As it has been shown, heterospecific mating of male sailfin mollies, Poecilia latipinna, to the gynogenetic Amazon molly, Poecilia formosa, results in an increased attractivity for conspecific females. Even previously rejected males can thus be made attractive enough to female P. latipinna to overrule other preferences. While it has already been shown how this benefits both the male sailfin and the female amazon, the question remains as to whether the female sailfin gains any advantage in copying the preference of a heterospecific female. To answer this it must first be proven whether the female P. latipinna is able to correctly discriminate between amazon and sailfin females.
While mate copying is a widely spread
mechanism it should be expected that this is due to the increased
success in terms of offspring resulting from mating with an already
proven and experienced male. If the mate copying leads to an overruling
of primal mate choice characteristics it should on the other hand
result in a decrease of fitness since "honest" phenotypical
signal of genetic quality are rejected over more manipulative
signals, and to a decline of the species. Thus the mate copying
of heterospecific females would put a challenge to basic evolutionary
principles.
Before we can start to find an explanation as to why females of
P. latipinna would copy P. formosa, I'll have to first determine
whether they are able to discriminate between conspecific females
and the rather similar looking Amazons.
For my test I take advantage of the shoaling behaviour that poecilides
show, working from the basic premise that shoaling preferences
work by a high phenotypical likeness of the individuals within
the shoal. If a female sailfin molly would show a significant
preference for a shoal of conspecific females over one of similar
size and makeup but of amazon mollies it can be deduced that they
are able to tell the two apart, thus showing that they would well
be able to discriminate against copying P. formosa's mate choices.
Experimental design
A large tank would be partitioned into three sections by clear
Plexiglas so that fish in one compartment would be able to observe
the other two. In the two side ones there are a shoal of 20 individuals
randomly taken from the stock tank of either P. formosa or P.
latipinna so that each shoal is unispecific. Exceedingly large,
small or otherwise unusual individuals should not be used for
the shoals as not to introduce any confounding factors as size
preference. Both partitions would be covered as to render them
opaque with additional sheets of Plexiglas. A female of P. latipinna
would be inserted into the middle compartment and left for 10
minutes to acclimate. Afterwards the covers would be removed so
that she can now observe both shoals. Over the next 20 minutes
it would be measured on which side of their compartment the middle
molly is. Afterwards both shoals would be swapped in place and
the test repeated to eliminate the confounding factor of a possible
size preference. A clear preference to the conspecific shoal would
show that female sailfin molly are able to discriminate between
the two species.
Fig 1, showing the test aquarium with the single P. latipinna
in the middle and the shoals of P. latipinna and P. formosa (colored
gray in the picture) in the respective side sections.
Methods
Test Subjects
Aquarium bred sailfin mollies (Poecilia latipinna) and amazon
mollies (Poecilia formosa), housed in a group of about 50 each
in a joint glass tank of about 200 liter. The sympatric housing
is on purpose to emulate natural conditions as both species life
sympatric in the wild as well. Light circle would be 12:12 hours,
water temperature at room temperature, about 21-23°C. A tablespoon
Marine salt per liter would be added to prevent fungal growth
and simulate the somewhat brackish waters of their natural habitat.
Test Tank
An Aquarium of 80x40x40 cm is partitioned by two pieces of clear
Plexiglas into three compartments of 25, 30 and 25 cm length.
The two partitions can be covered by sheets of opaque Plexiglas.
The middle compartment is further partitioned by marker into three
equal zones to help in evaluating which side the center molly
is preferring.
Statistical Analysis
The time spent on each side of the middle compartment would be
measured by a 0/1 assay with a check at every 30 seconds over
a 20 minute interval. Fish would be considered to show a clear
preference for one side if they spend more than 2/3 (66%) of their
time on this side. The sample size of tested fish should be 20.
Discussion of possible
results
If there is no significant preference for either shoals consisting
of P. latipinna or P. formosa that would show that female sailfin
molly seem indeed unable to properly discriminate between the
two species.
Thus the mate copying behaviour towards heterospecific fish can
be explained as a lack of discriminative ability. Natural selection
would presumably favor females that are more apt at discriminating
against mate copying heterospecifics, since they would rely rather
on primal, honest signs for male quality. It could be speculated
that in this way discriminating ability would be bred stronger,
which might eventually lead to a reduction of the advantage gained
by male Sailfins in heterospecific matings as shown by I. Schlupp.
