Pre-Existing Preferences of Fish

(An Introduction to Mate Selection)

As was proposed by Darwin, sexual selection by females could be an important selection pressure used to explain many of the elaborate characteristics males may display in their efforts to survive and reproduce.

The way an individual female chooses her associates is one important aspect of mate selection.� Whereas actual mating behavior itself involves only a small set of behaviors (or choices), the choices inherent in the choosing of �a mating partner is usually� complex.� A potential mate can be identified by nearly any sort of characteristic, but is usually something that is prominent or very noticeable.... size, color, odor, etc.���

Interestingly, it is common in cases where a female is selecting a male the characteristic discerned by the female may be somewhat �deleterious� to the overall lifespan of the male!� A classic example can be seen in the coloration differences in male and female cardinals.� Female cardinals are a drab, yellowish-beige color, with small amounts of red (often a pale red) displayed in the beak and feet.� Conversely, the male is brightly colored, with plumage, beak, and feet all a distinct red color.� Studies have shown that female cardinals show a strong preference for mating with male cardinals that are brightly colored.� Brightly colored male cardinals receive more mating opportunities than do less brightly plumed males.� However, this wonderful benefit of bright, bold plumage in the male turns out to be a double-edged sword.� Studies also shown that predation rates are much higher in bright, bold colored males than for either dull colored males or female cardinals!�

This seemingly harsh price to pay for the greater chance for �amour� is believed to be a cue understood by the female to indicate a tougher, more fit male... one that can survive and thrive even if burdened by the disadvantage of a higher chance of predation.� Females, of course, wish to copulate with males who are likely to have �strong� genes.... so her offspring will be more successful in their environment.�� Hence, selecting a male who is �burdened� by bright plumage... and is still able to survive... is a successful way for the female to produce the most fit offspring she can.�

Use of Regression Analysis in an Example

In the example below, you are given a sample set of data that compares the relative courtship time 84 female sword-tail fish spend with different sword-tail males based upon differences in their tail length.�

The longer (and hence more visible) the tail, the greater the predation rate that occurs on male sword-tails. In the image below, an extreme variant in sword tail length is shown.

Using the statistical program, Minitab (click on the link to open the program), copy the data (or subset of data) you wish to analyze into the Minitab spreadsheet.�

In the chart below, the numbers listed under each male represent the courtshiptime (in seconds) a female spent with a particular male.� Because not all malesreceived the same number of courtship opportunities (because of female choice),the number of episodes for each male can differ.

Questions to Ponder

1.��������� What is your hypothesis concerning sword-tail fish tail length in males and mating opportunities?

2.�������� In analyzing the above data, was your hypothesis supported or refuted?

3.�������� Identify problems or concerns inherent in the data-set provided.

4. ������� If you were to conduct this experiment, how would you design (or modify) the experiment to be more robust?