Behavioral Ecology Vol. 10 No. 4: 444-451
© 1999 International Society for Behavioral Ecology
Male rank, female breeding synchrony, and patterns of paternity in the boat-tailed grackle
a Curriculum in Ecology and Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3280, USA b T.H. Morgan School of Biological Sciences, 101 Morgan Building, University of Kentucky, Lexington, KY 40506-0225, USA
Address correspondence to J.P. Poston, who is now at the T.H. Morgan School of Biological Sciences, 101 Morgan Building, University of Kentucky, Lexington, KY 40506-0225, USA. E-mail: jppost2{at}pop.uky.edu
Received 22 September 1998; revised 6 January 1999; accepted 6 January 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Species in which males directly defend groups of breeding females often have extreme skew in observed male mating success. In only a few species, however, has a corresponding skew in fertilization success been confirmed. Furthermore, the ecological and social factors contributing to variation in fertilization success need investigation. This study examined competition for mates and paternity in the boat-tailed grackle (Quiscalus major). Observations at colonies of nesting females revealed that the topranking or alpha males performed more than 70% of the copulations. DNA fingerprinting indicated that alpha males sired less than 40% of nestlings. Nevertheless, analysis of band-sharing scores among nestlings from different nests suggested that alpha males sired more than three times as many offspring as any other individual male. Because few nestlings were sired by the nonalpha males that associated with colonies, females must have mated with other males while on trips away from colonies. Analysis of paternity within broods revealed that at least half of all females had their brood fertilized by more than one male. Alpha males' success at fertilizing eggs did not vary with the number of simultaneously receptive females within a colony. Our results suggest that male and female behavior in female-defense polygyny results from complex coevolution of the sexes.
Key words: boat-tailed grackle, DNA fingerprinting, female-defense polygyny, female synchrony, Quiscalus major.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Species in which males defend females directly, rather than the resources females require, are characterized by intense competition among males for access to females and extreme sexual dimorphism in size and in the timing of maturation (Robinson, 1986
;
Webster, 1992). Typically in
these species a small percentage of males achieve most matings, and many males
fail to copulate during a breeding season. Highly skewed mating success among
males creates a large opportunity for selection on males and has presumably
favored males that delay breeding until they have achieved large size or high
social rank (Clutton-Brock et al.,
1982; Le Boeuf,
1974; Post, 1992;
Poston, 1997a).
Among species with female-defense polygyny, three aspects of females'
breeding biology permit males to defend groups of females during a season:
females aggregate during the breeding season, they frequently copulate within
these aggregations, and they have a prolonged breeding season
(Robinson, 1986;
Webster, 1994a). In short,
fertile females are spatially clumped and temporally dispersed. Emlen and
Oring (1977) proposed that
increased synchrony or decreased aggregation among females would limit males'
ability to defend and to mate with more than one female, and comparisons among
species have confirmed their predictions
(Clutton-Brock, 1989;
Robinson, 1986;
Webster, 1994b). However, few
studies have either confirmed that males defending female aggregations
actually sire most of the young (see
Pemberton et al., 1992;
Webster, 1995) or examined the
effects of synchrony or aggregation on variation in male mating success within
a population.
Several studies of species with a variety of mating patterns indicate that
a female's choice of a mate is not necessarily limited by male behavior. In
many species that form pair bonds, females engage in extrapair copulations
(EPCs), and EPCs result in extrapair fertilizations (EPFs) in a wide range of
species (reviewed by Birkhead and
Møller, 1992; Westneat
and Sherman, 1997). Because EPCs are often difficult to observe,
fertilization success of males can differ significantly from observed mating
success. For species in which males defend females directly, male
fertilization success is usually inferred from observations of copulations.
The high frequency of EPFs in other species suggests that any study of mating
success should combine observation of the behavior of males and females with
determination of parentage of young by molecular techniques
(Westneat and Webster, 1994).
In addition, in many female-defense polygynous species, the number of sexually
receptive females varies during the course of the prolonged breeding season.
Thus these species offer opportunities to examine the effect of synchrony
among females on variance in male reproductive success.
