Behavioral Ecology Advance Access originally published online on October 13, 2006
Behavioral Ecology 2007 18(1):157-164; doi:10.1093/beheco/arl062
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No evidence for inbreeding avoidance in a great reed warbler population
a Department of Animal Ecology, Lund University, Ecology Building, S-223 62 Lund, Sweden b Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK c Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada d Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK
Address correspondence to B. Hansson. E-mail: bengt.hansson{at}zooekol.lu.se. L. Jack is now at Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand.
Received 16 March 2006; revised 24 August 2006; accepted 11 September 2006.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Inbreeding depression may drive the evolution of inbreeding avoidance through dispersal and mate choice. In birds, many species show female-biased dispersal, which is an effective inbreeding avoidance mechanism. In contrast, there is scarce evidence in birds for kin discriminative mate choice, which may, at least partly, reflect difficulties detecting it. First, kin discrimination may be realized as dispersal, and this is difficult to distinguish from other causes of dispersal. Second, even within small, isolated populations, it is often difficult to determine the potential candidates available to a female when choosing a mate. We sought evidence for inbreeding avoidance via kin discrimination in a breeding population of great reed warblers (Acrocephalus arundinaceus) studied over 17 years. Inbreeding depression is strong in the population, suggesting that it would be adaptive to avoid relatives as mates. Detailed data on timing of settlement and mate search movements made it possible to identify candidate mates for each female, and long-term pedigrees and resolved parentage enabled us to estimate relatedness between females and their candidate mates. We found no evidence for kin discrimination: mate choice was random with respect to relatedness when all mate-choice events were considered, and, after correction for multiple tests, also in all breeding years. We suggest that dispersal is a sufficient inbreeding avoidance mechanism in most situations, although the lack of kin discriminative mate choice has negative consequences for some females, because they end up mating with closely related males that lowers their fitness.
Key words: dispersal, inbreeding avoidance, inbreeding depression, kin recognition, pedigree, relatedness.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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It is well established that inbreeding reduces fitness in most outcrossing species (reviewed in Keller and Waller 2002
) and that it poses a real threat to the long-term persistence of small populations (Saccheri et al. 1998; Westemeier et al. 1998; Madsen et al. 1999). The severe consequences of inbreeding favor the evolution of inbreeding avoidance mechanisms (Waser et al. 1986; Jiménez et al. 1994; Pusey and Wolf 1996), and several mechanisms have been proposed (Blouin SF and Blouin M 1986; Ralls et al. 1986; Pusey and Wolf 1996). Inbreeding may be reduced passively by random dispersal, or sex-biased dispersal may evolve to minimize the risk that related individuals encounter each other at the breeding sites (Greenwood 1980; Pusey and Wolf 1996; but see e.g., Perrin and Mazalov [1999] for other causes of sex-biased dispersal). Furthermore, kin recognition and kin discrimination may evolve to avoid inbreeding. Kin-discriminative inbreeding avoidance can be realized in 2 ways. First, it may lead to increased dispersal in one or both sexes in response to the presence of related individuals of the opposite sex (Pärt 1996). Second, when related individuals reside in the population, relatives may be discriminated through active mate-choice decisions (Blouin SF and Blouin M 1986). Inbreeding avoidance through kin discrimination is known in several animals (Pusey and Wolf 1996), for example, in social mammals such as naked mole rat (Heterocephalus glaber; Clarke and Faulkes 1999) and prairie dog (Cynomys ludovicianus; Hoogland 1982, 1992). Finally, inbreeding avoidance may be achieved after copulation through sperm competition or sperm choice in animals (Birkhead and Møller 1993; Birkhead 1998) and self-incompatibility mechanisms in plants (Castric and Vekemans 2004) and after fertilization through abortion of inbred embryos (Zeh JA and Zeh DW 2006).
