Reproductive inequality among males in the genus Pan

Reproductive inequality, or reproductive skew, drives natural selection, but has been difficult to assess, particularly for males in species with promiscuous mating and slow life histories, such as bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Although bonobos are often portrayed as more egalitarian than chimpanzees, genetic studies have found high male reproductive skew in bonobos. Here, we discuss mechanisms likely to affect male reproductive skew in Pan, then re-examine skew patterns using paternity data from published work and new data from the Kokolopori Bonobo Reserve, Democratic Republic of Congo and Gombe National Park, Tanzania. Using the multinomial index (M), we found considerable overlap in skew between the species, but the highest skew occurred among bonobos. Additionally, for two of three bonobo communities, but no chimpanzee communities, the highest ranking male had greater siring success than predicted by priority-of-access. Thus, an expanded dataset covering a broader demographic range confirms that bonobos have high male reproductive skew. Detailed comparison of data from Pan highlights that reproductive skew models should consider male-male dynamics including the effect of between-group competition on incentives for reproductive concessions, but also female grouping patterns and factors related to male-female dynamics including the expression of female choice. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Reproductive benefits and reduced investment in parental care behavior associated with reproductive groups of males in Abudefduf troschelii

Fishes of the family Pomacentridae present a wide diversity of mating systems, ranging from polygyny to promiscuity and from individual territorial defense to the establishment of reproductive colonies of males. The damselfish species Abudefduf troschelii has a reproductive colony mating system, in which males temporarily aggregate in reproductive areas to court and attract females. Males defend an individual territory where they receive eggs and perform paternal care behaviors for their offspring. The present study evaluated the advantages of the colonial mating system in A. troschelii. During an entire reproductive period, in a breeding colony within a rocky reef, we located, marked, geo-referenced, and measured the distances between the territories of all males. We quantified the variance among males in their patterns of paternal care investment, eggs acquired, hatching success, reproductive success, body size, and changes in body coloration. We found that males spatially distributed their nests in groups or independently (i.e., solitary nests). Nesting groups are formed by larger males that show intense nuptial coloration during the entire receptivity period. They are located centrally to the colony and consist of three to six males whose territories overlap. In contrast, small solitary males that fail to acquire or maintain nuptial coloration during the receptivity period establish their nests peripherally to the colony, away from the territories of other males. Our results highlight that the reproductive benefits of colonial nesting are unequal for males, as the spatial distribution of nests within the colony determines the reproductive success of males. Group nesting confers the highest reproductive benefits to males regarding eggs obtained, hatching success, and relative fitness and also enables males to reduce their parental investment in brood care behaviors. The preference of females for oviposition could be associated with greater intrasexual competitiveness, defense ability, body condition, or experience of group-nesting males located at the center of the colony or because their progeny will have a lower probability of predation than they would in solitary nests males.

Star finches Neochmia ruficauda have a visual preference for white dot patterns: a possible case of trypophilia

Many animals have polka dot patterns on their body surface, some of which are known to have signalling functions; however, their evolutionary origins remain unclear. Dot patterns can trigger a fear response (trypophobia) in humans and are known to function as aposematic signals in non-human animals, suggesting that dots may deserve attention for biological reasons. Interestingly in many birds, plumage dot patterns serve for social/sexual signalling. To understand their evolution, we have focused on the sensory bias hypothesis, which predicts the role of pre-existing sensory preference driven by natural selection in shaping signal design. Our previous phylogenetic comparative study supported the hypothesis and showed that diet-driven visual preference promoted the evolution of plumage patterns, as there was an evolutionary correlation between termite-eating (white roundish gregarious prey) and the presence of plumage dot patterns in species of the family Estrildidae. This suggests that these species possess an intrinsic preference for dots. To test this, we compared the responses of an Estrildid species with dot plumage pattern (star finch Neochmia ruficauda) towards simultaneously presented monochrome-printed white dot vs white stripe patterns under both food-deprived and -supplied conditions. Overall, star finches preferred dots to stripes. They showed foraging-like behaviours almost only toward dots when hungry and gazed at dots frequently even when food was available, suggesting both hunger-related and hunger-neutral dot preferences. These results are rather surprising, given how strongly the subjects were attracted to abstract dot patterns without organic structure, but provided good support for the sensory bias hypothesis.

