Behavioral Ecology Advance Access originally published online on April 13, 2005
Behavioral Ecology 2005 16(4):747-754; doi:10.1093/beheco/ari048
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Manipulation of male quality in wild tits: effects on paternity loss
a Program for Experimental Behavioral and Population Ecological Research (EBE), Department of Biology, University of Oslo, P.O. Box 1066, Blindern, N-0316 Oslo, Norway, and b Zoological Museum, Natural History Museums and Botanical Garden, University of Oslo, Norway
Address correspondence to L.E. Johannessen. E-mail: l.e.johannessen{at}bio.uio.no.
Received 28 July 2004; revised 3 March 2005; accepted 15 March 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Extrapair paternity (EPP) has proved to be widespread and highly variable among birds and other taxa, including socially monogamous species. A multitude of hypotheses have been put forward to explain this variation, but its occurrence is not fully understood. Male age, social dominance rank, song and breeding density or synchrony have been among the suggested correlates of EPP, but results so far are inconclusive. We interspecifically cross-fostered blue tits (Parus caeruleus) and great tits (Parus major) in the wild, thus manipulating males to exhibit reduced social dominance rank, sing aberrant songs, and consequently be perceived as low-quality males as compared to controls. This allowed us to test if male quality had an influence on loss of paternity. We found no statistically significant differences between cross-fostered and control males of either species, neither with respect to levels of cuckoldry nor proportions of extrapair young (EPY) in the broods. Paternity levels were comparable to other studies on the same species. No effect of density could be detected on levels of EPP either, while an age effect seemed to be present at least in the blue tit, EPY being almost absent in broods of older blue tit males. We conclude that the effects of male quality on paternity loss are minor, if any, in these populations.
Key words: cross-fostering, early learning, extrapair paternity, microsatellite, sexual imprinting, social rearing conditions.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Extrapair paternity (EPP) has proved to be widespread among bird species (Griffith et al., 2002
; Jennions and Petrie, 2000; Westneat and Stewart, 2003). Considerable variation in the levels of EPP has been documented among species, populations, and even individuals, and numerous hypotheses have been put forward to explain the variation. Earlier studies have indicated a host of male correlates of EPP levels in birds, including male age (Chuang-Dobbs et al., 2001; Delhey et al., 2003; Dickinson, 2001; Hasselquist et al., 1996; Kempenaers et al., 1997; Krokene et al., 1998; Richardson and Burke, 1999; Verboven and Mateman, 1997), song (Forstmeier et al., 2002; Hasselquist et al., 1996; Kempenaers et al., 1997; Martin-Vivaldi et al., 2002; Otter et al., 2001; Slagsvold et al., 2001), social status (Double and Cockburn, 2003; Otter et al., 1994, 1998; Poston et al., 1999), and breeding density or synchrony (Johnson et al., 2002; Rätti et al., 2001; Thusius et al., 2001; Westneat and Sherman, 1997). However, results are equivocal, often with opposite effects found of the same factors in different studies, and the literature does not allow for any firm conclusions to be drawn.
One way to proceed may be to manipulate components of male quality (Burley et al., 1982; Ellegren et al., 1995; Johnsen et al., 2000; Otter et al., 2001). We have done so by interspecific cross-fostering as it has been proved in several studies that cross-fostering of birds alters the behavior and fitness of the cross-fostered individuals. Several effects have been found, all of which have been documented in our two study species, the blue tit (Parus caeruleus) and the great tit (P. major). These include reduced social dominance rank (Hansen and Slagsvold, 2004), increased levels of (interspecific) aggression (Hansen and Slagsvold, 2003), changes in song characteristics (Grant BR and Grant PR, 1996; Payne et al., 1998; ten Cate et al., 1993), and problems to obtain (conspecific) mates (Slagsvold and Hansen, 2001; Slagsvold et al., 2002).