In birds, random or sex-biased dispersal is considered to be an effective and often sufficient inbreeding avoidance mechanism (Greenwood 1980). Still, many birds are highly philopatric and have short natal and breeding dispersal distances (Arcese 1989; Pärt 1990; Negro et al. 1997; Wheelwright and Mauck 1998; Hansson et al. 2002; Markert et al. 2004), and in such species, it would be adaptive to discriminate against relatives to avoid inbreeding depression (Pärt 1996; Wheelwright et al. 2006). In a kin recognition system, individuals use sensory information to assess relatedness, which they then use to make decisions about social behaviors (Grafen 1990). Strong evidence for kin-discriminative mate choice in birds was found in an experimental study of Japanese quail (Coturnix japonica; Bateson 1982). In this experiment, females preferred mates of intermediate relatedness (cousins); thus, there was evidence for kin recognition and discrimination of both closely related individuals and unrelated individuals. It was suggested that the quails used a plumage phenotype matching mechanism (plumage cues) to recognize relatives. Another example of kin recognition ability in birds comes from a semiwild population of peacocks (Pavo cristatus; Petrie et al. 1999). In this study, male peacock brothers aggregated in leks, despite being raised separately and lacking social cues, suggesting that they recognized each other using some as yet unknown cue. Among passerines, both long-tailed tits (Aegithalos caudatus; Russell and Hatchwell 2001) and Seychelles warblers (Acrocephalus sechellensis; Komdeur et al. 2004) have been found to recognize kin. Both these species are cooperative breeders where the helpers preferentially help kin, and in the long-tailed tit, they achieve this by discriminating between familiar and unfamiliar vocalizations (Russell and Hatchwell 2001). These studies strongly suggest that some birds have kin recognition and discrimination abilities, which could be used in their mate-choice (Bateson 1982) and breeding decisions (Petrie et al. 1999; Russell and Hatchwell 2001; Komdeur et al. 2004).
There is, however, only scarce evidence for inbreeding avoidance via kin-discriminative mate choice in wild bird populations. A classical example is the superb fairy wren (Malurus splendens) where molecular analyses revealed that females avoided inbreeding by seeking extrapair fertilizations from unrelated males (Brooker et al. 1990), and in a recent study on Savannah sparrows (Passerculus sandwichensis), it was found that closely related individuals rarely mated and that father–daughter matings were avoided completely (Wheelwright et al. 2006). Nevertheless, several studies, for example, on great tit (Parus major; Greenwood et al. 1978; van Tienderen and van Noordwijk 1988), medium ground finch (Geospitza fortis; Gibbs and Grant 1989), collared flycatcher (Ficedula albicollis; Pärt 1996), and song sparrow (Melospiza melodia; Keller and Arcese 1998), have failed to show deviations from patterns expected under random mating. As pointed out by Pärt (1996), there are a number of difficulties involved in evaluating inbreeding avoidance via mate choice in wild populations. Kin discrimination may be realized as dispersal, and this is difficult to distinguish from other causes of dispersal (Perrin and Mazalov 1999). Furthermore, even within small, isolated populations, it is often difficult to determine the potential candidate mates available to a choosing individual. Males whose territories lie outside the search area of a mate-searching female might be erroneously treated as candidates, and when the timing of mate choice is uncertain, males might be included among the candidates even though they were absent at the time of mate choice. Moreover, relatively few studies have combined long-term pedigrees data with molecularly resolved parentage, which means that unresolved parentage, for example, caused by extrapair fertilizations, might have lowered the quality of the relatedness estimates and the power of the analyses. It may also be important to analyze specific types of incestuous relationships separately, as highlighted by the study of Wheelright et al. (2006) on Savannah sparrows, because females may be able to recognize their father's phenotype, thus excluding them as mates, whereas the adult phenotype of siblings and sons may be less easily recognized. Thus, a variety of interacting causes may explain why there is little evidence for inbreeding avoidance through kin recognition in wild populations (Pärt 1996; Wheelwright et al. 2006).
