Abstract
Poecilia reticulata (also known as the guppy) is a common tropical fish where female sexual preference has led to brightly colored males. We designed an experiment to determine if male P. reticulata also demonstrated sexual preference by placing randomly selected male guppies into a 1x2ft aquarium with two isolated but visible females with a size difference (one larger than the other). We measured the time the male attempted to court with either female over a ten minute period. We ran a total of 19 trials and then used JMP to average the time the males spent with each size using a paired t-test in JMP. There was no significant difference between the mean amount of time spent with either size. Based on these results, our original hypothesis that male Poecilia reticulate will show no preference in mating with females based on size was supported. Male P. reticulate may be demonstrating a lack of sexual preference because parental investment cost for males is very low. Because of the polyandry mating system used by guppies, it is advantageous for male guppies to mate with as many females as possible rather than being selective.
Introduction
Sexual selection is a process of natural selection in which organisms choose their mates. By choosing mates or competing with other organisms of the same species for mates, the overall genotype of a population can change over time, selecting for individuals that display phenotypes that are advantageous to interspecific or intraspecific competition (Evans & Magurran, 2000). Not all traits that are selected for as a result of sexual selection are fitness benefits. In fact, many such traits have fitness costs. In Poecilia reticulata (a widespread tropical fish commonly known as the guppy), female mate preference for brightly colored males has led to male guppies with color patterns that attract not only mates, but predators (Godin, 2003).
Because female guppies have higher parental investment than male guppies, female guppies are more selective than males in choosing mates (Rodd, Hughes, Grether & Baril, 2002). Because male parental investment in P. reticulata is low, male guppies may mate indiscriminately with females without any major preference for phenotypes. To test the hypothesis that male Poecilia reticulate will show no preference in mating with females based on size, we will conduct an experiment in which male P. reticulata are given equal opportunity to attempt to court with two females, one large and one small. We will measure the time the male guppies attempt to court with either size, and compare the averages between the two sizes. We predict that that if the male guppies have no preference for size than the males will spend equal amounts of time with each female.
Methods
To test the hypothesis that male Poecilia reticulate will show no preference in mating with females based on size, two lab grown female guppies with a visible size difference of .05cm in length or more were placed into a 1x2ft aquarium at room temperature. Clear bottles were placed around each female to limit movement and interaction with the male guppy. A randomly selected male guppy was placed into the aquarium with the two females. Two minutes were measured on a stopwatch while the male guppy became acquainted with the aquarium. After two minutes, data collection began. The amount of time the male guppy spent within 4cm of each female was recorded over a ten minute period using two additional stopwatches (one for each female). After ten minutes, the amount of time the male spent near each female was totaled and compared. A total of 19 ten minute trials were done. We compared the amount of time the male guppies spent with the larger or smaller females using a paired t-test in JMP.
Results
There was no significant difference between the amount of time the male guppy spent with the bigger female v. smaller female (paired t-test, s=5.0, df=18, p=0.7908).
Discussion
Our original hypothesis that male Poecilia reticulate will show no preference in mating with females based on size is supported by our data. The male guppies spent around the same amount of time with both large and small females, meaning they showed no obvious preference for one phenotype over the other. There are many explanations for this behavior. Because the parental investment cost for male guppies is much lower than female guppies, males may be less fastidious when choosing mates (Evans & Magurran, 2000). Poecilia reticulate engage in the mating style of polyandry, where one female will mate with multiple males, thus it is more advantageous for male guppies to mate with as many females as possible rather than being choosy and mating with fewer (Barbosa & Magurran, 2010). This type of behavior falls in line with many other species of aquatic animals, where males mate indiscriminately with females. To confirm that male guppies have a lack of preferences for mates because of a low investment cost, an additional, similar experiment could be run with aquatic species that have high parental investment costs for males such as Hippocampus erectus (seahorse) or Ariopsis felis (hardhead catfish). If male seahorses or hardheaded catfishes are more selective than male guppies in what females they mate with, it would likely be because their cost of investment is much higher. This data may be advantageous to fields such as fish farming, with species that have high investment costs. If this same experiment were to be run on species of farmed fish to determine which phenotypes males find desirable, exposing males to as many females with desirable phenotypes as possible could increase breeding rates and therefore the amount of fish produced. If fish farming efficiency is maximized, the massive fishing strain on the world’s oceans could be reduced, allowing wild fish populations to recover.
Literature Cited
- Barbosa, M., & Magurran, A. E. (2010). Evidence of female-promoted polyandry in Trinidadian guppies. Environmental Biology of Fishes, 90(1), 95-102. doi:10.1007/s10641-010-9721-y
- Evans, J. P., & Magurran, A. E. (2000). Multiple benefits of multiple mating in guppies. Proceedings of the National Academy of Sciences, 97(18), 10074-10076. doi:10.1073/pnas.180207297
- Godin, J. J. (2003). Predator preference for brightly colored males in the guppy: a viability cost for a sexually selected trait. Behavioral Ecology, 14(2), 194-200. doi:10.1093/beheco/14.2.194
- Rodd, F. H., Hughes, K. A., Grether, G. F., & Baril, C. T. (2002). A possible non-sexual origin of mate preference: are male guppies mimicking fruit? Proceedings of the Royal Society B: Biological Sciences, 269(1490), 475-481. doi:10.1098/rspb.2001.1891
Figures
Figure 1 Mean amount of time Male Poecilia reticulate spent with large or small females. Time (Y-Axis) is in seconds. Treatments are labeled for size (X-Axis). Error bar is one standard error from the mean