Behavioral Ecology Vol. 10 No. 3: 304-311
© 1999 International Society for Behavioral Ecology
Extrapair paternity and egg hatchability in tree swallows: evidence for the genetic compatibility hypothesis?
a Department of Biology, Queen's University, Kingston, Ontario, K7L 3N6, Canada b Austrian Academy of Sciences, Konrad Lorenz Institute for Comparative Ethology, KLIVV, Savoyenstrasse 1a, 1160 Vienna, Austria
Address correspondence to B. Kempenaers, Research Center for Ornithology of the Max Planck Society, Postfach 1564, D-82305 Starnberg, Germany. E-mail: b.kempenaers{at}erl.ornithol.mpg.de
Received 2 March 1998; revised 30 September 1998; accepted 3 November 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Tree swallows (Tachycineta bicolor) show one of the highest levels of extrapair paternity in birds, and there is evidence that females have control over who fathers their offspring. However, it is unclear which benefits female tree swallows obtain from mating with multiple males. Using microsatellite DNA fingerprinting, we studied extrapair paternity in relation to nesting success and male, female, and offspring characteristics. More than 70% of all nests contained extrapair young, and more than half of all offspring were extrapair. Within broods, the extrapair young were often fathered by several males. Despite screening all resident and some floater males, we could identify the biological father of only 21% of all extrapair offspring. All identified extrapair males were close neighbors. Extrapair males did not differ from within-pair males in any of the measured characteristics, except that the former had larger cloacal protuberances than the latter. Extrapair males were equally successful in gaining paternity in their own broods as males that did not father extra young. In nests with mixed paternity, extrapair young did not differ from within-pair young in body size or mass. However, nests with extrapair young had higher hatching success than nests without extrapair young. All examined unhatched eggs were fertilized and thus hatch failure resulted from embryo mortality. The available data are in accordance with the genetic diversity and the genetic compatibility hypothesis, but not with the good genes hypothesis.
Key words: extrapair fertilization, genetic compatibility, genetic diversity, good genes, microsatellite DNA fingerprinting, multiple paternity, Tachycineta bicolor, tree swallows.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Extrapair paternity is common in many socially monogamous birds, as shown by studies using a variety of molecular techniques (Gowaty, 1996
;
Westneat and Webster, 1994).
In several species females actively choose to engage in extrapair copulations
(e.g., Gray, 1996;
Houtman, 1992;
Kempenaers et al., 1992;
Smith, 1988;
Wagner, 1992;), and it is
generally agreed that they have at least some control over who fathers their
offspring (Birkhead and Møller,
1993). Because copulating with different males is costly (e.g.,
time and energy costs, increased predation risk), females must obtain some
benefits. Many hypotheses have been proposed to explain why females mate with
multiple males (reviews in Birkhead and
Møller, 1992;
Kempenaers and Dhondt, 1993;
Westneat et al., 1990), but
despite much study, the issue is still highly debated
(Brown, 1997;
Jennions, 1997;
Keller and Reeve, 1995;
Sheldon, 1994).
In general, it is suggested that females are more likely to obtain
indirect, genetic benefits because often there are no clear direct benefits
(but see, e.g., Gray, 1997;
review in Birkhead, 1998). The
most popular hypothesis suggests that, by mating with a high-quality extrapair
male, females can obtain "viability genes" or
"attractiveness genes" for their offspring (the good genes
hypothesis). Support for this hypothesis comes from recent studies showing a
relationship between male characteristics, paternity, and offspring condition
or survival (Hasselquist et al.,
1996; Kempenaers et al.,
1997; Møller,
1994; Petrie,
1994; Saino et al.,
1997; Sheldon et al.,
1997). Other studies, however, have failed to find a relationship
between male and/or offspring characteristics and paternity (e.g.,
Strohbach et al., 1998).
