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Song and female choice for extrapair copulations in the sedge warbler, Acrocephalus schoenobaenus
ANIMAL BEHAVIOUR, 2007, 73, 629e635 doi:10.1016/j.anbehav.2006.06.011
Song and female choice for extrapair copulations in the sedge warbler, Acrocephalus schoenobaenus RUPERT C. MAR SHALL* , KA TH E R IN E L . BUC HA NA N † & C LIVE K . C AT CH POLE*
*School of Biological Sciences, Royal Holloway, University of London ySchool of Biosciences, Cardiff University (Received 13 June 2005; initial acceptance 22 August 2005; final acceptance 27 June 2006; published online 13 March 2007; MS. number: 8581R)
Although 90% of passerine birds live in socially monogamous pair bonds, molecular studies have revealed that genetic polygamy occurs in 86% of surveyed passerines, because individuals engage in copulations outside the pair bond (extrapair copulations; EPCs). Most explanations for the occurrence of EPCs involve female gaining indirect benefits from the extrapair male. The sedge warbler is a socially monogamous species in which some offspring result from EPCs (8% in this study). Complex song is a sexually selected male trait used by females which select mates based on a variety of male qualities. We used microsatellite DNA profiling to detect extrapair young and assign paternity. ‘Good genes’ theory predicts that females should engage in EPCs with males of higher quality than their social mate, with resulting fitness benefits. Extrapair males had smaller song repertoires and smaller territories than the social mate. This apparent preference for small-repertoire males as extrapair mates conflicts with the predictions from previous studies of this species. Sudden cessation of song after pairing may mean that song cues are unavailable for later extrapair matings and females may switch to other cues. Such behaviour may lead to different patterns of female choice during social and extrapair mating in the sedge warbler. We conclude that multiple reasons underlie patterns of female choice in this species. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: Acrocephalus schoenobaenus; extrapair copulation; female choice; sedge warbler; song repertoire
While nearly 90% of passerine species maintain social monogamy in pair bonds (Lack 1968), only 14% are genetically monogamous (Griffith et al. 2002). Within passerine birds the proportion of broods containing extrapair young (EPY) varies greatly, from 0% in the wood warbler, Phylloscopus sibilatrix (Gyllensten et al. 1990) to 86% in the reed bunting, Emberiza schoeniclus (Dixon et al. 1994). Male mating success in species with a socially monogamous mating system is likely to vary more when individuals engage in mixed reproductive strategies such as extrapair copulations (EPCs). Furthermore, males may increase their
Correspondence and present address: R. C. Marshall, Institute of Biological Sciences, University of Wales, Aberystwyth, Edward Llwyd Building, Penglais, Aberystwyth SY23 3DA, U.K. (email: rmm@aber. ac.uk). K. L. Buchanan is at the School of Biosciences, Cardiff University, Main Building, Museum Avenue, Cardiff CF10 3TL, U.K. C. K. Catchpole is at the School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, U.K. 0003e 3472/07/$30.00/0
reproductive success by performing EPCs without expending effort on paternal care (e.g. Westneat 1993). The establishment of mixed reproductive strategies by females (Trivers 1972) and the absence of physical force in gaining EPCs in most avian species, other than waterfowl (McKinney & Evarts 1998), raise the question of what role active female choice plays in EPCs. In species where coercion by males is not suspected, attention has focused upon benefits accruing to the female or her offspring, rather than benefits to males. Females of only a few species receive direct benefits from extrapair males, such as foraging benefits (Wolf 1975), subsequent pairing advantage (Colwell & Oring 1989), reduced predation (Gray 1997) or insurance against male infertility (Wetton & Parkin 1991; Sheldon 1994). Females may also obtain indirect benefits in terms of ‘good genes’ for their offspring by mating with males of relatively high genetic quality (Hamilton 1990). Female blue tits Cyanistes caeruleus, engage in EPCs with older males who are good singers and have EPY that survive better than their half siblings fathered by the female’s social mate (Kempenaers et al.
629 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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1992). The importance of song in extrapair mate choice has been demonstrated in a number of species (e.g. Otter et al. 1998; Forstmeier et al. 2002; Mennill et al. 2002) including the great reed warbler, Acrocephalus arundinaceus, where females engage in EPCs with males that have a larger song repertoire than their pair mate (Hasselquist et al. 1996). However, data on other species suggest that the good genes hypothesis alone is not a general explanation for the occurrence of EPCs (Kempenaers et al. 1992, 1999; Blomqvist et al. 2002; Griffith et al. 2002). The sedge warbler is now established as a classic example of a species where sexual selection has targeted song complexity (reviewed in Catchpole 2000). Both in the field (Catchpole 1980; Buchanan & Catchpole 1997) and under controlled conditions in the laboratory (Catchpole et al. 1984) females have been shown to prefer males with larger repertoires. Repertoire size in this species is also indicative of parasite load (Buchanan et al. 1999), level of paternal offspring provisioning (Buchanan & Catchpole 2000b) and individual genetic diversity of the males (Marshall et al. 2003). However, although males use song to advertise for a mate, they cease to sing upon pairing (Catchpole 1973), so only females arriving before a male has paired can assess his repertoire size. Two studies have shown that about 8% of sedge warbler offspring are the result of EPCs (Langefors et al. 1998; Buchanan & Catchpole 2000a). In a Swedish population, Langefors et al. (1998) found that nests containing EPY had more neighbouring territories than nests without, suggesting that opportunity for EPC may be important. Langefors et al. (1998) also found that females with EPY in their nests had fewer singing males to choose from when they settled, and that extrapair males arrived earlier than other males and were not singing when the female paired. However, the authors did not investigate the role of song repertoires. A second study, of an English population of sedge warblers (a subsample of the current data set; Buchanan & Catchpole 2000a), did investigate song and found no difference in repertoire size between cuckolded and noncuckolded males, although paternity of extrapair offspring was not assigned in that study. Despite a wealth of recent studies, there remains no overall explanation for the occurrence of EPCs (Griffith et al. 2002). Using data from a field study of sedge warblers in southern England, we addressed the question of whether females engage in EPCs with males of higher quality, measured by song repertoire size, as reported for the closely related great reed warbler (Hasselquist et al. 1996).
the day on which the final egg was laid. We calculated hatch date from this date and thereafter monitored nests daily for indications of hatching from provisioning behaviour of the adults. Once this was observed, we visited nests once, within 2 days of hatching, and then again for blood sampling. Visit rate was minimized to reduce predation risk, while maintaining accuracy of ageing. Approximately 10 ml of blood was taken from the brachial vein of chicks when they were 5e8 days old. All sampled chicks were still alive when checked the following day, suggesting no adverse affect of blood sampling. We analysed hatching failure only for to those broods (N ¼ 67) in which the total number of eggs and young was equal to the maximum recorded clutch size for that nest. Where the number of chicks was lower than the number of eggs laid, we were unable to confirm whether the cause was hatching failure or chick mortality.
