| Strangers
and Sex - the role of familiarity in guppy
mate choice
Nirvana
Naidoo
Over the summer, I participated
in the UNSW
Faculty of Science Undergraduate Vacation Research
Scholarship program, open to science students in second
year and above. I completed a research project on sexual
selection in guppies with Dr Rob Brooks of the School
of Biological, Earth and Environmental Sciences, gaining
valuable practical experience as well as insight into
what is involved in scientific research.
Survival of the fittest often summons images of the
strongest or fastest. Yet even such an individual is evolutionarily
insignificant if he or she fails to pass on any of his
or her genes. Natural selection occurs because there is
genetic variation between individuals causing some, for
example, to live longer or be better at gathering food.
The fittest individuals leave the most descendants, and
to do that they must be better than their competitors
at finding resources and escaping predators. In many species,
however, we find traits that do not increase an individual’s
chance of surviving to reproduce but may in fact hinder
it. Traitsm such as the brightly coloured and ornate tails
of some male birds and fish, make them easier targets
for predators. Yet these traits persist.
The
theory of natural selection cannot explain the existence
of seemingly exaggerated traits, such as the peacock’s
tail, leading Darwin to the idea of sexual selection,
a special case of natural selection. Under sexual selection,
phenotypic
variation in a trait in one of the sexes, usually male,
results in differential mating success. Sexual selection
can occur either by intrasexual selection, where members
of the one sex compete for mates, usually in the form
of male fights or contests, or intersexual selection (mate
choice) where certain individuals are found to be more
attractive than others. In either case, some individuals
leave more descendants than other individuals.
The
guppy, Poecilia reticulata, is a small tropical
fish, from the freshwater streams of Trinidad, known for
their conspicuous and immensely varied male colour patterns.
Mature males possess larger tails than females and a variety
of polymorphic
markings while females are of uniform colour. This sexual
dimorphism (difference) is often indicative of sexual
selection.
It
is well established that male colour patterns are highly
heritable and the basis of mate choice. Males court females
incessantly, but females primarily choose whom to mate
with. As a trait under sexual selection, the level of
variation in male colour patterns is especially surprising
as selection is expected to deplete variation, as only
the males with few most appealing patterns will procreate.
However, though their conspicuous colour patterns were
more than likely the attribute that initially led to sexual
selection studies in guppies, we are still unable to satisfactorily
explain why such extreme levels of polymorphism persist.
In
the simplest scenario, the balance between mutations introducing
new variants and natural selection removing other variants
determines the level of genetic variation. Antagonistic
effects caused by conflicting forms of selection may maintain
greater than expected variation as the trade-off could
result in different combinations of variants of equal
fitness. This is thought to be the case in guppies as
it is well established that brightly coloured male guppies
are favoured by females but are also at greater risk from
predators. Frequency-dependent mate choice by females,
where they prefer rare or novel (infrequent) males, is
a common proposal because it has the potential to maintain
extensive variation. The suggestion is that females prefer
males who have unusual colour patterns, a phenomenon known
as the “rare-male effect”.
There
have been few studies on frequency-dependent mate choice
in guppies. Farr1 and Hughes
et al.2 both found
a female preference for novel or unfamiliar (‘unfamiliar’
meaning new to the female but not necessarily rare in
nature) males in guppies, while Kelley. et al3
found a male preference for unfamiliar females. We tested
for preferences for an unfamiliar mate as a factor contributing
to the maintenance of the variation in male colour patterns.
A preference for unfamiliar males is argued to be equivalent
to a preference for rare males. Though female mating preferences
in guppies are considered more significant, a male preference
for unfamiliar females would also contribute to high levels
of variation.
Our
experimental protocol was similar to that of Hughes et
al.2 We familiarized males
and females and then conducted behaviour trials with familiar
and unfamiliar individuals. Female responses to male sigmoid
displays were scored according to how much interest
the female seemed to show. This method allowed us to compare
the number of displays by the unfamiliar and familiar
males to determine if males preferred unfamiliar females.
It also allowed us to compare the scores of the displays
to determine if females preferred unfamiliar males.
Based
on earlier studies, we expected to find a female preference
for unfamiliar males due to the ‘rare-male effect’. Yet,
we found no significant difference in female response
to unfamiliar and familiar males. Since none of our fish
were allowed to mate prior to the trials, we were less
surprised to find no male preference for unfamiliar females
as there was no biological advantage for males discriminating
against females they have not previously mated with. These
results indicated that familiarity alone was not important
in mate choice. Thus a preference for unfamiliar mates
could not explain the extreme variation in male colour
patterns. Alternatively, it is possible that we did not
allow sufficient time for the fish to become familiarized.
However, it is unlikely that wild populations of guppies
would experience such an intensive familiarization. Thus,
it would still be unlikely that familiarity is an important
component of mate choice. In either case, a preference
for unfamiliar mates cannot explain the extreme variation
in male colour patterns.
This
result suggests the ‘rare-male effect’ may not be as significant
as once believed since the quantity of variation in itself
means every male is likely to be rare under some criteria.
So, my summer’s effort did not yield an earthshaking discovery.
However, I did learn that careful experimentation, like
what I was involved in, can challenge specialised scientific
theories.
References
1. Farr,
J. A. (1977). Male rarity or novelty, female choice behaviour,
and sexual selection in the guppy, Poecilia reticulata.
Evolution, 31, 162-168
2. Hughes,
K. A., Du, L., Rodd, F. H. and Reznick, D. N. (1999).
Familiarity leads to female preference for novel males
in the guppy, Poecilia reticulata. Animal Behaviour,
58, 907-916
3. Kelley,
J L., Graves, J. A. and Magurran, A. E. (1999). Familiarity
breeds contempt in guppies. Nature, 401,
661-662.
Further reading
Brooks,
R. (2002). Variation in female mate choice within guppy
populations: population divergence, multiple ornaments
and the maintenance of polymorphism. Genetica,
116, 343-358
Houde,
A. E. (1997). Sex, Color and Mate Choice In Guppies. Princeton
University Press
Glossary
Heterozygote:
A diploid organism with two different alleles of the one
gene.
Homozygote:
A diploid organism with two of the same alleles of the
one gene.
Phenotype:
The physical appearance resulting from the genotype.
Polymorphism:
Variants, in this case, in the colour patterns of the
male guppies.
Sigmoid
display:
The mating displays male guppies perform to attract mates.
|