Several alternative hypotheses have been proposed that do not require females
to show preferences for particular extrapair males, but that still lead to
genetic benefits in the form of good genes (e.g.,
Madsen et al., 1992), diverse
genes (Birkhead and Møller,
1992), or compatible genes (e.g.,
Zeh and Zeh, 1996,
1997).
The socially monogamous tree swallow (Tachycineta bicolor) is
among those species with the highest levels of extrapair paternity: 50-92% of
all broods contained extrapair young, and extrapair males fathered 38-76% of
all nestlings (see review of studies in
Barber et al., 1996). There is
also strong evidence that female tree swallows can control which male
fertilizes their eggs through active selection and rejection of copulation
partners (Lifjeld and Robertson,
1992). The question of why females mate with multiple males is
thus particularly relevant in this species.
The aim of this study was to investigate how female tree swallows can
benefit from engaging in extrapair copulations. A previous study failed to
find any male morphological or behavioral trait that correlated with paternity
in the nest (Dunn et al.,
1994a), but concluded that the data were consistent with both the
good genes and the genetic diversity hypothesis. An important step is to find
out who the extrapair fathers are, which allows pairwise comparisons of
characteristics of within- and extrapair males. Dunn et al.
(1994a) attempted to identify
the extrapair fathers on their study grid using multilocus DNA fingerprinting,
but they found the biological fathers of only 21% of all the extrapair young.
They suggested that floater males could be fathering extrapair young. We
extended Dunn et al.'s (1994a)
study by using a larger sample of nests and by including floater males to our
sample of potential fathers. We studied nesting success and male, female, and
offspring characteristics in relation to paternity. We used microsatellite DNA
fingerprinting, which is a more powerful tool to assign paternity. We discuss
our data in relation to the different hypotheses explaining female choice for
extrapair paternity.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Study area and field procedures
The study was carried out in 1995 near the Queen's University Biological Station (44°34' N, 76°19' W), Chaffeys Locks, Ontario, Canada. From April to July, we studied tree swallows breeding on different nest-box grids (RF, TG, and JG) and in solitary boxes along the road from the grids to the biological station (for details, see Kempenaers et al., 1998
).
Distances between nest-boxes were measured using maps produced with a global
positioning system with accuracy to <1 m.
Adults were caught with mist nets or inside their boxes, either before or
after the females' fertile period to avoid any effect on paternity. We banded
each individual with a metal band and, in case the sex could be determined
unequivocally, with a red (female) or a blue (male) color band. We also marked
each individual with a unique combination of colored spots on wings and/or
tail using acrylic paint. We determined sex based on plumage characteristics
and wing chord (Hussell, 1983;
Stutchbury and Robertson,
1987) and checked the sex by the presence of a brood patch
(females) or cloacal protuberance (males) and via behavioral observations. For
females, we also noted the percentage of iridescent blue plumage on the
upperparts. Second-year females have a subadult brown-blue plumage (less than
50% blue). Females with 100% iridescent blue plumage are after-second-year
birds (Hussell, 1983).
For each individual, we measured left and right wing and left and right
outermost tail feather to the nearest 0.1 mm using a ruler and the left and
right tarsus length and beak length to the nearest 0.01 mm using digital
calipers. We also measured male cloacal protuberance size (height and
diameter) to the nearest 0.01 mm using calipers. As an index of parasite load,
we counted the number of holes in wing and tail feathers caused by feather
mites (see Dunn et al.,
1994a). We weighed adults with a Pesola spring balance to the
nearest 0.1 g and chicks with an electronic balance (accurate to 0.1 g). We
caught all adults (whether already banded or not) when they fed 3-day-old
chicks and measured their weight and, for males, their cloacal protuberance.
We calculated cloacal protuberance volume as V =
(d/2)2(h) (assuming a cylindrical shape).
When chicks were 14 days old, we measured their left and right wing and tarsus
and weighed them.