Song Analysis To record males, we used a Marantz CP430 cassette recorder with a Sennheiser MD211U microphone mounted at the focal point of a fibreglass parabolic reflector 50 cm in diameter. All recordings were made within the first 5 h of daylight. We recorded at least 15 min of continuous song for each male (Catchpole 1980; Buchanan & Catchpole 1997). Recordings were analysed with Avisoft SASLab Pro for Windows, Avisoft, Berlin, Germany; Fast Fourier transform (FFT, length at 256, 50% frame in a Hamming window and 75% overlap). Songs were initially played into Avisoft using the RealTime Spectrograph window (display parameters FFT length 128, 100% frame in the Hamming window). We measured song complexity in terms of repertoire size by counting the total number of unique syllables in 20 complete songs sung by each male (Catchpole 1976). The appearance of novel syllables reaches an asymptote at approximately 10 songs (Catchpole 1980). Recordings of repertoire size from the same male over several days have shown that this measure of repertoire size has more than 98% repeatability (Buchanan et al. 1999). We estimated male territory size from observed aggressive encounters with neighbours and from the location of song posts. Territory size decreases through the breeding season and was therefore measured at the time of pairing. Males were identified as paired when they ceased singing (Catchpole 1980; Buchanan & Catchpole 1997). Full details of these methods are in Buchanan & Catchpole (1997).
METHODS We studied a population of individually colour-ringed sedge warblers at Wraysbury, U.K., between 1993 and 1998. Adults were caught, within a few days of their arrival on the field site, with mist nests under U.K. Government licence by K.L.B, R.C.M and members of the Runnymede Ringing Group. Each adult received a unique combination of colour rings. We took 50e100 ml of blood from the brachial vein of each adult. Nests were visited regularly during nest building until
DNA Analysis Standard phenol/choloroform DNA extraction (Sambrook et al. 1990) and polymerase chain reaction (PCR) techniques were used to amplify microsatellite DNA. We used four to nine primers to assign paternity, with PCR annealing temperatures as follows: Fhu2 (Ellegren 1992) 55 C, Ppi2 (Martinez et al. 1999) 53 C, Pca3 (Dawson et al. 2000) 59 C and Ase18 (Richardson et al. 2000) 57 C. Ase37 and Ase48 (Richardson
MARSHALL ET AL.: SONG AND PATERNITY
to 74% of EPY (25/34). EPY for which paternity could be assigned were located in 14 broods. Data on song characteristics were available for social and genetic fathers in 12 of these broods. However, two females had two consecutive broods with the same social mate in one breeding season, both broods containing EPY, although in each case the extrapair male in the first brood was not the extrapair male in the second brood. To avoid pseudoreplication, we used the mean values of characteristics from these two extrapair males in our comparison with the traits of the female’s social mate. Another brood contained two extrapair chicks, each fathered by a different extrapair male. Again, we took the mean values of these extrapair males when comparing the social versus extrapair males. Combining data in this manner allowed us to make 10 pairwise statistical comparisons. The song repertoire of a female’s extrapair partner (the genetic father) was significantly smaller than that of her social partner (Wilcoxon signed-ranks test: Z ¼ 2.7, N ¼ 10, P ¼ 0.007; Fig. 1a). The territory size of extrapair males was also smaller than that of social males in all cases (Z ¼ 2.67, N ¼ 9, P ¼ 0.008; Fig. 1b; no data available for one pair). Genetic and social males showed no significant difference in any other behavioural or anatomical measure (arrival date, pair date, wing length, mass; Table 2). The extrapair male arrived later than the male he cuckolded in
90
RESULTS
No. of broods
1993 1994 1995 1996 1997 1998
7 18 20 19 13 8
All years
85
*EPY: extrapair young.
No. of young
No. of EPY* (%)
(29) (22) (20) (16) (23) (0)
31 86 93 86 69 38
4 (13) 5 (6) 8 (9) 7 (8) 10 (14) 0 (0)
16 (19)
403
34 (8.4)
2 4 4 3 3 0
60
40 2600
Territory size (m2)
Table 1. Annual levels of extrapair paternity
Year
70
50
We successfully profiled 85 broods from 73 pairs for paternity analysis during the 6 years of the field study. The level of extrapair paternity was similar between years (Table 1) and within the combined data set (1993e1998) EPY young occurred in 19% of clutches (16/85), accounting for 8.4% of offspring (34/403). Paternity was assigned
No. of broods (%) containing EPY*
(a)
80 Repertoire size
et al. 2000), Pca5 (Dawson et al. 2000), Pocc1 and Pocc 8 (Bensch et al. 1997) were all used with a Touchdown cycle of reduced annealing temperatures, with two cycles at each of 60, 57, 54 and 51 C followed by 27 cycles at 48 C (Don et al. 1991). Four of the loci displayed 17 alleles, and the remainder had 10e15 alleles (N ¼ 55 presumably unrelated adult sedge warblers). These primers provided a minimum combined probability of false inclusion of 0.00021 (Chakravarti & Li 1983; Marshall et al. 1998), based on combination of allele frequencies and assuming successful profiling at the six weakest loci in the same 55 individuals. Null alleles prevented us from profiling all individuals at all nine loci (Pemberton et al. 1995). We did not use a likelihood-based approach to paternity as these approaches have difficulty coping with loci having null alleles (Marshall et al. 1998). Furthermore, Marshall et al. (2003) found a link between individual genetic diversity and song repertoire size. If limited numbers of loci are used and males appear homozygous for rare alleles shared with extrapair offspring, likelihood analysis may then erroneously indicate that males of low individual genetic diversity are preferred and/or father more offspring than heterozygous males. We defined as an extrapair offspring any chick having two or more allelic mismatches with their social father. Three offspring showed only one mismatched allele, one repeat unit from the nearest matching parental allele, suggesting a mutation. All offspring were profiled at a minimum of four loci. For offspring not matching their putative social father, we assigned paternity by matching alleles with those of all other males present in the population during the female’s fertile period. Extrapair males were required to match putative offspring at all loci. EPY to whom paternity could not be assigned might have been fathered by males from outside our study area.
