Ontogenetic effects of a sexually selected trait

Sexually selected traits have long interested biologists because they seem to be disadvantageous to survival. Because of this, many studies have sought to determine the costs of these traits. Numerous methods have been used to detect costs, including experimentally manipulating the trait, comparing natural variation of the trait and comparisons to the sex that lacks the trait. In collaboration with Kristine and Paulina (undergrads at UCR) a few years ago, we employed a novel approach to detect costs of sexually selected traits, by examining their effect ontogenetically as the sexually selected trait develops. The results from this work have recently been published online at Physiological and Biochemical Zoology (click here to go to the online version).

Male green swordtail (Xiphophorus hellerii)

In this study we used male green swordtails (Xiphophorus hellerii), and measured sprint speed (a measure of anaerobic performance) and critical swimming speed (a measure of aerobic performance) as the sexually selected sword develops. In many species of swordtails, males posses an elongation of the lower margin of the caudal fin, termed the sword. These fish are common in the hobby industry and aqua-cultured variants of green swordtails can often be found in pet stores. The sword is the result of a pre-existing female bias, where females tend to prefer males with longer swords. There are several unique features of the sword that make it the ideal system to examine the ontogenetic costs. First, males are born without the structure, mature, then the sword grows. Second, the sword is directly linked to the thrust producing system, the caudal peduncle and caudal fin. Therefore, we can compare mature males with and without the structure without any experimental manipulations that may alter functional relationships. We can also examine the sword’s affect on locomotion, since it is associated with the structures used to propel the fish through water. What we expected was a reduction in swimming performance as the sword grew longer. However, this is not what we found.

We used a combination of model comparison techniques and a repeated measures design to determine the effect of the sword on swimming performance over a nine week period and to compare statistical models with various predictors of swimming performance. What we found is that in the best fit models, sword length was not included. Furthermore, in less supported models, sword length had no effect on swimming performance throughout its development. Instead of performance decreasing, we found an increase; although this increase was not consistent with increases in size alone. What this suggests is that as the sexually selected sword develops it has minimal impact on a male’s ability to swim. Furthermore, an individual’s consistency in performance does not change in relation to the development of the trait. Fast swimming individuals remain fast with the sword.

Ontogenetic changes in A) standard length, B) sword length, C) critical swimming speed (Ucrit), and D) sprint speed. Note that the pattern of change in performance does not parallel the change in either body size or sword length. Points represent means ± 1 S.E.

Just because the sword doesn’t affect swimming performance doesn’t mean there is no cost, just a lack of a functional cost. This may be due to a minimal amount of drag to the structure, or not requiring much energy to swim with the structure, or that compensatory traits develop in conjunction to offset the potential swimming costs. However, there may be other associated costs, such as a reduction in survival. Our approach, may be useful in studies where the sexually selected trait can’t be altered and may provide additional insight into how sexually selected traits interact with the rest of the organism. To read more see our paper:

Oufiero, C.E., K. Jugo*, P. Tran* and T. Garland, Jr. 2012. As the sword grows: individual variation and ontogenetic effects of a sexually selected trait on locomotor performance in Xiphophorus hellerii. In Press. Physiological and Biochemical Zoology.

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