4. Imagine that you are studying a species of guppy in which mature males are ob
ID: 255252 • Letter: 4
Question
4. Imagine that you are studying a species of guppy in which mature males are observed to have bright orange tails, while females and immature fish have grey tails. You decide to test whether sexual selection can explain this dimorphism.
a) Describe an experiment to test this hypothesis. Include details about what you will measure. Draw and label two diagrams: one showing potential data that would refute your hypothesis, the other showing potential data that would support it.
b) A competing colleague discovers that some mature males in this population have grey tails, and publishes a paper claiming that this evidence shows that sexual selection could not explain the dimorphism in tail color in this species. Do you agree or disagree? What observations or experiments could help you refute your colleague’s conclusion?
Explanation / Answer
4). Guppies are popular pets and make attractive additions to a home aquarium. Male and female guppies will reproduce quickly once they mate, though guppies are known to eat their young.[1] You can identify male and female guppies fairly easily once they are at least one week old. The sex of the guppy can be confirmed by examining the fish’s body shape, fins, and coloring.
a). Experiment:
Females were placed in the tank in the absence of males to control for side preference. We found that females preferred to stay in the middle of the tank, under the cover of a plastic plant.
Two Choice tests were used in the Experimental Trials:
The first confined the male movement by placing them in test tubes to prevent display behaviors, and prompting females to respond solely to visual stimuli.
The second allowed males to move freely prompting females to respond to both visual and behavioral stimuli.
In both experiments females preferred male guppies over brighter endlers; this conflicts with previous research on guppy mate choice, which suggests that females prefer bright orange males to other mates.
We found no significant difference between female preference for guppies in the visual and behavioral choice tests. However, all endlers and some male guppies failed to exhibit typical display behavior this likely influenced female responses.
These results indicate that females primarily use visual cues to differentiate between “foreign” males and males of their ancestral lineage, causing sexual selection that may eventually lead to reproductive isolation.
b). Here I disagree my colleague’s concept and agree that the sexsual selection will explain the dimorphism of these guppy.
Although females prefer to mate with brightly colored males in numerous species, the benefits accruing to such females are virtually unknown. According to one hypothesis of sexual selection theory, if the expression of costly preferred traits in males is proportional to the male's overall quality or reveals his quality, a well-developed trait should indicate good condition and/or viability for example.
A female choosing such a male would therefore stand to gain direct or indirect fitness benefits, or both. Among potential phenotypic indicators of an individual's quality are the amount and brightness of its carotenoid-based colors and its boldness, as measured by its willingness to risk approaching predators without being killed.
All females were raised separately from males. Our females were allowed indirect experience with courting males by viewing them through the glass panes of adjacent holding aquaria prior to testing. Guppies in this population are exposed to relatively high fish predation pressure, occur in mixed-sex shoals, and have been observed to approach fish predators in the wild.
To quantify boldness and escape potential in male guppies under predation hazard, we used either a live pike cichlid fish (Crenicichla alta) or a model of it as a predatory threat depending on the particular experiment. Experimental males were allowed to court and mate with other females in their home aquaria prior to being used in the current study.
Fish holding aquaria contained a corner water filter, a plastic plant, and gravel substratum, and were maintained at 24-26°C under a 12 h light/12 h dark illumination cycle provided by Sun-Glo fluorescent tubes, which simulate the energy spectrum of sunlight. Guppies were fed ad libitum flake food (NutraFin) three times daily, supplemented with live brine shrimp nauplii. No food was available to the fish during any of the experimental trials.
This experiment tested the hypothesis that the boldness of brightly colored male guppies, measured as their frequency of approaches initiated towards , is greater than that of less colorful individuals.
The tank was illuminated overhead with a Sun-Glo fluorescent tube (24 ,uE/ m2/s at the water surface), and water temperature was maintained at 24-26°C. We quantified the level of boldness of paired bright and drab males in the presence and absence of the predator when females were present.
As the predator and guppies were physically separated by a clear Plexiglas partition, no actual predatory attacks occurred. Males were tested in pairs to provide each male the choice of either initiating an inspection visit, following the other male who initiated the approach, or not inspecting at all.
Paired males were intentionally chosen from their home aquaria to differ in their overall color patterns, but otherwise matched for body length. The brighter male in a pair always possessed more and relatively larger, more saturated, and darker color patches than the drabber male and was thus more visually conspicuous at least to our eyes.
For a given trial, a preassigned pair of males and two nongravid) adult females were placed into the larger compartment and allowed to acclimate for 2 h, during which time the opaque partition blocked their view into the predator compartment.
Following this period, the opaque partition was raised remotely, allowing the guppies to view the predator compartment, which either contained the live cichlid or was empty.
The number of approaches toward the predator or the empty predator compartment in the tank initiated separately by the bright and drab males was then recorded from behind a blind for 30 min. Fifteen different pairs of males were similarly tested for each of the two predator treatments separately, and different females were used for each trial.
Individual fish were thus used only once in the experiment. When females were present nearby, the more brightly colored males approached the live fish predator significantly more often than the paired drab males.
However, in the absence of the predator, the levels of boldness of the paired males were similar, owing to a significant reduction in the inspection frequency of the more brightly colored males.
Differences in the boldness of paired males differing in coloration were therefore only realized when a threat of predation was imminent.
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