1
2
3
4
5
6
7
8
9
10
(b)
2100 1600 1100 600 100
1
2
3
4
5
6
7
9
10
Brood Figure 1. (a) Repertoire size (number of syllables) and (b) territory size of social (-) and genetic (,) fathers. Mean values of the two genetic fathers were used in analyses when a nest contained offspring fathered by more than one extrapair male.
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Table 2. Paired comparisons of social and extrapair males Characteristic Repertoire size (no. of syllables) Territory size (m2) Arrival date (days)z Pair date (days)z Wing length (mm) Mass (g)
Difference
Z*
N
Py
7.61.83
2.703
10
0.007
519.5124 7.13.79 0.672.36 0.930.50 0.640.27
2.666 1.601 0.178 1.693 1.836
9 9 9 9 9
0.008 0.11 0.86 0.09 0.07
Mean difference SE is given. A positive Z value indicates that the social male had a larger value than the extrapair male. *Wilcoxon signed-ranks test. yTwo-tailed P. zNumber of days from arrival of first sedge warbler on the field site.
seven of the nine cases for which data were available and also paired later in five of these nine cases (Table 2). However, the extrapair male was still unpaired on the day that the social male paired in three of four cases. There was no difference in the number of neighbouring males singing during the fertile period of females whose nests contained EPY ðX SD ¼ 1 1:3Þ and those that did not (2 1.7). Similarly, there was no difference in the number of neighbours during the fertile period between nests containing EPY (7 1.9) and those that did not (8 2.3). While a high level of nest predation was recorded in all years of the study (an average of 65% of all nest attempts were predated during the course of the study), females suffering nest loss engaged in EPCs with a similar frequency in subsequent nesting attempts (7/34) as did those that did not suffer nest loss (9/29). Since fewer than 3% of offspring returned to breed as adults, in this study (data not shown), assessment of relative survivorship between EPY and their half siblings was not possible. Finally, a small number of both cuckolded and noncuckolded nests contained unhatched eggs (2/10 and 2/57 nests, respectively), but their occurrence was too low for statistical comparison.
DISCUSSION Song is an established male cue used by females when choosing pair mates in several Acrocephalus species (reviewed in Catchpole 2000) and repertoire size, compared to other measures of song output, is a robust and reliable measure of male quality in the sedge warbler (Catchpole 2000). Despite this evidence, our results suggest that female sedge warblers engage in EPCs with males that have smaller song repertoires and territories. No other male, female or detectable environmental trait predicted this trend. We suggest several reasons for this result. In this species, females apparently mate socially with phenotypically attractive males to gain direct benefits (high offspring provisioning rate, Buchanan et al. 1999; low parasite level, Buchanan & Catchpole 2000b). However, cessation of male song upon pairing (Catchpole 1973) constrains the female’s mode of choice of extrapair mate in this species, making choice of an extrapair male
on the basis of song more difficult. Song-flighting time, another independent correlate of female choice for social mate in the sedge warbler (Buchanan & Catchpole 1997), similarly ceases along with all singing activity upon pairing (Catchpole 1980; Buchanan & Catchpole 1997). We found no significant difference in the relative arrival or pairing date in our paired comparison of social and extrapair mates, suggesting that the availability of late-pairing males with small repertoires does not determine the skew in extrapair mate choice. Alternatively, females may seek different qualities from a pair mate than from an extrapair mate, suggesting the existence of multiple male strategies with respect to fitness. Male sedge warblers with larger repertoires may be healthier and provision their offspring at a higher rate (Buchanan et al. 1999; Buchanan & Catchpole 2000b). However, extrapair males do not provision offspring of young at nests where they gain EPY (Buchanan & Catchpole 2000b), so females may not choose an extrapair partner for the same reasons as they choose a social partner. As extrapair males had significantly smaller repertoires than the social male at the nest, a nonrandom choice of partners clearly occurs, raising the possibility that males with small repertoires are superior in some respect. The recent finding that repertoire size in the sedge warbler appears to increase with male age (J. Nicholson, K. L. Buchanan, R. C. Marshall & C. K. Catchpole, unpublished data) may be relevant. If so, females may be sampling younger males as possible mates for future breeding attempts, or gaining genetic variety or fertility within their clutch, by including genetic material from young males. As the choice of male for EPCs is nonrandom, but related to song repertoire size, the reasons underlying this behaviour are presumably correlated with song production. We found no significant difference in hatching failure rates between cuckolded and noncuckolded broods in our sedge warbler population, suggesting that fertility insurance (Trivers 1972; Gibson & Jewell 1982; Sheldon 1994) may not be a reason for engaging in EPCs in this species; however, the power of this test is low, so this conclusion should be treated with caution. However, if EPCs do decrease hatching failure, then failure rates of cuckolded and noncuckolded nests might be expected to be similar if EPCs brought a given nest’s hatching failure into line with the population mean (Kempenaers & Dhondt 1993). A final possibility is that our results occurred because of rapid mate switching, as females ‘trade up’ to find a male with a larger repertoire as he is discovered or becomes available. Although we have little information about female behaviour before nest initiation, we find this possibility unlikely, because, over several years of observation, we have never observed females switching partners within a season, except as a result of nest predation. Studies of extrapair fertilizations in a Swedish population of sedge warblers (Hasselquist & Langefors 1998; Langefors et al. 1998) suggested that female sedge warblers do engage in EPCs with attractive male neighbours, although the researchers did not include any measures of song. In these studies, females that engaged in EPCs had fewer males to
MARSHALL ET AL.: SONG AND PATERNITY