Based on the altered behavior of cross-fostered individuals, we may predict that they will be perceived by conspecifics as being of lower quality than normally raised individuals and that females paired to such males consequently might be more prone to seek extrapair copulations (EPCs). We have thus asked whether cross-fostered males suffer greater loss of paternity than controls. We first investigate the general levels of paternity loss in our study populations of blue tits and great tits, and then compare the paternity loss of cross-fostered males to these levels. We predict that broods of cross-fostered males more often will contain extrapair young (EPY) than broods of control males and/or that the proportions of EPY will be higher in broods of the former.
Older males seem to lose less paternity than younger males (Dickinson, 2001; Richardson and Burke, 1999; Verboven and Mateman, 1997), and breeding density has also been found to influence paternity levels (Foerster et al., 2003; Johnson et al., 2002; Rätti et al., 2001; Thusius et al., 2001; Westneat and Sherman, 1997). We therefore also consider male age and distance to nearest conspecific neighbor (the latter as a proxy to breeding density) in our analyses.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study site and species
Data were collected in a 1.6-km2 nest-box plot located outside Oslo, Norway (59° 56' N, 10° 33' E), during 1999–2003. Throughout the predominantly deciduous forest, about 400 nest-boxes have been provided. Each year, approximately 70–80 great tit and 80–100 blue tit pairs nest in these nest-boxes, along with similar numbers of pied flycatchers (Ficedula hypoleuca) and a few nuthatches (Sitta europaea) and coal tits (Parus ater). Boxes and territories were checked on a regular basis, at least twice a week, from early spring to the end of the nestling period, to ensure correct identification of owners and to record breeding success.
Treatment groups
In the study site, interspecific cross-fostering has been carried out between several passerine species since 1995 under license from the Directorate for Nature Management and from the National Animal Research Authority in Norway (for further details on the cross-fostering procedure see Slagsvold et al., 2002). The present study makes use of a subset of these individuals, focusing on blue tit and great tit males raised in the study area with either conspecific (control) or heterospecific (cross-fostered) parents of the two species. Some of the latter grew up with conspecific siblings only in the brood, while others had one or two heterospecific siblings, that is, of the same species as the foster parents. However, no significant differences were found between these two groups with respect to the proportion of cuckolded broods (Fisher's Exact tests) or proportion of EPY in broods (Mann-Whitney U tests) in either species (four tests, all p > .20), and they were thus combined in all analyses. This study includes families where the male belongs to one of the focal groups mentioned above and from which blood samples were obtained of nestlings and at least the male parent (blue tit: 19 controls, 28 cross-fostered; great tit: 15 controls, 10 cross-fostered). The actual number of males included in the various analyses will, however, vary depending on the selection criteria for the particular tests. A small proportion of the males had mates that were themselves cross-fostered (blue tit control 2/19, cross-fostered 5/28; great tit control 0/15, cross-fostered 2/10), but exclusion of these did not alter the results qualitatively.
In an attempt to characterize the behavior of the cross-fostered males included in this study of paternity, we looked at data on various behavioral aspects collected as part of other studies on the same populations. These included data on social dominance rank (Hansen and Slagsvold, 2004), song (Johannessen LE, Slagsvold T, Hansen BT, unpublished data), and aggression towards intruders of both species at the nest (Hansen and Slagsvold, 2003). Dominance data were collected by video filming of interactions with other tits on feeding tables during preceding winters (see Hansen and Slagsvold, 2004, for details). Basically, a score was calculated as the proportion won of all interactions seen with individuals of the same species and sex (for blue tits the sexes were pooled due to difficulties in sexing of unbanded individuals on video). Song was recorded in the field during the breeding season, and the composition of individual repertoires and temporal and frequency qualities of songs were subsequently digitally analyzed. Aggression was assessed by presenting caged intruders (a great tit and a blue tit, in randomized order) close to the nest-box during late nest building or egg laying and by noting the proportion of time spent within 2 m of the cage (see Hansen and Slagsvold, 2003, for details). A score could then be calculated expressing the proportional response to heterospecifics relative to total response (identical response to the two intruders thus gave a score of 0.5, while response only to the heterospecific intruder gave 1.0). As meaningful statistical testing of these data was difficult due to small samples, we present a summary of all the analyses, highlighting the results that could possibly shed light on the behavior of the cross-fostered males included in the paternity analyses.