The great reed warbler (Acrocephalus arundinacues) is a long-distance migratory passerine that breeds polygynously in lakes and marshes in Eurasia (Cramp 1992). In the present study, we use pedigree and mate-choice data from an ongoing long-term study of great reed warblers at Kvismaren, Sweden, initiated in 1983 (Bensch and Hasselquist 1992; Hasselquist et al. 1996; Hansson, Bensch, and Hasselquist 2000), to evaluate whether females avoid related individuals when choosing mates. Two previous molecular-based studies of the same population failed to find support for a major histocompatibility complex (MHC)–based mating preference (Westerdahl 2004) or a preference for genetically dissimilar (measured at 19 microsatellites) extrapair mates (Hansson et al. 2004a). For several reasons, we believe that this population is particularly well suited for evaluating inbreeding avoidance via mate choice. First, philopatric and immigrant individuals of both sexes reside in the population, which implies that many females face the risk of mating with relatives if they choose randomly. Second, close relatives do occasionally mate, which results in substantial inbreeding depression (Bensch et al. 1994; Hansson 2004). Thus, individuals that avoid relatives may be expected to benefit significantly. Third, a previous radio tracking experiment has produced detailed knowledge of how female great reed warblers behave when choosing mates in the study population (Bensch and Hasselquist 1992), which allows a high temporal and spatial accuracy when defining the pool of candidate mates. This, together with data on the arrival dates and spatial location of all males and females from detailed, daily field observations, make it possible to determine accurately candidate mates for each female in the population. Finally, we have banded all adults and almost all nestlings (100% in most years and >95% in a few years) in the population (Bensch et al. 1998; Hansson et al. 2004b) and resolved parentage for all offspring since 1987 (Hasselquist et al. 1995; Arlt et al. 2004; Hansson et al. 2004a; Hansson B, unpublished data), which enable us to build an accurate pedigree and obtain high-quality estimates of relatedness between individuals. We did not find support for kin-discriminative mate choice in great reed warblers, and this result is discussed in the context of the evolution of inbreeding avoidance mechanisms.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Study species and population
The great reed warbler breeds in eutrophic lakes and marshes over large parts of the Palaearctic and has a facultative polygynous social mating system (Dyrcz 1986
; Hasselquist 1998; Hansson, Bensch, and Hasselquist 2000). It spends the winter in sub-Saharan Africa (Cramp 1992).
The species colonized the Scandinavian Peninsula in the 1960s, and currently about 85% of the approximately 450 Scandinavian males hold territories in less than 15 larger subpopulations (Hansson et al. 2002). The great reed warbler population at 2 well-separated but closely located (ca. 0.5 km apart; see map in Hansson et al. 2002) marshes at Kvismaren, southern Central Sweden (59°10'N, 15°25'E), was founded by a few individuals in 1978, followed by a gradual increase in numbers until 1988. Since 1988, the population size has remained relatively constant with about 50–60 breeding birds per year (Hansson, Bensch, Hasselquist, Lillandt, et al. 2000). There is relatively frequent exchange of individuals between this and nearby breeding sites (Hansson et al. 2002). We have been studying the breeding ecology of the birds at Lake Kvismaren every year between 1983 and 2006 (Bensch et al. 1994; Hasselquist et al. 1996; Bensch et al. 1998; Hasselquist 1998; Hansson, Bensch, and Hasselquist 2000). Since 1985, population data have been gathered on a daily basis during each breeding season (early May to mid-July), and all reproducing adults have been banded with unique combinations of one aluminum band and several colored plastic bands. The majority of nestlings (100% in most years and >95% in a few years) were banded and examined at an age of 9 days (Bensch et al. 1998). At Lake Segersjö, the nearest other breeding site, 12 km east of Lake Kvismaren, most breeding birds and many nestlings have been ringed since 1987 (Hansson et al. 2002).
