Sexual selection in guppies
Authors: Brian D. Wisenden, Linda C. Fuselier
Institutional address:
Biosciences Department, Minnesota State University Moorhead, 1104 7th Ave S, Moorhead,
MN, USA, 56563, [email protected], [email protected], 218-477-2572
1. Overview:
Objectives: In this lab we will test one prediction from Intersexual Selection and one hypothesis
from Intrasexual Selection.
Intersexual selection occurs when one sex exerts selection on the other sex through differential
mate choice. Because the cost of reproduction in usually greater for females than for males (i.e.
anisogamy), females are typically the choosy sex. However, here we exploit the correlation
between female body size and fecundity in fishes to test for mate choice by males. Large females
have the capacity to produce more offspring per brood than small females therefore males should
preferentially direct courtship toward large females.
Intrasexual selection results from competition within one sex for access to the other sex. Again,
because of anisogamy, intrasexual selection typically takes the form of males competing with
other males. Guppies have internal fertilization. The intromittent organ is the gonopodium,
which is formed by modification of the male‟s anal fin. On average, male guppies court and
attempt copulation 13 times every five minutes (Farr 1976). They do this because females mate
with more than one male, and sperm from previous males is partially displaced by sperm
deposited by a new male. Thus, only the most ardent males achieve reproductive success. These
peculiar aspects of their breeding biology make guppies an ideal study animal for the study of
sexual selection (Houde 1997).
Courtship behaviors
Males use several behaviors for courtship but we will consider only a few here to test our
hypotheses quantitatively. One general form is a lateral fin spread where males position
themselves beside the female and extend all their fins. A second is a sigmoid display in which
the male curves its body into a “C” or an „S‟ shape and then swims backward with its fins
extended in a series of rapid hopping jerks. Males may engage in gonopodal thrusts where the
gonopodium is swiveled into the forward position accompanied by a rapid approach toward the
female‟s genital opening.
Time frame: This lab exercise is easily completed within a 3-hour laboratory period.
Number of students in the group: The hypothesis is tested using class data. If students work in
pairs then about 8 pairs (16 students) is needed to make the data work.
Type of lab presentation or report: The major product is a formal lab report in standard scientific
format.
2. Materials and methods:
Wild-type guppies (i.e., ones that have not been selectively bred for colorful and elongated fins
for the ornamental fish trade) are the ideal test subjects. However, domestic strains still serve as
reliable test subject for studies of sexual selection because sexual behavior is well conserved.
Avoid “feeder” guppies sold at pet stores unless they have been nursed back to health. Pet store
stocks of feeder guppies are usually diseased.
Test apparatus:
Set up one 37 L aquarium per station with dechlorinated tap water. Filtration and a heater are
required. On the front viewing pane (long side) mark three zones with a black permanent marker
to demark two end zones equivalent to ¼ of the tank length each, leaving a section of about ½
the tank in the center. The end zones are the choice zones and the middle section is the no-choice
zone.
Experiment 1: Intersexual selection (mate choice)
Select two females from the stock tank that differ in size (or whatever quality you wish to test).
Place two inverted flat-sided jars into the test tank. Place one jar in one of the choice zones and
place one of the females in it. Place a second identical jar leave the other jar empty (full of water,
but with no fish). Add a male to the center of the tank. After 5 min acclimation time, record the
time the male spends near each jar for 5 min. Put the first female aside (in a beaker) and put the
second female (different in size from the first) in one of the jars and record the time the male
spends near each jar. Now put both females in the tank, one in each jar and record the time spent
near each jar.
a. Does the male spend more time with the larger female when the females are presented
sequentially?
b. Does the male spend more time with the larger female when the females are simultaneously?
c. What do these data indicate about male sexual behavior?
Instructor note: sometimes the size distribution of guppies sold in pet store is somewhat uniform
making it very difficult to test the effect of female size on male behavior. In this event, let the
males‟ behavior guide the result. Females differ in many ways other than size. Compare the
magnitude of the difference in time spent with each female when the females were presented
sequentially (no opportunity to compare and show preference) to the difference in time spent
with each female when the females were presented simultaneously.
Experiment 2: Intrasexual selection (male-male competition)
Males can adjust the frequency of their courtship display. Males are predicted to increase the rate
of courtship display in the presence of rival males (sexual competition). Place 3 females in a tank
and allow them to swim freely. Add one male and record the frequency of courtship behaviors
for 5 min. Add a second male and record the frequency of courtship behaviors of each male for 5
min. Use variation in markings to keep track of individual males. Add a third male and record
the frequency of courtship behaviors for each of the three males.
a. Does the first male increase the frequency of courtship behavior in response to the presence
of sexual competitors?
b. How can you eliminate the alternative hypothesis that an increase in male courtship is simply
males increasing courtship behaviors over time?
Sample results (from real class data)
Here is an example of using females that were uniform in length.
Mate choice
Alone
Team
Fem1
Fem2
Eric/Jen
29
205
Supun/Phil/Kyle
123
123
Kyle/Sarah
266
18
Anish/Susan
0
20
Kiaja/Michelle/Tara
125
138
Cheryl/Nikki
38
47
Kris/Jess/Ashley
133
139
Differences
Alone
Simult
Sim>Alone?
176
212
36
0
51
51
248
264
16
20
44
24
13
133
120
9
60
51
6
6
0
Simultaneous
Fem1
Fem2
229
17
49
100
268
4
9
53
39
172
60
0
69
63
Expression of mate preference was significantly stronger when females were presented
simultaneously than when presented sequentially (Wilcoxon matched-pairs signed ranks test: T+
= 21, N = 6, P = 0.031).
Intrasexual selection
Team
Eric/Jen
Supun/Phil/Kyle
Kyle/Sarah
Anish/Susan
Kiaja/Michelle/Tara
Cheryl/Nikki
Kris/Jess/Ashley
Total
Behavior of male #1
Alone 1 Extra M 2 Extra M
3
16
35
10
0
6
5
3
3
0
13
1
8
9
6
1
0
0
0
0
0
27
41
51
The frequency of finspread displays increased significantly in the presence of sexual competitors
(Chi-Sq = 7.34, df = 2, P < 0.05).
Acknowledgements:
We are grateful to funding from NSF CCLI DUE Award No. 0736872 for stimulating the
development and publication of this self-directed open-inquiry lab exercise.
References
Farr, J.A. 1976. Social facilitation of male sexual behavior, intrasexual competition, and sexual
selection in the guppy, Poecilia reticulata (Pisces: Poeciliidae). Evolution 30, 707-717.
Houde, A.E. 1997. Sex, color and mate choice in guppies. Princeton University Press. Princeton,
NJ.