In total, we studied 60 nests (all first broods), of which 11 failed for various reasons. We collected blood samples (10-180 µl) for DNA fingerprinting from all offspring and social parents of 49 nests. Young were bled when 8 days old. Because one male was polygynous (two nests), we sampled 48 resident males. We also collected blood samples from 9 known male floaters and from 33 suspected male floaters for which sex could not be determined with certainty (n = 42 floaters). A floater was defined as an individual that was caught on one of our study grids but did not breed in a nest-box. We cannot exclude the possibility that these birds bred elsewhere in natural cavities.
We checked all nests daily during the nest-building, egglaying, and early incubation stages and numbered each egg on the day it was laid. On the day the first egg was laid, we measured nest height (from the bottom of the box to the rim of the nest) to the nearest 0.5 cm using a ruler. After 4-5 days of incubation, we candled all the eggs. Normally, clear signs of embryonic development (small embryo and blood vessels) can be observed. Eggs that showed no sign of development (in contrast to the other eggs of the same clutch) were collected. We collected a total of 19 eggs and stored them at 4°C within a few hours after collection. At the end of the season (late July) we sent 17 eggs (two broke before shipping) to Sheffield University (UK) to determine whether they were fertilized. We continued to check nests daily around and after the time of hatching, and we collected all unhatched eggs that remained in the nest when the young were 4 days old.
Fertility determination
Fertility of eggs can be determined by investigating the perivitelline
layers around the ovum (see Birkhead et
al., 1994, 1995,
for details). Sperm penetrating the inner perivitelline layer at the germinal
disc leave relatively large holes which can be easily observed with a
microscope. Also, sperm present in the infundibulum is trapped when the outer
perivitelline layer is formed just after fertilization and can be made visible
by staining. Thus, the presence of holes and/or sperm provide evidence that
fertilization has occurred. Eggs were opened and the content soaked with
phosphate-buffered saline. If the yolk was intact, the perivitelline layers
were removed. The tissue was placed on a microscope slide, a drop of
fluorescent Hoechst dye 33342 was added to stain sperm nuclei, and the
presence of holes and sperm was recorded. The perivitelline membranes were
damaged by drying and storing and because of the few days of incubation. Nine
eggs were completely dried out and could not be used. For the other eight
eggs, it was possible to qualitatively assess the presence of sperm and
holes.
Tissue sampling
We stored the collected blood in 10 volumes of Queen's lysis buffer
(Seutin et al., 1991) at
4°C. A 10-µl aliquot of blood was suspended in 90 µl of Queen's
lysis buffer and digested with 20 U proteinase K in 750 µl of buffer
containing 100 mM Tris-Cl, pH 8.0, 10 mM EDTA, 100 mM NaCl, and 1% sodium
dodecyl sulfate (Kocher et al.,
1989). Digestions were incubated at 65°C for 3-5 h. Genomic
DNA was extracted using two equal volume phenol/chloroform:isoamyl alcohol
(24:24:1) washes, and one equal volume of chloroform:isoamyl alcohol (24:1)
wash. DNA was precipitated using 1/10 volume of 2.5M sodium acetate and 1.5
volumes 95% ice-cold ethanol. Precipitated DNA was looped, washed with 70%
ethanol, pelleted, dried, and resuspended in 200 µl of ultrapure water.
PCR amplification
We used four sets of European barn swallow (Hirundo rustica)
microsatellite primers (HrU3, HrU5, HrU6 and HrU7;
Primmer et al., 1995) and one
set of North American tree swallow primers (IBI MP 3-31;
Crossman, 1996) for polymerase
chain reaction (PGR) amplification. PCR reactions were performed in 10 µl
volumes which contained 50-100 ng of genomic DNA, 0.5 mol forward primer
labeled with P33- dATP, 0.5 mol unlabeled forward primer, 1.0 mol
reverse primer, 100 µM dNTPs, 0.35U Taq DNA polymerase, 0.2M Tris-Cl, pH
8.4, 0.5 M KCl, and 2.5 mM MgCl2. PCR amplification was performed
using a Perkin Elmer Gene Amp PCR system 9600 Thermocycler. The following
thermal cycling conditions were used for all HrU primers: 1 cycle 94°C, 2
min (denaturation); 35 cycles 94°C, 30 s (denaturation), 59°C, 30 s
(annealing), 72°C, 40 s (extension); 1 cycle 72°C, 5 min (final
extension). Thermal cycling conditions for IBI MP 3-31 were identical, except
that an annealing temperature of 56°C was used. Amplification products
were resolved on 5% polyacrylamide denaturing gels containing 7.0 M urea. Gels
were run at 40 W (20 cm) or 70 W (40 cm) depending on gel width. Dried gels
were exposed to BIOMAX (Dupont) X-ray film overnight.
