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Kathryn Knight

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Most humans have a strong preference for the hand that they write with. But humans are not the only animals that favour one side of their body. Masaki Yasugi and Michio Hori from Kyoto University, Japan, explain that the behaviour of many fish is biased to one side or the other. ‘This antisymmetry is defined as a dimorphism in which one side of the body is structurally and/or functionally more developed than the other’, the duo explains, leading some fish to turn preferentially in one direction while other fish prefer the opposite. But what impact could a fish's preference for a dominant side have on the interaction between fish predators and their prey? For example, would predators that preferentially turned in one direction have more success in capturing prey that turned toward or away from them? And if so, could the predator and prey's preferences for particular turning directions result in a natural cross-predation pattern, with right-dominant predators selectively picking off left-handed prey and vice versa (p. 2390)?

Yasugi and Hori filmed encounters between largemouth bass and their freshwater goby victims as the predators approached from the rear. Recording the distance, direction, speed and success of the bass strike, the duo also noted the point during the attack when the goby began to take evasive action and the direction in which it turned. Finally, knowing that left-biased individuals tend to be more strongly developed on the left side (and vice versa), the duo measured the size of the bass lower jaws, looking for the telltale asymmetry that would confirm their directional preference.

Correlating the bass approach direction with the direction preference, Yasugi and Hori realised that when approaching from the rear the left-biased bass always circled in a clockwise direction while the right-biased bass circled anti-clockwise.

Meanwhile, the left-biased gobies reacted earlier to the approach of left-biased bass and right-biased gobies escaped more quickly from right-biased bass. This suggests that right-biased gobies are more at risk from left-biased bass approaching from behind while left-biased gobies are more vulnerable to attacks from right-biased bass approaching from the rear. ‘We believe that the lateral biases in approach direction and in evasive response corresponding to morphological antisymmetry are the principal mechanism causing the predominance of cross-predation’, say Yasugi and Hori.