choose from when they arrived and chose extrapair males that arrived earlier in the season than their neighbours. We found no such correlations in our English study population, although four of nine extrapair males paired before the social male, eliminating any opportunity for assessment of song by later-arriving females. However, no differences in any physical, behavioural or territorial trait measured were found between cuckolded and noncuckolded males, when compared as independent groups, in either the Swedish study (Langefors et al. 1998) or a subsample of data from our study population (Buchanan & Catchpole 2000a). Furthermore, some males with larger repertoires neighbouring the focal female paired before her arrival, so random selection of a neighbouring male as an extrapair partner would not automatically result in extrapair males having smaller repertoires than social males. This finding implies some level of choice by the female. Our results for song repertoire appear to contradict those found for the closely related great reed warbler (Hasselquist et al. 1996; Leisler et al. 1997). Of the three main clades identified in the phylogeny of the genus Acrocephalus based upon mtDNA sequences, these two species fall into different clades, with a relative distance of 10% between them (Leisler et al. 1997). They also differ in many aspects of their ecology, including mating system, feeding pattern and parental care. In sedge warblers repertoire size indicates a male’s ability to provision nestlings (Buchanan & Catchpole 2000b), whereas in great reed warblers it reflects genetic quality affecting offspring survival in a Swedish population (Hasselquist et al. 1996), although not in a longer established German population (Forstmeier & Leisler 2004). In addition to repertoire size, territory size (but not quality) was also correlated with pair date when sedge warblers paired socially (Buchanan & Catchpole 1997), although there was no independent correlation with repertoire size. However, as with repertoire size, extrapair males had smaller territories than a female’s social mate. There was no difference in cuckoldry between holders of large and small territories. Many studies have shown equivocal results when assessing the female’s choice of an extrapair mate (reviewed in Griffith et al. 2002; Westneat & Stewart 2003). For example, in the pied flycatcher, Ficedula hypoleuca, male colour, where darker shades indicate higher levels of paternal care, affects initial mate choice (Saetre et al. 1994). However, the most attractive males were the most likely to be cuckolded in this Norwegian population (Lifjeld et al. 1997), although no such pattern to cuckoldry was found in a Finnish population of the same species (Ratti et al. 1995). Although only three extrapair males were identified, two of them were darker (and thus more attractive) than the social mate that they cuckolded (Lifjeld et al. 1997). To our knowledge, our study is the first to identify a significant negative relation between sexually selected traits of social and genetic fathers in socially monogamous mating systems where females are known to engage in EPCs. The extent of female engagement in EPCs may be limited by their ability to assess male phenotypic quality either before or after copulation (Colegrave et al. 2002), even where initial choice of social mate is based on
multiple cues (Sullivan 1994; Johnstone 1996), as in the sedge warbler (Buchanan & Catchpole 1997). Territory size decreases after pairing in the sedge warbler and all singing activity ceases at this time (Buchanan & Catchpole 1997). In initial mate choice, song-flighting time correlates with pairing date only on the day before pairing (Buchanan & Catchpole 1997), suggesting that songflighting time is a measure of current health and energy rather than of long-term fitness. Song repertoire size itself may not be heritable, and may also represent current male health rather than good genes. If females choose extrapair males using cues other than those used when choosing their social mate, these as yet unidentified characteristics must be negatively correlated with repertoire size, as the extrapair males would otherwise be chosen randomly in relation to this cue. Females in our study showed an active preference for males with small repertoires as extrapair mates, demonstrating that this result was not due to random mate choice. Males that are unattractive as social mates are preferred as extrapair mates, suggesting that males can maximize their fitness by multiple routes. Female choice for any trait may be constrained by the magnitude of differences in quality between males in a population (Møller 1992), so females should be choosy and seek extrapair sires only in populations where variability in heritable male fitness is high (Petrie & Lipsitch 1994). In our English study population, sedge warbler females appear to engage in EPCs with males of lower phenotypic quality than their social mate. Sudden cessation of song after pairing may require female sedge warblers to switch to a different pattern of mate choice, using as yet unidentified cues during extrapair mating.
Acknowledgments We thank T. Burke and the NERC Sheffield Molecular Genetics Facility, NERC (U.K.) for financial support, H.E. Nice for advice, members of the Runnymede Ringing Group for assistance in the field and B. Hatchwell and two anonymous referees for comments on the manuscript. References Bensch, S., Price, T. & Kohn, J. 1997. Isolation and characterization of microsatellite loci in a Phylloscopus warbler. Molecular Ecology, 6, 91e92. Blomqvist, D., Andersson, M., Kupper, C., Cuthill, I. C., Kis, J., Lanctot, R. B., Sandercock, B. K., Szekely, T., Wallander, J. & Kempenaers, B. 2002. Genetic similarity between mates and extra-pair parentage in three species of shorebirds. Nature, 419, 613e615. Buchanan, K. L. & Catchpole, C. K. 1997. Female choice in the sedge warbler, Acrocephalus schoenobaenus: multiple cues from song and territory quality. Proceedings of the Royal Society of London, Series B, 264, 521e526. Buchanan, K. L. & Catchpole, C. K. 2000a. Extra-pair paternity in the socially monogamous sedge warbler Acrocephalus schoenobaenus as revealed by multilocus DNA fingerprinting. Ibis, 142, 12e20. Buchanan, K. L. & Catchpole, C. K. 2000b. Song as an indicator of male parental effort in the sedge warbler. Proceedings of the Royal Society of London, Series B, 267, 321e326.
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Hasselquist, D., Bensch, S. & Schantz, T. V. 1996. Correlation between male song repertoire, extra-pair paternity and offspring survival in the great reed warbler. Nature, 381, 229e232. Johnstone, R. A. 1996. Multiple displays in animal communication: ‘backup signals’ and ‘multiple messages’. Philosophical Transactions of the Royal Society of London, Series B, 351, 329e338. Kempenaers, B. & Dhondt, A. A. 1993. Why do females engage in extra-pair copulations: a review of hypotheses and their Predictions. Belgian Journal of Zoology, 123, 93e103. Kempenaers, B., Verheyen, G. R. & Van den Broek, M. 1992. Extra pair paternity results from female preference for high quality males in the blue tit. Nature, 357, 494e496. Kempenaers, B., Congdon, B., Boag, P. & Robertson, R. J. 1999. Extrapair paternity and egg hatchability in tree swallows: evidence for the genetic compatibility hypothesis? Behavioral Ecology, 10, 304e311. Lack, D. 1968. Ecological Adaptations for Breeding Birds. London: Chapman & Hall. Langefors, A., Hasselquist, D. & von Schantz, T. 1998. Extra-pair fertilizations in the sedge warbler. Journal of Avian Biology, 29, 134e144. Leisler, B., Heidrich, P., Schulze-Hagen, K. & Wink, M. 1997. Taxonomy and phylogeny of reed warblers (genus Acrocephalus) based on mtDNA sequences and morphology. Journal fu¨r Ornithologie, 138, 469e496. Lifjeld, J., Slagsvold, T., Dale, S. & Ellegren, H. 1997. A sexually selected paradox in the pied flycatcher: attractive males are cuckolded. Auk, 114, 112e115. McKinney, F. & Evarts, S. 1998. Sexual coercion in waterfowl and other birds. In: Avian Reproductive Tactics: Female and Male Perspectives (Ed. by P. Parker & N. Burley), pp. 163e195. Washington, DC: American Ornithologists’ Union. Marshall, R. C., Buchanan, K. L. & Catchpole, C. K. 2003. Sexual selection and individual genetic diversity in a songbird. Proceedings