Parentage analyses
At each locus, we determined allele product sizes for a set of reference
individuals by comparing these individuals to a sequencing reaction of known
template. Unknown alleles were sized by comparison to these reference
individuals. For highly variable loci, it was necessary to run multiple gels
of different duration and to rerun individuals so that the stutter patterns
from adjacent alleles overlapped to form an allele size ladder. Allele sizes
that fell between widely spaced reference individuals could then be determined
by comparing adjacent individuals.
The four barn swallow markers were used for all parentage analyses, and the polymorphism data for these four loci are shown in Table 1. The four markers combined give an exclusion power of p =.999; the combined identity probability is p = 3.4 x 10-7. For each nestling, we compared the allele sizes at each of the four loci with those of the putative parents. The genotypes of all the nestlings matched those found in the female at the nest; thus we concluded that there were no cases of intraspecific brood parasitism. The genotypes of many offspring were not compatible with the genotype of the male at the nest. When this was the case for one or more of the four loci, we considered this offspring an extrapair young. Of 117 offspring, 11 (9.4%) showed a mismatch at 1 locus, 27 (23.1%) at 2 loci, 43 (36.8%) at 3 loci, and 36 (30.8%) at 4 loci. Even if there was a mismatch at only one locus, we did not consider this the result of a mutation because (1) we found no mutations in the maternally inherited alleles, (2) the single mismatches did not occur more often at the most variable locus (three occurred at HrU3, four at HrU5, two at HrU6, and two at HrU7), and (3) in five offspring with a single mismatch, the paternal alleles were similar to those in other extrapair offspring in the same nest, suggesting these offspring had the same (extrapair) father. It cannot be excluded that some of the single mismatches are caused by mutation, but this would have little effect on the analyses presented here.
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To assign paternity, we screened all sampled males (residents and floaters)
for the set of paternally inherited alleles found in the extrapair young. With
the four HrU primers, the probability that a randomly chosen male would share
the same genotype as the extrapair offspring was calculated as
(2p1—p12)
(2p2—p22)
(2p3—p32)
(2p4—p42), where
pi is the frequency of the paternally transmitted allele
at locus i (Jeffreys et al.,
1992). These probabilities ranged from 8 x 10-6
to 0.0043 (mean ± SD: 0.0010 ± 0.0014, n = 24). Thus,
it is likely that a male with a matching genotype was the true biological
father. Moreover, when we found such a male in the population, we used the
tree swallow primer IBI MP 3-31 to confirm our assignment.
Data analyses
For all analyses, we used averages of the left and right measurements of
wing and tarsus length. If individuals were measured more than once, average
values were used, except for cloacal protuberance size and body mass. The
latter two variables change within individuals over the season, so we used
only those measurements taken when the individual was feeding 3-day-old chicks
(measured for all individuals). The disadvantage is that we measured cloacal
protuberance after the fertile period of the female, but we assume that
differences during the chick feeding stage reflect differences during the
fertile period when cloacal protuberance volume has reached its maximum
size.