Despite being a long-distance migrant, the great reed warbler shows a high level of philopatry to our study sites, and the majority of birds that do disperse settle at nearby breeding sites (Bensch et al. 1998; Hansson et al. 2002). On average, 55% of the breeding birds and approximately 15% of the nestlings (i.e., ca. 50% of the nestlings surviving the first winter) return to breed at Kvismaren or Segersjö in successive years (Bensch et al. 1998; Hansson et al. 2002). The level of molecular genetic variation is relatively low in the great reed warbler population at Lake Kvismaren, probably a consequence of the recent founder event in the region and the birds' philopatric habit (Bensch et al. 1994; Bensch and Hasselquist 1999; Bensch et al. 2000; Hansson, Bensch, Hasselquist, Lillandt, et al. 2000). Occasional immigration of long-distance dispersing birds to the population has changed the genetic variation over time (Bensch et al. 2000; Hansson, Bensch, Hasselquist, Lillandt, et al. 2000; Hansson et al. 2003). A strong negative correlation between the genetic similarity (assessed by DNA fingerprinting and microsatellites) of pair mates and egg hatchability indicates the existence of significant genetic load and substantial inbreeding depression in the population (Bensch et al. 1994; Hansson 2004; see also Hansson et al. 2004b). The presence of significant genetic load in the population is also supported by evaluations of within-family heterozygosity–fitness correlations (Hansson et al. 2001, 2004).
Pedigree building, relatedness, and candidate males
The high level of philopatry and rate of survival in great reed warblers make it possible to build detailed pedigrees and obtain high-quality relatedness estimates from field data. Parentage was resolved for the whole population in all years between 1987 and 2002 by DNA fingerprinting and microsatellite analyses (Hasselquist et al. 1995; Arlt et al. 2004; Hansson et al. 2004a; Hansson B, unpublished data), including identifying the fathers of extrapair young (extrapair young were detected in 6.4% of the nests and constituted 3.3% of the nestlings; Arlt et al. 2004). A pedigree was drawn for the population using PEDIGREE VIEWER v5.1.a (http://www-personal.une.edu.au/
bkinghor/pedigree.htm). This program displays a full pedigree structure using a data file containing the identity, maternity, and paternity of individuals. It also estimates inbreeding coefficients for these individuals. In order to generate pairwise relatedness values between all males and females, we created dummy offspring for each possible male–female combination in each year. By building a data set including the maternity and paternity of real individuals known from field data and these dummy offspring, inbreeding coefficients for the "offspring" of all possible mate combinations were generated. The relatedness (R) between 2 individuals was then calculated as twice the inbreeding coefficient. With this logic, PEDIGREE VIEWER was used to generate pairwise relatedness estimates for all possible mate combinations in the study population.
To investigate whether great reed warblers avoid inbreeding, for each year, we calculated the relatedness between each female and her true chosen mate (RTRUE) and the relatedness between her and any candidate males. Candidate males were defined on the basis of the female's breeding ecology. The birds arrive at our study site from their wintering grounds from early May to late June, and we have gathered data on the arrival dates and spatial location of both males and females through detailed daily field observations (Bensch et al. 1994; Hasselquist et al. 1996; Bensch et al. 1998; Hasselquist 1998; Hansson, Bensch, and Hasselquist 2000). At arrival, the male great reed warbler takes up a territory and starts singing a complex mate attraction song (long song). When a female settles in the territory, the male initiates mate guarding and switches from the long song to the relatively simple short song (Hasselquist and Bensch 1991). During this period, the male is less accessible, although additional females do sometimes settle during such circumstances. Within 1–2 days, the female initiates nest building and the nest is usually completed 4–5 days later. During the nest building period, the male gradually spends more time singing long song (i.e., gradually becoming increasingly accessible for mating). A previous radio tracking experiment has evaluated the movements and strategies of mate-seeking female great reed warbler (Bensch and Hasselquist 1992), and the data from this study allowed us to define the mate search area for the females. In brief, it was found that females, within a few days, visit the territorial males in the marsh in which they were experimentally introduced before choosing one of them (Bensch and Hasselquist 1992). Females regularly select already mated males despite the conspicuous presence of nesting females and despite the availability of unmated males (each year ca. 40% of the males mate with 2–5 females; ca. 20% remain unmated; Hasselquist 1998). Based on the above facts, candidate males are defined in this study as all males with established territories at the time and in the same marsh in which the female settled. Because both males and females arrive asynchronously over the breeding season (males from beginning of May to mid-June, females from mid-May to late June; e.g., Bensch and Hasselquist 1991a), most settling females had different combinations of males from which to choose.
