Behavioral Ecology Vol. 13 No. 5: 670-675
© 2002 International Society for Behavioral Ecology
Increased cuckoldry as a cost of breeding late for male house wrens (Troglodytes aedon)
Department of Biology, Towson University, Towson, MD 21252, USA
Address correspondence to L.S. Johnson. E-mail: sjohnson{at}towson.edu.
Received 15 May 2001; revised 11 January 2002; accepted 17 January 2002.
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ABSTRACT |
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One factor hypothesized to influence the reproductive behavior of individuals is the degree to which reproductive efforts are synchronized with others in the population. We asked whether the timing of a pair's breeding cycle, relative to cycles of pairs on neighboring territories, affected rates of extrapair mating over 2 years in a Wyoming population of house wrens (Troglodytes aedon). Extrapair young (identified using 5 microsatellite loci) occurred in 74% of nests of 19 pairs whose cycles began later than cycles of one or more neighbors compared to only 26% of nests of 27 pairs whose cycles began earlier than, or simultaneously with, cycles of all neighbors. Extrapair offspring occurred in 65% of 17 nests belonging to males who initially settled and began nesting early relative to neighbors but who were forced to renest late after we removed their first mates. Rates of cuckoldry were not significantly different for forced-late and naturally late males. Our experimental approach controlled for possible effects of male quality, clearly demonstrating an effect of timing of breeding on extrapair mating activity.
Key words: breeding synchrony, extrapair mating, house wrens, Troglodytes aedon.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Notable variation in mating success occurs in many animal species, especially among males. One factor long hypothesized to influence the skew in male mating success in a population is the temporal distribution of fertilizable females. In their seminal paper, Emlen and Oring (1977
) argued that, with an
increasing degree of breeding asynchrony in a population, the potential for
males to mate multiply should increase. This is especially true when males are
freed from some or all parental duties.
Emlen and Oring's (1977)
prediction has been tested in a variety of animals, including birds. In most
bird species, individuals form long-term, monogamous pair-bonds. This
arrangement requires that for males to mate multiply, they usually must secure
copulations with extrapair females. Much debated in recent years is how
breeding synchrony normally affects extrapair mating (EPM) activity and hence
the skew in male mating success. In contrast to Emlen and Oring's
(1977) predictions, Stutchbury
and Morton (1995;
Stutchbury, 1998) proposed
that rates of EPM should increase with increased breeding synchrony. They
assume that females participate in EPM primarily to obtain high-quality
alleles for offspring. As synchrony increases, the proportion of potential
extrapair male partners engaged in sexual display simultaneously, and under
similar conditions, increases, providing females with more opportunity to
compare males and copulate with higher quality males. Note that in this
scenario, EPM is most likely to increase with increasing breeding synchrony
when females actively pursue EPM, either through off-territory forays or by
signaling their fertility status and location to nearby males (e.g.,
Gray, 1996;
Kempenaers et al., 1992;
Otter et al., 1994;
Sheldon, 1994;
Stutchbury and Neudorf,
1997).
Other researchers have followed Emlen and Oring
(1977) in arguing that EPM
should decrease as breeding synchrony increases
(Birkhead and Biggins, 1987;
Buitron, 1983;
Møller, 1985;
Westneat et al., 1990). These
researchers have focused on species in which females do not actively pursue
EPM, but rather, EPM occurs primarily when males intrude into foreign
territories and solicit resident females (e.g.,
Buitron, 1983;
Currie et al., 1998;
Westneat, 1988;
Yezerinac and Weatherhead,
1997). If it is primarily males, not females, who pursue EPMs,
then when breeding is synchronous, few nests are expected to contain extrapair
young. This is because at the time that most females are fertile, most
potential extrapair male partners will be occupied constructing nests,
gathering food for females, and/or guarding their own fertile females, leaving
little time for pursuit of EPMs. When breeding is asynchronous, EPM should be
more common, but it is also expected to occur more on some territories than
others. If males pursue EPMs primarily after their own mates are no longer
fertile, then pairs who begin nesting earlier than other pairs will, like
synchronous pairs, be unlikely to have extrapair young in their nests. In
contrast, when breeding is asynchronous, later-breeding females should
encounter relatively more potential extrapair partners, and their nests should
be more likely to contain extrapair offspring. This will be most true when
males do not participate in incubation.
