Evolution, 57(9), 2003, pp. 2139–2146

MUTATION AND SEXUAL SELECTION: A TEST USING BARN SWALLOWSFROM CHERNOBYL
A. P. MøLLER1 AND T. A. MOUSSEAU2
1 Laboratoire de Parasitologie Evolutive, CNRS UMR 7103, Université Pierre et Marie Curie, 7 quai St. Bernard,

Case 237, F-75252 Paris Cedex 05, France
E-mail: amoller@snv.jussieu.fr
2 Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208
E-mail: mousseau@sc.edu
Abstract. Secondary sexual characters have been hypothesized to be particularly susceptible to the deleterious effects
of mutation because the expression of such characters is usually influenced by many more metabolic pathways thanare ordinary morphological characters. We tested this hypothesis using the elevated mutation rates in the barn swallow(Hirundo rustica) of the Chernobyl region of Ukraine as a model system. A great deal is known about the relative
importance of different characters for male mating success in this species. The importance of phenotypic charactersfor male mating success was quantified based on a long-term study of a Danish breeding population, by expressing
phenotypic differences between mated and unmated males as the difference between log-transformed mean values.
For field samples from Ukraine we likewise expressed the difference in male phenotype between individuals living
in a relatively uncontaminated area and individuals from the Chernobyl region as the difference between log-transformed mean values. The standardized difference in male phenotype between the two regions in Ukraine for the 41
different characters was strongly positively correlated with the standardized difference in male phenotype between
mated and unmated males from Denmark. The standardized difference in male phenotype between the two regionsin Ukraine was significantly positively associated with sexual size dimorphism. However, the standardized differencein male phenotype between mated and unmated males was a much better predictor of standardized difference in malephenotype between the two regions in Ukraine than was the standardized difference in sexual size dimorphism,
expressed as the difference between log-transformed mean values for males and females. These findings are consistent
with the hypothesis that traits most important for sexual selection are particularly susceptible to the effects of deleterious
mutations.
Key words.

Antioxidants, Chernobyl, condition dependence, Hirundo rustica, mate choice, mutation, sexual selection.
Received January 20, 2003.

The advantages of strong mate preferences in sexual selection remain largely undetermined (Andersson 1994). Thereis good evidence of direct, material benefits of mate choicein many taxa, but others show direct benefits generally beingof little or no importance (reviewed in Møller and Jennions
2001). Although there is ample evidence for females obtaining genetic benefits for their offspring (reviewed in Møller
and Alatalo 1999), the mechanisms generating such benefits
remain illusive. Although mutation-selection balance likely
plays a role in maintaining additive genetic variance in fitness
and the expression of secondary sexual characters, whether
such effects are sufficient is unknown. A Fisherian mechanism of sexual selection could arise from biased mutations
affecting the expression of male secondary sexual characters(Pomiankowski et al. 1991), allowing females to avoid mutants from inspection of male phenotype. Male secondary
sexual characters are exaggerated traits that for this reason
are likely to be affected by many different metabolic pathways. Therefore, mutations may more readily be assessedfrom such characters than from ordinary phenotypic traits
that are primarily influenced by mutations through a single
or few metabolic pathways. Similarly, biased mutation simultaneously differentially affecting viability and the expression of male traits may render continuous sexual selection feasible (Iwasa et al. 1991). Both these ideas are basedon the untested assumption that deleterious mutations constantly reduce the expression of male traits and maximum
viability. Estimates of the additive genetic variance in viability have suggested that deleterious mutations may reduce

Accepted March 18, 2003.

viability by 20% or more (Charlesworth 1987; Kondrashov
1988; Rice 1988; Lynch et al. 1999). Such effects may account for the small, but highly significant heritability of fitness (Burt 1995) and fitness components (Mousseau and Roff
1987; Roff and Mousseau 1987).
We quantified the effects of mutation on phenotype byexpressing the difference in phenotype between male barnswallows (Hirundo rustica) from a little-contaminated region
in Ukraine (Kanev) and conspecifics from the Chernobyl region, which has high levels of radioactive contamination(mainly Cs 137 and Sr 90; Shestopalov 1996). Ellegren etal. (1997) reported an increase in germline mutation rates inbarn swallow microsatellite DNA by a factor of two to 10
when comparing families from Kanev with families from
Chernobyl. Barn swallows from Chernobyl have markedly
increased frequency of partial albinism in the plumage (Ellegren et al. 1997; Møller and Mousseau 2001). Previous
studies have shown significant parent-offspring resemblancein partial albinism, indicating that such albinism is partly of
germline origin (Ellegren et al. 1997). Individuals with partial
albinism are less likely to survive than other individuals (Ellegren et al. 1997), and they generally have a smaller but
more variable phenotype (Møller and Mousseau 2001). These
patterns of phenotypic variation are consistent with the hypothesis that radioactive contamination has increased the rateof mutations, and that such mutations have deleterious effects. Thus, this area has the potential for providing insights
into the effects of mutation on sexual selection and other
evolutionary phenomena.

