Abstract
Simple SummaryMale ghost crabs (Ocypode spp.) are known to build sand structures near the entrance to their burrows. It has always been assumed that the structure played a role in sexual selection and mate attraction. We hypothesized that the larger males would make relatively larger pyramids in order to enhance fitness. We studied Red Sea ghost crabs at Eilat, Israel, and found that the larger crabs actually did not put much effort into building pyramids, and it was the medium-sized crabs that invested the most in building the pyramids.Display, wherein males attempt to maximize fitness by attracting sexually mature females to mate, is known to drive speciation by Sexual Selection. We researched the Red Sea Ghost Crab (Ocypode saratan; RSGC), in which males build display pyramids to attract females. The study was conducted at the beach in Eilat, Israel. At each session, we measured the height (in cm) of all pyramids and the dimensions (height, breadth; in cm) of the burrow entrance. We assumed that the size of the entrance represented the relative size of the carapace width of the occupant. The mean (± SE) entrance volume was 230.8 ± 11.7 cm, and the height of the pyramid was 11.8 ± 0.49 cm (n = 54). The results of our study did not support our hypothesis because we had expected to find a linear correlation between body size and pyramid height, i.e., the larger the male, the larger the pyramid. However, our results show that the largest males in the population either built small pyramids or not at all, and the cut-off of the larger crab’s body size appears to be around 350 cm3. We discovered a step-wise function in the data in that crabs with the smallest body size of ca. 250 cm3 constructed the highest pyramids, with a declining tendency between 250–350 cm3 and extremely low pyramids beyond 350 cm3. However, our findings need to be further studied with a stress on the ambiance and elucidate whether the habitats differ in temperature, humidity, prey-base, etc., before concluding as to why the larger males desist from building pyramids. This study underwrites the importance of studying the mating systems of the macro-fauna of the beaches that are fast disappearing owing to anthropogenic development.
Highlights
Since the theory of speciation and natural selection were co-proposed [1,2], the subject is still debated, and many subsequent models either support or refute their theory [3,4]
Our findings need to be further studied with a stress on the ambiance and elucidate whether the habitats differ in temperature, humidity, prey-base, etc., before concluding as to why the larger males desist from building pyramids
They found that the RSGC is the most conspicuous member of the sandy beach communities in the Red Sea and that male burrow complexes consisted of a sand-pyramid (Figure 1), a pathway leading to it and around it, a vestibule at the entrance and a spiral burrow, which are only constructed by sexually mature adults
Summary
Since the theory of speciation and natural selection were co-proposed [1,2], the subject is still debated, and many subsequent models either support or refute their theory [3,4]. Trying to impress females to choose themselves in a lek mating system requires males to outdo their competition by being either more vocal or colorful, executing a more elaborate dance and/or having exemplary ornamentation [15]. This has not been clarified for any of the Ocypode species studied to date globally, from our observations in the field, we are convinced that the ghost crabs have a lek mating system, wherein the males display and build their pyramids in proximity to each other, and attract females from the surrounding areas of the beach [16]. Our speculations are supported indirectly by biased sex ratios in other ghost crabs and in New World fiddler crabs [17,18,19]
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