Paternity analysis
Blood samples (10–50 µl) were collected in capillary tubes after puncture of the brachial vein and suspended in 1 ml of Queen's lysis buffer. In the laboratory, 200 µl of the buffer solution was used for DNA extraction with a QIAamp DNA Blood Mini Kit (Qiagen, Venlo, The Netherlands) according to the provided blood and body fluid protocol. For a few dead nestlings, DNA was extracted from tissue samples, using a QIAamp DNA Mini Kit (Qiagen) and following the provided tissue protocol. Further analyses of extracted DNA were identical for DNA extracted from blood and tissue samples.
Six microsatellite markers were used for each of the species to resolve the paternity of 339 blue tit nestlings from 47 broods and 153 great tit nestlings from 25 broods. Pat-MP2-43 (Otter et al., 1998), Phtr3 (Fridolfsson et al., 1997), PK12 (Tanner SM, Richner H, and Schuenperli D, unpublished data; EMBL accession no. AF041466), and POCC6 (Bensch et al., 1997) were used for both species, Pca8 and Pca9 (Dawson et al., 2000) for blue tits only, and Pdoµ5 (Griffith et al., 1999) and Ppi2 (Martinez et al., 1999) for great tits only.
A 10.0-µl reaction mix consisting of 1.0 µl of genomic DNA, 0.2 µl of Dynazyme II DNA Polymerase (2.0 U/µl; Finnzymes), 0.3 µl deoxynucleoside triphosphate (20 mM), 0.5 µl each of reverse and fluorescently labeled forward primers (10 mM), 1.0 µl 10x buffer (10 mM Tris-HCl, 1.5 mM MgCl2, 50 mM KCl, 0.1% Triton X-100), and 6.5 µl dH2O was prepared. The samples were subjected to a standard polymerase chain reaction (PCR) protocol in thermal cyclers (GeneAmp PCR System 9700, Applied Biosystems [Foster City, CA] or PTC-100 Thermal Cycler, MJ Research, Inc. [Watertown, MA]). The cycles included an initial denaturizing step at 94°C for 5 min, followed by 22–35 cycles of 94°C for 30 s, annealing temperature (see Table 1) for 30 s and 72°C for 30 s, and ended with an elongation step at 72°C for 7 min. PCR products were run on an ABI PRISM 3100 Genetic Analyzer (Applied Biosystems). Data on fragment lengths were collected with Genescan 3.7 and analyzed in GeneMapper 3.0.
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A summary of characteristics for the microsatellite markers employed are given in Table 1. The combined probabilities of exclusion of >0.99 for both species indicate that the chosen suites of six loci should be sufficient to resolve paternity. A nestling was classified as EPY if it did not match the genotype of the social male at two or more loci. Single-locus mismatches were interpreted as mutations or mistyping, and such nestlings were conservatively classified as withinpair young. Nestlings showing multiple mismatches with the social female were considered as resulting from intraspecific brood parasitism.
We are unfortunately not able to identify the extrapair (EP) sires of the EPY as we have not performed any total blood sampling of all candidate males in our populations. However, to investigate the possibility that the cross-fostered males sired offspring outside their own nests by cuckolding other males, we checked whether any of the cross-fostered males could be the sire of any of all conspecific EPY genotyped throughout the project (using the software Cervus 2.0). For a male to be regarded as the likely EP sire of a particular EPY, he was required to be present in the study area, in his second calendar year or later, in the year when the EPY hatched, and the two genotypes were to match completely at all six loci.