of the Royal Society of London, Series B, 270, S248eS250. doi:10.1098/rsbl.2003.0081. Marshall, T. C., Slate, J., Kruuk, L. E. B. & Pemberton, J. M. 1998. Statistical confidence for likelihood-based paternity inference in natural populations. Molecular Ecology, 7, 639e655. Martinez, J. G., Soler, J. J., Soler, M., Møller, A. P. & Burke, T. 1999. Comparative population structure and gene flow of a brood parasite, the great spotted cuckoo (Clamator glandarius), and its primary host, the magpie (Pica pica). Evolution, 53, 269e278. Mennill, D. J., Ratcliffe, L. M. & Boag, P. T. 2002. Female eavesdropping on male song contests in songbirds. Science, 296, 873. Møller, A. P. 1992. Frequency of female copulations with multiple males and sexual selection. American Naturalist, 139, 1089e1101. Otter, K., Ratcliffe, L., Michaud, D. & Boag, P. T. 1998. Do female black-capped chickadees prefer high-ranking males as extra-pair partners? Behavioral Ecology and Sociobiology, 43, 25e36. Pemberton, J. M., Slate, J., Bancroft, D. R. & Barrett, A. 1995. Nonamplifying alleles at microsatellite loci: a caution for parentage and population studies. Molecular Ecology, 4, 249e252. Petrie, M. & Lipsitch, M. 1994. Avian polygyny is most likely in populations with high variability in heritable male fitness. Proceedings of the Royal Society of London, Series B, 256, 275e280. Ratti, O., Hovi, M., Lundberg, A., Tegelstrom, H. & Alatalo, R. V. 1995. Extra-pair paternity and male characteristics in the pied flycatcher. Behavioral Ecology and Sociobiology, 37, 419e425. Richardson, D. S., Jury, F. L., Dawson, D. A., Salgueiro, P., Komdeur, J. & Burke, T. 2000. Fifty Seychelles warbler (Acrocephalus sechellensis) microsatellite loci polymorphic in Sylviidae species and their cross-species amplification in other passerine birds. Molecular Ecology, 9, 2226e2231. Saetre, G. P., Dale, S. & Slagsvold, T. 1994. Female pied flycatchers prefer brightly coloured males. Animal Behaviour, 48, 1407e1416.
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Sambrook, J., Fritsch, E. F. & Maniatis, T. 1990. Molecular Cloning: a Laboratory Manual. New York: Cold Spring Harbor Laboratory Press. Sheldon, B. 1994. Male phenotype, fertility, and the pursuit of extra-pair copulations by female birds. Proceedings of the Royal Society of London, Series B, 257, 25e30. Sullivan, M. S. 1994. Mate choice as an information gathering process under time constraint: implications for behaviour and signal design. Animal Behaviour, 47, 141e151. Trivers, R. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man (Ed. by B. Campbell), pp. 136e 179. Chicago: Aldine.
Westneat, D. F. 1993. Polygyny and extra-pair fertilizations in eastern red-winged blackbirds (Agelaius phoeniceus). Behavioral Ecology, 4, 49e60. Westneat, D. F. & Stewart, I. R. K. 2003. Extra-pair paternity in birds: causes, correlates, and conflict. Annual Review of Ecology, Evolution and Systematics, 34, 365e396. Wetton, J. & Parkin, D. 1991. An association between fertility and cuckoldry in the house sparrow. Proceedings of the Royal Society of London, Series B, 245, 227e233. Wolf, L. L. 1975. Prostitution behavior in a tropical humming-bird. Condor, 77, 140e144.
635
Song and female choice for extrapair copulations in the sedge warbler, Acrocephalus schoenobaenus RUPERT C. MAR SHALL* , KA TH E R IN E L . BUC HA NA N † & C LIVE K . C AT CH POLE*
*School of Biological Sciences, Royal Holloway, University of London ySchool of Biosciences, Cardiff University (Received 13 June 2005; initial acceptance 22 August 2005; final acceptance 27 June 2006; published online 13 March 2007; MS. number: 8581R)
Although 90% of passerine birds live in socially monogamous pair bonds, molecular studies have revealed that genetic polygamy occurs in 86% of surveyed passerines, because individuals engage in copulations outside the pair bond (extrapair copulations; EPCs). Most explanations for the occurrence of EPCs involve female gaining indirect benefits from the extrapair male. The sedge warbler is a socially monogamous species in which some offspring result from EPCs (8% in this study). Complex song is a sexually selected male trait used by females which select mates based on a variety of male qualities. We used microsatellite DNA profiling to detect extrapair young and assign paternity. ‘Good genes’ theory predicts that females should engage in EPCs with males of higher quality than their social mate, with resulting fitness benefits. Extrapair males had smaller song repertoires and smaller territories than the social mate. This apparent preference for small-repertoire males as extrapair mates conflicts with the predictions from previous studies of this species. Sudden cessation of song after pairing may mean that song cues are unavailable for later extrapair matings and females may switch to other cues. Such behaviour may lead to different patterns of female choice during social and extrapair mating in the sedge warbler. We conclude that multiple reasons underlie patterns of female choice in this species. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: Acrocephalus schoenobaenus; extrapair copulation; female choice; sedge warbler; song repertoire
While nearly 90% of passerine species maintain social monogamy in pair bonds (Lack 1968), only 14% are genetically monogamous (Griffith et al. 2002). Within passerine birds the proportion of broods containing extrapair young (EPY) varies greatly, from 0% in the wood warbler, Phylloscopus sibilatrix (Gyllensten et al. 1990) to 86% in the reed bunting, Emberiza schoeniclus (Dixon et al. 1994). Male mating success in species with a socially monogamous mating system is likely to vary more when individuals engage in mixed reproductive strategies such as extrapair copulations (EPCs). Furthermore, males may increase their
Correspondence and present address: R. C. Marshall, Institute of Biological Sciences, University of Wales, Aberystwyth, Edward Llwyd Building, Penglais, Aberystwyth SY23 3DA, U.K. (email: rmm@aber. ac.uk). K. L. Buchanan is at the School of Biosciences, Cardiff University, Main Building, Museum Avenue, Cardiff CF10 3TL, U.K. C. K. Catchpole is at the School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, U.K. 0003e 3472/07/$30.00/0
reproductive success by performing EPCs without expending effort on paternal care (e.g. Westneat 1993). The establishment of mixed reproductive strategies by females (Trivers 1972) and the absence of physical force in gaining EPCs in most avian species, other than waterfowl (McKinney & Evarts 1998), raise the question of what role active female choice plays in EPCs. In species where coercion by males is not suspected, attention has focused upon benefits accruing to the female or her offspring, rather than benefits to males. Females of only a few species receive direct benefits from extrapair males, such as foraging benefits (Wolf 1975), subsequent pairing advantage (Colwell & Oring 1989), reduced predation (Gray 1997) or insurance against male infertility (Wetton & Parkin 1991; Sheldon 1994). Females may also obtain indirect benefits in terms of ‘good genes’ for their offspring by mating with males of relatively high genetic quality (Hamilton 1990). Female blue tits Cyanistes caeruleus, engage in EPCs with older males who are good singers and have EPY that survive better than their half siblings fathered by the female’s social mate (Kempenaers et al.
