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Sexual Selection in Potomac Sculpins (Cottus
girardi) Determinants of Male Mating
Success Kimberly J. BolyardDepartment of
Biology, James Madison University Harrisonburg,
Virginia 22807
Potomac Sculpin Two conditions in the Potomac
sculpin make it a likely candidate for sexual
selection. First, Potomac sculpin males mature
at one year of age and these young males are up
to 40mm smaller in standard length than two and
three year old males (Jenkins and Burkhead 1994).
Thus, there are reproductively mature males of
many different size classes present during the
breeding season. Variation in male size opens up
the possibility that female mate choice could be
operating in this species. Females should choose
to mate with larger males as large males would be
better able to defend a nest and eggs from
predators. Thus, larger males would get a
disproportionate number of matings relative to
small males. Second, in the spring, males defend
a territory and nest site consisting of a cavity
underneath a rock or other similar object, court
females using head nodding and shaking, body
undulations, and gill cover erections (Savage
1963). After spawning has occurred, females
leave the nest site and males defend the eggs
(Savage 1963). If nest sites are limited, or
even if only high quality nest sites (e.g., large
or more protected sites) are limited, there is
the possibility that males compete for nests.
Males that lack a nest or have only a small nest
would probably get fewer matings and have zero to
low reproductive success. Males with a nest,
especially if it is large or well-protected,
would be expected to have the opportunity to mate
with more than one female, assuming an equal
operational sex ratio, and therefore have high
reproductive success. Past research on fish in
the genus Cottus supports these predictions
(e.g., Cottus bairdi, Downhower et al. 1983
Cottus cognatus, Mousseau and Collins 1987
Cottus gobio, Bisazza and Marconato 1988 Cottus
pollux, Natsumeda 1998)
Sexual Selection Sexual selection is a process
whereby competition for mates or selectivity in
mate choice leads to the evolution of traits in
individuals that do not enhance, and indeed may
hinder, their survival and reproduction. Darwin
(1871) developed the theory of sexual selection
to explain the presence of these traits, which he
believed were not accounted for by his theory of
natural selection. Sexual selection consists of
competition among members of the same sex for
mates (usually male-male competition) and mate
choice decisions by members of one sex for the
opposite sex (usually female mate choice). Both
of these behaviors can lead to nonrandom
variation in reproductive success. Thus, a
skewed distribution of reproductive success among
males is a tell-tale sign of sexual selection.
This research proposes to investigate sexual
selection in Potomac sculpins (Cottus girardi) by
examining variability in male mating success.
Broader Objectives If I find that larger males
have higher reproductive success, it may be
because females prefer to spawn with larger
males. A concurrently running laboratory study
is investigating the possibility of female mate
choice in Potomac sculpins. Further, these
studies will be considered in light of
demographic and movement data currently being
collected by members of Mark Hudys laboratory.
Ultimately, these data can allow us to develop a
more complete picture of the reproductive ecology
of Potomac sculpins. Information about the
reproductive ecology of Potomac sculpin will be
used to predict how they will be affected by the
reintroduction of brook trout (Salvelinus
fontinalis) into Smith Creek. Brook trout are
apex predators and generalists though adults tend
to be primarily piscivorous. Thus, I would
expect that brook trout will prey on Potomac
sculpin, potentially increasing male-male
competition for large nest sites and making egg
defense by males more critical, thereby, skewing
reproductive success towards larger males.
Methods In this field study, I plan to examine
the reproductive success of male Potomac sculpin
by measuring the number of discrete egg clutches
a male guards in his nest. Egg clutches will
likely be identifiable as uniquely from one
female based on their coloration, size, and
number as they are for mottled sculpins (e.g.,
Downhower and Brown 1977). To explore the
possibility that males with larger nest sizes
have higher reproductive success than males with
smaller nest sizes, I will place tiles of two
different sizes into Smith Creek following the
methodology of Downhower and Brown (1977). I
will sample the tiles frequently to check for
males that adopt these tiles as their nest sites
and to check for the presence of egg masses. To
evaluate the role of male size in reproductive
success, I will measure the standard length and
the weight of males found underneath the
tiles. I predict that large males will have more
egg clutches than small males and that males
under large tiles will have more egg clutches
than males under small tiles.
Literature Cited Bisazza, A. and Marconato, A.
1988. Female mate choice, male-male competition
and parental care in the river bullhead, Cottus
gobio L. (Pisces, Cottidae). Animal Behaviour.
36(5) 1352-1360. Darwin, C. 1871. The Descent
of Man and Selection in Relation to Sex. London
John Murray. Downhower, J.F. and Brown, L.
1977. A sampling technique for benthic fish
populations. Copeia. 2 403-406. Downhower,
J.F., Brown, L., Pederson, R., and Staples, G.
1983. Sexual Selection and Sexual Dimorphism in
Mottled Sculpins. Evolution. 37(1)
96-103. Mousseau, T.A. and Collins, N.C. 1987.
Polygyny and nest site abundance in the slimy
sculpin (Cottus cognatus). Canadian Journal of
Zoology. 65 2827-2829. Natsumeda, T. 1998.
Size-assortative nest choice by the Japanese
fluvial sculpin in the presence of male-male
competition. Journal of Fish Biology. 53
33-38. Savage, T. 1963. Reproductive behavior
of the mottled sculpin, Cottus bairdi Girard.
Copeia. 2 317-325.