As part of an experiment on the relationship between paternity and parental
care, we temporarily locked the resident female in her nest-box on the morning
she laid her second and third egg for 25 nests (see
Kempenaers et al., 1998, for
details). However, it is unlikely that our experiment had a strong influence
on paternity: 64% of experimental nests (n = 25) and 83% of control
nests (n = 20) contained at least one extrapair young (Fisher's Exact
test: p =.196). Also, the proportion of extrapair young per brood did
not differ between experimental and control pairs (data not shown). Control
and experimental nests did not differ in laying date, clutch size, hatching
success, and fledging success (Kempenaers
et al., 1998).
Data were analyzed using Statistica V5.1 (StatSoft, Inc.), StatXact-Turbo
(StatXact, 1992), and GLIM
V3.77. Proportional data were analyzed in GLIM using binomial errors with
Williams's adjustment for overdispersion
(Crawley, 1993). Data shown are
mean ± SE, unless specified otherwise.
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RESULTS |
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Patterns of extrapair paternity
Table 2 summarizes the pattern of extrapair paternity in the population. Because breeding density or the social structure of the population might influence paternity, we compared colonial (grids) and solitary nests. There was no difference in the proportion of nests containing extrapair young (colonial: 0.76, n = 29; solitary: 0.70, n = 20; Fisher's Exact test: p =.747), nor in the proportion of extrapair young per nest (only nests with at least one extrapair young considered; colonial: 0.68 ± 0.05, n = 22; solitary: 0.62 ± 0.08, n = 14; Mann-Whitney U test, U = 138.5, p =.611).
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Biological fathers
We determined the biological fathers of 21% (24 out of 117) of the
extrapair young. In 33% (12 out of 36) of the nests with extrapair young, we
found the biological father of at least one extrapair young, but in only 4
nests (11%) the biological father(s) of all extrapair young was identified. In
only 3 of the 12 nests (25%) were all extrapair young fathered by a single
extrapair male. In total, 11 males were identified as the father of one or
more extrapair young, and two of them (18%) fathered extrapair young in two
nests. All these males were residents that bred nearby. The median distance
between the extrapair male's own nest and the nest where he fathered extra
young was 93 m (range: 30-535 m). Males that fathered extrapair young lost an
equal amount of paternity in their own nest (proportion of extrapair young per
nest: 0.44 ± 0.11) as males that did not father extra offspring (0.50
± 0.06, U = 185.0, p >.5).
We identified the biological father of at least one extrapair young in 36% of 22 nests on the grids and in 29% of 14 solitary nests. Thus, we were equally successful in determining the biological fathers on grids and in solitary nests (Fisher's Exact test: p =.73). However, the males fathering extrapair offspring on the grids had their own nest significantly closer (median: 58 m, n = 8) than those fathering extra young in solitary boxes (median: 475 m, n = 5; Mann-Whitney U test, U = 40, p <.005).
The extrapair males nested earlier than the within-pair males they cuckolded (Sign test: p <.05, n = 13), but the median difference in laying date was only 1 day (range: from 16 days earlier to 5 days later).
Extrapair paternity and nesting success
Nests with or without extrapair young did not differ in nest height, clutch
size, laying date, or in the proportion of hatched young that fledged
(Table 3). However, hatching
success was significantly higher for nests with extrapair young than for those
without (Figure 1).
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Of 49 nests, 25 (51.0%) contained at least one unhatched egg (range: 1-6). In total, 43 (15.8%) of 272 eggs failed to hatch. Of all unhatched eggs, only 19 (44.2%) did not show any sign of development after 4 days of incubation (see Methods). Of the analyzed unhatched eggs (n = 8), five showed holes in the inner perivitelline layer, and all had sperm trapped between the inner and outer perivitelline layer. Thus, there is no evidence that any of the unhatched eggs were infertile. We conclude that a minimum of 74.4% (8+24) of the unhatched eggs failed because of embryo mortality.