629 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 73, 4
1992). The importance of song in extrapair mate choice has been demonstrated in a number of species (e.g. Otter et al. 1998; Forstmeier et al. 2002; Mennill et al. 2002) including the great reed warbler, Acrocephalus arundinaceus, where females engage in EPCs with males that have a larger song repertoire than their pair mate (Hasselquist et al. 1996). However, data on other species suggest that the good genes hypothesis alone is not a general explanation for the occurrence of EPCs (Kempenaers et al. 1992, 1999; Blomqvist et al. 2002; Griffith et al. 2002). The sedge warbler is now established as a classic example of a species where sexual selection has targeted song complexity (reviewed in Catchpole 2000). Both in the field (Catchpole 1980; Buchanan & Catchpole 1997) and under controlled conditions in the laboratory (Catchpole et al. 1984) females have been shown to prefer males with larger repertoires. Repertoire size in this species is also indicative of parasite load (Buchanan et al. 1999), level of paternal offspring provisioning (Buchanan & Catchpole 2000b) and individual genetic diversity of the males (Marshall et al. 2003). However, although males use song to advertise for a mate, they cease to sing upon pairing (Catchpole 1973), so only females arriving before a male has paired can assess his repertoire size. Two studies have shown that about 8% of sedge warbler offspring are the result of EPCs (Langefors et al. 1998; Buchanan & Catchpole 2000a). In a Swedish population, Langefors et al. (1998) found that nests containing EPY had more neighbouring territories than nests without, suggesting that opportunity for EPC may be important. Langefors et al. (1998) also found that females with EPY in their nests had fewer singing males to choose from when they settled, and that extrapair males arrived earlier than other males and were not singing when the female paired. However, the authors did not investigate the role of song repertoires. A second study, of an English population of sedge warblers (a subsample of the current data set; Buchanan & Catchpole 2000a), did investigate song and found no difference in repertoire size between cuckolded and noncuckolded males, although paternity of extrapair offspring was not assigned in that study. Despite a wealth of recent studies, there remains no overall explanation for the occurrence of EPCs (Griffith et al. 2002). Using data from a field study of sedge warblers in southern England, we addressed the question of whether females engage in EPCs with males of higher quality, measured by song repertoire size, as reported for the closely related great reed warbler (Hasselquist et al. 1996).
the day on which the final egg was laid. We calculated hatch date from this date and thereafter monitored nests daily for indications of hatching from provisioning behaviour of the adults. Once this was observed, we visited nests once, within 2 days of hatching, and then again for blood sampling. Visit rate was minimized to reduce predation risk, while maintaining accuracy of ageing. Approximately 10 ml of blood was taken from the brachial vein of chicks when they were 5e8 days old. All sampled chicks were still alive when checked the following day, suggesting no adverse affect of blood sampling. We analysed hatching failure only for to those broods (N ¼ 67) in which the total number of eggs and young was equal to the maximum recorded clutch size for that nest. Where the number of chicks was lower than the number of eggs laid, we were unable to confirm whether the cause was hatching failure or chick mortality.
Song Analysis To record males, we used a Marantz CP430 cassette recorder with a Sennheiser MD211U microphone mounted at the focal point of a fibreglass parabolic reflector 50 cm in diameter. All recordings were made within the first 5 h of daylight. We recorded at least 15 min of continuous song for each male (Catchpole 1980; Buchanan & Catchpole 1997). Recordings were analysed with Avisoft SASLab Pro for Windows, Avisoft, Berlin, Germany; Fast Fourier transform (FFT, length at 256, 50% frame in a Hamming window and 75% overlap). Songs were initially played into Avisoft using the RealTime Spectrograph window (display parameters FFT length 128, 100% frame in the Hamming window). We measured song complexity in terms of repertoire size by counting the total number of unique syllables in 20 complete songs sung by each male (Catchpole 1976). The appearance of novel syllables reaches an asymptote at approximately 10 songs (Catchpole 1980). Recordings of repertoire size from the same male over several days have shown that this measure of repertoire size has more than 98% repeatability (Buchanan et al. 1999). We estimated male territory size from observed aggressive encounters with neighbours and from the location of song posts. Territory size decreases through the breeding season and was therefore measured at the time of pairing. Males were identified as paired when they ceased singing (Catchpole 1980; Buchanan & Catchpole 1997). Full details of these methods are in Buchanan & Catchpole (1997).
METHODS We studied a population of individually colour-ringed sedge warblers at Wraysbury, U.K., between 1993 and 1998. Adults were caught, within a few days of their arrival on the field site, with mist nests under U.K. Government licence by K.L.B, R.C.M and members of the Runnymede Ringing Group. Each adult received a unique combination of colour rings. We took 50e100 ml of blood from the brachial vein of each adult. Nests were visited regularly during nest building until
DNA Analysis Standard phenol/choloroform DNA extraction (Sambrook et al. 1990) and polymerase chain reaction (PCR) techniques were used to amplify microsatellite DNA. We used four to nine primers to assign paternity, with PCR annealing temperatures as follows: Fhu2 (Ellegren 1992) 55 C, Ppi2 (Martinez et al. 1999) 53 C, Pca3 (Dawson et al. 2000) 59 C and Ase18 (Richardson et al. 2000) 57 C. Ase37 and Ase48 (Richardson
MARSHALL ET AL.: SONG AND PATERNITY
to 74% of EPY (25/34). EPY for which paternity could be assigned were located in 14 broods. Data on song characteristics were available for social and genetic fathers in 12 of these broods. However, two females had two consecutive broods with the same social mate in one breeding season, both broods containing EPY, although in each case the extrapair male in the first brood was not the extrapair male in the second brood. To avoid pseudoreplication, we used the mean values of characteristics from these two extrapair males in our comparison with the traits of the female’s social mate. Another brood contained two extrapair chicks, each fathered by a different extrapair male. Again, we took the mean values of these extrapair males when comparing the social versus extrapair males. Combining data in this manner allowed us to make 10 pairwise statistical comparisons. The song repertoire of a female’s extrapair partner (the genetic father) was significantly smaller than that of her social partner (Wilcoxon signed-ranks test: Z ¼ 2.7, N ¼ 10, P ¼ 0.007; Fig. 1a). The territory size of extrapair males was also smaller than that of social males in all cases (Z ¼ 2.67, N ¼ 9, P ¼ 0.008; Fig. 1b; no data available for one pair). Genetic and social males showed no significant difference in any other behavioural or anatomical measure (arrival date, pair date, wing length, mass; Table 2). The extrapair male arrived later than the male he cuckolded in
90
RESULTS
No. of broods
1993 1994 1995 1996 1997 1998
7 18 20 19 13 8
All years
85
*EPY: extrapair young.