2139
q 2003 The Society for the Study of Evolution. All rights reserved.

2140

A. P. MØLLER AND T. A. MOUSSEAU

One aim of this study was to investigate the extent to which
the phenotypic characters that vary in importance for malemating success are susceptible to the deleterious effects ofmutation. We predicted that the standardized difference inmale phenotype between mated and unmated males, reflectedby the difference between log-transformed mean values,
should represent an estimate of the importance of a characterfor sexual selection. Likewise, we expressed the standardized
phenotypic difference between individuals from the contaminated and the control area. If secondary sexual characters
were particularly susceptible to the effects of mutation, compared to other phenotypic characters, we would predict a
positive association between standardized difference in malephenotype with respect to mutation and standardized difference in phenotype with respect to male mating success. A
second aim of this study was to investigate the relationshipbetween sexual size dimorphism and effects of radiation on
phenotype. We standardized sexual size dimorphism as described above. Sexual dimorphism evolves over extremely
long time periods (Lande and Arnold 1985), whereas females
are supposed to assess phenotypic characters that currently
reveal benefits from mate choice (Andersson 1994). Therefore, we can predict that the standardized difference in phenotype related to mating success is mainly related to thestandardized difference in phenotype related to radiation, andmuch less to the difference in phenotype related to sexual
size dimorphism.
MATERIALS AND METHODS
Study Species
The barn swallow is an approximately 20-g aerial insectivorous passerine bird (Møller 1994a). Males arrive before
females from the African winter quarters to the breeding
grounds and attract mates by aerial displays of their red throat
patches, long outermost tail feathers, and elaborate song
(Møller 1994a). After a male is mated, the pair builds a nest,
where the female lays eggs and incubates. Both males andfemales feed nestlings and often raise a second brood before
returning to Africa in September or October (Møller 1994a).
Barn swallows from Chernobyl have increased asymmetry
in their outermost tail feathers compared to males from acontrol area and to males predating the Chernobyl accident(Møller 1993). Barn swallow males from Chernobyl also havemuch paler throat coloration than males from a control area,with long-tailed males in particular being very pale comparedto long-tailed control males (Camplani et al. 1999). The frequency of partial albinism is elevated by a factor of 10 in
the Chernobyl area compared to a control area (Møller andMousseau 2001). Microsatellite mutation rates in barn swallows increased by a factor of two to 10 in the Chernobyl
region compared to controls (Ellegren et al. 1997).
Field Procedures and Definition of Characters
We studied barn swallows just outside the exclusion zoneof the Chernobyl area (Shestopalov 1996) by visiting villages
and checking collective farms for the presence of barn swallows. When such a farm had been located, we recorded radiation levels and captured adult barn swallows for mea-

