Sound-sensitive organisms are abundant on coral reefs. Accordingly, experiments suggest that boat noise could elicit adverse effects on coral reef organisms. Yet, there are few data quantifying boat noise prevalence on coral reefs. We use long-term passive acoustic recordings at nine coral reefs and one sandy comparison site in a marine protected area to quantify spatio-temporal variation in boat noise and its effect on the soundscape. Boat noise was most common at reefs with high coral cover and fish density, and temporal patterns reflected patterns of human activity. Boat noise significantly increased low-frequency sound levels at the monitored sites. With boat noise present, the peak frequencies of the natural soundscape shifted from higher frequencies to the lower frequencies frequently used in fish communication. Taken together, the spectral overlap between boat noise and fish communication and the elevated boat detections on reefs with biological densities raises concern for coral reef organisms.
Animals compete in contests over limited resources. Contestants forfeit once they ascertain that their opponent has greater resource‐holding potential (RHP; mutual assessment) or once they reach a threshold of costs (self‐assessment). Functional scaling studies of contest behaviour performance can inform how assessment signals, offensive capacity and endurance scale with RHP and thereby elucidate the mechanisms through which each of these assessment types operates. Here, we performed behavioural contest analyses to determine the assessment strategies used in snapping shrimp (Alpheus heterochaelis) contests. Then, we used biomechanical measurements of a common contest behaviour to inform how assessment might operate. We were specifically interested in the snapping behaviour during which snapping shrimp fire imploding cavitation bubbles—hereafter, ‘snaps’—at their opponents. We showed that A. heterochaelis use mutual assessment early in contests. Then, when they fire snaps, they switch to cumulative assessment—a type of self‐assessment where contestants endure costs from their own behaviours (e.g. energy) and their opponent's (e.g. injury). Because larger individuals tend to win contests, we then tested how the maximum performance and endurance of snaps scaled with size. We measured the average angular velocity of the snapping dactyl, cavitation bubble duration and pressure of snaps as metrics of performance. We measured 10 snaps per individual (n = 76 individuals). From this series of 10 snaps, we calculated the maximum of each metric as the maximum performance and the attrition of each metric over the course of 10 snaps as a measure of endurance. Maximum performance increased with size, but endurance did not. This suggests that cumulative assessment in snapping shrimp is driven by opponent‐imposed costs. Our results are not consistent with self‐assessment based on endurance; however, the experiment could not fully replicate the quick succession of snaps fired in real contests. Future experiments should better replicate the rapid firing of snaps to test if endurance matters in a more ecologically relevant context. Our framework of integrating biomechanics and behavioural ecology provides a pathway to identify precise mechanisms of contest assessment and animal behaviour more broadly. Read the free Plain Language Summary for this article on the Journal blog.
The cost-minimization hypothesis proposes that positive allometry in sexually selected traits can be explained if the proportional energetic maintenance costs of weapons decrease as traits increase in size. Energetic maintenance costs are the costs of maintaining homeostasis. They are slow, persistent energy sinks that are distinct from ephemeral costs of growth. Because some tissues expend more energy on maintenance than others, energetic maintenance costs can be inferred from proportional tissue composition. For example, soft tissues require more energy for maintenance than exoskeleton, so an arthropod claw that is 50% soft tissue and 50% exoskeleton would have higher energetic maintenance costs than one that is 30% soft tissue and 70% exoskeleton. I tested the cost-minimization hypothesis using proportional tissue composition as a proxy for energetic maintenance costs in snapping shrimp ( Alpheus heterochaelis and Alpheus estuariensis ) and fiddler crabs ( Uca pugilator ). As predicted, larger weapons comprised proportionally less soft tissue mass and more exoskeleton mass than smaller weapons. Furthermore, I extended cost-minimization to explain trait exaggeration: individuals might exaggerate traits by investing more mass in exoskeleton. As predicted, exoskeleton mass proportional to weapon mass increased as exaggeration increased. These results support and extend the cost-minimization hypothesis to explain positive allometry and weapon exaggeration.
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