Statistical analyses
To analyze the data, we adopted a randomization procedure because the relatedness values had a skewed distribution, with an overrepresentation of values close to zero (see Figure 1), and because the pool of candidate mates differed for most females, both in terms of number of males and the relatedness values. The randomization was run in SAS 6.12 (SAS Institute 1990). For each randomization, a random mate-choice event was created for each female by selecting one male from the female's pool of candidate males (which included her true mate as well as the other males available to her). The mean relatedness for all mate-choice events was calculated for each randomization, and this process was repeated 10,000 times to generate the distribution of mean relatedness under the null hypothesis that females mated with males at random. The mean relatedness among true mate pairs (mean RTRUE) was compared with this distribution using a 2-sided test to determine whether mean RTRUE was less than that expected under random mating (evidence of inbreeding avoidance) or whether it was greater than expected (evidence of outbreeding avoidance). Mean RTRUE was considered significantly different from random if it was below the 2.5th percentile or above the 97.5th percentile of the randomly generated distribution. We chose to consider also the possibility of outbreeding avoidance because previous studies have shown evidence for low pairing success of immigrant male great reed warblers (Bensch et al. 1998; Hansson et al. 2004b).
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Only females that had at least one related candidate male were included in the analyses because females with no related males were not informative with regard to mate choice. The randomizations were run for different data sets. First, we analyzed all females in all years (N = 183 female years; see Figure 1), where several females were included in more than 1 year, that is, allowing pseudoreplication. This should be a minor problem because female great reed warblers lack mate and territory fidelity across years (Bensch and Hasselquist 1991a
). Second, to definitely rule out pseudoreplication, we analyzed a data set where each female was included only once, in the first year that she bred in the presence of at least one related candidate male (N = 95 females). Third, we analyzed each year separately, again only including females with at least one related candidate male. We treated relatedness in different ways: (A) R, ranging between 0 and 1, (B) as a categorical variable, where R-CAT is 0 if R = 0, or else 1, and (C) as a categorical variable, where R-CAT-1/8 is 0 if R < 0.125, or else 1. By treating R as a categorical variable, we aimed to account for the possibility that the birds could distinguish only related from unrelated individuals (R-CAT) or that they could separate more related individuals (first cousins or higher) from unrelated and less related individuals (R-CAT-1/8). The null distributions for mean R, R-CAT, and R-CAT-1/8 were calculated using the same set of 10,000 randomizations.
Finally, we evaluated mate choice for the highest levels of relatedness, that is, R 
0.5. In these analyses, R-CAT-1/2 was 0 if R < 0.5, or else 1. Three data sets were evaluated: 1) all females with at least one candidate male with R 0.5, 2) only females that had their fathers among the candidate males, and 3) only females that had sons or brothers among the candidate males. We performed 10,000 randomizations for each of these data sets.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Relatedness of 519 pairs of great reed warblers breeding between 1986 and 2002 at Lake Kvismaren, Sweden, ranged between 0 and 0.503 (mean relatedness was 0.015; Table 1). In several mate-choice situations (N = 336 female years), the pool of candidate males consisted only of males with zero relatedness to the female (Figure 1).
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On 183 occasions, the females had at least one candidate male with nonzero relatedness (Figure 1). In this data set (N = 183 female years), mean RTRUE was 0.042 (Table 1) and ranked 4,190 out of the 10,000 randomized values (median of randomized R = 0.044). Thus, there was no inbreeding avoidance under the null hypothesis of random mate choice among the candidate males (P = 0.84 for a 2-tailed test). In the analyses of the categorized data, R-CATTRUE ranked 7,685–8,182 (P = 0.36–0.46) and R-CAT-1/8TRUE ranked 3,911–4,719 (P = 0.78–0.94) out of the 10,000 randomized values. For R-CATTRUE and R-CAT-1/8TRUE, we give the rank as a range because many randomizations yielded the same value due to the categorization process (there are only 2 potential values for each female, 0 and 1, and relatively few potential combinations of mate-choice events).
