Thursday, July 7, 2016

The Sinful Side of Guppies

Female (front) and male (back) guppies

First, congrats to my colleague Dale Broder for accepting an offer for a postdoctoral position with the Interdisciplinary Research Incubator for the Study of (In)Equality at Denver University! In celebration of her impending defense, I thought I would post about what I've learned from her over the past few years regarding the dark side of guppies, that is, reproduction. Also, since I took some photos for her to use and wanted to post them, it seemed like the right place.

Baby guppies (domestic strain)

Guppies are a small livebearing fish from the Caribbean and South America. This means that they have internal fertilization and the young develop inside the females, similar to pregnancy in humans. Males inseminate females with an organ called a gonopodium, which is a modified anal fin. Since females have a large investment into these offspring, they are choosy about who they mate with. The idea is that they should give their offspring the best chance of also surviving and reproducing, increasing the female's fitness. However, males should also do their best to pass on their genes, which leads to an interesting paradigm where males and females potentially have conflicting goals (if the female is not interested in the male). This is the primary force behind the vast differences in size, color, body shape, and behavior between males and females.

A male showing off his colors

To compete with each other, males develop unique color patterns that are attractive to females. Females are attracted to the color orange. This may be because of a sensory bias since orange is also associated with tasty food items, but it may also be an indicator of male quality since orange pigment is derived from diet. Females also prefer uniquely patterned males, which leads to the most extreme polymorphism I am aware of, such that no two males (even brothers) are exactly alike.

Variation of color pattern among males
from the same population

One interesting way that males try to encourage females to mate with them is through courtship displays. This involves males swimming around females to ensure they can see their colors, as well as displays that look like a whole-body spasm, called sigmoids. Males are also fairly pre-occupied with females, and follow her everywhere just in case she finally wants to mate. Based on my observations in the lab, I'm pretty sure that, given the choice between food or a female, he will always pick the female.

Male doing a sigmoid display for a female

Close-up of a sigmoid display
Male following a female

So what does he do if she doesn't want to mate? This is where it gets a little more sinister. At this point, male guppies engage in a behavior called forced copulation, in other words, they don't give the females a choice (I didn't get photos of this). This can become particularly stressful for females since there are often several males around, all of which are trying to mate with her. For this reason, once they are mature, females are continuously pregnant. Females can also store sperm (in my experience, up to 1 year!), which is presumably an adaptation for colonizing new areas of streams. Because of these two mechanisms, multiple matings and sperm storage, the offspring from one female may be sired by several fathers.

Male swinging his gonopodium

Close-up of a gonopodium swing

This scenario that I've just explained becomes more complicated in high predation environments, where color patterns and displays make males highly conspicuous to predators. Here, males have minimum color and do not display as much, engaging in more forced copulations. This means that their gonopodia tend to be longer than males in environments without predators (sometimes up to 1mm longer!). See some examples below (note the colorful guppies are from Dale's personal tank and are extremely colorful):

These may be small fish, but they sure do have a lot for us to learn about. The more I learn the more I am intrigued by them! Also, the more I am glad I'm not one of them...

Male harassing a female

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