Small Neighbours Make You Look Good...

Male fiddler crabs will wave their enlarged claw to attract a female to inspect their burrow
Image credit: Tanya Detto

... at least if you're a fiddler crab.

When shopping around for the best mate, female fiddler crabs (Uca mjoebergi) like to know what's on offer. According to the researchers of a recent paper in Behavioural Ecology, females make decisions based on comparisons of competing males. 

Densely packed along the shorelines, the burrows of male fiddler crabs are fiercely defended by their occupant. Males are easily distinguished by their large claw used in male competitions for the best territories and for attracting the females to their burrow. 

Observations of male behaviour have noted that some larger males like to keep smaller neighbours nearby. In some cases they even 'lend a claw' to help small males when threatened by competitors. 

To test the idea that females make comparisons within close groups of males, the researchers created and painted robotic crab arms to tempt the females. These arms were placed in the sand at equal distances to the female (see image [2]). 

Females are known to be impressed by big claws and by the eager males who start their waving displays first. To make sure female preference was swayed only by claw size, all robotic arms waved in synchrony at the same rate. 

As expected in most cases, 65-70% of females chose the largest males in the group. Interestingly though, females chose the focal 'medium sized' male more often when he had small males either side of him: 

"In the small neighbors treatment, 22.5% of females chose the focal male. In the large neighbors treatment only 5% of females chose the focal male"

As the largest male is not always chosen the researchers suggest that females may struggle to assess all 5 males at once; or that it is easier to perceive the difference between small and medium claw sizes, than medium and large. Hmm.. to me this seems unlikely (for this experiment at least), as the size difference between small and medium claws was 5.9 mm, and between medium and large was 6 mm.

Image [2] Male robotic arms were arranged in an arc and 3 different sizes were used in both scenarios (small, medium & large). The focus was on the focal 'medium sized' male and whether females chose to mate with him more often when he had direct large neighbours or small.
Image courtesy of Callander et al. 2012

Note-worthy: Female choice is not only based on male claw size. The quality of the burrow is very important for reproductive success and a smaller male with a fantastic burrow may have as good a mating chance as a large male who's burrow isn't up to scratch. 

In this experiment the robotic crab claws were not all the same colour. They were painted to within "the natural colour variation" of male fiddler crab claws. Other research has shown that females can use this colour to make sure she mates with a male of the correct species. Whilst she shows a distinct preference for yellow claws, the colour intensity plays no role.

Find Out More:

VIDEOS: Robotic crab arms -  BBC Nature has a couple of good videos showing the robotic crab arms in action plus another summary of the study

Source:

• CALLANDER, S., HAYES, C. L., JENNIONS, M. D. & BLACKWELL, P. R. Y. 2012. Experimental evidence that immediate neighbors affect male attractiveness. Behavioural Ecology,  730-733. 
• DETTO, T. 2007. The fiddler crab: Uca mjpebergi uses colour vision in mate choice. Proceedings of the Royal Society B-Biological Sciences, 274, 2785-2790.