Many fish make the intrepid migration from sea to freshwater to breed; however, some of these visitors enjoy their trips so much that they stay and evolve to live full time in freshwater. But how do marine sticklebacks make this evolutionary transition? Intrigued, Matthew McGee, a PhD student from the University of California, Davis, USA, captured marine sticklebacks from a nearby bay to begin to find out. While filming them snacking on crustaceans, he noticed something unusual: ‘The males and females were striking at their prey in different ways’, remembers McGee. This observation was surprising and when McGee told his advisor, Peter Wainwright, McGee recalls, ‘He didn't believe me at first because sexually dimorphic feeding movements had not been seen before in any fish species’. Nonetheless, McGee persisted and decided to investigate further the different feeding habits of the two sexes ().
Feeding in fish mainly occurs by sucking prey into an open mouth, which involves many bones and muscles explains McGee: ‘Fish can protrude their jaws forward from their head to move their suction flow closer to a prey item and hyoid [bone] depression causes the floor of the fish's mouth cavity to drop down to expand the area of the cavity and suck more water in’. To characterise differences in feeding mechanisms between the sexes, he filmed the feasting fish at 500 frames s–1 before analysing the exact jaw and hyoid movements. He found that females were able to both protrude their jaw and depress their hyoid more than males. However, males struck prey at a much closer distance and reached maximum jaw protrusion 11 ms faster than their female counterparts. When he measured some of the same traits in preserved sticklebacks from another marine location in Washington, USA, he again saw jaw protrusion was larger in female fish.
McGee believes that these differences between the sexes mean that males are less capable of capturing prey by suction feeding, as their mouths open less than females'. Instead, he suggests that their jaws are more suited to biting. Explaining that males construct nests, and the ability to bite is particularly useful for collecting building materials from the lake floor, McGee suggests that it is possible that the males have traded their prowess as hunters to become homemakers.
But McGee also wondered whether this sexual dimorphism might also help sticklebacks make the transition from being a marine inhabitant to a freshwater resident. As the male-like jaw is better for biting rather than sucking, it is also well suited to capturing non-evasive prey on the lake floor, which rely on armour and burrowing to evade predators, whereas a female-like jaw would be suited to capturing evasive free-swimming prey from a further distance. McGee reasoned that if one particular prey was more abundant in a new freshwater environment, it would be useful to already have the two different feeding traits within the population for selection to act upon. Luckily, McGee had already scouted out freshwater sticklebacks in Canada, and was able investigate his theory by measuring the jaw protrusions in these sticklebacks. Sure enough, males and females that inhabited open water away from the sides and bottom of the lake had female-like jaw protrusions irrespective of sex, whereas another population, which forages along the lake floor, had a more male-like jaw protrusion.
McGee suspects that probably many fish species display sexual dimorphism in feeding movements and that this dimorphism provides them with the important ability to adapt to new situations or tasks, be it at sea or in a lake.