In the data set where each female was included only once, in the first year that she bred in the presence of at least one related candidate male (N = 95 females), mean RTRUE was 0.028 and ranked 643 out of the 10,000 randomized values (P = 0.13; median of randomized R = 0.044; Figure 2). In the analyses of the categorized data, R-CATTRUE ranked 990–1,545 (P = 0.20–0.31) and R-CAT-1/8TRUE ranked 878–1,549 (P = 0.18–0.31) out of the 10,000 randomized values.
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In the analyses of each year separately, again only including females with at least one related candidate male (sample size ranged between 3 females in 1986 and 1987 and 19 females in 1995), there was, without any correction for multiple tests, significant inbreeding avoidance in 1991 (mean RTRUE = 0.002; P = 0.020; N = 17 females) and significant outbreeding avoidance in 1996 (mean RTRUE = 0.038; P = 0.032; N = 11 females). This analysis was performed only for uncategorized R. However, both these values are nonsignificant after accounting for testing in 17 separate years (Pcritical = 0.003).
Finally, we evaluated whether females discriminated highly related males, that is, R 0.5. On 8 occasions, females chose close relatives as mates (Table 1), which did not differ from the random expectation (R-CAT-1/2TRUE ranked 4,644–6,149 out of the 10,000 randomized values; P = 0.77–1.00 for a 2-tailed test; N = 80 female years). Neither did the number of observed cases with daughter–father matings (Table 1) nor the number of observed cases with mother–son or sister–brother matings (Table 1) differ from the random expectation (daughter–father: R-CAT-1/2TRUE ranked 800–2,923, P = 0.16–0.58, N = 29 female years; mother–son/sister–brother: R-CAT-1/2TRUE ranked 6,879–8,217, P = 0.36–0.62, N = 55 female years).
Thus, none of our analyses gave statistical support for inbreeding (or outbreeding) avoidance under the null hypothesis of random mate choice among the candidate males.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There was no evidence of inbreeding avoidance through mate choice in female great reed warblers, despite substantial strengths in experimental design compared with previous work. There are several potential explanations for why we did not find evidence for inbreeding avoidance via mate choice in female great reed warblers. One explanation relates to problems with defining candidate mates. The pool of candidate males often differs between females for temporal and spatial reasons. In many species, for example, in resident passerines, it is difficult to know when mate choice is taking place and therefore to correctly define the pool of candidate mates for the females. One way of circumventing this problem could be to use different sets of candidate males, which are based on different mate-choice assumptions (Ralls et al. 1986
; Keller and Arcese 1998). We believe that our analysis is relatively unaffected by problems with defining candidate mates, as previous work has produced detailed knowledge of how female great reed warblers behave when selecting mates in our study population (Bensch and Hasselquist 1992), enabling a high level of temporal and spatial accuracy when defining the candidate males. Another potential methodological problem is that pedigree errors caused imprecise relatedness estimates. Absence of inbreeding avoidance has been claimed from several studies where pedigrees were established by social partnership only (e.g., Craig and Jamieson 1988). In at least one case, the superb fairy wren, subsequent genetic analyses showed that inbreeding avoidance was achieved by extrapair copulations with unrelated males (Brooker et al. 1990). We have banded almost all adults and nestlings (Bensch et al. 1998; Hansson et al. 2004b) and resolved parentage for almost all offspring in the population (Hasselquist et al. 1995; Arlt et al. 2004; Hansson et al. 2004a; B. Hansson, unpublished), which enabled us to draw accurate pedigrees and make accurate estimates of relatedness between individuals. If anything, the pedigree may have been somewhat shallow in the first study year(s) included in this study, but it should have been of high quality in most years and particularly so in later years.