Statistical analyses
Occurrence of EPP was analyzed with contingency tables, using a 
2 adjusted for small sample sizes (V2, as implemented in the software Statistica 6.1 for Windows). Proportions of EPY in own brood (i.e., paternity loss) were analyzed using generalized linear models (GLMs) with binomial errors and logit links (as implemented in the software GLMStat for Macintosh). Number of EPY was used as the response variable and total number of young genotyped as the binomial denominator.
Due to similar proportions of second-year males among the two treatment groups within the species, age is not likely to confound our analyses (blue tit control 74% second year, cross-fostered 86%, 
p = .31; great tit control 87%, cross-fostered 60%, 
p = .13). However, as evidence is accumulating that older males tend to lose less paternity, a possible age effect was tested for, on two slightly different data sets. First, the same analyses as described above were extended by adding male age (second year or older) as a covariable in the GLM analyses (the interaction term was never statistically significant and is thus not reported), or as an extra grouping level in the contingency tables. However, especially the blue tit sample was heavily biased towards young males, and lack of paternity loss among the few older males prevented efficient statistical testing of the estimates. Therefore, for all males of which we had data for two or more breeding seasons, we replaced the data of their first-recorded breeding attempt with data of their last-recorded breeding attempt. We thus obtained a better balance between younger and older males in the samples, especially for blue tits, without altering the total sample sizes.
Further, we also made use of the repeated samplings of a subset of males by performing pairwise tests within individual males as such analyses may provide interesting results despite small samples. Due to the limited number of males available for these analyses, we analyzed both treatment groups within each species together. It should be noted that only one data point was used for each male in any one analysis, except of course for the pairwise testing within those males for which we had repeated samplings.
Distance to nearest conspecific neighbor was analyzed with t tests and linear regressions on the log-transformed distance variable as tests of normality (Shapiro-Wilk W test) and homogeneity of variances (Levene test) indicated that the variable met the criteria of parametric analyses (normality, six tests, all p > .12; homogeneity of variances, six tests, all p > .21).
Means ± SD are reported, all tests are two-tailed, and a significance level of 0.05 has been applied throughout.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Blue tit
Of blue tit control broods, 29% (4/14) contained at least one EPY (0–3 EPY per brood; Table 2). Of all young from control broods, 7% (6/88) were EPY, while 21% (6/28) of young from cuckolded control broods were EPY. In all cases a single male may have sired all EPY (1–2 nonpair alleles detected). In two families we found a nestling having multiple mismatches with both parents. These were regarded as resulting from intraspecific brood parasitism.
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Of broods belonging to cross-fostered blue tit males, 46% (11/24) contained at least one EPY (1–7 EPY per brood). The proportions of EPY were 16% (29/177) and 36% (29/81) for all broods and cuckolded broods, respectively. In 9 of 11 cases one male may have sired all EPY, while in the remaining two broods at least two EP males must have been involved as there were more than two paternal alleles present at individual loci.
Comparing cross-fostered blue tit males to controls, we neither found a significant difference in the proportion of broods cuckolded (
p = .30) nor in the proportion of EPY in all broods (GLM, odds ratio = 2.68, 95% confidence interval [CI] = [0.56;12.78], F = 1.74, df = 1, p = .20) or in cuckolded broods only (GLM, odds ratio = 2.05, 95% CI = [0.35;12.04], F = 0.67, df = 1, p = .43).
Great tit
Of great tit control broods, 31% (4/13) contained at least one EPY (0–3 EPY per brood; Table 2). Of all young from control broods, 9% (7/82) were EPY, while 32% (7/22) of young from cuckolded control broods were EPY. Also, here a single male may have sired all EPY.
Of broods belonging to cross-fostered great tit males, 17% (1/6) contained EPY, and in that single brood two or more EP males had sired all the nestlings. The proportion of EPY in all broods was 15% (5/33).