No. of young
No. of EPY* (%)
(29) (22) (20) (16) (23) (0)
31 86 93 86 69 38
4 (13) 5 (6) 8 (9) 7 (8) 10 (14) 0 (0)
16 (19)
403
34 (8.4)
2 4 4 3 3 0
60
40 2600
Territory size (m2)
Table 1. Annual levels of extrapair paternity
Year
70
50
We successfully profiled 85 broods from 73 pairs for paternity analysis during the 6 years of the field study. The level of extrapair paternity was similar between years (Table 1) and within the combined data set (1993e1998) EPY young occurred in 19% of clutches (16/85), accounting for 8.4% of offspring (34/403). Paternity was assigned
No. of broods (%) containing EPY*
(a)
80 Repertoire size
et al. 2000), Pca5 (Dawson et al. 2000), Pocc1 and Pocc 8 (Bensch et al. 1997) were all used with a Touchdown cycle of reduced annealing temperatures, with two cycles at each of 60, 57, 54 and 51 C followed by 27 cycles at 48 C (Don et al. 1991). Four of the loci displayed 17 alleles, and the remainder had 10e15 alleles (N ¼ 55 presumably unrelated adult sedge warblers). These primers provided a minimum combined probability of false inclusion of 0.00021 (Chakravarti & Li 1983; Marshall et al. 1998), based on combination of allele frequencies and assuming successful profiling at the six weakest loci in the same 55 individuals. Null alleles prevented us from profiling all individuals at all nine loci (Pemberton et al. 1995). We did not use a likelihood-based approach to paternity as these approaches have difficulty coping with loci having null alleles (Marshall et al. 1998). Furthermore, Marshall et al. (2003) found a link between individual genetic diversity and song repertoire size. If limited numbers of loci are used and males appear homozygous for rare alleles shared with extrapair offspring, likelihood analysis may then erroneously indicate that males of low individual genetic diversity are preferred and/or father more offspring than heterozygous males. We defined as an extrapair offspring any chick having two or more allelic mismatches with their social father. Three offspring showed only one mismatched allele, one repeat unit from the nearest matching parental allele, suggesting a mutation. All offspring were profiled at a minimum of four loci. For offspring not matching their putative social father, we assigned paternity by matching alleles with those of all other males present in the population during the female’s fertile period. Extrapair males were required to match putative offspring at all loci. EPY to whom paternity could not be assigned might have been fathered by males from outside our study area.
1
2
3
4
5
6
7
8
9
10
(b)
2100 1600 1100 600 100
1
2
3
4
5
6
7
9
10
Brood Figure 1. (a) Repertoire size (number of syllables) and (b) territory size of social (-) and genetic (,) fathers. Mean values of the two genetic fathers were used in analyses when a nest contained offspring fathered by more than one extrapair male.
631
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ANIMAL BEHAVIOUR, 73, 4
Table 2. Paired comparisons of social and extrapair males Characteristic Repertoire size (no. of syllables) Territory size (m2) Arrival date (days)z Pair date (days)z Wing length (mm) Mass (g)
Difference
Z*
N
Py
7.61.83
2.703
10
0.007
519.5124 7.13.79 0.672.36 0.930.50 0.640.27
2.666 1.601 0.178 1.693 1.836
9 9 9 9 9
0.008 0.11 0.86 0.09 0.07
Mean difference SE is given. A positive Z value indicates that the social male had a larger value than the extrapair male. *Wilcoxon signed-ranks test. yTwo-tailed P. zNumber of days from arrival of first sedge warbler on the field site.
seven of the nine cases for which data were available and also paired later in five of these nine cases (Table 2). However, the extrapair male was still unpaired on the day that the social male paired in three of four cases. There was no difference in the number of neighbouring males singing during the fertile period of females whose nests contained EPY ðX SD ¼ 1 1:3Þ and those that did not (2 1.7). Similarly, there was no difference in the number of neighbours during the fertile period between nests containing EPY (7 1.9) and those that did not (8 2.3). While a high level of nest predation was recorded in all years of the study (an average of 65% of all nest attempts were predated during the course of the study), females suffering nest loss engaged in EPCs with a similar frequency in subsequent nesting attempts (7/34) as did those that did not suffer nest loss (9/29). Since fewer than 3% of offspring returned to breed as adults, in this study (data not shown), assessment of relative survivorship between EPY and their half siblings was not possible. Finally, a small number of both cuckolded and noncuckolded nests contained unhatched eggs (2/10 and 2/57 nests, respectively), but their occurrence was too low for statistical comparison.