Alternatively, perhaps we failed to detect the importance of relatedness as a mate-choice criterion because its effect was masked by the effects of other important factors (cf. Mays and Hill 2004). In great reed warblers and some other songbirds, males that have a highly variable song attract several females to their territories (Catchpole 1986; Buchanan and Catchpole 1997; Hasselquist 1998) and are more successful in obtaining extrapair fertilizations (Hasselquist et al. 1996; but see Forstmeier and Leisler 2004). Furthermore, female great reed warblers distinguish between territories of different quality (with respect to rate of nest predation) and prefer to settle in high-quality territories (Hansson, Bensch, and Hasselquist 2000). Males are also less accessible when they have newly settled females in their territory because they spend much time mate guarding the female and thus less time singing mate attraction songs (Hasselquist and Bensch 1991; Bensch and Hasselquist 1992). Thus, females might partly base their choice of males on song and territory quality, mating status, and possibly also partly on other parameters, for example, other male characteristics. Therefore, it is likely that factors contributing to mate choice are multiple and substantially more complex than pairwise relatedness alone and that more detailed and advanced analyses are needed to disentangle their relative importance.
Still it is possible that inbreeding avoidance was not detected simply because female great reed warblers do not have kin recognition abilities. What potential cues are available for distinguishing between related and unrelated individuals? One obvious suggestion is the song as shown, for example, for zebra finches (Taeniopygia guttata; Miller 1979), great tits (McGregor and Krebs 1982), large ground finches (Gospiza magnirostris; Grant 1984), and long-tailed tits (Russell and Hatchwell 2001) and which might also be valid for Savannah sparrows, where females avoid fathers, but not brothers or sons, as mates (Wheelwright et al. 2006). It is, however, uncertain whether the song contains family-specific elements in great reed warblers because the song is mainly culturally inherited in passerines (Marler and Tamura 1964; Liu et al. 2004) and because great reed warbler chicks hatched in the second half of the breeding season will not experience their father's song because most males stop singing when feeding nestlings late in the season (Hasselquist and Bensch 1991). On the other hand, it may be that female great reed warblers are able to distinguish philopatric individuals and immigrants using song (Bensch et al. 1998; Hansson et al. 2004b). Even closely located great reed warbler populations are differentiated with respect to song (Fischer et al. 1996), and thus, the song might contain population-specific elements (cf. West et al. 1981; McGregor and Krebs 1982; Hegelbach 1986; Baker et al. 1987). Indeed, our previous analyses suggest that philopatric males are preferred as breeding partners (Bensch et al. 1998; Hansson et al. 2004b) and that long-distance dispersers are the least preferred ones (Hansson et al. 2004b). Still, this would not allow discrimination at a more fine-tuned scale of relatedness. Another possibility is that birds use odor cues for kin discrimination. An explicit hypothesis is the suggestion that compounds specific to the MHC genes are present in an individual's odor and that this enables individuals to find mates with complementary MHC genotypes. This hypothesis seems to apply in some fish (Reusch et al. 2001) and mammals (Potts et al. 1991; reviewed in Piertney and Oliver 2006). A study of Savannah sparrows suggests that MHC-based mate preference may occur in some passerines (Freeman-Gallant et al. 2003), but previous studies in our great reed warbler study population (Westerdahl 2004) and another Acrocephalus warbler, the Seychelles warbler (Richardson et al. 2005), did not find support for such preference. Furthermore, even if MHC was a main cue in mate choice, it is uncertain whether this would reflect overall relatedness on its own, despite its high molecular variability because pairwise similarity at a single locus, or in this case a single-linked gene complex, will be highly affected by random segregation and therefore not correlate strongly with relatedness (Lynch 1988; Lynch and Ritland 1999; Csilléry et al. 2006). Another potential cue is the location of a male's territory (Wheelwright et al. 2006), but although adult great reed warblers are highly faithful to their breeding lakes (Bensch et al. 1998; Hansson et al. 2002), they have low territory fidelity between years (Bensch and Hasselquist 1991a). Finally, the cues might be attained by learning the precise characteristics (e.g., plumage, body size, and shape) of early associates, parents, and nest mates, so that those particular individuals can be avoided as mates (Hoogland 1992; Komdeur and Hatchwell 1999; Wheelwright et al. 2006). Such a kin recognition mechanism was recently detected among Seychelles warbler helpers (Komdeur et al. 2004; cf. Bateson 1982; but see Petrie et al. 1999). However, analyses based on categorical relatedness values and separate analyses of specific types of incestuous relationships neither suggested that the female great reed warblers discriminated highly related candidates from unrelated and less related individuals nor that, for example, daughters avoided their fathers (the sample size was, however, low in this analysis).
