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How to Tell the Difference Between a Male or Female Red Crab

Christmas Island has about 120 million land-dwelling red crabs (Gecarcoidea natalis), which play an essential role in the island's rain forest ecosystem. They recycle nutrients and influence what vegetation grows where. These crabs are famous for their mass annual migrations, when they move near the ocean to breed. Distinguishing the genders is easy enough. Just watching a crab provides several clues as to whether it is a male or female. To confirm the gender, however, you need to turn the crab upside down.

Instructions

- 1
  Compare the size of the crab to others nearby. Males tend to be slightly larger than females. However this is not a determining characteristic because the smaller crabs could be male juveniles.
- 2
  Examine the pincers. Males normally have proportionally larger pincers than females. Compare the last segment of the pincers to the width of the crab's head. In males this is about the same length.In females the last segment of each pincer is roughly half the width of the head.
- 3
  Watch the crab behavior during mating season. At this time crabs gather near the ocean. The males dig holes for the females, wait near the entrance and pull females into the holes.
- 4
  Pick the crab up carefully, avoiding the pincers, and turn it upside down. Crabs have a folded tail here, which on males is narrow and pointed and on females is broad and rounded. This applies to most crabs.

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