Male Green swordtails (Xiphophorus helleri) are well known for the long extensions on the lower part of the caudal fins (the sword) that they use to attract females. But do they increase the risk of predation?
Common sense suggests that these fin extensions come with the cost of making the fish more visible to predators and could slow the fish down in fleeing, since the drag imposed by the 'sword' in escape could ostensibly be significant enough to do so.
However, Alex Baumgartner, Seth Coleman and Brook Swanson of Gonzaga University demonstrate in a recent issue of the journal PLoS ONE that having a long sword does not compromise the escape response swimming performance in the male swordfish.
Fishes escape from a threat (e.g. a predator), by primarily utilising what is known as a C-start escape response.
When a predator attacks, the prey fish responds to changes in water pressure (arising from the attack) by activating motor neurons along the side of the fish's body opposite the stimulus.
This causes the muscles here to contract almost simultaneously and the fish bends into the shape of the letter C.
When this bend moves towards the caudal fin, the fish propels itself away from its resting position.
Given that studies have shown that a faster C-start greatly improves the chance of a fish escaping a predator, the authors use C-start performance as a measure of the swordfish’s ability to escape predation.
The authors test the hypothesis that variation in sword length predicts variation in escape response performance in male green swordtails. If a longer sword increases drag, then males with shorter swords should have a better (i.e. faster) C-start performance than those with longer swords. If such a relationship existed, then removing the swords from the males should increase their escape performance, with a greater improvement expected in males with longer swords.
The researchers conducted their studies on 24 male Green swordtails (with swords of varying length). In each case, a glass rod was suddenly thrust into the water in front of the experimental subject to induce the C-start response, whose performance was documented using a high-speed camera. The swords of the fishes were then cut off with razor blades, and the experiments repeated.
The authors found no significant relationship between sword presence or length and escape performance. Sword length did not predict escape velocity or any of the other escape performance variables they measured. Even after their swords were removed, fish that originally had longer swords did not perform significantly better or worse than those that originally possessed shorter swords. This indicates that fish with longer swords are not intrinsically better escape performers than fish with short swords.
To explain their results, the authors hypothesise that either the hydrodynamics of the escape response may not depend on the presence of a sword, or that fishes with a longer sword are able to compensate for this drag physiologically. However, the results of their experiments do not support the second hypothesis, since they would have expected fish with longer swords to show increased improvement upon removal of the sword (which they did not).
Although this suggests that the sword is an evolutionarily 'ideal' ornament that allows male swordfishes to gain attractiveness with little or no increase in predation risk, other studies have shown that male swordtails bear other costs for having a long sword.
Males with longer swords expend more energy during continuous and courtship swimming, and it also costs more resources to grow long swords.
Furthermore, having a long sword makes the male swordtails more conspicuous to predators, so that they have to endure more attacks (although they would be equally good as males with short swords in escaping these attacks).
The authors conclude that the sword in male swordtails is what they expect natural selection to produce—an ornament that exploits a female preference, and yet not increase vulnerability to predation.
For more information, see the paper: Baumgartner A, S Coleman and B Swanson (2011) The cost of the sword: escape performance in male swordtails. PLoS ONE 6, e15837. doi:10.1371/journal.pone.0015837