In this study we examined mating and fertilization success of male
boat-tailed grackles (Quiscalus major). Boat-tailed grackles nest in
colonies that range in size from 2 to more than 90 nests. Colonies are usually
in islands of emergent vegetation in marshes or in isolated trees, sites that
presumably offer protection from terrestrial predators. Female grackles build
nests and rear offspring unaided by males. Females nest asynchronously, so the
number of receptive females per day in a colony ranges widely
(Post, 1992;
Poston, 1997b). Females make
frequent trips away from colonies to forage and to gather nest material. Males
pursue alternative strategies for access to females: some males compete for
access to females in and near colonies ("colony males"), and other
males ("noncolony males") display to females at sites away from
colonies (Post, 1992). Colony
males form age-dependent dominance hierarchies that represent queues for
access to colonies (Post,
1992; Poston,
1997a). The top-ranked male is the alpha male, the second-ranked
male is the beta male, and so forth. The alpha male has unrestricted access to
the colony; he spends more time in the colony than does any other male, and he
interrupts other males' attempts to copulate within the colony
(Poston, 1997a,
b). As a result, alpha males
perform the majority of copulations in a colony
(Post, 1992; this study).
Non-alpha colony males display to females within 150 m of the colony, and they
venture into the colony when the alpha male is absent. Position in the queue
is thus correlated with access to breeding females. This study was designed to
determine the mating success of colony males, the conditions under which
colony males escort females on trips away from colonies, and the fertilization
success of males.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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General
We observed boat-tailed grackles at Magnolia Gardens near Charleston, South Carolina, during January-June, 1991-1993. The study site adjoined the area used for previous studies of this species by Post (1992
,
1994). The grackles nested on
islands of emergent vegetation in a 30-ha marsh (see
Post and Seals, 1991, and
Poston, 1997a, for more
details of this site). The study focused on colonies of grackles at seven
islands on the northeastern corner of the marsh. Two groups of colony males
competed for these seven colonies. Ten males competed for the 4 easternmost
colonies, and 12 males competed for the 3 westernmost colonies. Males
frequently interacted with members of their group but rarely interacted with
males from the other group. Ad libitum observations of agonistic interactions
between males revealed linear dominance hierarchies among colony males in each
group. The western group was further divided: two high-ranking males (alpha
and beta) defended one of the three islands, and three high-ranking males
(alpha, beta, and gamma) defended the other two islands. Lower-ranking males
in the western group moved between the three islands (see also
Poston, 1997a).
By, 1992, 86% of the adult males and 83% of the adult females had been
marked with unique combinations of colored aluminum bands. The grackles were
observed in their colonies from blinds on wooden platforms, from a canoe, and
from foot on the dike that surrounded the marsh. Each season, observations
rotated among three to four colonies. Midday attendance at colonies was low,
so observations were made for 3-4 h in the morning and 2-3 h in the evening.
We visited each colony at least twice per week. Frequencies of courtship
displays, copulations, and escorting behavior were based on ad libitum records
(Altman, 1974) of events during
observation sessions at colonies. A subset of observations based on focal
samples of males and colonies (Poston et al., unpublished data) revealed
similar patterns of mating success and consorts among males, so we used the
larger set of data based on ad libitum observations. In addition to the
results presented here, Poston investigated spatial relationships and queuing
among males (1997a) and mate
choice among females
(1997b).
Male mating success and escorts of females
To measure mating success of males at colonies, whenever a male performed a
courtship display to a female we recorded the location, identity, and behavior
of the male and the female. We observed 12 courtship displays that resulted in
copulation (the male mounted the female and made cloacal contact). We assumed
that an additional 110 displays resulted in copulation because the pair was
hidden from view for at least 10 s, and the male produced the high-pitched
vocalization (Post et al.,
1996; Selander and Giller,
1961) characteristic of all observed copulations but not heard in
other contexts (see also Post,
1992). To measure the incidence of escorting behavior by males,
each time we observed a male depart a colony within 5 s of a female and follow
her away from a colony, we recorded the participants, date, time and
location.
Synchrony of female receptivity
Previously, Post (1992)
found that the females in this population were receptive for 3 days, beginning
4 days before the first egg appeared in the nest. Therefore, we could
determine when each female was receptive by back-dating from the day she laid
her first egg. Female grackles lay one egg per day, and the modal clutch is
three eggs (76% of 1400 clutches; Post et
al., 1996). By checking each nest in each colony every 3-4 days,
we determined the number of receptive females per day for each colony.