surements and subsequent banding. Our own field measurements of radiation at the ground level using a hand-heldInspector dosimeter (SE International, Inc., Summertown,TN) revealed average levels of radiation of 0.390 mR/h (SE5 0.317) at 14 breeding sites in the Chernobyl region. As acontrol area we used Kanev, about 220 km southwest of Kiev,
which has a relatively low level of contamination. Mean levels of radiation were 0.025 mR/h (SE 5 0.002) at five breeding sites. We studied barn swallows in the two study areas
10–29 June 1991 (A. P. Møller), 16–24 June 1996 (A. P.
Møller), 6–12 June 2000 (A. P. Møller, T. A. Mousseau) and
4–11 June 2002 (A. P. Møller, T. A. Mousseau).
We captured barn swallows with mist nets across open
doors and windows in farm buildings. This method is highly
efficient during the main breeding season since mark-recapture studies have shown a capture probability of 98% (Møller
and Szép 2002).
We measured a total of 41 phenotypic characters in adultmales and females from the two samples. Twenty morphological characters (beak length, beak width, beak depth, winglength, wing asymmetry, tail length, tail asymmetry, lengthof central tail feathers, wingspan, wing area, wing loading,aspect ratio, tarsus length, keel length, feather tip width,feather base width, tip vane asymmetry, base vane asymmetry, difference in width between tip and base, and bodymass) were measured with a ruler, a digital caliper, or a Pesola
AG (Baar, Switzerland) spring balance as described in Møller
(1994a) and Møller et al. (1995). Six color variables (chroma,hue, ultraviolet reflectance, lutein-based reflectance, visible
light reflectance, and albinism) were measured as describedin Camplani et al. (1999). Ten immunological and hematological variables (monocytes, heterophils, lymphocytes, eosinophils, basophils, heterophil:lymphocyte ratio, immunoglobulins, hematocrit, buffy coat, and spleen mass) were measured as described in Camplani et al. (1999). Three parasitological variables (abundance of chewing lice, louseflies,
feather mites) were estimated as described in Møller (1994a).
Testes mass was measured as described in Møller (1994b).
Fault bars were quantified as described in Møller (1989). All
these variables have high and statistically significant repeatabilities as recorded from double measurements of the same
individuals during later recaptures the same season. All characters were not measured for all individuals, and sample sizes
therefore differ considerably among characters. We see noreason why this should cause any bias in the analyses. The
only effect would be that some estimates should be less reliable than others, obscuring any true relationships.
A second sample of barn swallows was captured, measured
and banded in Denmark during the years 1984–2002, using
the same procedures as in Ukraine. This sample consisted ofadult barn swallows captured the first time, when aged one
year. Thus, all mated and unmated males were of a similar
age when first captured and assessed for phenotypic characters. Mating status was determined from the presence (mated) or absence (unmated) of a female in the territory of banded males in the breeding sites (for further information see
Møller 1994a).
We classified phenotypic characters as feather characters
produced during the annual molt in Africa, when the linear
measurements included feathers. The justification for this dis-