DISCUSSION Song is an established male cue used by females when choosing pair mates in several Acrocephalus species (reviewed in Catchpole 2000) and repertoire size, compared to other measures of song output, is a robust and reliable measure of male quality in the sedge warbler (Catchpole 2000). Despite this evidence, our results suggest that female sedge warblers engage in EPCs with males that have smaller song repertoires and territories. No other male, female or detectable environmental trait predicted this trend. We suggest several reasons for this result. In this species, females apparently mate socially with phenotypically attractive males to gain direct benefits (high offspring provisioning rate, Buchanan et al. 1999; low parasite level, Buchanan & Catchpole 2000b). However, cessation of male song upon pairing (Catchpole 1973) constrains the female’s mode of choice of extrapair mate in this species, making choice of an extrapair male
on the basis of song more difficult. Song-flighting time, another independent correlate of female choice for social mate in the sedge warbler (Buchanan & Catchpole 1997), similarly ceases along with all singing activity upon pairing (Catchpole 1980; Buchanan & Catchpole 1997). We found no significant difference in the relative arrival or pairing date in our paired comparison of social and extrapair mates, suggesting that the availability of late-pairing males with small repertoires does not determine the skew in extrapair mate choice. Alternatively, females may seek different qualities from a pair mate than from an extrapair mate, suggesting the existence of multiple male strategies with respect to fitness. Male sedge warblers with larger repertoires may be healthier and provision their offspring at a higher rate (Buchanan et al. 1999; Buchanan & Catchpole 2000b). However, extrapair males do not provision offspring of young at nests where they gain EPY (Buchanan & Catchpole 2000b), so females may not choose an extrapair partner for the same reasons as they choose a social partner. As extrapair males had significantly smaller repertoires than the social male at the nest, a nonrandom choice of partners clearly occurs, raising the possibility that males with small repertoires are superior in some respect. The recent finding that repertoire size in the sedge warbler appears to increase with male age (J. Nicholson, K. L. Buchanan, R. C. Marshall & C. K. Catchpole, unpublished data) may be relevant. If so, females may be sampling younger males as possible mates for future breeding attempts, or gaining genetic variety or fertility within their clutch, by including genetic material from young males. As the choice of male for EPCs is nonrandom, but related to song repertoire size, the reasons underlying this behaviour are presumably correlated with song production. We found no significant difference in hatching failure rates between cuckolded and noncuckolded broods in our sedge warbler population, suggesting that fertility insurance (Trivers 1972; Gibson & Jewell 1982; Sheldon 1994) may not be a reason for engaging in EPCs in this species; however, the power of this test is low, so this conclusion should be treated with caution. However, if EPCs do decrease hatching failure, then failure rates of cuckolded and noncuckolded nests might be expected to be similar if EPCs brought a given nest’s hatching failure into line with the population mean (Kempenaers & Dhondt 1993). A final possibility is that our results occurred because of rapid mate switching, as females ‘trade up’ to find a male with a larger repertoire as he is discovered or becomes available. Although we have little information about female behaviour before nest initiation, we find this possibility unlikely, because, over several years of observation, we have never observed females switching partners within a season, except as a result of nest predation. Studies of extrapair fertilizations in a Swedish population of sedge warblers (Hasselquist & Langefors 1998; Langefors et al. 1998) suggested that female sedge warblers do engage in EPCs with attractive male neighbours, although the researchers did not include any measures of song. In these studies, females that engaged in EPCs had fewer males to
MARSHALL ET AL.: SONG AND PATERNITY
choose from when they arrived and chose extrapair males that arrived earlier in the season than their neighbours. We found no such correlations in our English study population, although four of nine extrapair males paired before the social male, eliminating any opportunity for assessment of song by later-arriving females. However, no differences in any physical, behavioural or territorial trait measured were found between cuckolded and noncuckolded males, when compared as independent groups, in either the Swedish study (Langefors et al. 1998) or a subsample of data from our study population (Buchanan & Catchpole 2000a). Furthermore, some males with larger repertoires neighbouring the focal female paired before her arrival, so random selection of a neighbouring male as an extrapair partner would not automatically result in extrapair males having smaller repertoires than social males. This finding implies some level of choice by the female. Our results for song repertoire appear to contradict those found for the closely related great reed warbler (Hasselquist et al. 1996; Leisler et al. 1997). Of the three main clades identified in the phylogeny of the genus Acrocephalus based upon mtDNA sequences, these two species fall into different clades, with a relative distance of 10% between them (Leisler et al. 1997). They also differ in many aspects of their ecology, including mating system, feeding pattern and parental care. In sedge warblers repertoire size indicates a male’s ability to provision nestlings (Buchanan & Catchpole 2000b), whereas in great reed warblers it reflects genetic quality affecting offspring survival in a Swedish population (Hasselquist et al. 1996), although not in a longer established German population (Forstmeier & Leisler 2004). In addition to repertoire size, territory size (but not quality) was also correlated with pair date when sedge warblers paired socially (Buchanan & Catchpole 1997), although there was no independent correlation with repertoire size. However, as with repertoire size, extrapair males had smaller territories than a female’s social mate. There was no difference in cuckoldry between holders of large and small territories. Many studies have shown equivocal results when assessing the female’s choice of an extrapair mate (reviewed in Griffith et al. 2002; Westneat & Stewart 2003). For example, in the pied flycatcher, Ficedula hypoleuca, male colour, where darker shades indicate higher levels of paternal care, affects initial mate choice (Saetre et al. 1994). However, the most attractive males were the most likely to be cuckolded in this Norwegian population (Lifjeld et al. 1997), although no such pattern to cuckoldry was found in a Finnish population of the same species (Ratti et al. 1995). Although only three extrapair males were identified, two of them were darker (and thus more attractive) than the social mate that they cuckolded (Lifjeld et al. 1997). To our knowledge, our study is the first to identify a significant negative relation between sexually selected traits of social and genetic fathers in socially monogamous mating systems where females are known to engage in EPCs. The extent of female engagement in EPCs may be limited by their ability to assess male phenotypic quality either before or after copulation (Colegrave et al. 2002), even where initial choice of social mate is based on
multiple cues (Sullivan 1994; Johnstone 1996), as in the sedge warbler (Buchanan & Catchpole 1997). Territory size decreases after pairing in the sedge warbler and all singing activity ceases at this time (Buchanan & Catchpole 1997). In initial mate choice, song-flighting time correlates with pairing date only on the day before pairing (Buchanan & Catchpole 1997), suggesting that songflighting time is a measure of current health and energy rather than of long-term fitness. Song repertoire size itself may not be heritable, and may also represent current male health rather than good genes. If females choose extrapair males using cues other than those used when choosing their social mate, these as yet unidentified characteristics must be negatively correlated with repertoire size, as the extrapair males would otherwise be chosen randomly in relation to this cue. Females in our study showed an active preference for males with small repertoires as extrapair mates, demonstrating that this result was not due to random mate choice. Males that are unattractive as social mates are preferred as extrapair mates, suggesting that males can maximize their fitness by multiple routes. Female choice for any trait may be constrained by the magnitude of differences in quality between males in a population (Møller 1992), so females should be choosy and seek extrapair sires only in populations where variability in heritable male fitness is high (Petrie & Lipsitch 1994). In our English study population, sedge warbler females appear to engage in EPCs with males of lower phenotypic quality than their social mate. Sudden cessation of song after pairing may require female sedge warblers to switch to a different pattern of mate choice, using as yet unidentified cues during extrapair mating.
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