For National Fishing Week, Dispatches from the Field welcomes Tim Hain, a biologist at the University of Western Ontario to tell us about his fieldwork studying the not-so-fancy-looking (but very cool for evolutionary studies) guppies in Trinidad. To find out more about Tim and his fieldwork stores, check out the end of this blog for a link to a book he recently published!
Many North Americans have heard of guppies – perhaps because they or a friend had guppies as pets, perhaps because they have watched Bubble Guppies on television. Aquarium hobbyists have an enthusiasm for guppies because these fish have natural variation in colouration and fin size or shape that breeders have exploited to develop many different beautiful strains with descriptive names like tuxedo, sunrise, mosaic, snakeskin, or swordtail. Although “guppies” have name recognition with the public, many people do not realize that these little fish are a favourite among researchers in evolutionary ecology. In fact, guppies are one of the best vertebrate species for studying evolution in the wild, particularly on the island of Trinidad in the West Indies. Because guppies have short generation times and waterfall barriers that restrict migration, there is variation in behaviour, life history, physiology, and appearance among populations that can often be attributed to variation in the local predator community.
The classic story is that guppies below waterfalls are subjected to predation by large vision-oriented predators, so male guppies in these populations tend to be cryptic in colour to avoid being eaten.
Above waterfalls, the predators are either smaller and cannot eat adult guppies or eat using tactile or chemical signals,
so male guppies from these populations are free to evolve conspicuous bright colours to attract females.Many evolution students will have heard all about this. But “science-world famous” is very different from “world famous.”
In fact, most people do not know how important these insignificant-looking fish are. When I first went to Trinidad in 2006 as a Ph.D. student, I had been studying guppies for three years. At its most fanciful, my imagination pictured monuments to guppies at important sites around the country.
Of course, I did not truly expect to find statues of guppies, but I was amazed by how common guppies were in the country. My first ‘wild’ guppy sighting was in a sewer along the major east-west road, and this was not unusual. In fact, they are so common that many locals were surprised that someone would travel from Canada to study them. In some locations where I collected guppies, I would attract a small crowd. Because I neither looked or sounded like I was from around there, local people would ask me what I was doing. One middle-aged Trinidadian that I spoke to was confused when I mentioned guppies, but when I described them, he said “Oh, you mean canalfish.” In Trinidad, guppies have this common name because they are frequently found in sewers and ditches alongside roads. Several times I used this name with Trinidadians to refer to guppies, and they knew what I meant.
Because female guppies give birth to live young, a single pregnant female can establish a population. This makes guppies master colonizers, and I saw them in a huge range of environments. The best-studied guppies are native to the Northern Range of forested mountains, where waterfalls break up narrow streams, but they are also founder in wider, dirtier rivers and some unique geographical features, like Pitch Lake in the southern part of the country.
Pitch Lake was formed when pitch – a resin once used for waterproofing ships – bubbled out of the ground and now covers 40 hectares of area . It resembles a naturally-formed parking lot, but without lines and full of fissures that give the tarmac area structure. Rain filled these fissures, and guppies have found their way to the lake and become established. The unusual water chemistry of Pitch Lake and the black substrate (leading to high water temperatures) means that it is very difficult to rear these guppies in the lab.
One environment where I did not find guppies was in the brackish estuaries along the northern coast. Guppies can tolerate light saline environments, but in one tea-coloured estuary that I visited, I instead found the congenic Poecilia picta fish. The low visibility in the water of that river might explain two unusual observations I made: low colouration of P. picta males, and transparent bodies of their predator, a prawn.
Transparent bodies are also found in deep-sea fish, which live in low-light (or no-light) environments: an interesting example of convergent evolution.
Perhaps the reason why guppies are such an appealing textbook example of evolution is in how intuitive and simple the explanation is: the same geographical barriers that restrict predator presence also restrict gene flow, and predation as a selection pressure drives trait differentiation. Guppy researchers know that the story is a little more complex than that, but these wrinkles in the story seldom make it into textbooks. So, I was left to independently discover these things for myself.
One variation on the story is that waterfalls are not the only feature that restrict large predators. For example, I found one ‘low predation’ environment located between two ‘high predation’ environments because the water in one stretch of the river was too shallow for the larger predators to enter. Male guppies in this stretch were more colourful than males I found upstream.
A second under-discussed variation on the story is the presence of avian predators. I often saw striated herons or little egrets walking alongside narrow streams, looking for guppies to eat. I also saw or heard kingfishers around my collection sites. These birds were skittish and difficult to photograph in the act of feeding, but their intention was clear. What is less clear is if they exert a selection pressures on guppies to be more cryptic in colour, or if their feeding habits are random with respect to colour. I do think that avian predators are important to guppy evolution – I suspect that guppies colonize new environments by escaping these flying predators after being given a short trip.
My fieldwork in Trinidad taught me many things about guppy evolution that I could not have learned from a textbook. Who knew that such small and common fish could be so interesting?!
Tim Hain is a biologist at the University of Western Ontario in London. He completed his PhD on kin recognition and multiple mating in guppies and bluegill sunfish, and he did his fieldwork in Trinidad and at the Queen’s University Biological Station. His first trip to Trinidad was for eight months, and he recently published his memoirs (Fieldwork: Stories from Trinidad) of his time living in the country on Amazon. Tim currently teaches at UWO. You can follow him on Twitter (@tjahain).
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