her body. After several months of gestation the young are produced viviparously and the female carries them around on her back for a week or so, beyond their first instar stage. During this time she provides them with food and water. After one to two weeks the young become independent.

In 1996 Shaffer and Formanowicz* reported the findings from their research on the common striped scorpion, Centruroides vittatus. Their data relates to seventeen pregnant scorpions they collected from the Kickapoo Caverns State Park, Texas, United States of America.

1. Figure 2 shows the sprint speed of females in three reproductive states: when pregnant, when carrying young and in the post-reproductive phase when the young have dispersed. Approximately how much faster is the sprint speed of a pregnant female compared to that of a female carrying young? (1)
2. The mean mass of a pregnant female was 1.05 g, that of a post-dispersal female was 0.73 g. What would be the mean percentage weight loss for a female scorpion after pregnancy? (1)
3. The mean number of offspring in a litter was 31.9 and the mean offspring mass was 6.7 mg. What would be the total mass (g) of offspring a female would carry
4. on her back? (2)
5. Figure 3 and Figure 4 show the relationships between the sprint speed of female scorpions carrying young and the number in the litter and the litter mass (g) respectively. Describe the relationship between the two variables in each of the two graphs. (4)
6. Briefly outline two possible disadvantages to scorpion females if they tried to sprint away from danger when carrying young on their backs. (4)
7. All the scorpions used in this study were tested on three occasions: firstly when they were pregnant, secondly when the young were being carried on the female's back and thirdly after their young had dispersed. Outline one factor that may influence their sprint speed on their second and third trial compared with their performance on the first trial. (2)
8. The scorpions used in this study were collected in the field but they were probably not a random sample. Explain what a random sample is and why it would be difficult to collect a random sample of mobile animals, such as female scorpions. (2)
9. Female scorpions are viviparous. Explain what the term 'viviparous' means. (2)
10. This was a laboratory study, not one caused out in the field. Identify one advantage and one disadvantage of laboratory studies of animals. (2)

* Shaffer, L. R. and Formanowicz, Jr., D. R. 1996. A cost of viviparity and parental care in scorpions: reduced sprint speed and behavioural compensation. Animal Behaviour, 5 1, 10 1 7 - 1024.