2141

MUTATION AND SEXUAL SELECTION

tinction is that these characters are developed in the African
winter quarters during the annual molt many months after
having left the contaminated breeding grounds. Characters
were classified as physiological if they were immune response variables, hematological variables, or variables associated with coloration. The latter variables have a physiological basis in terms of either color pigments or microstructure of feathers. Physiological characters may change ona short-term basis, and this may increase their susceptibility
to rapid changes in environmental conditions such as radioactive contamination. Hence, if the differences in phenotype
were mainly due to phenotypic plasticity rather than the effects of mutations, we should expect to see a stronger effecton physiological variables than on other characters. In contrast, we should expect to see a weaker effect on feather
characters grown in Africa than on other characters if thedifferences in phenotype were mainly due to phenotypic plasticity rather than the effects of mutations.
Statistical Analyses
We based our analyses on standardized differences in phenotype expressed in terms of differences in log-transformed
mean values for the two samples of the 41 phenotypic characters (40 characters in the analyses of sexual size dimorphism). Since characters differed dramatically in size andvariance, we had to use standardization.
We quantified the importance of phenotypic characters formale mating success by quantifying the difference in mean
phenotype between mated and unmated males in the Danish
sample. This sample is based on the phenotypes of 1354
mated males and 99 unmated males from Denmark. The difference in phenotype was expressed as differences in logtransformed mean values. This standardization eliminatedany covariance between standardized difference and mean or
standard deviation of the variables under investigation.
We quantified the difference in phenotype between males
from Kanev (154 males) and males from Chernobyl (206males) as differences in log-transformed mean values.
Finally, sexual size dimorphism was quantified as the difference in log-transformed mean values between mean malephenotype and mean female phenotype in Denmark. Testes
size had to be excluded from this analysis because the trait
is only expressed in males. We calculated sexual size dimorphism for all characters, including characters that were
not morphological. This is justified since differences in physiology, immunology, or hematology are likely to form thephysiological basis for the morphological characters.
We conservatively assessed the association between pairsof standardized sets of variables using the phi rank correlation
for a two-by-two table in which the two states for each character are the negative and positive standardized values, respectively (Siegel and Castellan 1988).
We tested for the effect of outliers on the robustness of
the reported relationships using Kendall rank correlation
analysis (Siegel and Castellan 1988).
We tested for a difference in relationships between pairsof standardized sets of characters separately for feather characters that are formed during the annual molt in Africa andother characters, using analysis of covariance. A statisticallysignificant factor-by-covariate interaction would indicate thatthe relationships for the two kinds of characters are statistically different. We made a similar test for physiological andother characters.
RESULTS
Mean and standard errors for all samples are reported inTables 1–3. The standardized difference in male phenotype
between Kanev and Chernobyl was strongly positively related
to the standardized difference in male phenotype between
mated and unmated males (Fig. 1; linear regression: F 5
167.196, df 5 1,39, r2 5 0.811, P , 0.0001, slope [SE] 5
0.903 [0.070]). The slope did not differ significantly fromone (t 5 1.386, df 5 39, nonsignificant). Since both dependent and independent variables were measured with error, areduced major axis may provide a better estimate (Sokal andRohlf 1995). The slope in this case is 1.083 (SE 5 0.109),
which still does not differ significantly from one (t 5 0.763,
df 5 39, nonsignificant). The relationship between standardized difference in male phenotype with respect to radiationand the standardized difference in male phenotype with respect to mating was robust since a rank correlation gave asimilar conclusion (t 5 0.644, z 5 5.930, P , 0.0001). Thus,
effects of environmental conditions in Chernobyl on malephenotype closely matched differences in male phenotype
associated with mating success.
The effects of environmental conditions around Chernobylon male phenotype could be due to physiological effects on
phenotype or to effects of mutations. We know that the frequency of partial albinism is increased in the Chernobyl region compared to the control area, and that this difference
was absent before the nuclear accident in 1986 (Ellegren etal. 1997; Møller and Mousseau 2001). If mutations accounted
for differences in male phenotype in Ukraine, we should also
expect that feather characters formed during the annual molt
in the African winter quarters (where birds from Ukraine mix
and share a common environment) would be good predictorsof the importance of male phenotype for mating success. Barnswallows are less likely to be affected by the direct physiological effects of radiation in Africa, several months after
having left the Chernobyl region. Indeed, feather characters
were as good predictors as all phenotypic characters with nosignificant difference between feather characters and othercharacters, and the covariate by factor interaction was notstatistically significant (Fig. 1; Table 4). Although physiological characters were no better predictors of susceptibility
to radiation than other characters, the covariate-by-factor interaction was statistically significant (Fig. 1; Table 5). Thisimplies that the mating success effect was weaker for physiological characters than for other characters.
If the expression of male secondary sexual characters wasparticularly affected by mutations, we would predict a positive relationship between effects of mutations on phenotypicvariables and sexual size dimorphism. However, this relationship was not significant (Fig. 2; linear regression: F 5
0.547, df 5 1,38, r2 5 0.014, P 5 0.464, slope (SE) 5 0.466[0.631]). However, a rank correlation gave a significant positive relationship (t 5 0.385, z 5 3.495, P 5 0.0005).
A more conservative method to assess the association be-

2142

A. P. MØLLER AND T. A. MOUSSEAUTABLE 1.

Mean (SD) phenotypic values for adult male barn swallows from Chernobyl and Kanev, Ukraine. n, sample size.
Chernobyl

Kanev

Character

Mean

SD

n

Mean

SD

n

Albinism
Aspect ratio
Base vane asymmetry
Base width
Basophils
Bill heightBill length
Bill width
Body mass
Buffy coat
Chewing lice
ChromaEosinophils
Fault bars
Feather mites
H/L ratio
Heterophils
Hematocrit
Hue
Immunoglobulins
Keel length
Louseflies
Lutein color
Lymphocytes
Monocytes
Spleen mass
Short tail length
Tail length
Tail asymmetry
Tarsus length
Testes mass
Tip vane asymmetry
Tip width
Tip-base difference
Ultraviolet
Visible light
Wing length
Wing asymmetry
Wing area
Wing loading
Wingspan

0.146
7.300
2.780
6.980
0.002
2.558
7.928
11.75918.5540.215
14.092
0.255
0.045
0.116
31.3270.376
0.230
54.60035.77613.20021.006
0.017
1.111
0.686
0.031
20.00044.206110.097
4.752
11.287
0.434
0.090
1.860
5.120
3.116
3.889
124.975
0.219
0.015
12.200330.747

0.355
0.335
0.465
0.542
0.003
0.297
0.553
0.721
1.220
0.135
11.841
0.034
0.031
0.495
28.610
0.266
0.108
4.159
2.171
2.062
0.890
0.057
0.144
0.116
0.020
13.194
1.771
9.607
4.452
0.942
0.100
0.155
0.232
0.542
0.138
0.092
3.289
0.443
0.001
0.939
6.899