We compared the frequency with which extrapair young occurred in nests of pairs breeding earlier than, or simultaneously with, all immediate neighbors and nests of pairs breeding substantially later than immediate neighbors in a predominantly monogamous songbird, the house wren (Troglodytes aedon). Male house wrens, which do not help females incubate, appear to pursue EPMs most readily after their own mates are no longer fertile (Johnson, unpublished data). We predicted that late-breeding pairs would be more likely to have extrapair young in nests than would early or simultaneously breeding pairs based on the expectation that late females will have more encounters with potential extrapair partners than will early or simultaneously breeding females.
Several studies have examined effects of timing of breeding on EPM activity
in predominantly monogamous birds (e.g.,
Chuang et al., 1999;
Dunn et al., 1999;
Li and Brown, 2000;
Saino et al., 1999;
Thusius et al., 2001;
Yezerinac and Weatherhead,
1997). Our study was unique, however, in that we used an
experimental approach to control for possible confounding effects of male
quality. Males who arrive early on the breeding grounds or who are for other
reasons ready to breed earliest may be older than later-settling males. Early
males may also be in better condition and more fit, regardless of age (e.g.,
Marra et al., 1998;
Merilä and Svensson,
1997; Nyström,
1997). Early-breeding males may therefore be cuckolded less, and
late males cuckolded more, because higher quality early-breeding males are
preferred as copulation partners by both their own mates and by mates of their
later-nesting, lower quality neighbors. To assess the relative effects of
timing of breeding and male quality on male paternity, we removed first mates
of some early males, forcing them to breed late relative to their neighbors.
If unmanipulated early males and early-but-forced-to-nest-late males have near
equal and low frequencies of cuckoldry, we could conclude that male quality
more strongly influences the probability of a mate engaging in EPM than does
timing of breeding relative to close neighbors. We would conclude the reverse
if early-but-forced-late males and naturally late males are cuckolded to an
equal and high degree.
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METHODS |
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Study species
House wrens are small (10-12 g), drab, sexually monomorphic, cavity-nesting passerines found in open woodlands (Johnson, 1998
). Males begin
arriving on breeding grounds in late April, and first-arriving females appear
a few days thereafter. Females visit territories of different males and choose
mates at least partly based on nest-site quality
(Johnson and Searcy, 1993).
The extent to which prospecting females also consider male characteristics is
unknown. Females usually lay one egg per day, producing a clutch of four to eight eggs. Full incubation usually begins with the laying of the penultimate egg. Males do not incubate eggs but provide extensive help feeding nestlings, especially right after hatching.
Territorial males often intrude into territories of immediate neighbors,
apparently to obtain EPMs. Significantly more intrusions into a territory
occur during the resident female's presumed fertile period than before or
after this period. Intruders typically approach fertile females and adopt a
copulation-associated tail raise display
(Johnson and Kermott, 1989).
Genetic evidence for extrapair mating has been obtained in populations
elsewhere (e.g., Soukup and Thompson,
1997). No observations in over 15 years of work with our study
population have suggested that females routinely pursue extrapair copulations
off-territory or advertise their fertility.
Study site and general field procedures
We conducted this study in 1998 and 1999 on cattle ranches near Big Horn,
Wyoming, USA. We erected >90 nestboxes on greased poles on site each year
with most boxes 35-55 m from the next nearest box, mimicking inter-nest
distances observed when wrens on site are using only natural cavities. Almost
all wrens nesting on the site used boxes; only two and three pairs used
natural cavities within the confines of the study site in 1998 and 1999,
respectively. For each focal pair in this study, the nearest resident males or
resident pairs within 
200 m in any direction constituted the focal pair's
set of immediately adjacent neighbors.
