By Anton, our first guest writer!
The guppy (Poecilia reticulata) is a small fish that originates from the Caribbean. Incredibly resilient, these little fish can now be found everywhere from the River Lee in Essex to the Moscow sewage system (guppies have even been sent into space!). Scientists love guppies. However, they love Trinidadian guppies most of all for three important reasons. Firstly, guppies have a short life cycle, which allows the study of genetic changes across generations. Secondly, on the island of Trinidad, there are many small populations of guppies isolated from each other by natural barriers (e.g. different rivers or either side of waterfalls). Finally, each population of guppy occur under slightly different environmental pressures such as varying amounts of predation. This unique arrangement of isolated populations under different ecological stressors results in various physical and behavioural traits being selected across populations. The most striking example is the massive variance in male guppies colouration found across populations. It is likely that eventually, guppies from different populations will evolve into separate species (Magurran 2005).
One of the great puzzles that surround the Trinidadian guppies is that where predation is high, males and females live in a kind of artificial segregation. Female guppies choose to live in deeper water where there are fewer male guppies and more predators, whilst the males play in safe in the shallow end. So the question is why are female guppies risking their lives? There are no physical barriers to stop female guppies from living in shallow water, and there are no apparent advantages of feeding in deeper water. The answer lies in an underlying tension between male and female reproductive desires.
Male guppy colouration. The guppies in this picture have been caught from different areas of Trinidad. Female guppies find orange males attractive because their favourite fruit the Sloanea laurifolia is orange|credit for photo Andrew Hendry of McGill University
The female guppy is less colourful than a male guppy and tends to be larger. However, there is a lot of variation in guppy size across Trinidad. Near the coast male and female guppies grow mature at 9mm while further inland female guppies grow much larger at 15mm to 18mm in length.|Wiki Commons
Let’s first address male guppies. Male guppies are smaller than the females and brightly coloured. Males spend half their time looking for food and the other half of their time trying to mate with a female. A male guppy employs two different tactics to obtain a mate. The first, of these is to make a mating display; this involves becoming more intensely coloured and creating a sigmoid or “s” shape with its body. A displaying male’s colour changes so dramatically that to the human eye they appear like a completely different guppy! The display tactic is advantageous when a female is receptive to advances, and there are no other more attractive males in the vicinity. If the male guppy is successful in attracting a mate with a display, the male is likely to successfully fertilise the female guppy. However, there are disadvantages in displaying for the male guppies. Brightly coloured displaying males are more vulnerable to predators. Further, most of the time female guppies are just not interested, so displaying ends up as a complete waste of time for the males.
Male guppy displaying|Youtube
The alternative mating tactic is gonopodial thrusting, aka sneaky mating. Sneaky mating attempts involve sneaking up on a female guppy when they are not paying attention and attempting to mate with them. Sneaky copulations are advantageous for the male guppies because there is less risk from predators and it takes less time than displaying for female attention. However, it is debatable just how successful the sneaky mating strategy is. Many authors suggest that sneaky matings work occasionally, but on the whole, they are unsuccessful.
Male guppy attempting a sneaky copulation|Youtube
They only also store sperm this means that after one successful copulation a female can become pregnant seven times without needing to mate with a male. Female reproductive success is highly correlated with body size, foraging success and age. So, to succeed as a female guppy, they have to feed at a rate that is six times faster than males and spend two to three times longer feeding.
For a female guppy to succeed they require relatively few mating opportunities and lots of food. For a male guppy to succeed, they require much less food and lots of mating opportunities. The disparity between male and female optimal mating strategies is a sexual conflict in the guppy mating system. Females have to fend off sneaky male copulation attempts continuously, up to one a minute, which can have serious consequences. This constant barrage of sexual harassment interferes with female guppies feeding and may reduce the size of their brood. Further, the sneaky mating attempts may distract females and make them more vulnerable to predation (Magurran and Seghers 1994, Darden and Croft 2008).
If you have fallen head over heels for the guppy as I have, I suggest you read “Evolutionary Ecology: The Trinidadian Guppy” by Magurran (2005). I would also like to refer you to “Male harassment drives females to alter habitat use and leads to segregation of the sexes” by Darden and Croft (2008) about segregation of the sexes in guppies, this is a nicely written published journal article that is freely available.
Darden S K, Croft D P (2008) Male harassment drives females to alter habitat use and leads to segregation of the sexes. Biology Letters 4:449-451
Magurran, A. (2005) Evolutionary Ecology: The Trinidadian Guppy, Oxford University Press
Magurran, A. E. & Seghers, B. H. 1994 Sexual conflict as a consequence of ecology: evidence from guppy, Poecilia reticulata, populations in Trinidad. Proc. R. Soc. Lond. B 255, 31-36
lemon fairy. (2012, April, 25). Guppy mating dance! [Video file]. Retrieved November, 28, 2017 from https://www.youtube.com/watch?v=1tKOIc0qReQ
FishyBizzz Channel. (2016, May, 16). How do guppies breed [Video file]. Retrieved November, 28, 2017 from https://www.youtube.com/watch?v=cG-ZD41oqmg
Hendry, A. (UNK). Guppies [Online image]. Retrieved November, 28, 2017 from http://redpath-staff.mcgill.ca/hendry/guppy.html
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