206
45
60
60
21
206
206
206
206
33
206
33
21
189
153
21
21
33
33
17
205
205
33
21
21
17
204
190
190
204
51
60
60
60
33
33
141
204
45
45
204

0.013
7.120
2.710
6.960
0.001
2.751
8.010
11.99518.8350.242
17.112
0.267
0.028
0.125
50.2890.264
0.192
54.35235.407
12.26921.320
0.015
1.159
0.744
0.028
29.90044.392116.687
0.232
11.079
0.502
0.040
1.787
5.397
3.063
3.838
126.289
0.179
0.015
12.020331.039

0.114
0.940
0.453
4.469
0.002
0.391
0.550
0.605
1.305
0.191
12.773
0.044
0.025
0.419
36.797
0.091
0.054
3.170
2.930
3.098
0.808
0.173
0.162
0.066
0.022
15.105
1.806
9.004
2.757
0.653
0.150
0.113
0.611
0.481
0.201
0.118
2.928
0.335
0.007
0.940
6.900

154
73
8
8
17
153
153
153
154
24
152
24
17
128
83
17
17
24
24
15
153
153
24
17
17
15
153
140
140
153
8
8
8
8
24
24
123
154
73
73
154

tween effects of radiation on phenotype and the differencein phenotype between mated and unmated males is to calculate the rank correlation for a two-by-two table in which
the two states for each character are the negative and positive
values, respectively (Siegel and Castellan 1988). Althoughstandardized variables may be affected both by mean andstandard deviation values, we can be sure that the sign of thephenotypic measures does not change with standardization.
Hence, a test for similarity in sign of two categories of variables provides a clear, but conservative test of covariation.
The phi correlation coefficient for the association between
effects of radiation on phenotype and the difference in phenotype between mated and unmated males was r (w) 5 0.664,
x2 5 16.739, df 5 1, P , 0.001. The coefficient for radiationand sexual dimorphism was r (w) 5 0.323, x2 5 2.947, df
5 1, P , 0.10, which did not reach statistical significance.
If characters subject to sexual selection are particularly
susceptible to mutations, we should expect the association
between effects of radiation on phenotype and the differencein phenotype between mated and unmated males to be muchstronger than for the difference in phenotype related to sexual
dimorphism. A multiple regression analysis revealed a highly
significant model that explained 81% of the variance (Table
6). Whereas the partial regression coefficient for standardized
difference in phenotype related to radiation was highly significant and accounted for 25.2% of the variance, the partial
regression coefficient for standardized difference in phenotype related to sexual dimorphism was not significant andaccounted for almost none of the variance.
DISCUSSION
We tested the hypothesis that phenotypic characters of thegreatest importance in terms of sexual selection are the traits
particularly susceptible to the effects of radiation. The test
consisted of comparison of the phenotype of male barn swallows from Chernobyl, in which mutation rates of microsatellites are increased by a factor two to 10, with a sample ofmales from a less-contaminated area in Ukraine. The standardized difference in male phenotype between these two

2143

MUTATION AND SEXUAL SELECTIONTABLE 2.

Mean (SD) phenotypic values for mated and unmated male barn swallows from Denmark. n, sample size.
Mated male

Unmated maleCharacter

Mean

SD

n

Mean

SD

n

Albinism
Aspect ratio
Base vane asymmetry
Base width
Basophils
Bill heightBill length
Bill width
Body mass
Buffy coat
Chewing lice
ChromaEosinophils
Fault bars
Feather mites
H/L ratio
Heterophils
Hematocrit
Hue
Immunoglobulins
Keel length
Louseflies
Lutein color
Lymphocytes
Monocytes
Spleen mass
Short tail length
Tail length
Tail asymmetry
Tarsus length
Testes mass
Tip vane asymmetry
Tip width
Tip-base difference
Ultraviolet
Visible light
Wing length
Wing asymmetry
Wing area
Wing loading
Wingspan

0.001
7.239
2.628
6.820
0.001
2.806
7.649
11.97419.0910.610
10.667
0.286
0.044
0.151
48.7400.266
0.228
52.82033.76713.92021.556
0.015
1.123
0.775
0.028
33.60044.028108.786
0.298
11.266
0.502
0.074
1.797
5.374
3.028
3.792
126.533
0.216
0.015
12.477333.936

