Measuring the costs of alternative reproductive tactics in horseshoe crabs, Limulus polyphemus

Trade-offs are inherent to alternative reproductive tactics (ARTs), and identifying the costs and benefits of tactics is essential to understanding their evolution and maintenance within a population. Male horseshoe crabs exhibit two condition-dependent ARTs: males that are in better condition arrive on spawning beaches attached to a female, while males in poorer condition join spawning pairs as satellites and engage in sperm competition. Previous research has identified several benefits to the attached tactic, but the costs are less well understood. We examined a previously uninvestigated potential cost to the attached male tactic: nutritional stress caused by a restricted ability to feed. We found that field-caught attached males produced 57% less faeces in a 12 h period than satellite males, and had 2.5 times emptier digestive tracts than satellite males. We further examined this cost using stable isotopes because nutritionally stressed animals are predicted to have higher δ15N levels. We found that field-caught attached males had higher δ15N values than satellite males. However, higher δ15N values could result from nutritional stress or from feeding on higher trophic levels. We tested this experimentally and found that starved animals had higher post-treatment δ15N values compared to animals that were fed. Furthermore, the digestive tracts of field-caught attached males contained three times more sea grass (lower tropic levels have lower δ15N values) than satellite males. These findings mean that the higher δ15N values of field-caught attached males likely result from fasting rather than differences in diet. Taken together, our results indicate that a period of nutritional stress caused by reduced food consumption is a novel cost of the attached tactic. This study provides a key piece of information to explain why ARTs in horseshoe crabs take the form they do and provides a novel method for studying costs associated with ARTs in other species.