The Relevance of Colour Patch Size and Reproduction in Eurema Hecabe

Abstract

Female Eurema Hacabe (the grass yellow butterfly) has been found to have a bias when selecting males to mate with. They more often select males that have more yellow on their wings to breed with, the reason for why this selection process is done is not yet completely understood (Kemp 07). Males on the other hand do not show this bias as females do, they do not have a preference on the amount of yellow that their mating females have. In this experiment we are supporting theories that the colour of males truly is a standard in a females selecting process when she is choosing a stronger and healthier mate. This would be done, by finding an allometric relationship in the colours of the male’s wings. Which would indicate that certain males have sexual advantages to other males with less yellow. We did this by running a linear regression of the total amount of yellow and total wing area of the butterflies. We found that there was a significant relationship between the yellow and the total wing area, however the analysis showed that they were still slightly allometric.

Key Words: Allometry, Eurema Hecabe, Sexual Selection, butterfly, Reproduction

Introduction and Hypothesis

Throughout evolutionary time males of all species of animals have evolved more abstract and exotic secondary sexual traits, this is a result of females sexual preferences of these more decorated males (Andersson 1994). The individuals that have the greatest fitness have the ability to have the greatest amounts of these characteristics that will make them more attractive to mating partners, because these characteristics are usually costly in resources. (Nur and Hasson 1984; Rowe and Houle 1996). Colour as a sexual attraction has been studied in many different types of animal varying from guppies (Endler 1980, 1983) to finches (Hill 1990, 1991).

Butterflies are some of the planets most diversely coloured animals and because of this they are the chosen animals for many different research projects to test many different things (Kemp 2007). However as far as the relevance of colouration and sexual selection of butterflies, there are few studies. It is known that butterfly wing marking are tools that butterflies use to be able to recognize their mates (Sweeney 2003). Studies have also shown that female Eurema Hecabe (the grass yellow butterfly) do prefer to mate with males that have more yellow in there wings. This is a result of the higher UV iridescence. (Kemp 2007). There has also been other research that indicates females’ preference to higher amounts of UV not only in E. Hecabe but also in other species (Silberglied and Taylor 1973, 1978). The question would then be why do females favour this higher output of UV from males? In other species it has been found that males that express a higher amount of UV are better mates for the females. Because males that express poor quality larval environments would not express as bright of a UV reflectance and happen to be less fit individuals (Kemp and Rutowski 2007). So females of E. Hecabe might use the higher UV reflectance as an indication of the condition of that male and ability to successfully reproduce (Kemp 2007). Males of this species do not show any bias to which females that they breed with based on the their colouration.

The purpose of this experiment was to show that the males E. Hecabe yellow colour was indeed a signal for sexual preference for the females. I examined the ratios of the area of yellow oppose the total area of the males wings, to indicate an allometric relationship between the two. Allometry is when there is a greater than a one to one relationship between the colour patches and the total body size, and for every unit of size that the body increases the colour patches increase by more than that one unit. I also calculated females to see what type of relationship their colour patches had. If they were allometric it would indicate that there was not a one to one relationship between the two areas. Which would mean that males that were healthier and that were better mates for the females would have more yellow even though they may have had the same total size or even smaller. If this was not the case and the males happened to have an isometric relationship that would indicate that the colour patches had a one to one relationship and that the amount of yellow on the wings was just dependent on the over all size of the butterfly. An allometric relationship would help support the findings that the yellow colour patches of a male E. Hecabe are an indication or attraction for females E. Hacabe as a selection process of choosing the best mate (Kemp 2007). Therefore, I Hypothesized that the colours on the E. Hacabe wings would have an allometric relationship.

Sampling

The sampling and Data collection took place at Macquarie University (biology laboratory), in New South Wales, Australia.

The Experiment began by obtaining twenty-three samples of males and females sets of wings of Eurema Hecabe (the grass yellow butterfly). These wings were laboratory reared at, Macquarie university in 2010 (using standard methods). Then proceeded using a specific measuring instrument (called mitotic images) on the computer using the microscopes camera, the microscope was set to the lowest possible magnification to be able to fit the entire wing of the butterfly on the screen and was calibrated. To calculate the area of the yellow and black patches of the butterfly’s dorsal side and also the total area of the butterfly wings. This process was repeated 3 times on each wing and the results for each of the three areas of measurements were than averaged and then the results were averaged for both of the wings of that individual organism together. The Males and Females measurements were recorded and analysed separately.