0.165
0.406
0.510
0.612
0.002
0.331
0.511
0.603
1.141
0.426
10.660
0.039
0.030
0.540
46.711
0.093
0.112
4.315
2.636
3.200
0.726
0.147
0.043
0.069
0.022
18.800
1.959
8.858
4.849
0.625
0.100
0.153
0.510
0.969
0.118
0.079
3.017
0.413
0.001
1.161
7.396

991
942
26
26
25
1351
1351
1351
1354
37
1246
43
25
1141
724
37
25
352
43
16
861
1355
43
25
25
16
1102
1351
885
1351
51
26
26
26
43
43
1354
980
944
944
1150

0.030
7.280
2.730
6.840
0.002
2.778
7.683
11.97818.1900.517
13.872
0.275
0.059
0.276
40.1250.300
0.273
52.98034.47415.20021.448
0.020
1.096
0.742
0.031
24.00043.08099.3332.259
11.310
0.436
0.120
1.950
4.890
3.081
3.817
124.6260.175
0.015
12.693333.280

0.044
0.351
0.621
0.587
0.002
0.298
0.497
0.587
1.085
0.449
13.864
0.040
0.029
0.663
39.055
0.089
0.112
4.540
2.421
2.970
0.768
0.139
0.053
0.066
0.021
7.967
1.386
62.754
0.198
0.617
0.149
0.162
0.479
0.929
0.106
0.082
2.915
0.873
0.001
1.002
5.451

65
60
12
12
6
99
99
99
99
6
86
6
6
76
56
6
25
18
6
6
59
99
15
6
6
3
75
99
58
99
8
12
12
12
6
6
99
63
60
60
77

areas was strongly positively correlated with the standardized
difference in phenotype between mated and unmated malesof the same age from a distant population in Denmark. The
slope was not significantly different from one, and although
the reduced major axis had a larger slope, it did not differ
from one. This suggests that differences in phenotype between barn swallows from Chernobyl and a control area are
as large as differences in phenotype between mated and unmated males. We found similarly strong relationships formorphological characters formed during the annual molt in
Africa and for physiological characters. Sexual selection theory proposes that secondary sexual characters may be particularly susceptible to the negative effects of mutation because many different metabolic pathways contribute to theirexpression (Iwasa et al. 1991). Indeed, we found that characters with greater influence on male mating success wereaffected to a greater extent by mutation (Fig. 1), whereas theeffects of radiation on phenotype were only weakly predictedby sexual size dimorphism (Fig. 2).
We found evidence of a strong positive relationship be-

tween the standardized difference in phenotype associatedwith radiation and the standardized difference in phenotype
associated with mating success. We used a strict definitionof sexual selection arising from social mating success in thissocially monogamous bird species. Several studies haveshown that the differences in phenotype between mated andunmated male barn swallows provide reliable information on
the characters associated with sexual selection as verified byexperiments (Møller 1994a). Although many other mechanisms of sexual selection exist, and many of these have been
demonstrated in the barn swallow (reviewed in Møller
1994a), we do not have information on the relationship between sexual selection and phenotypic characters for morethan a few traits. Thus, we were forced to use strict mating
success as an estimate for sexual selection to maximize oursample sizes.
We used linear regression analyses to investigate how differences in phenotype related to male mating success wereassociated with differences in phenotype related to standardized difference in phenotype associated with radiation. Our

2144

A. P. MØLLER AND T. A. MOUSSEAUTABLE 3.

Mean (SD) phenotypic values for adult male and female barn swallows from Denmark. n, sample size.
Male

FemaleCharacter

Mean

SD

n

Mean

SD

n

Albinism
Aspect ratio
Base vane asymmetry
Base width
Basophils
Bill heightBill length
Bill width
Body mass
Buffy coat
Chewing lice
ChromaEosinophils
Fault bars
Feather mites
H/L ratio
Heterophils
Hematocrit
Hue
Immunoglobulins
Keel length
Louseflies
Lutein color
Lymphocytes
Monocytes
Spleen mass
Short tail length
Tail length
Tail asymmetry
Tarsus length
Tip vane asymmetry
Tip width
Tip-base difference
Ultraviolet
Visible color
Wing length
Wing asymmetry
Wing area
Wing loading
Wingspan

0.032
7.243
2.730
6.840
0.001
2.794
7.655
11.96819.0680.610
11.035
0.286
0.047
0.163
46.5740.271
0.237
52.89934.62614.85121.535
0.019
1.080
0.769
0.029
32.08443.954108.1292.257
11.255
0.120
1.950
4.890
3.078
3.809
126.4110.545
0.015
12.497330.734