For all these methodological and biological reasons, our current finding should be viewed as provisional. An alternative approach to study inbreeding avoidance, or rather to judge whether inbreeding avoidance should be expected, is to contrast the costs of avoiding and tolerating close inbreeding (Pärt 1996). In resident and monogamous species, avoiding inbreeding might be costly because this could lead to delayed breeding, breeding in territories of lower quality or not breeding at all (Pärt 1996; Keller and Arcese 1998). We expect that the cost of avoiding inbreeding is relatively low in great reed warblers because mating is more or less unconstrained in this polygynous species (Bensch and Hasselquist 1991b; Bensch 1996; Hasselquist 1998; Hansson, Bensch, and Hasselquist 2000), whereas the cost of tolerating inbreeding is substantial due to significant inbreeding depression (Bensch et al. 1994; Hansson 2004). Thus, if relatives regularly reside together, kin discriminative mate choice would be expected to evolve in great reed warblers. Still, we did not detect this. Instead, it might be suggested that dispersal is a sufficient inbreeding avoidance mechanism in most situations.
It should be pointed out that we have studied the great reed warbler at the Scandinavian Peninsula that is at the edge of the species' breeding range. The species is rare in this region (ca. 450 pairs; Hansson et al. 2002), whereas it is widely distributed on the European continent (>106 pairs; Cramp 1992; Hansson and Richardson 2005). In Sweden, the habitat is patchily distributed, and great reed warblers are highly philopatric and disperse very short distances (Hansson et al. 2002). On the continent, for example, in Hungary, philopatry is less pronounced and dispersal rates are higher, probably because populations are less isolated which facilitates interpopulation dispersal (Moskát C, Hansson B, Barabás L, Cherry MI, Bártol I, Honza M, and Karcza Z, unpublished data). Thus, the majority of great reed warblers are unlikely to encounter relatives, and under such circumstances, kin discrimination will not evolve. It is therefore plausible that gene flow from the numerous and often large continental populations swamps the potential for this type of local adaptation in Sweden (cf. Kirkpatrick and Barton 1997), even if long-distance gene flow is rare (Hansson et al. 2003). A similar reasoning might apply to the song sparrow population at Mandarte Island, Canada. In this population, one would expect kin discrimination on the basis of strong inbreeding depression and presence of relatives (although avoiding inbreeding might be costly), but occasional gene flow from the large mainland populations might prevent such local adaptation (Keller et al. 1994; Keller 1998; Keller and Arcese 1998; Keller et al. 2001). To conclude, despite that the lack of kin discrimination has negative consequences on fitness for females in some populations (Bensch et al. 1994; Hansson 2004), our study might suggest that dispersal is a sufficient inbreeding avoidance mechanism in most great reed warbler populations.
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ACKNOWLEDGEMENTS |
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We would like to thank all field workers, and especially B. Nielsen and M. Stervander, for their assistance in the field. The work was supported by a postdoctoral grant from the Swedish Research Council and STINT (623-2003-191; B.H.), by a Marie-Curie postdoctoral fellowship (EC-MEIF-CT-2003-501256; B.H./J.M.P.), and by research grants from the following: Swedish Research Council (S.B.), Swedish Council for Environment, Agricultural Sciences and Spatial Planning (Formas; D.H.), Royal Swedish Academy of Science (Ahlstrands, Hierta-Retzius; D.H. and S.B.), Lunds Djurskyddsfond (B.H. and D.H.), Carl Tryggers Stiftelse (D.H.), Crafoordska Stiftelsen (D.H.), Magnus Bergvalls Stiftelse (D.H.), National Geographic Society (D.H.), Stiftelsen Olle Engkvist Byggmästare (S.B.), Berggrens Stiftelse (B.H.), Schwartz Stiftelse (B.H.), and Kvismare Bird Observatory (report no. 142).
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