DNA fingerprinting
The techniques for DNA fingerprinting followed those of Westneat
(1990,
1993). Blood was sampled from
the brachial vein of adults and nestlings and mixed with an equal volume of
TNE buffer (0.01 M Tris, 0.001 M NaCl, 0.002 M EDTA) and frozen at -20° to
-70°C for up to 3 years. DNA was extracted from blood with a modified
phenol-chloroform procedure (Quinn and
White, 1987; Westneat,
1993). Approximately 10-15 µg of DNA was incubated with an
excess of the restriction enzyme AluI. The amount of digested DNA was
measured in a spectrophotometer and about 6 µg was subjected to
electrophoresis in a 0.8% agarose gel at 33 volts for 48 h. After it was
denatured and neutralized (Westneat et
al., 1988), DNA was transferred to a nylon membrane with a vacuum
blotter. Each membrane was hybridized with a radioactively labeled probe and
exposed to film for 1 week. Membranes were stripped of the probe and then
reprobed twice for a total of three probings (probes: M13, M2.5 per,
Jeffreys 33.15; Jeffreys et al.,
1985a, b).
We organized gels with the alpha male in the center lane, nestlings in lanes on either side of the alpha, and other males that attended the colony in the outer lanes. All nestlings were within eight lanes of alpha males, and the majority (69%) were within four lanes. For each of 10 nestlings we also had a sample from the female that attended its nest. Each of these nestlings was placed in a lane between the female and the alpha male. Density of nests in the thick vegetation of colonies made association of other marked females with particular nests uncertain.
Scoring autoradiographs
With an acetate overlay, we marked bands and measured the distance each
band had migrated to the nearest 0.5 mm. Bands that had migrated within 0.5 mm
of one another were scored as shared. In lanes that hybridized weakly,
indicating less DNA, we scored all visible bands; in lanes hybridizing
strongly, indicating much more DNA, we only scored distinct bands (see also
Westneat, 1993).
We summed the results from all three probes to produce one coefficient of
band sharing for each pair of birds on each gel. The proportion of bands
shared between a pair of individuals is 2N/ (A+B),
where N is the number of bands in common and A and
B are the number of bands for each individual
(Wetton et al., 1987).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Male mating success and response to female synchrony
The majority of copulations (83 of 122; 74%) was performed by the three alpha males (Figure 1). These three males represented 14% of the 22 colony males. Beta males performed 14% of the observed copulations. The lowest-ranking male observed to copulate was ranked seventh in his hierarchy. Four additional copulations involved males that we did not see well enough to identify, and no copulations involved unbanded males.
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Although each alpha male spent the majority of his time at the colony (see
Post, 1992;
Poston, 1997a), each also
escorted females on trips away from the colony. There was a significant
positive correlation between the number of receptive females per day and the
likelihood that we observed the alpha male perform at least one escort that
day (logistic regression; 
2 = 6.99, df = 1, p
<.01). In addition, the number of trips per day on which an alpha male was
observed escorting a female was correlated with the number of receptive
females (Figure 2). However,
the slope of the relationship (± 95% confidence intervals; dotted lines
in Figure 2) was much less than
one. Consequently, as the number of simultaneously receptive females
increased, each female was less likely to be escorted by the alpha male.
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The number of different males observed performing a courtship display in a
colony per day was positively correlated with the number of receptive females
per day (Spearman's r =.253, n = 77, p <.05).
However, the correlation between the number of males observed to copulate in a
colony and the number of receptive females was not significant (Spearman's
r =.094, n = 49, p >.50), perhaps because alpha
males frequently interrupted the displays of other males (see
Poston, 1997b).