We visited all territories every 1-3 days, noting nestbox contents, behavioral activities of any wrens present, and bird identity as revealed by unique combinations of colored leg bands. For each nest included in this study, we obtained DNA from the male and female attending the nest and from all nestlings surviving 7-8 days after hatching. We also attempted to obtain DNA from all males on all immediately adjacent territories. We extracted DNA from 50-100 µl samples of blood taken from a brachial blood vessel. To obtain blood, adults were trapped in nestboxes or mist nests. We did not trap unmarked focal males from which we had no DNA until about 2-48 h after their mates had completed egg laying. To identify cases of rapid mate switching, we would have preferred to capture, color-band, and bleed all unmarked males immediately after they settled on territories. We avoided capturing still-unmated males, however, because such males often desert territories if caught. We also avoided capturing mated males with potentially fertile mates because, in past years, we have witnessed intruders soliciting females paired with males we had in hand for banding.
Exact ages of some adults were known because we banded them as nestlings. Ages of other adults were estimated. Because adults tend to return to the same territory each year, or a territory nearby, we assumed that all unmarked adults on site each year were yearlings.
Experimental manipulation and categorization and comparison of
males/pairs
Our objective was to compare rates of cuckoldry of males that settled and
bred earlier than, or simultaneously with, all adjacent neighbors (hereafter
called "early" males) and males who bred later than adjacent
neighbors. We included a male in the early category if his breeding cycle was
ahead of, exactly synchronous with, or only slightly behind cycles of all
immediate neighbors such that on no neighboring territory did a female begin
incubation until the focal male's own female was at least halfway through
laying (i.e., until her presumed fertile period had ended or had nearly
ended). We assumed that females were potentially fertile from 5 days before
laying through the day before the penultimate egg was laid and full incubation
began.
We categorized a male as a "late" male if the female mated to one or more of his immediately adjacent neighbors began incubation (i.e., was no longer fertile) before the focal male's female was at least halfway through laying. Late males included those who, on their own, settled and began breeding later than one or more adjacent neighbors (hereafter called "naturally late" males), and those who settled early but who we manipulated into breeding late (hereafter called "forced late" males). Each season we removed females from a randomly selected set of early pairs. Females were transported to suitable habitat at a distant location from which they would not return. Affected males attracted new mates and commenced nesting substantially later than their closest neighbors, and often other neighbors as well. Five and 12 males were forced to nest late in 1998 and 1999, respectively. On average, these males acquired new mates in 4 days (range: 0-11 days).
We stress that our categorization of pairs as early or late refers only to the pair's timing of breeding relative to neighbors and not to the time of breeding within the season. All pairs involved in this study nested during the period when first broods of the season were being raised. Mean first egg dates for early, naturally late and forced late pairs were 27 May, 7 June, and 12 June, respectively. We did not assess paternity during the second wave of breeding that occurs in July and August.
Circumstances were such that for all naturally late and forced late males,
the closest neighbor's nest was within 70 m of their own nest. Among early
males, distance to nearest neighbor ranged up to 202 m. To help control for
any effect of distance to neighbors on extrapair mating activity, we included
in analyses only early males with nearest neighbors 
70 m away. We compared
rates of cuckoldry for 27 early, 17 forced late and, 19 naturally late males
observed over 2 years. Two focal males from the 1998 breeding season were
included in the study as focal males in 1999. However, in both cases, males
changed categories between years.
Except where noted, we used logistic regression in all analyses to identify factors that influenced the presence/absence of extrapair young within nests, including time-of-breeding category, number of neighbors (1, 2, or 3-4), distance to nearest neighbor, and male and female age. House wrens are short-lived; fewer than 30% of breeding adults in our study were >1 year old, and fewer than 10% were >2 years old. We therefore categorized bird age as 1 or >1.