Comparing cross-fostered great tit males to controls, we neither found significant difference in the proportion of broods cuckolded (
p = .53) nor in the proportion of EPY in all broods (GLM, odds ratio = 1.91, 95% CI = [0.20;17.92], F = 0.31, df = 1, p = .58) or in cuckolded broods only (GLM, odds ratio = 68,000, 95% CI = [0.00; 
], F = 3.47, df = 1, p = .16).
Identity of EP sires
Of blue tits, 15 control and 23 cross-fostered males were checked as possible EP sires of a total of 68 EPY identified among 545 offspring from 80 nests. None of the control males were identified as sires of any of the EPY, while two of the cross-fostered males most likely had sired one and three offspring, respectively, in one nest each. In both cases, the putative EP sire was a close neighbor, the females receiving the EPCs were themselves not cross-fostered, and the females of the cuckolder and the cuckoldee commenced egg laying at the same time. The proportion of males identified as EP sires did not differ statistically between control and cross-fostered males (Fisher's Exact p = .51).
Of great tits, 13 control and 6 cross-fostered males were checked as possible EP sires of a total of 27 EPY identified among 208 offspring from 37 nests. Two of the control males had most likely sired one and two offspring, respectively, in one nest each. Again, the EP sires were close neighbors of the cuckolded males, the females receiving the EPCs were not cross-fostered, and in one case the females commenced egg laying almost simultaneously. In the other case, the cuckolder was present in his territory at the time when the female of the cuckoldee started egg laying, even though his female did not commence egg laying until about 3 weeks later. Of the cross-fostered males, none could be identified as EP sires. The proportion of males identified as EP sires did not differ statistically between control and cross-fostered males (Fisher's Exact p = .54).
Behavior of the cross-fostered males
For 13 of the 24 blue tit and two of the six great tit cross-fostered males in this study, social dominance rank in winter could be evaluated. Dominance scores obtained by these blue tits (0.33 ± 0.36) were similar to those of other cross-fostered males (0.33 ± 0.40; z = 0.21, p = 1.0) but tended to be lower than those of controls (0.67 ± 0.42, z = 2.01, p = .13). Of the two great tits, one was dominant to all other individuals he was seen interacting with. The other had a dominance rank similar to other cross-fostered great tits, that is, subdominant to conspecific controls.
Results of the song analyses are to be published elsewhere (Johannessen LE, Slagsvold T, Hansen BT, unpublished data). In short, the song of the cross-fostered males was found to be altered so as to be more similar to that of their foster species, compared to the song of control males. Both repertoire composition and temporal and frequency qualities of songs were affected. The effects were most pronounced in the great tits, but similar patterns could be seen also in the blue tits. There were no tendencies for the song of the cross-fostered males included in the present study to be less, or more, affected than that of other cross-fostered males for any of the measured variables (details not shown).
Aggression towards caged intruders was tested in the breeding season. Of the cross-fostered great tits, two males did not approach the cage in any of the two trials, even though they clearly had noted the intruder on both occasions. One male responded only to the conspecific intruder, while the remaining three males responded as much to the heterospecific blue tit as to the conspecific great tit intruder. Other cross-fostered great tit males generally responded more to blue tit than to great tit intruders, while the opposite was true for controls. For the blue tits, data on aggression existed for 23 of the 24 cross-fostered males. These males responded about as much to heterospecific as to conspecific intruders (score 0.51 ± 0.20). Their response was significantly different from that of controls (0.15 ± 0.19, z = 4.94, p < .0001) but not from that of other cross-fostered males (0.41 ± 0.25, z = 1.33, p = .55).
Distance to nearest conspecific neighbor
In both species, cross-fostered males had on average neighbors closer to their nest-box than control males. This difference was very near statistical significance in great tits (67 versus 90 m; t = 2.07, df = 17, p = .05) but not in blue tits (66 versus 78 m; t = 0.54, df = 36, p = .59). However, the distance to nearest neighbor did not differ for cuckolded and noncuckolded males, neither when all individuals of one species were analyzed together nor when they were split with respect to treatment groups (six tests, all |t| < 1.28, all p > .23; details not shown).
