2.2. Natural observations: females

We collected 59 females (32 wanderers; 27 residents), measured them, and released them individually onto the mudflat at least 2 m away from their capture point, and observed their behaviour until they had secured a new burrow. Immediately after release, the female would often dart into the nearest burrow. To avoid documenting this escape response, we did not begin recording the female's behaviour until the female emerged from this burrow and began moving across the mudflat. We followed the females from at least 1.5 m away to avoid disrupting their normal behaviours [10]. From the time of release to the securing of a new burrow, we noted the number of times that a female visited (approached, put her legs into, or fully entered the burrow) a waving male, a non-waving male or an unguarded burrow. We defined an unguarded burrow as one with no surface-active crab; these burrows may be empty (which is rare) or they may be occupied by a resident that is underground. To an approaching female, the burrow would appear unguarded until she reaches (and briefly enters) the burrow shaft. We reserve the term ‘empty burrow’ for those that were unoccupied when the female visited them. We also noted the time it took for the female to secure her new burrow.

A female could secure a new burrow in one of two ways: (i) she could visit a courting male and remain underground with him after he sealed the burrow entrance with a sand plug (this indicates a mating); (ii) she could find and occupy an empty burrow. Females very rarely dig new burrows and, unlike males, they do not fight residents and evict them in order to steal their burrows.