General fingerprinting results
We had scorable fingerprints from 184 individuals on 11 gels. The mean
(±SD) bands scored per individual, by probe, was: M13, 16.5±3.8;
M2.5 per, 13.6±4.1; 33.15, 17.9±4.8; sum of three
probes, 44.0±14.1. To estimate the background level of band sharing for
unrelated birds, we determined the proportion of bands shared between an adult
female and nestlings from nests other than that female's (n = 116,
mean = 0.19±0.07). We expected that band sharing among all adults could
be biased because we tested samples from many more males than females, and
males did not disperse (Post and Poston, unpublished data); thus some males
were possibly related to one another (although low band sharing between males,
0.20±0.06, does not support this view). Band-sharing scores among
females and nondescendent nestlings was slightly less than that among all
adults (mean = 0.20±0.06, n = 154). The results outlined below
do not differ if we use scores from adults as our estimate of background band
sharing instead of scores from females and nondescendant nestlings.
Distributions of band sharing between related birds (an adult female and her nestlings) and between unrelated birds overlapped (Figure 3). Coefficients of band sharing between unrelated birds averaged 0.19, and those between females and nestlings averaged 0.48 (Table 1).
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Paternity results
We had scorable samples from 109 nestlings from 74 broods (44 broods with 1
nestling sampled, 28 broods with 2 nestlings sampled, and 2 broods with 3
nestlings sampled). Three nestlings could not be assigned to a nest because
they were captured within colonies after they had fledged but before they
could fly well enough to leave the colony. These 109 nestling samples came
from four of the seven colonies for which we had observed behavior. These
colonies were controlled by two alpha males. Samples were collected during two
seasons.
We had samples from both putative parents for 10 offspring. For these 10,
we assigned parentage based on the presence of novel bands and the proportion
of bands shared (e.g., Westneat,
1993). Of the 10, 2 had few novel bands (1 and 3) and also had
high band-sharing scores with both putative parents (all > 0.40). The other
8 nestlings had several novel bands (8-21). Each had high band-sharing scores
with the female (0.38-0.58) and low band-sharing scores with the alpha males
(0.11-0.30). Thus for this small subset of nestlings, the alpha males sired
20% of nestlings.
The remainder of the nestlings had to be assigned a sire based on band-sharing proportions alone. We had to decide on a threshold for assigning parentage because the distribution of band-sharing values for mother-offspring dyads overlapped the distribution for unrelated dyads (Figure 3). We calculated that the value that balanced the chance of a false alarm (unrelated individuals thought to be related) and a missed detection (related individuals thought to be unrelated), based on the mean and SD of both distributions, was 0.334. Less than 2% of true parent-offspring dyads should have band-sharing scores that were below that value, and less than 2% of unrelated dyads should have scores that exceeded that value. With this criterion, the two alpha males sired 33 of the 99 nestlings assigned by band-sharing proportions alone.
Alpha males thus sired 32.1% of the total sample of nestlings, and they sired all the nestlings from 24.3% of the broods (Table 2). These percentages are much lower than the observed percentage of copulations by alpha males (see Figure 1).
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Fertilization success of nonalpha males
Of the 109 nestlings sampled, 74 were not sired by alpha males. Our data
indicated that few of these nestlings were sired by other colony males. Even
though we had DNA samples from many of the other males observed to copulate
within the colonies (Table 2),
only four nestlings were sired by another male that attended that colony. In
one colony at which 46 nestlings were sampled, the beta male and 3 other males
were not sampled, so our results could underestimate the success of colony
males. However, only 1 of 63 nestlings from colonies at which the beta males
were sampled was sired by them. The low success of nonalpha colony males
suggests that many if not nearly all offspring that were not sired by the
alpha male were sired by males not present in the colony.
After omitting nestlings sired by the alpha male, the mean proportion of
bands shared between nestlings from different nests (0. 21) was slightly
greater than the mean proportion of bands shared between unrelated birds
(0.19; Table 1). To evaluate
whether this slight difference was significant, we compared these means with a
randomization test (Manly,
1991) because dyads represent nonindependent data points
(Danforth et al., 1996). We
pooled the observed values and drew two random samples (without replacement)
from the pool (N1 = 116; N2 = 219). We
measured the mean for each sample and the difference between the means. This
procedure was repeated 10,000 times. The observed difference between unrelated
birds and nestlings from different nests was greater than the difference we
obtained in 9939 out of 10,000 simulations (p =.0061). Thus, the
proportion of bands shared between nestlings that were in different nests was
slightly but significantly higher than the proportion shared between unrelated
birds.
