Size Matters to Female Fiddler Crabs

Imagine it took you over 100 different people to find the one for you, but looks and personality were not the criteria. What if you determined your partner solely on the size of their home? If humans did this they would be considered “gold diggers,” but to female crabs this is essential for the survival of their offspring.

Many females in the animal kingdom choose a male according to his size , as a larger male’s genes are preferred for the female’s offspring. In a study led by Dr. Catherine deRivera, from the University of California San Diego, she and her research team discovered that female fiddler crabs, Uca crenulata, have the highest standers when it comes to selecting a mate. In one case, a female selected a male after going through 106 other potential suitors; however, it wasn’t necessarily the crab itself that the female was interested in.

Female fiddler crabs are unique to other females, as they have been identified as the choosiest animal when it comes to selecting a mate. They determine a mate on the size of the male’s burrow, as the burrow in turn determines the size of the male crab; therefore, larger crabs have larger homes.

In order for males to attract females to their home, male fiddler crabs signal to the female fiddler crabs by waving their larger claw, which is 1-2 inches in length, as if telling the female to “come here.” If she shows interest she will investigate the home of the male crab, most of the time rejecting the male without ever entering his burrow. Even if she desires further investigation by entering the burrow, that does not guarantee she will stay. However, once she finds the home that fits her standards, the male and female crab mate, and a few days later the female will exit the burrow leaving her eggs behind.

The reason a female crab is so finicky when locating a perfectly sized burrow is because the size of the burrow determines the amount of heat it will receive. Therefore, certain sized burrows allow for better incubation of the larvae before they are swept away by the next incoming tide.

Hence, the female fiddler crab is not necessarily a “gold digger” by human standards, but rather a concerned mother; since the size of the male’s home can have a significant impact on her offspring’s survival.

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