Once all of the wings, three different areas were collected a simple linear regression was used to analyse the data that was collected between wing colour patch size and total wing area. Before the linear regression could be ran the raw data had to be log transformed (natural log transformation was used) to fulfil the linear assumption of linearity. Scatter plots were also made with the raw data to see the relationship of the slope with out being transformed.

Results

The area of colour patch size was significantly related to the wing size of the butterflies in males (t=24.43, df=21, P<0.0001). The 95% confidence limits of the slope estimate are (0.9872-1.171), and the (r squared=.966). The projected slope of the line was 1.08. The area of colour patch size in females was also significantly related to their wing size (t=9.64, df=21, P<0.00101). The 95% confidence limits of the slope estimate are (0.7702-1.194) and the (r squared=0.8157 ). The projected slope of the line was 0.98. Figures 1 and 2 are scatter plots with the raw data and just as with the transformed data males trend line slope was (0.7126) and the females was (0.7221)

Discussion

The results provided evidence that the male E. Hecabes colour patches were in an allometric relationship and were able to support the hypothesis, however they were only slightly allometric with a slope of (1.08). The females slope was below one (0.98) so indicating an isometric relationship because when rounded it becomes a one to one ratio. There was a significant relationship between the yellow patches and the total wing area for both males and females, this was expected because the larger the wings were, the larger the area of yellow was able to be, this did not have to indicate an isometric relationship. The 95% confidence limits indicate the confidence that the estimated slope would be between the two limits. The r-squared values tell us the variation that is based on the relationship of the two variables. Which was vey high for males (.966).

Obtaining preferable sexual characteristics takes up a lots of the organisms’ resources, which allows well-fit individuals to have a larger amount of these characteristics making, them the best choice for potential mates. (Nur and Hasson 1984; Rowe and Houle 1996). This means that it would take less energy and resources for the butterflies to have more black, because the colour black has nothing to do with sexual signaling in E. Hecabes (Kemp et al. 2005). So the females must have a purpose for investing resources on being yellow appose to being all black. Sexual selection is not the reason for this because males had no sexual preferences based on the colour of the females. There are a few possible explanations for why the females are still significantly yellow. One possible explanation is that males actually prefer to mate with larger females (Kemp 07) and because of the isometric relationship that was found in females the larger they are the more yellow that they will have. Having the sexual advantage of being chosen by a male for being bigger leads to them growing larger and consequently having more yellow. Another possible explanation is that the females of E. Hecabes need to be yellow so males could recognize them as potential mates, this was found to be the case in Coliadinae (Silberglied and Taylor 1973, 1978).

There are a few explanations of why the slope for the males was not significantly higher than (1.08) and did not have a clearer distinction of having an allometric relationship, one could be the low number of replicates in the experiment with 23 males tested, with a larger number of replicates a more definite trend would have been able to be found. Another reason could be because we had two outliers, which changed our findings however with a larger number of replicates the outliers would not have a significant influence on our data. Another major factor to this experiment that should have been calculated that I did not include because of limitations of resources, was the UV reflectance of each wing. The majority of the studies done about these butterflies indicate that higher UV has a major role in sexual selection. The amount of yellow has the potential to not be a big factor on sexual selection but the brightest of the UV might be the confounding factor.

These results even though they may not be overwhelmingly supportive do show that the male E. Hecabe colour patches have an allometric relationship and the amount of yellow on the butterflies wings, could potentially be a main distinction factor for who the females decide to breed with. For future studies scientists should attempt to expand this research project in a few different ways. One using this same experiment however largely increasing the sample size so that there could be a more distinctive pattern of what was found. Another option that someone could go about doing is expanding this experiment and adding UV reflectance as another possible variable to this experiment and see if there is a relationship between the amount of yellow that a wing has and the brightness of the UV reflection. Lastly an experiment could go about trying to find a sexual preference that males have when selecting females to mate with.

Ackowledgments

I want to thank the Department of Biology at Macquarie University for allowing me to use their equipment to conduct this experiment. I will like to thank people in my lab for helping me collect the data. I also thank Dr. Darrell Kemp and Dr. Kate Umbers for supervising me through the process of this experiment.

Reference

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Appendix

Fig 1. This a scatter plot of raw untransformed data with its trend line and the equation of the line. For the males yellow wing area vs total wing area.

Fig 2. This a scatter plot of raw untransformed data with its trend line and the equation of the line. For the females yellow wing area vs. total wing area.

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