Microsatellite and paternity analyses
We used the following five primer pairs: HRU3 and HRU6
(Primmer et al., 1995), POCC1
(Bensch et al., 1996), FHU2
(Ellegren, 1992), and PCAµ3
(Dawson et al., 2000). DNA was
isolated from blood samples following Bruford et al.
(1992). Polymerase chain
reaction (PCR) amplification was carried out following Primmer et al.
(1996) with modifications.
Reactions were carried out in 15-µl volumes containing 100 µM dNTPs, 2.5
µM MgCl, 1 x PCR Buffer II (PE Biosystems), 0.67 mM forward and
reverse primers, and 0.5 units of AmpliTaq DNA polymerase (PE Biosystems).
Reactions were performed in a Perkin Elmer AMP PCR System 2400 Thermocycler
using the following touchdown protocol: one cycle at 95°C for 3 min,
followed by two cycles at 94°C for 30 s, 60°C for 45 s, 72°C for
45 s; two cycles at 94°C for 30 s, 57° C for 45 s, 72°C for 45 s;
two cycles at 94°C for 30 s, 54°C for 45 s, 72°C for 45 s; two
cycles at 94°C for 30 s, 51°C for 45 s, 72°C for 45 s; then 25
cycles at 94°C for 30 s, 48°C for 45 s, and 72°C for 45 s and
finally 1 cycle at 72°C for 5 min. PCR products were electrophoresed
through 6% denaturing acrylamide gels and visualized by silver-staining
following Bassam et al. (1991;
see also Promega Corp., 1993).
To facilitate scoring, attendant males and females, nestlings, and neighboring
males were always run in adjacent lanes.
In all cases, nestling genotypes were compatible with genotypes of
attendant females at all loci with the exception of HRU3. In five nests,
females were apparently homozygous for an HRU3 allele that was different from
that found in some of her apparently homozygous nestlings. This observation
was consistent with the presence of a null allele at the HRU3 locus in this
population. Deviation of heterozygosity from Hardy-Weinberg expectations at
this locus provides an estimated frequency of 0.09 for the null allele
(Brookfield, 1996).
Heterozygosity levels for all other loci were consistent with Hardy-Weinberg
expectations. Because nestling genotypes were otherwise consistent with the
genotypes of attendant females, we assumed that brood parasitism did not occur
and that new mutations were not significantly frequent. We considered
nestlings with paternally derived alleles that were incompatible with
attendant males at any of the five loci analyzed to be extrapair young (note
that the presence of a null allele meant that males homozygous at the HRU3
locus could not be excluded from paternity of nestlings homozygous at the same
locus even when respective alleles did not match). The average exclusion
probability for the population was estimated following Jamieson
(1994). The probability that a
random (non-sire) male in the population would possess the alleles to be
included as a possible sire for a particular nestling (i.e., the probability
of false paternal inclusion), was estimated by 
— (1 —
xi)2 where
xi is the allelic frequency for the paternally
contributed allele for each of the five loci analyzed. Allelic frequencies
were based on all males typed in each year, which we estimate represented
>75% of all males in the study area. The probability of any of a group of
males (e.g., immediate neighbors) being falsely included as a sire of a
nestling was estimated by 1 — (1 — probability of false paternal
inclusion)n where n is the number of males in the
group.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Frequency of cuckoldry versus relative timing of breeding
Frequency of cuckoldry differed significantly for early, forced late, and naturally late males (logistic regression:
2 = 11.01,
p = .004; Figure 1).
Early males were cuckolded significantly less than forced late males
(2 = 10.73, p = .001). Forced late and naturally late
males, however, were not cuckolded to a significantly different degree
(2 = 1.0, p > .30).
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Observed differences in the frequency of cuckoldry were not an artifact of differences in our ability to detect extrapair young in nests of different types of males. The mean percentage of offspring typed in a nest did not differ substantially for early, forced late and naturally late pairs (88%, 81%, and 91%, respectively). Also, the mean probability of falsely including the pair male as the sire of offspring in his nest was not higher for early males (0.041, range: 0.005-0.11) than it was for forced late males (0.038, 0.001-0.12) or naturally late males (0.043, 0.001-0.18).
