Comparison of barn owl (Tyto alba) sex ratios between
northeastern and southeastern regions of Pennsylvania
Kaitlyn M. Pontuti* & Karl Kleiner
Department of Biological Sciences, York College of Pennsylvania, York, PA 17403
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INTRODUCTION
• Natural selection usually favors equal investment in
male and female offspring leading to a 50/50 sex
ratio.
• However deviations in parity do arise and there are
two explanations for this.
o
Fisher (1958) postulates that parents will
invest equally in both sexes so that at the
population level, the sex ratio should be
50/50. However sex bias is likely to be found
in reverse sexually size-dimorphic birds such as
barn owls (Tyto alba). The smaller males are
less energetically expensive and would be
expected to be overproduced. Over time
however frequency dependent selection will
maintain a 50/50 sex ratio.
o
Trivers & Willard (1973) propose that bias in
sex ratios can occur at the family level
• There is evidence of environmentally induced sex
ratio bias in some avian species caused by factors
such as territory quality or paternal
attractiveness/quality (Brommer et al. 2003).
• Food shortages and times of low breeding success
favor an overproduction of males rather than the
more energetically costly females.
• The availability of food sources commonly differs
with landscape or geographic location. The state of
Pennsylvania is comprised of different physiographic
provinces (see map) and consequently different
landscape types; therefore, food availability may
differ across regions.
• Differences in food availability could affect barn owl
sex ratios at the clutch (family) level between regions,
but the population-wide sex ratio should be
maintained at parity.
METHODS
RESULTS
DISCUSSION
indicates W chromosome
DNA extracted from feathers using
Chelex
F
F
F
F
M
indicates Z chromosome
Agar gel displaying PCR products. A single band indicates a male (ZZ)
and a double band indicates a female (WZ).
Gender determined using PCR & Gel
electrophoresis
Sex ratio data analyzed using a
Wilcoxon Signed Rank test
WORKS CITED
Brommer, Jon E., Karell, Patrik, Pihlaja, Tuomo, Painter,
Jodie N., Primmer, Craig R., Pietiainen, Hannu. 2003
Ural owl sex allocation and parental investment under
poor food conditions. Behavioural Ecology. 137:140-147.
Fauth, Shawn E. 2010. Sex ratios of barn owls in south-central
Pennsylvania. Senior thesis paper. York College of PA.
Fisher, R.A. The Genetical Theory of Natural Selection. 1958
2nd edn. Dover, New York.
Trivers, Robert L. and Willard, Dan E. 1973. Natural
selection of parental ability to vary the sex ratio of
offspring. Science. 179: 90-91.
OBJECTIVE
• To compare the sex ratios of clutches located in the
agriculturally rich Piedmont region of southeastern
Pennsylvania to clutches located in the predominately woody
habitat of the ridge and valley & Allegheny regions of
northeastern Pennsylvania.
• Our prediction was to expect a male biased sex ratio in the
northeastern regions of Pennsylvania given the absence of
agriculturally cultivated land which is more favorable to the
barn owl’s prey type and hunting habits.
• There is evidence of sex ratio bias at the
family and population level
• The increased percentage of males in
the northern region was expected, as
the prey population was hypothesized
to be less abundant (Fig. 1). This
supports sex ratio adjustment on the
family level as Trivers & Willard (1973)
suggest with their theory.
• The percentage of males (64.53%) for
the combined data for the northern
and southern regions does not
support Fisher’s (1958) theory of a
50/50 ratio at the population level for
eastern Pennsylvania.
• There was a greater percentage of
males produced in 2009 (Fig. 2). We
would have expected an increase in
number of females in 2010 due to
frequency-dependent selection for
the rarer sex. However, this type of
selection may operate on a longer
time scale.
www.dcnr.state.pa.us
• The percentage of males in the northern
region differed from parity (p<0.05)
• The percentage of males in the southern
region did not differ from parity (p>0.05)
• The sex ratio for the combined regions
differed from parity (p<0.05)
ACKNOWLEDGEMENTS
I would like to thank Dr. Karl Kleiner for all of his help
throughout this project