Regression analyses of the proportion of EPY in the brood as a function of distance to nearest neighbor did not reveal any consistent pattern either; however, a counterintuitive tendency of a positive correlation existed for cross-fostered blue tits (R2 = .13, p = .09; all other p > .28, six tests).
Age effect
Analyses of the original data set showed a tendency for young cross-fostered blue tits to be cuckolded more often than older ones (
p = .10), and the same was true when all blue tits were analyzed together (
p = .09). For all other comparisons (four tests; within treatment groups and for all individuals of one species together), 
and p > .16. When the original data set was balanced by swapping the first-recorded breeding attempt with the last for all multiply-sampled males (see Methods; Table 3), the trend for cross-fostered blue tits remained similar (
p = .11) while that for all blue tits together weakened considerably (
p = .20). A similar trend now appeared also for control great tits (
p = .08), while for the remaining five comparisons, 
and p > .32.
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Inclusion of male age in the GLM analyses indicated that older males tend to have a lower proportion of EPY in their broods than second-year males. The estimated age effect was the same for both data sets in blue tits (GLM, F = 3.41, df = 1, p = .07), while in the great tit the tendency was present only in the original data set (GLM, original, F = 3.07, df = 1, p = .10; balanced, F = 1.10, df = 1, p = .31).
Directions of change in the paternity levels for the subset of males sampled over two breeding seasons are presented in Table 4. No consistent pattern seemed to exist with respect to risk of cuckoldry or proportion of EPY in one year, given the situation in the preceding year. Statistical testing did not reveal any significant effects in any of the species (blue tit, risk of cuckoldry, Fisher's Exact p = 1.0; proportion of EPY, Spearman rank correlation, rs = .47, p = .21; great tit, risk of cuckoldry p = 1.0; proportion EPY rs = .10, p = .84). However, given the small sample sizes, one should be careful regarding concluding on these results.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Contrary to expectation, cross-fostered blue tit and great tit males did not suffer significantly more from cuckoldry than did controls. A nonsignificant trend could be seen in blue tits, as cross-fostered males experienced a 50% increase in risk of losing paternity in own brood as compared to controls, and a similar difference was present in the proportion of EPY nestlings. For the great tits, no such tendencies could be found. It should, however, be mentioned that three males (two blue tits and one great tit), which lost all their paternity, made a disproportionately large impact on the figures. Removal of these males weakened, though not eliminating, the tendency for cross-fostered blue tits to be slightly more prone to cuckoldry than controls, but the five remaining great tits actually avoided cuckoldry completely. However, the small sample size of cross-fostered great tit males might still prevent the detection of any real effects in this material. This is reflected in the wide CIs of the odds ratios and the huge odds ratio for the test of cuckolded broods only. The small number of cross-fostered great tit males in the sample is due to their heavily misimprinted sexual preferences, resulting in most of them ending up in heterospecific pairs with blue tit females (36%; see Slagsvold et al., 2002
) or as bachelors (49%). However, all such males that paired up conspecifically throughout the whole project period (1999–2003) have been included here, and the sample size is thus as large as possible. Our search for the EP sires of all EPY identified throughout the project among all sampled males did not reveal any statistically significant differences in EPP gain among the treatment groups in either species. It is, however, necessary to consider the limited extent of these analyses as only a fraction of the families and candidate males were sampled each year. Still, the results do not indicate any large discrepancies in potential for EPP gain among the treatment groups.
Control levels of EPP found in our study populations are within the ranges found for other populations of blue tits and great tits, even though they are among the lower estimates recorded for the former: blue tits 29–68% of broods/6–25% of young (Charmantier et al., 2004; Delhey et al., 2003; Gullberg et al., 1992; Kempenaers et al., 1997; Krokene et al., 1998; Leech et al., 2001) and great tits 8.5–50% of broods/3.5–15% of young (Gullberg et al., 1992; Krokene et al., 1998; Lubjuhn et al., 1999; Strohbach et al., 1998; Verboven and Mateman, 1997).