We also asked whether early, forced late, and naturally late pairs differed
in some factor other than relative timing of breeding that could have caused
or contributed to a lower frequency of EPY in nests of early versus late
males. These factors included number of neighbors, distance to neighbors, and
female and male age. Early pairs actually had, on average, more neighboring
males present during part or all of their mate's fertile period than did
forced late or naturally late pairs (2.8, 2.5, and 2.4 neighbors,
respectively). Because neighboring males were not always present during all of
a focal female's fertile period, we also calculated for each focal pair the
total days of the female's fertile period that each neighboring male was
present and summed for all neighbors (= total neighboring male-days). Early
pairs averaged more, not fewer, total neighboring male-days (28.1) than did
forced late pairs (20.2) and naturally late pairs (22.4) during the female's
fertile period. The probability of cuckoldry was also unrelated to total
neighboring male-days (2 = 0.1, p > .70).
Distances to nearest, second nearest, and third nearest neighbors were, on
average, greater for early pairs (46, 79, and 92 m, respectively) than for
forced late pairs (42, 71, 88 m) and naturally late (44, 76, 80 m). However,
differences were probably too small (i.e., 4-12 m) to have caused different
rates of extrapair mating between groups. A univariate analysis did show that
as distance to nearest neighbor increased, the probability of cuckoldry tended
to increase (2 = 2.79, p = .098). However, when we
included both timing-of-breeding category and distance to nearest neighbor in
a single analysis, probability of cuckoldry was significantly related to
timing of breeding (2 = 10.45, p = .005) but not to
distance to nearest neighbor (2 = 2.21, p = .14)
Older females tend to settle on territories earlier than younger females.
As such, the mean age of females in early pairs (1.7 years; range: 1-4, 50%
>1 year old) was greater than that of females in both forced late pairs
(1.1 years; 1-2, 0.6% >1 year old) and naturally late pairs (1.4 years; 1-5
years, 21% >1 year old). Having older mates, however, probably did not
cause early males to be cuckolded less than late males. The probability of
cuckoldry was unrelated to female age (i.e., 1 vs. >1 year old:
2 = 0.52, p > .45). Moreover, within all three
timing-of-breeding categories, females with EPY were, on average, older, not
younger, than females without EPY.
Finally, we examined effects of male age. Our objective in randomly
choosing a group of early settling males and forcing them to breed late was to
create a set of early and late-breeding males that did not differ in quality.
One potential indicator of male quality is age. By chance, the mean age of
early males (1.7 years; range: 1-4, 48% >1 year old), exceeded that of
forced late males (1.4 years; 1-3, 30% >1 year old). The mean age of
naturally late males was 1.2 year (range: 1-3, 10% >1 year old). That early
males were, on average, older probably cannot explain why early males were
cuckolded less than forced late males. The probability of cuckoldry was
unrelated to male age (i.e., 1 vs. >1 year old: 2 = 0.1,
p > .9). Moreover, within all three timing-of-breeding categories,
cuckolded males were, on average, older, not younger, than noncuckolded
males.
Frequency of neighboring males as extrapair sires
Extrapair sires were frequently males on immediately adjacent territories.
In the 32 instances where we detected extrapair young (EPY) in a nest, we
obtained DNA from all males on adjacent territories (30 cases, including all
nearest neighbors) or from all but one such male (2 cases). We could exclude
all typed neighbors as sires of EPY in 8 of 32 cases. In 15 cases, one or more
neighbors had a high probability of siring one or more EPY (probability of any
neighbor being falsely included as a sire was <.05, see Methods). In the
remaining 9 cases, one or more neighbors could not be excluded as a sire, but
our confidence that those neighbors being sires was less (probability of false
inclusion: .09-.35).