The difference between band sharing of unrelated dyads and that between nestlings at different nests could be affected by three situations: (1) relatedness among females, (2) relatedness among different sires, or (3) some individual (nonalpha) males siring nestlings in more than one nest. Options 1 and 2 seem unlikely, given that band sharing between females was 0.199 (SD = 0.072, n = 13) and that between males was 0.202 (SD = 0.064, n = 74).
Half-sibs should have an average band-sharing score of 0.39 (see below).
The observed value of 0.21 is clearly much below 0.39, so one nonalpha male
was not the sire of all unassigned offspring. To estimate the bounds on
success of noncolony males, we calculated the proportion of dyads of half sibs
that would be necessary to raise the proportion of bands shared between
non-nestmates from 0.19 to 0.21. We make the simplifying, assumption that the
distribution of band-sharing scores for nestlings from different nests is a
composite of dyads of unrelated birds and dyads of between-nest half-sibs.
This assumption will provide an upper limit estimate of the success of
nonalpha males. The mean proportion of bands shared between nestlings from
different nests (xt) is the sum of the mean for half sibs
(xh) and the mean for unrelated pairs
(xu), each multiplied by the proportion of dyads of each
type [nh/nt and
(nt - nh) / nt,
respectively]:
 | (1) |
Solving equation 1 for nh yields
 | (2) |
From above, nt = 219, xt = 0.21,
and xu = 0.19. To estimate xh we
applied a rearranged form of equation 8 from Reeve et al.
(1992), which provides an
estimate of relatedness within groups. If w is the mean band-sharing
proportion within groups and b is the mean band-sharing proportion
between groups, then w = r (1 - b) + b. In
the present case, r = 0.25 (half sibs) and b = 0.19. The
expected proportion of bands shared between half-sibs is thus w (or
xh) = 0.39. Entering these values into equation 2 gives
nh = 21.9 (95% confidence intervals = 0-41.7) dyads. This
means that out of 219 cross-brood dyads, about 22 dyads were half sibs. The
number of nestlings that these 22 dyads represent depends on the number of
sires involved. If we assume that each dyad of half-sibs was sired by a
different male, then the 22 dyads would represent 44 of the 74 nestlings not
sired by an alpha male. Each male would have sired at most 2 offspring (1.9%
of all nestlings), and 44 (95% CI = 0-74) of the nestlings sired by other
males would be cross-brood half sibs. If at the other extreme we assume that
all half-sib dyads were sired by a single male (an unlikely assumption, but
one which provides an upper bound on the success of individual noncolony
males), then the 22 dyads of cross-brood half-sibs would represent at most 8
(95% CI = 0-10) nestlings. This one male would have sired 7.3% (95% CI =
0-9.2) of all nestlings. Thus, the maximum estimate of fertilization success
of an unsampled colony or a noncolony male (9.2%) is less than a third of the
measured fertilization success of alpha males (32.1%). Note that this analysis
included nests from four different colonies. Although additional nearby
colonies that were not sampled could contain young sired by the same male(s)
siring offspring in our sample, it is unlikely that one nonalpha male gained
as much success as most alpha males.
Mixed paternity within broods
To estimate the proportion of females that mated with more than one male,
we determined the proportion of clutches with mixed paternity. For this
analysis we included nests with more than one nestling for which the sire of
at least one nestling had been identified. Sixteen nests fit these criteria,
and from each of these nests we had sampled two nestlings. In 8 of the 16
nests the nestlings did not have the same sire. Thus, at least 50% of these 16
females had mated with more than one male.
Factors influencing paternity by alpha males
The number of receptive females per day within these colonies ranged from 0
to 16 over the course of a breeding season (see also
Poston, 1997b). There was no
significant correlation between the number of receptive females and the
likelihood that a nestling or an entire brood was sired by the alpha male
(Table 3). Similarly, there was
no significant effect of date in the season
(Table 3).