Extrapair paternity and characteristics of males and females
Males that lost paternity did not differ in tarsus, wing, tail, and beak
length, nor in body mass, number of mite holes, and cloacal protuberance
volume from those males that had full paternity in their nest
(Table 4). Pairwise comparisons
of the characteristics of extrapair and within-pair males showed no
differences in tarsus, wing, and tail length, nor in body mass and number of
mite holes (Table 5). Extrapair
males had significantly larger cloacal protuberance volumes than the
within-pair males they cuckolded (Table
5).
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Females with extrapair young in their nest did not differ in any of the
measured characteristics from those without extrapair young
(Table 4). Younger females were
less likely to have extrapair young in their nest than older females, but this
difference was not significant. However, female plumage score (related to age;
see Methods), explains a significant part of the variation in proportion of
extrapair young in the nest (generalized linear model with plumage score as
independent variable, 
2 = 4.57, df = 1, p
<.05).
Extrapair paternity and offspring characteristics
For nests with mixed paternity, we found no difference between the
extrapair and within-pair offspring in body mass, wing, and tarsus length at
age 14 days (Table 6). For
seven nests with mixed paternity and partial brood mortality (between day 8
and fledging), the extrapair young were not more likely to survive than the
within-pair young (extrapair young: 64% ± 39, within-pair young: 61%
± 31; common odds ratio for seven 2 x 2 contingency tables =
0.95, p = 1.00).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The socially monogamous tree swallow shows one of the highest rates of extrapair paternity found in birds (see Petrie and Kempenaers, 1998
).
In our 1995 population, more than 70% of all nests contained extrapair young,
and about half of all the offspring produced were fathered by an extrapair
male. Almost one out of five nests contained exclusively extrapair young
(Table 2). Females are highly
promiscuous, as evidenced by the fact that the offspring in a single nest are
often fathered by more than two males
(Table 2). This promiscuous
behavior must be costly because of time and energy invested, increased
predation risk, and increased risk of obtaining parasites or sexually
transmitted diseases (Keller and Reeve,
1995).
Because female tree swallows have control over whom they copulate with and
who fathers their offspring (Lifjeld and
Robertson, 1992; see also
Venier et al., 1993), they
must obtain some benefits from having some or all of their young fathered by
one or more extrapair males. Direct benefits can almost certainly be excluded.
Female tree swallows do not receive any food nor do they receive any parental
assistance from extrapair males. Unlike in some other species, male tree
swallows do not respond aggressively if females refuse to copulate
(Venier et al., 1993), so
avoidance of the risk of injury is an unlikely explanation for engaging in
extrapair copulations. On a few occassions, we observed an extrapair male
entering the nestbox when the female was inside during the egg-laying period.
Inspection of the nestbox showed that these males attempted to copulate. In
such cases, refusing to copulate might be costly because the extrapair male
could react aggressively and the female would risk damage to her eggs.
However, for two broods where we observed this behavior and where we obtained
blood samples from the extrapair male and all the chicks, these extrapair
copulations did not lead to extrapair paternity (our unpublished data).
Avoidance of infertility is also an unlikely reason why female tree swallows
frequently engage in extrapair copulations. Although our sample size is small,
all examined eggs which showed no sign of development were fertilized.
Is there evidence that females obtain genetic benefits? The patterns of
extrapair paternity found in this study and in a previous study
(Dunn et al., 1994a) do not
support the traditional good genes hypothesis (for general predictions, see
Kempenaers and Dhondt, 1993;
Westneat et al., 1990). Both
studies show that (1) most females had extrapair young in their nest, (2) in
most nests, the majority of young were extrapair, and these young were usually
fathered by several extrapair males, (3) extrapair males did not differ in any
of the measured characteristics from within-pair males (except for cloacal
protuberance size, see below) and males that lost paternity were not different
from those that did not, (4) there is no indication that extrapair young grow
better or have a higher probability of survival than within-pair young. Dunn
et al. (1994a) also found one
case where two males "exchanged" paternity. Our study does not
allow us to refute the good genes hypothesis because it is possible that
females do select males with a particular phenotype and that we did not
measure the relevant male trait (e.g., plumage color). It is also possible
that extrapair young do better in the long term (e.g., survival), but this is
hard to test because of the low local recruitment rate.