On the other hand, continual inability to discriminate might lead
to a decrease in fitness in sailfins and maybe to an eventual
end of the species.
If on the other hand it were proven that female P. latipinna are
well able to tell conspecifics apart from P. formosa more questions
would be raised as to why they would willingly copy a female they
know not to belong to the own species. It could simply be concluded
that the mate copying mechanism is strong enough to not need rely
on conspecific females, which could be tested by repeating the
experiment conducted by I. Schlupp (Benefit to male sailfin mollies
of mating with heterospecific females) while replacing the females
of P. formosa with increasingly dissimilar models) Otherwise field
studies would have to be undertaken to check for a possible advantage
that sailfins could gain by an increased population of amazons
in their habitat. Hypotheses might include a decrease of predatory
pressure from a larger variety of species.
References used:
1. Ingo Schlupp, Cathy Marler,
Michael J. Ryan; Benefit to Male Sailfin Mollies of Mating
with Heterospecific Females (Science, Vol 263, 21 Jan 1994,
373-374) :
Female gynogens reproduce clonally but rely on sperm from heterospecific
males to initiate embryogenesis. It had been assumed that males
gain no benefit from such matings; thus selection would favor
males that avoid them. Studying on the sympatric species of sexually
reproducing sailfin mollies (Poecilia latipinna) and gynogenic
amazon mollies(Poecilia formosa) it is shown that males do nevertheless
get a benefit by mating with female gynogenes in an asuexual-sexual
complex of fish. The sexual females increase their preference
for males whom they observe consorting with female gynogenes by
following mate copying behaviour. Thus it can be shown how gynogenic
species might persist because selection favors males to be sexually
parasitized.
2. Scott P. McRobert, Joshua Bradner; The influence of coloration
on shoaling preferences (Animal Behaviour, 1998, 56, 611-615):
Shoaling behaviour provides antipredator benefits that rely to
some extent on a high degree of phenotypic homogeneity between
the individuals making up the shoal. Therefore it is advantageous
for fish to be able to discriminate between potential shoalmates
to choose to associate with fish of similar appearance. The effects
of a single phenotypic character, here body coloration, was studied
on association choices made by black and white mollies (P. latipinna).
Individual fish were shown to spend significantly higher time
near shoals of their own body coloration than near those of a
different color. Thus it was indicated that fish can use visual
cues to actively discriminate between potential shoalmates.
3. Anne Schlüter, Jakob Parzefall, Ingo Schlupp; Female
preference for symmetrical vertical bars in male sailfin mollies.
(Animal Behaviour, 1998, 56, 147-153):
Fluctuating asymmetry is defined as random deviation from perfect
bilateral symmetry, originating in developmental errors during
ontogenesis. It is thought to reflect the inability of the genotype
to buffer itself effectively against environmental perturbations.
This way the measure of symmetry would be a honest phenotypical
expression of genotypical qualities in a potential partner. It
has been proposed that females use the degree of asymmetry in
males as a sign of male quality in choosing mates. It was studied
whether symmetrical vertical bars are a criterion in the sailfish
molly Poecilia latipinna, as it had been previously shown in a
swordtail, Xiphophorus cortezi. It was shown that it is indeed
that case and it is concluded that symmetrical vertical bars in
males may well have influenced the evolution of courtship display.
4. Catherine A. Marler, Christy Foran, Michael J Ryan;
The influence of experience on mating preference of the gynogenetic
Amazon Molly (Animal Behaviour, 1997, 53 (3), 1035-1041)
The role of experience in shaping individual female mating preferences
had been relatively neglected before. This study examined how
female mating preferences are shaped through male/female interactions
in the clonally reproducing Poecilia formosa. In visual choice
tests some preferred P. latipinna males and some preferred males
of another poecilid species Xiphophora multilinneatus. After interacting,
females originally preferring X. multilinneatus switched their
preference to P. latipinna. Females therefore can change their
preference based on experience. It was also shown that this experience
was generalized to other males and not confined to specific individuals
and was also somewhat consistent after more time had passed.
5. Margaret B. Ptacek; Interspecific mate choice in sailfin
and shortfin species of mollies (Animal Behaviour, 1998, 56,
1145-1154)
Links of interest
The university of Texas' fish index (great pictures)
Biosis' Homepage (publishers of the Biological abtracts) There
is also a "interesting facts" page there :)