Of the 32 nearest neighbors typed, 9 (28%) had a high probability (as defined above) of siring EPY, and 19 such neighbors were completely excluded as sires. Nearest neighbors were the only neighbors for 4 of 32 pairs. Five of 28 (18%) second-nearest neighbors were likely fathers of EPY (19 were completely excluded), compared to 2 of 17 (12%) third-nearest neighbors (11 completely excluded). One focal pair with EPY had a fourth neighbor who was excluded as a sire.
Rapid mate switching
Rapid mate switching (RMS) is the replacement of a male by a new male
before the resident female's fertile period ends. RMS can create nests that
falsely appear to contain EPY (young sired by the first resident).
"Floating" male house wrens do occasionally challenge resident
males for their territories, and the resident female (if one is present)
sometimes remains on the territory after a successful takeover
(Johnson and Kermott, 1990).
We observed one case of RMS over 2 years and excluded that nest from analyses.
RMS could be ruled out on 29 of the 63 territories included in this study
because territories were occupied continuously by one marked male. We saw no
behavioral evidence of RMS on any of the remaining 34 territories.
Nevertheless, we categorized these nests according to whether RMS was
"unlikely" or "more likely." We considered RMS to be
unlikely when the initial male settling the territory had a single aluminum
leg band, received as a nestling. Only 10-20% of males on site in the 2 years
of study had single aluminum bands, so the probability of one single-band male
replacing another was low. We considered RMS to be more likely when the male
that initially settled the territory was unmarked. If we consider only nests
where RMS could either be ruled out or was unlikely, the proportion of early
pairs with EPY (39%, n = 18) remains significantly lower than the
proportion of forced late pairs with EPY (67%, n = 17;
2 = 4.3, p < .038). The latter, however, is not
significantly lower than the proportion of naturally late pairs with EPY
(100%, n = 5; 2 = 0.12, p > .7), but note
that the number of naturally late pairs in this analysis is now small.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The probability that a pair's nest would contain extrapair young was strongly associated with the timing of that pair's breeding cycle relative to the cycles of pairs on adjacent territories. Over 2 years, 26% of pairs whose cycles began either earlier than, or at approximately the same time as, cycles of pairs on all adjacent territories had extrapair young in their nests, compared to 74% of pairs whose cycles began later than cycles of pairs on one or more adjacent territories.
Males in early pairs may be cuckolded less than males in late pairs not because of any effect of timing of breeding, but because early males are of higher quality and are preferred as mates by both their own females and females in later-breeding pairs on adjacent territories. To control for possible effects of male quality, we removed the first mates of a randomly selected group of early-settling males, forcing these males to attract a new mate and ultimately nest late relative to their adjacent neighbors. Sixty-five percent of early-but-forced-late males in our study were cuckolded, a rate which was not significantly different from the rate of cuckoldry for naturally late males. We conclude that timing of breeding relative to neighbors has a stronger influence on female EPM activity than do any consistent differences between males who breed early and late relative to neighbors.
Our experimental design was such that early, naturally late, and forced late pairs did not differ substantially in mean number of neighboring males present during the female's fertile period or in mean distance to those neighbors, two factors that could influence levels of cuckoldry. We could not control for differences between females that paired with early and late males, such as age, experience, and physical condition. Because younger females tend to settle on territories later than older females, the mean age of females paired to early males was greater than that of females paired to forced late and naturally late males. However, the probability that a nest would contain EPY was unrelated to female age, and cuckolded males tended to have older mates than noncuckolded males within each treatment group. Although we can exclude female age as a factor, we cannot rule out the possibility that some other consistent difference between females paired with early and late males contributed to our results.
We suggest that the primary reason extrapair young occur more often in
nests of pairs breeding late relative to close neighbors is that females in
such pairs have more contact with potential extrapair partners. Female house
wrens do not appear to pursue extrapair matings off territory, nor do they
advertise their fertility and location to neighboring males. Rather, extrapair
matings result from males intruding into other males territories to seek
copulations (Johnson and Kermott,
1989). In our study, the majority of extrapair young were sired by
males on immediately adjacent territories. Nearest neighbors were most likely
to be sires, but the risk of being cuckolded by second and third nearest
neighbors was not insignificant. Although unmated males and males with fertile
mates may pursue extrapair matings to some degree, those males whose mates
have begun incubating and are no longer fertile should have the most time to
pursue extrapair matings (and are readily observed doing so), especially
because male house wrens do not incubate. Unlike early pairs, late pairs had,
by definition, one and often several male neighbors whose mates had begun
incubating well before their own mate's fertile period had ended.