0.167
0.403
0.510
0.612
0.002
0.344
0.501
0.598
1.131
0.408
11.032
0.039
0.030
0.535
45.547
0.093
0.111
4.366
2.131
3.144
0.737
0.155
0.105
0.068
0.022
4.688
1.932
9.003
4.511
0.625
0.153
0.510
0.969
0.098
0.079
3.031
0.388
0.001
1.154
7.346

1113
1008
26
26
31
1507
1507
1507
1510
34
1389
43
31
1274
831
43
31
370
43
22
971
1511
43
31
31
19
1234
1506
995
1507
26
26
26
43
43
1510
1100
1010
1010
1284

0.037
7.153
2.550
6.570
0.002
2.763
7.731
11.831
20.1500.461
11.884
0.270
0.065
0.141
42.8370.306
0.272
51.40433.767
16.27120.771
0.015
1.048
0.737
0.033
29.27144.55790.4951.700
11.285
0.180
2.080
4.520
3.028
3.792
124.347
0.464
0.015
13.443326.022

0.203
0.412
0.635
0.688
0.003
0.348
0.509
0.589
1.969
0.182
11.673
0.034
0.031
0.472
49.184
0.101
0.118
5.208
2.756
3.067
0.766
0.157
0.110
0.066
0.022
4.825
1.934
6.254
2.953
0.635
0.159
0.370
0.741
0.123
0.082
2.941
0.607
0.001
1.627
6.738

1145
1055
28
28
34
1536
1536
1536
1539
37
1504
47
34
1321
843
44
34
351
47
22
982
1541
47
34
34
21
1283
1528
1053
1535
28
28
28
47
47
1538
1138
1055
1055
1341

regression approach was verified in a second series of analyses based on conservative nonparametric correlations using
the phi-correlation procedure. Even with that approach we
found a very strong positive association for mating success,but not for sexual size dimorphism.
Previous studies of the barn swallow in Chernobyl and acontrol area have shown an increase in germline mutationsof microsatellite loci by a factor two to 10 (Ellegren et al.
1997), similar to increases in mutation rates in wheat andhumans (Dubrova et al. 1996; Kovalchuk et al. 2000). Likewise, studies of partial albinism in barn swallows have shown
an increase in their frequency by a factor of ten when compared to control areas (Møller and Mousseau 2001). Partial
albinism is caused by mutations, and barn swallows from
Chernobyl showed that these were germline mutations (Ellegren et al. 1997). We do not have direct information onwhether the expression of other phenotypic traits are affectedby mutations, although we see no reason why that should not
be the case. Indeed, we found very similar patterns for morphological traits produced in the winter quarters of the barnswallows in Africa during the annual molt and for other char-acters (Fig. 1; Table 4). In fact, the patterns for physiologicalcharacters recorded during the breeding season in Ukraine
and other characters differed significantly, with physiologicalcharacters showing weaker relationships for mating successas compared to other variables (Fig. 1; Table 5).
We compared the standardized difference in male phenotype between two areas in Ukraine and related these differences to standardized differences in phenotype between mated and unmated males in Denmark more than 2000 km away.
This approach was adopted to ensure that the two datasets
were independent. The effects of elevated mutation rates in
Ukraine are unlikely to show up in Denmark, since natal andbreeding dispersal distances are on average less than 5 km
per generation (Møller 1994a). This approach also assumes
that similar traits are important for sexual selection in different populations of the barn swallow. Indeed, long-term
studies of populations of barn swallows in Denmark, Estonia,
Hungary, Italy, and Spain have shown similarly strong effects
of male tail length and red coloration of the plumage of thehead on mating success (Møller 1994a; F. de Lope, M. Kose,
A. P. Møller, P. Ninni, P. Pap, and N. Saino, unpubl. data).

2145

MUTATION AND SEXUAL SELECTIONTABLE 5. Analysis of covariance with standardized difference inphenotype between Chernobyl and Kanev as the dependent variable,
physiological character (P) as a factor, and standardized differencein phenotype between mated and unmated males (S) as a covariate.
Model: F 5 96.174, df 5 3,37, r2 5 0.886, P , 0.0001.

FIG. 1. Relationship between standardized difference in phenotype
between mated and unmated male barn swallows and standardized
difference in phenotype between male barn swallows from Kanev
and Chernobyl.