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The preceding analysis could be confounded by several factors, such as differences between the individual males competing for a colony, physical aspects of the colony and its surroundings, and environmental factors that change from one season to the next. The proportion of young sired by each of the two alpha males did not differ (nestlings: 33.3% and 30.4%, Fisher's Exact test, p =.84; broods: 24.4% and 24.1%, p = 1.0). Similarly, the percentage of young sired by the alpha male did not vary among colonies (nestlings: G test, G = 0.97, df = 3, p =.81; broods: G = 0.61, df = 3, p =.90). Finally, the percentage of young sired by the alpha male did not vary with the year in which the samples were collected (nestlings: 29.6% and 32.9%, Fisher's Exact test, p =.82; broods: 20.0% and 25.9%, p =.76). Although based on small samples, these results suggest that fertilization success was the same for these two alpha males and that effects of colony site or year did not influence paternity within this study.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Boat-tailed grackles differ from most birds in having a social mating system of female-defense polygyny. The most conspicuous feature of this system is the competition between males over groups of nesting females, in which the top-ranking males perform 74% of the copulations (see also Post, 1992
). Just as in many
socially monogamous birds (e.g., Westneat
and Sherman, 1997), we have found that the actual pattern of
fertilizations deviated from the observed mating relationships. Despite the
high proportion of matings by alpha males in colonies, alphas sired only 32%
of the nestlings. Only two other studies have measured paternity with DNA
fingerprinting in species with similar mating systems. Webster
(1995) found a similar pattern
of mating and fertilization success in Montezuma's oropendola (Psarocolius
montezuma): alpha males performed at least 90% of the copulations within
nesting colonies, but they sired only 7 of 21 nestlings (33%). In a study of
red deer (Cervus elaphus) by Pemberton et al.
(1992), alpha (or
harem-holding) males sired all or nearly all offspring. In oropendolas and
grackles, observations of mating success overestimate variance in male
fertilization success, whereas in red deer algorithms for assigning paternity
based on gestation length and harem membership of females underestimate
variance in male mating success.
One or more of three possibilities might explain the surprisingly low
success of alpha male grackles. First, alpha males could have transferred
fewer sperm per copulation than males that copulated away from colonies.
Passerines produce sperm during a short period each night (citations in
Birkhead et al., 1994), and
Birkhead et al. (1994)
calculated that male house sparrows (Passer domesticus) use nearly
all their available sperm each day. Males in highly polygynous species could
be particularly prone to sperm limitation. For example, in a captive group of
the harem-polygynous hamadryas baboons (Papio hamadryas), females
that came into estrus synchronously were less likely to conceive than females
that came into estrus asynchronously
(Zinner et al., 1994). We did
not find an effect of synchrony on paternity in this study. However, alpha
males sometimes refuse to mount females that give solicitation displays
(Poston, unpublished data). Male refusals to copulate might indicate low sperm
reserves (Hatchwell and Davies,
1992; Hunter et al.,
1993). Our data do not allow us to test whether alpha male
grackles refuse copulations more than other males or whether rates of refusal
are linked to synchrony.
Second, paternity by alpha males could be limited by their ability to guard females away from colonies. Alpha males occasionally follow females off the colony, and they interrupt other males' courtship displays on such trips. We would expect synchrony among females to affect alpha males' success in guarding them, and indeed we found that when several females were receptive simultaneously each female was less likely to be escorted by the alpha male than when only one female was receptive. However, the lack of a correlation between synchrony and paternity suggests that alpha males have relatively low success in preventing other males from copulating even during periods of asynchrony. We hypothesize (but as yet cannot test) that male mate-guarding is limited by the combination of females' pursuit of matings with other males and conflicting demands on alpha males. An alpha male must balance guarding females away from his colony with defending his colony from other males in the queue. Thus, although alpha males probably have the most access to females and consequently sire the most offspring, they cannot monopolize matings.
Third, females could make it difficult for males to guard them. Females often leave colonies, and some trips might be attempts to mate with other males. At least 8 of 16 females we tested in this study had mated with more than one male. We have limited information on female behavior or potential benefits of multiple mating, and we cannot reject the possibility that females are forced into mating with other males, although all copulations we observed involved the cooperation of the female.
Our results raise some interesting questions about male and female
reproductive tactics. Because we have more data on male grackles, we will
focus on males. Post (1992)
observed that male grackles adopt alternative strategies of competing for
colonies or displaying to females away from colonies. Individual males do not
switch between strategies (Post,
1992; Poston,
1997a). In the present study, we documented that many nestlings
were sired by males away from colonies, but that no one male sired more young
than an alpha male. One alpha male in our study maintained his status at least
4 years and defended at least two colonies per year. At the colonies we
observed, there were an average of 37 nests. Assuming that 60% of nests fledge
one young (Post et al., 1996)
and alpha males sire 32% of the nestlings, then an alpha male would sire more
than 50 fledglings during a 4-year tenure.
