Walk into almost any aquatic pet store and it’s likely that some of the first animals to catch your eye will be male guppies. Domestic male guppies come in all shapes and colors; creating shimmering masses of blue, green, purple, red, and everything in between. However, if you have a chance to view wild guppies (Poecilia reticulata), you’ll see something far different. Wild male guppies are smaller, less ornate, and their color is limited almost entirely to orange. Now, that’s strange. Why the lack of pigmented panache in the wild?
Well, for one, being conspicuous is a dangerous game to play in the wild. Unless you’re trying to make a statement about your unpalatability (as in the case of aposematic coloration), being brightly colored and conspicuous is often a one way ticket to a predator’s digestive tract. Animals need to balance the cost of being conspicuous with the benefits; which are typically expressed in terms of mating success. The second reason, and perhaps the primary driving force behind the lack of color diversity in guppies, is that female guppies have a particular affinity for orange. Furthermore, it’s not just any orange, but a particular hue of orange and thus male guppies have evolved to present the same color orange in wild populations. But there’s a catch here – male guppies are incapable of biologically synthesizing the carotenoids necessary to produce this particular color orange and thus must derive them from the environment.
Carotenoids are a group of organic pigments that are well expressed in plants and appear to have evolved as a response to the interactions of lifeforms with light. They fill key roles in photosynthesis and photoprotection and are often responsible for the red, yellow, and orange colors of various fruits and vegetables that we are all familiar with. Unfortunately for guppies, almost all animal life is incapable of synthesizing carotenoids, so they must rely on dietary intake to obtain these chemicals. In this case, the guppies appear to derive most of their carotenoid pigments from a diet of benthic algae. Once consumed, the guppies are able to utilize these foreign pigments to produce the color that females find so appealing.
But again, that’s not the end of the story – that would be too easy. Apparently just consuming carotenoids and turning orange isn’t good enough and female guppies like to put a little more pressure on their prospective mates. It turns out that if the male is too orange, it isn’t appealing. The same goes for males that aren’t orange enough. To get just the right orange, male guppies balance their levels of carotenoids (environmentally derived) with levels of drosopterin pigments (produced by the guppies themselves). The desired combined effect is a sort of yellow-orange hue within the range that females find most attractive. Only the guppies that can create the right balance of “diet and drosopterins” are able to occupy the niche of highest attractiveness.
I’m sure you’re wondering where this inordinate preference for orange came from in the first place? Well, one of the prevailing theories is that it originated from guppy foraging behavior. In the South American and Caribbean habitats where guppies originated, nutritious orange fruits will occasionally fall into the water. Observational studies have shown that guppies avidly consume these fruits when available and thus have developed a sensory preference for orange items. It thus would appear that this sensory preference for the color orange eventually made it’s way into the reproductive strategies of male guppies looking to court females. It makes you wonder what guppies would like if the fruit weren’t orange and provides us with a great example of how seemingly disjoint evolutionary occurrences, like the color of fruit and guppy sex, can be so intimately connected.
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