We acknowledge that the statistical power pertaining to the analyses of cross-fostered great tits is low. However, the results may still be of some biological relevance. All previous findings have indicated a decline in the "value" or position of the cross-fostered males, be it with respect to social dominance rank, mating success, song, or other measures of their behavioral performance. Hence, we expected these males also to lose more paternity than controls. However, in case of the great tit the results were in fact the opposite as only one out of six cross-fostered males lost any paternity at all.
The apparent lack of differences in loss of paternity was surprising based on earlier findings, both from our own project and by other authors, generally indicating that such cross-fostering and the consequences it has for the social rearing conditions of the young may influence greatly the social life of the adult birds. For instance, it has been shown (also for the focal species) that cross-fostered individuals rank lower socially than controls (Hansen and Slagsvold, 2004) and may produce aberrant song (Grant BR and Grant PR, 1996; Payne et al., 1998; ten Cate et al., 1993; Johannessen LE, Slagsvold T, Hansen BT, unpublished data), often resulting in the individuals having trouble attracting (conspecific) mates (Slagsvold et al., 2002). Further, it has been shown that dominance rank and song may be important for the protection of paternity in own brood and/or the achievement of EPP in the broods of others (Double and Cockburn, 2003; Forstmeier et al., 2002; Hasselquist et al., 1996; Kempenaers et al., 1997; Martin-Vivaldi et al., 2002; Otter et al., 1994, 1998, 2001; Poston et al., 1999; Slagsvold et al., 2001). Thus, the cross-fostered males in our study, whose social dominance rank (Hansen and Slagsvold, 2004) and song performance (Johannessen LE, Slagsvold T, Hansen BT, unpublished data) had been manipulated to an inferior level experimentally, were expected to suffer more from cuckoldry than controls.
A likely explanation could be that there is some kind of quality threshold (based on song performance, social status, courting behavior or other traits) that cross-fostered and all other males have to pass: a first level to be perceived as a suitable mate, and a second one to avoid being cuckolded. If the cross-fostered males are perceived as being of inferior quality, they might not succeed in passing even the first of these threshold levels and hence end up as unmated bachelors or potentially direct their mating effort towards other species. This is in fact the case with the cross-fostered great tit males as only 15% of them manage to pair up conspecifically during their first breeding season (as opposed to 95–100% of control males. However, some of them manage to obtain mates in later years). The rest pair up heterospecifically (36%) or go through the breeding season as unmated bachelors (49%). Of blue tit males, virtually all obtained mates and only two cross-fostered males (3.8%) have engaged in heterospecific pairings. The cross-fostering thus severely reduced the mating opportunities of the males in one of the species, the great tit, but not the other. However, the risk of cuckoldry of the males succeeding in obtaining a mate was not significantly affected in any of the species. Consequently, the cross-fostering seems to have an influence on the female choice of a social partner but not on her propensity to seek EPCs.
If this hypothesis holds true, one might argue that the behavior of the males included in this study really does not deviate significantly from that of control males and thus throw doubt on our classification of them as being of inferior quality due to the cross-fostering treatment. To elucidate this possibility, we made some further investigations into the general behavior of the cross-fostered males. Data on social dominance rank generally indicated that they were not any different from other cross-fostered males, that is, they were subdominant to conspecific controls. The song analyses did not reveal any difference between the song of the cross-fostered males included in the present study and that of other cross-fostered males and thus seem to have been similarly affected by the cross-fostering treatment. Also, the tendency to respond aggressively towards intruders at the nest seemed to be comparable to other cross-fostered males and was quite different from that of controls. However, for all of these variables some males clearly behaved more like controls than other cross-fostered males. The assumption that there might be variation in the degree of misimprinting within the group of cross-fostered males thus seems justified. Still, the variation in behavior exhibited by the males in the present study definitely seemed to expand well into that of other cross-fostered males that never succeeded in obtaining any conspecific mates. We thus find it justified to regard the cross-fostered males of this study as actually being deviant males, that is, of reduced quality, as compared to controls.