Such success appears to come only after a long wait. A striking result of
our study was the rarity of paternity by beta and lower-ranking colony males
in the queue. Young males apparently face a decision between joining a queue
for a colony and not reproducing until they reach alpha status at 6 or more
years of age (see Poston,
1997a) or displaying to females away from colonies and perhaps
reproducing at an earlier age but at a lower rate. These alternatives are
similar to those faced by individuals in species with other social systems
that form queues for access to reproductive opportunities
(Apollonio et al., 1992;
Ens et al., 1995;
Wiley, 1981;
Wiley and Rabenold, 1984).
Theory predicts that the proportion of individuals adopting each strategy
should produce equal expected payoffs for the two alternatives
(Fretwell, 1972). In the case
of grackles, we do not know what proportion of individuals choose to queue or
to display away from colonies, so we cannot test Fretwell's prediction. The
high reproductive success of alpha males indicates that many males do not
reproduce at all during their lives, either because they unsuccessfully
attempt to mate with females away from colonies or because they die in the
queue for a colony before they reach alpha status (see
Post, 1992;
Poston, 1997a).
We know little about the behavior of noncolony males. The genetic results
reveal that this gap in our knowledge is much more important than we would
have thought, and filling that gap could reveal new patterns of male
competition and fertilization success. For example, males off the colony could
be displaying to females in a mating congregation similar to the leks
razorbills (Alca torda) form away from their breeding sites
(Wagner, 1991). However, the
behavior of females and the patterns of paternity we uncovered suggest this
behavior is unlikely. Females leave a colony in a variety of directions
(Poston, unpublished data). In addition, we found no evidence that any single
noncolony male had sired large numbers of young. Neither of these patterns fit
the expectation that males are congregating to display to females in a
traditional site with no resources and with considerable skew in male success.
Further research on noncolony males is needed to estimate mating skew among
noncolony males and to establish the patterns of male competition for access
to mates. Our results best fit the possibility that males away from colonies
engage in scramble competition for access to fertilizable females foraging or
collecting nesting material at a variety of sites.
This study demonstrates that even when males compete intensely for matings,
females have opportunities to mate with several males. Indeed, whether females
actively seek copulations from different males or simply set the conditions
under which they copulate, their behavior has important consequences for the
success of male competition for mates
(Poston, 1997b;
Wiley and Poston, 1996).
Previous studies have often overlooked opportunities for females to influence
paternity in species in which males compete for access to mates
(Ahnesjö et
al., 1993; Wiley and Poston,
1996). Because of the mix of female and male behavior in
boat-tailed grackles, their mating system is much more complex than the easily
observed female-defense polygyny. The present study also emphasizes the
growing need for sophistication in mating system theory. These grackles
confirm the predictions of Emlen and Oring
(1977) that clustered females
and low male parental care favor female defense. Yet female mating behavior
clearly limits males' ability to monopolize females. Recent theory (e.g.,
Davies, 1991;
Reynolds, 1996;
Wiley and Poston, 1996) has
adopted a more balanced view of the ways in which each sex influences
selection on the other, and thus of the coevolution of female and male
behavior.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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We thank Aaron Milleson and Allison Poston for their help with the field work and Richard Hanschu, Herman Mays, Birch Rambo, and Tammy Roush for help with the DNA analyses. Darrel Stafford and Jon McMurray provided lab facilities for DNA extractions at UNC-CH. The manuscript was improved by the comments of L. Oring and an anonymous reviewer. Drayton Hastie kindly permitted us to conduct the work at Magnolia Gardens, South Carolina. The work was supported by the Behavioral Research Fund of the University of North Carolina at Chapel Hill, the Charleston Museum, the Frank Chapman fund of the American Museum of Natural History, and Sigma Xi Grants-in-Aid of Research. D.F.W. was supported by grants from the National Science Foundation and the University of Kentucky.
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