From Armament to Ornament: Performance Trade-Offs in the Sexual Weaponry of Neotropical Electric Fishes

Kory M. Evans, Maxwell J. Bernt, Matthew A. Kolmann, Kassandra L. Ford, James S. Albert

Preprint posted on 11 January 2018

Some electric fishes fence with long jaws for mates! They sacrifice bite force for a longer jaw, which makes a better weapon.

Selected by Cassandra Donatelli

Background: Most of the work on electric fishes has focused on their namesake: the ability to produce electricity. There have been countless studies on their electric organs, communication using electricity, and the role of electricity in navigation and prey capture. However, these fish are more than just their electric organs. They also engage in complex behaviors unrelated to electricity. One such behavior is battling over mates using weapons built in to their faces.

Kory Evans and colleagues discuss the potential consequences of having weaponry used in male contests over mates. They use two species of electric fishes, Apteronotus rostratus and Compsaraia samueli, with elongated jaws as their study system; they are sexually dimorphic, and the elongation of the jaw varies between individuals.

What they did: The researchers took micro-CT scans and created 3D computer models of each individual. They then quantified skull variation using three-dimensional geometric morphometrics. This technique uses anatomical landmarks placed on the 3D models to analyze differences in shape between subsets of a group. Finally, they calculated mechanical advantage of the jaw. Mechanical advantage is an estimate of the force produced by different systems. In this case, the researchers used the relationship between the length of the adductor mandubulae muscle (closes the jaw), and the length of the jaw itself as a proxy for bite force [1].

The discovery: In C. samueli males, the researchers found that fish with longer jaws have a lower mechanical advantage (MA in Fig. 1 below). The relationship is opposite in females, though the variation in jaw length is much lower than in males. In contrast, there was no difference in mechanical advantage between males and females in A. rostraus, and no effect of jaw length on mechanical advantage. The authors conclude that this trade-off between mechanical advantage and jaw length is due to sexual selection. In C. camueli, it may be more important for the fish to have a longer jaw for mating contests than it is for them to produce large amounts of bite force.

Reproduced from Evans et al., Figure 2B

Open questions and possible future studies: It would be interesting to take the study further by measuring the kinematics of the fish during a contest. 3D printed physical models based on the CT scans could also be tested to provide more concrete evidence of mechanical advantage.

[1] Westneat, M.W. 2003 A biomechanical model for analysis of muscle force, power output and lower jaw motion in fishes. Journal of Theoretical Biology 223, 269-281.

Tags: biomechanics, electric fish, fish, sexual dimorphism, sexual selection, weaponry

Posted on: 13 February 2018 , updated on: 19 February 2018

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