The patterns of extrapair paternity do agree with the predictions from the
genetic diversity hypothesis (Kempenaers
and Dhondt, 1993). However, this hypothesis seems an unlikely
explanation for female promiscuity because meiosis and the fusion of gametes
already result in genetically diverse offspring
(Williams, 1975).
Our results are also in accordance with the genetic compatibility
hypothesis. One version of this hypothesis proposes that females would benefit
from increased heterozygosity in their offspring because the probability that
lethal or deleterious recessive alleles are expressed is reduced
(Brown, 1997). Genetic
incompatibility between partners can also arise as a consequence of various
agents of intragenomic conflict and other forces acting at the suborganismal
level leading to inviable or less viable zygotes
(Zeh and Zeh, 1996).
As found in sand lizards Lacerta agilis
(Olsson et al., 1994), female
tree swallows that copulated with multiple males had a greater egg-hatching
success than those that did not. Examination of the unhatched eggs suggests
that this is the result of a lower frequency of embryo mortality in females
that mate with multiple males. Similarly, multiple mating by females reduced
the number of stillborn offspring in adders Vipera berus
(Madsen et al., 1992) and may
have reduced the mortality of unweaned infants or the frequency of abortions
in Gunnison's prairie dogs Cynomys gunnisoni
(Hoogland, 1998). A study on
house sparrows Passer domesticus shows the opposite pattern:
unhatched eggs were more common in broods with extrapair offspring
(Wetton and Parkin, 1991).
The observed relationship between the presence of extrapair offspring and
the proportion of unhatched eggs (Figure
1) can be explained in other ways. For example, because our data
suggest that 1-year-old females are less likely to have extrapair young in
their nest, the lower hatching success in nests without extrapair young could
result from younger females being less experienced in incubation compared to
older females. However, fewer 1-year-old females had unhatched eggs in their
nest (44% of 18) than older females (55% of 31; Fisher's Exact test,
p =.56). Eggs fathered by extrapair males may also be more likely to
fail than eggs fathered by the social male. This might be the case if the last
laid eggs are more likely to be fathered by extrapair males (as in the house
sparrow; Wetton et al., 1995)
and less likely to hatch. However, in the tree swallow, neither paternity nor
egg hatchability is related to the order of laying (Kempenaers et al.,
unpublished data). Although our results are not conclusive (e.g., we do not
know who fathered the unhatched eggs), the hypothesis that female tree
swallows copulate with multiple males to avoid the costs of genetic
incompatibility deserves further investigation.
We only identified the biological fathers of one-fifth of the extrapair
young, despite using microsatellite DNA fingerprinting, which allowed us to
screen the entire sample of males (including all residents and some floaters),
and despite studying a large population (including birds nesting on grids and
in solitary boxes). Surprisingly, Dunn et al.
(1994a) were equally
successful in assigning extrapair young to known males, despite a smaller
scale study on a single grid where only resident males were sampled. The
missing extrapair fathers could be residents on other nearby study grids or
breeding in natural cavities, or they could be among the large numbers of
floater males present in the area. A comparative study of resident and floater
tree swallows showed that floaters have fully developed reproductive organs,
and there is indirect evidence that they copulate (Peer et al., unpublished
manuscript). None of the floater males in our sample (n = 42)
fathered offspring, but we caught only a small proportion of the floaters, and
the majority of them were caught long before laying began.