An alternative explanation could be that female EP matings occur independently of the qualities of the social male. This is true for the fertility insurance hypothesis (Gibson and Jewell, 1982; Krokene et al., 1998; Sheldon, 1994), stating that females seek EPCs as an insurance against potential infertility of their social mate. If this holds true, one would not expect cross-fostered males to be cuckolded more often than controls. The fact that young produced in the broods of heterospecific pairs (between blue tit females and great tit males, where one might suspect a reduced fertilization success) always seemed to be conspecific with the female (i.e., EPY; see Slagsvold et al., 2002), indeed illustrates that a process similar to the one predicted by this hypothesis might be active in this system. Our data are thus also in concordance with the findings of Krokene et al. (1998), who found no effects of male quality on EPP in blue tits or great tits.
Most of the males included in this study were mated to control females, while a few had mates that where themselves cross-fostered. However, the paternity loss of the families including cross-fostered females was highly variable (0–100%) and did not seem to differ systematically from that of families with control females. We still cannot completely disregard the possibility that female quality might have had an effect on the patterns of paternity loss, but we desist from making any firm conclusions on this topic as the cross-fostered females are severely underrepresented in our samples.
Lack of opportunities for EPC is a less likely explanation of our results. EPY occurred in about 30% of control broods in the study populations, and females thus seemed to have ample access to EP males. Breeding density has also been found to correlate with risk of losing paternity in several species (Johnson et al., 2002; Rätti et al., 2001; Thusius et al., 2001; Westneat and Sherman, 1997), and EP sires are often close neighbors (Foerster et al., 2003; Hasselquist et al., 1996; Webster et al., 2001; Westneat, 1987a), also in the focal species (Foerster et al., 2003; present study). However, distances to nearest conspecific neighbor were generally short and no shorter in cases with than without EPY.
An age effect on loss of paternity might indeed be present. A trend in the expected direction, that is, older males losing less paternity, was found, but limited sample size (great tit) and/or a bias towards young males (blue tit) may have prevented statistical significance. Our findings thus seem to be in accordance with a growing number of studies indicating effects of age on the likelihood of losing paternity in own brood and/or gaining EPP in other broods (Delhey et al., 2003; Dickinson, 2001; Kempenaers et al., 1997; Møller and Ninni, 1998; Richardson and Burke, 1999; Wagner et al., 1996; Weatherhead, 1997; Westneat, 1987b).
We manipulated social behavior, and thus male quality, by interspecific cross-fostering in the wild. Surprisingly, our study does not demonstrate any major effect of this treatment on loss of paternity, although total reproductive success cannot be fully assessed as we have only very limited information on the success of the males in gaining EPP in other males' broods. Small sample sizes are, however, a concern, and we might thus not have been able to detect subtle effects of the treatment. Still, our results suggest that the cross-fostered males apparently are not faced with heavily increased levels of cuckoldry in spite of them behaving aberrantly in several ways. Perhaps females do not pay as much attention to the social skills of potential partners as we have believed so far? More work is clearly needed on how variation in male quality might influence the female choice of social as well as EP partners.
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ACKNOWLEDGEMENTS |
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We thank Per Kristian Slagsvold and Eirik Grønningsæter for invaluable help in the field and the Haakonsen, Johnsen, Johnsrud, and Vestgård families for kind permission to work on their premises. The bird study group at the Zoological Museum at Tøyen and Øistein Haugsten Holen have critically read various drafts of this paper and provided highly valuable discussions. Their suggestions, together with those of two anonymous reviewers, have greatly improved the manuscript. The study was supported financially by a grant (to L.E.J.) from the Research Council of Norway.
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REFERENCES |
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