All data of the present study are comparable because the same
person (A. P. Møller) measured all morphological charactersin a standardized way, ensuring a high degree of repeatability(Møller 1994a). Likewise, other characters analyzed here
showed a high repeatability based on repeat measurements(see Materials and Methods for references).
In our statistical analyses we treated the different characters as independent. We have already emphasized that suchvalues cannot necessarily be considered statistically independent. Previous analyses of phenotypic correlations in thebarn swallow have shown relatively weak positive associations for most characters, with correlation coefficients generally less than 0.5 (Møller 1994a). These associations aremuch weaker than usually found in studies of birds (e.g.,
Grant and Grant 1989). Thus, our conclusions are unlikely
to be strongly affected by these weak phenotypic correlations.
We also note that numerous other scientists have used estimates of different phenotypic traits as statistically independent observations in their analyses, even when phenotypiccorrelations are much stronger than in our case (e.g., Alatalo
et al. 1988; Grant and Grant 1989; Houle 1992; Merilä and
Sheldon 2000).
Sexual size dimorphism evolves over very long time scales(Lande and Arnold 1985), and the relationship between mating success and sexual size dimorphism is thus generally
believed to be relatively weak (Andersson 1994). Female

Source

Slope

SE

t

P

P
S
P3S

20.0351.331
20.607

0.034
0.103
0.123

1.03
12.94
4.95

0.308
0.00010.0001mate preferences are supposed to have evolved toward benefits that can be accrued from current differences in malephenotype. If traits involved in sexual selection are particularly susceptible to mutation, we can expect that sexual size
dimorphism should be a poorer predictor of mutation effects
on phenotype than traits that have been directly shown to be
associated with mating success. We tested this prediction ina multiple regression analysis (Table 6). The effects of radiation proved to be many times more important for the standardized difference in phenotype related to mating successthan the standardized difference related to sexual size dimorphism, confirming our prediction. This observation suggests that characters that previously have been important insexual selection, as evidenced from their current sexual size
dimorphism, are less susceptible to the effects of mutationcompared to characters that are currently of great importancefor sexual selection. This also provides evidence of the dynamic nature of sexual selection, with sexually dimorphic
traits that previously may have been of importance for sexual
selection now showing less susceptibility to mutation thantraits that differ in size between mated and unmated individuals.
Finally, we should consider whether the present results areof general interest, since the radiation levels in Chernobyl
are perhaps extreme. Chernobyl barn swallows were used in
the present study because we can assume with a high degree

TABLE 4. Analysis of covariance with standardized difference inphenotype between Chernobyl and Kanev as the dependent variable,feather character (F) as a factor, and standardized difference inphenotype between mated and unmated males (S) as a covariate.
Model: F 5 54.636, df 5 3,37, r2 5 0.816, P , 0.0001.
Source

Slope

SE

t

P

F
S
F3S

20.0291.164
20.283

0.040
0.342
0.350

0.74
3.40
0.81

0.465
0.0016
0.423

FIG. 2. Relationship between standardized difference in phenotype
between male barn swallows from Kanev and Chernobyl and standardized sexual size dimorphism.

2146

A. P. MØLLER AND T. A. MOUSSEAUTABLE 6. Multiple linear regression with standardized differencein phenotype between Chernobyl and Kanev as the dependent variable and the standardized difference in phenotype between matedand unmated males (S) and the standardized sexual size dimorphism(D) as independent variables. Model: F 5 78.748, df 5 2,37, r2 5
0.810, P , 0.0001.
Variable

Slope

SE

t

P

S
D

0.901
0.024

0.072
0.283

12.4390.085

0.00010.933

of certainty that mutation rates are considerably elevatedwhen compared to other areas. Indeed, that was the case formicrosatellites in the barn swallow and for several other organisms. Deleterious mutations are invariably occurring everywhere, although rates will vary depending on local conditions. The extent to which the phenotype of organisms is
disrupted will depend on local mutation rates. However, there
is no reason to believe that the positive relationships illustrated in Figures 1 and 2 will not be general. It is only theslope of these relationships that will differ among populations.
ACKNOWLEDGMENTS
We thank G. Milinevski, A. M. Peklo, E. Pysanets, N.
Szczerbak, and A. A. Tokar for considerable help during ourvisits to Ukraine, and two anonymous reviewers for comments on the manuscript. We acknowledge funding from theCentre National de la Recherche Scientifique (France), theUniversity of South Carolina School of the Environment, theSamuel Freeman Charitable Trust, the National Science
Foundation, and the National Geographic Society.
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Corresponding Editor: T. Smith