There are still several puzzling facts about extrapair paternity in tree
swallows. If the genetic diversity or genetic compatibility hypotheses explain
multiple paternity, why do not all females engage in extrapair copulations,
and why are some females selective in their choice of copulation partner as
shown by Lifjeld and Robertson
(1992)? Precopulatory female
choice is not necessarily inconsistent with the genetic compatibility
hypothesis because male phenotype may reflect individual heterozygosity at key
loci or at many loci (Brown,
1997), but it is still unclear what male characteristics (if any)
female tree swallows choose. Our data suggest that young females are less
likely to have extrapair young in their nest. If females perform extrapair
copulations with high-quality males, as predicted by the good genes
hypothesis, and if older females are more likely to be paired to high-quality
males, the opposite trend would be expected (as found in, e.g., hooded
warblers Wilsonia citrina,
Stutchbury et al., 1997).
Perhaps older females are more likely to obtain extrapair copulations simply
because they are more experienced.
If females benefit from extrapair copulations, it is also difficult to
understand why so few of the extrapair fathers are local residents and why the
nearest neighbors are more likely to be the father than more distant
neighbors. Reduced paternal investment is a potential cost for females
engaging in extrapair copulations. However, in tree swallows, the social males
provide full parental care independent of paternity
(Kempenaers et al., 1998;
Lifjeld et al., 1993;
Whittingham et al., 1993).
Males might not be aware of the extrapair activities of their mates. Extrapair
copulations are rarely observed in tree swallows relative to within-pair
copulations (Venier et al.,
1993), and this is surprising given the high frequency of
extrapair paternity. Perhaps females perform most extrapair copulations away
from the nesting grids to avoid losing male help, which would explain why so
few of the local residents are fathering extrapair young.
Female extrapair behavior could also explain why the social structure and
the local breeding density (colonial versus solitary boxes) did not influence
the frequency of extrapair paternity (Dunn
et al., 1994b; this study). In most other colonial and dispersed
breeders, the frequency of extrapair paternity is positively correlated with
density (review in Westneat and Sherman,
1997). Such an effect is expected if females are limited in their
choice of copulation partners. However, if female tree swallows copulate away
from the nesting grids and/or copulate with floaters, local breeding density
may be less important.
If females copulate frequently with their social mate
(Venier and Robertson, 1991)
and if they copulate with one or more extrapair males, which factors determine
male fertilization success? We still do not have enough information on the
patterns of copulation or on the possibilities of postcopulatory female choice
to understand the mechanisms of sperm competition in this species, but we
found that the extrapair fathers had significantly larger cloacal
protuberances than the withinpair males they cuckolded. This suggests that
males that produce more sperm are more likely to father offspring. On the
other hand, these extrapair fathers were not more successful in gaining
paternity in their own brood.
To solve the tree swallow paternity puzzle, we need largescale paternity
studies over several breeding seasons. These are needed to confirm some of the
patterns reported here— in particular the relationship between multiple
paternity and the presence of unhatched eggs. Such a study would allow us to
assess the reproductive success of individual males and females over several
years (see also Dunn et al.,
1994b). According to the genetic compatibility hypothesis, male
quality depends on the female he mates with. Thus, we would expect low
repeatabilities in male fertilization success. A preliminary analysis for tree
swallows seems to support this
(Møller, 1998). A
longer term study would also allow tests of whether individual females are
more likely to obtain multiple paternity with increasing age. Finally, an
effort should be made to identify the extrapair males. This might be achieved
by following females away from nesting grids using radio-tracking and/or by
capturing floaters males in the population during the fertile period.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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We thank Colleen Barber, Ellen Bentley, An Blieck, Vicky Boomgaardt, Lara Edwards, and Richard Lanctot for excellent assistance in the field, Susie Everding and Denise Michaud for assistance in the lab and Tim Birkhead and Bobbie Fletcher for analyzing the unhatched eggs. We are grateful to Hans Ellegren and Chris Primmer for providing information about the microsatellite primers. Thanks also to the QUBS staff for being patient with demanding guests and especially to Frank Phelan and Floyd Connor for making our stay at the station so enjoyable. This study was supported by a Natural Sciences and Engineering Research Council (NSERC) International Fellowship to B.K., by NSERC research grants to R.J.R. and P.T.B., and by the Austrian Academy of Sciences.
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