Nests of late-breeding pairs may alternatively (or additionally) contain
extrapair young if females in pairs that are breeding later than neighboring
pairs are simply more receptive to extrapair mating attempts than females in
early breeding pairs. Females might alter receptivity according to some cue
that would indicate the lateness of their cycle relative to cycles of
neighboring pairs, such as the nature of song output by males on neighboring
territories (see Johnson and Kermott,
1991) or the rate at which males are intruding into the territory.
Females could also alter receptivity with some external cue that indicates
lateness in the season in general (e.g., day length). Either way, by altering
receptivity, females probably enhance their chances of securing high-quality
alleles for their offspring. Later-settling males are, on average, younger
than early-arriving males. Given that there is undoubtedly strong selection to
arrive on the breeding grounds early (e.g., this increases the probability of
securing a nest site, raising a second brood, and siring young in one's own
nest and neighbors' nests), later-settling males may also be of low quality,
regardless of age. If early-settling males typically pair quickly (which,
because of limited nest sites, is the norm), then most males who are still
unpaired at mid-season will be young, lower quality birds. Moreover, if males
primarily seek extrapair matings after their mate begins incubating, then most
males seeking extrapair copulations with a late-breeding female will have
begun nesting earlier than her own mate. Increased receptivity to extrapair
matings as the season progresses will most enhance a female's fitness in
species where the frequency of nest failure and mate loss for early males is
low. In our population, under natural conditions (i.e., no nestboxes present),
only about 30% of early nests fail, and males sometimes retain mates for
renesting (Johnson and Kermott,
1994).
In conclusion, our results provide experimental support for a long-standing
hypothesis that, under certain conditions, pairs that begin breeding earlier
than, or simultaneously with, nearby pairs will be less likely to have
extrapair young than pairs who begin breeding late relative to nearby pairs.
We note that the same may hold true not only in the many other species of
birds with territorial and mating systems similar to that of house wrens, but
also in other types of animals, including certain arthropods, rodents,
ungulates, and primates in which one male and one female reside together for
all or much of breeding periods in proximity to other pairs (e.g.,
Brotherton et al., 1997;
Eduard and Linsenmair, 1971;
Fietz et al., 2000;
Goossens et al., 1998;
Palombit, 1994;
Reichard, 1995;
Ribble, 1991;
Shellman-Reeve, 2001). We
encourage researchers who investigate the links among breeding synchrony,
extrapair mating activity, and male mating success to not just compare rates
of extrapair mating for "synchronous" and
"asynchronous" pairs but to explore the consequences of breeding
asynchronously early as opposed to asynchronously late.
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ACKNOWLEDGEMENTS |
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Sara Campbell, Steve Czapka, Joy McCall, Jennifer Leyhe, Erin O'Brien, Brian Morrison, Lori Erb, and Kathleen Ross provided assistance in the field and/or laboratory. Ken Schuster, the Bradford Brinton Memorial, the Northern Trust Company, and Beatrice Beuf gave permission to work on their property. Beatrice Beuf generously provided living accommodations and much moral support. Joel Snodgrass assisted with statistical analyses. Deb Buitron, Sheryl Soukup, Steve Yezerinac, David Westneat, and an anonymous reviewer provided comments on the manuscript, enhancing its clarity considerably. Financial support came from grants from the Graduate Students' Association, the College of Graduate Education and Research, and the Faculty Development and Research Committee, all of Towson University, and grants from the American Philosophical Society, the Sigma-Xi Scientific Research Society, and the National Science Foundation (IBN-9904823). To all we are grateful.
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REFERENCES |
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