Fitness costs of female choosiness are low in a socially monogamous songbird

Female mate choice is thought to be responsible for the evolution of many extravagant male ornaments and displays, but the costs of being too selective may hinder the evolution of choosiness. Selection against choosiness may be particularly strong in socially monogamous mating systems, because females may end up without a partner and forego reproduction, especially when many females prefer the same few partners (frequency-dependent selection). Here, we quantify the fitness costs of having mating preferences that are difficult to satisfy, by manipulating the availability of preferred males. We capitalize on the recent discovery that female zebra finches (Taeniopygia guttata) prefer males of familiar song dialect. We measured female fitness in captive breeding colonies in which one-third of females were given ample opportunity to choose a mate of their preferred dialect (two-thirds of all males; "relaxed competition"), while two-thirds of the females had to compete over a limited pool of mates they preferred (one-third of all males; "high competition"). As expected, social pairings were strongly assortative with regard to song dialect. In the high-competition group, 26% of the females remained unpaired, yet they still obtained relatively high fitness by using brood parasitism as an alternative reproductive tactic. Another 31% of high-competition females paired disassortatively for song dialect. These females showed increased levels of extra-pair paternity, mostly with same-dialect males as sires, suggesting that preferences were not abolished after social pairing. However, females that paired disassortatively for song dialect did not have lower reproductive success. Overall, females in the high-competition group reached equal fitness to those that experienced relaxed competition. Our study suggests that alternative reproductive tactics such as egg dumping can help overcome the frequency-dependent costs of being selective in a monogamous mating system, thereby facilitating the evolution of female choosiness.

Sexual selection and introgression in avian hybrid zones: Spotlight on Manacus

Hybrid zones offer a window into the processes and outcomes of evolution, from species formation or fusion to genomic underpinnings of specific traits and isolating mechanisms. Sexual selection is believed to be an important factor in speciation processes, and hybrid zones present special opportunities to probe its impact. The manakins (Aves, Pipridae) are a promising group in which to study the interplay of sexual selection and natural hybridization: they show substantial variation across the family in the strength of sexual selection they experience, they readily hybridize within and between genera, and they appear to have formed hybrid species, a rare event in birds. A hybrid zone between two manakins in the genus Manacus is unusual in that plumage and behavioral traits of one species have introgressed asymmetrically into populations of the second species through positive sexual selection, then apparently stalled at a river barrier. This is one of a handful of documented examples of asymmetric sexual trait introgression with a known selective mechanism. It offers opportunities to examine reproductive isolation, introgression, plumage color evolution, and natural factors enhancing or constraining the effects of sexual selection in real time. Here, we review previous work in this system, propose new hypotheses for observed patterns, and recommend approaches to test them.

Demography, genetics, and decline of a spatially structured population of lekking bird

Understanding the mechanisms underlying population decline is a critical challenge for conservation biologists. Both deterministic (e.g. habitat loss, fragmentation, and Allee effect) and stochastic (i.e. demographic and environmental stochasticity) demographic processes are involved in population decline. Simultaneously, a decrease of population size has far-reaching consequences for genetics of populations by increasing the risk of inbreeding and the strength of genetic drift, which together inevitably results in a loss of genetic diversity and a reduced effective population size ([Formula: see text]). These genetic factors may retroactively affect vital rates (a phenomenon coined 'inbreeding depression'), reduce population growth, and accelerate demographic decline. To date, most studies that have examined the demographic and genetic processes driving the decline of wild populations have neglected their spatial structure. In this study, we examined demographic and genetic factors involved in the decline of a spatially structured population of a lekking bird, the western capercaillie (Tetrao urogallus). To address this issue, we collected capture-recapture and genetic data over a 6-years period in the Vosges Mountains (France). Our study showed that the population of T. urogallus experienced a severe decline between 2010 and 2015. We did not detect any Allee effect on survival and recruitment. By contrast, individuals of both sexes dispersed to avoid small subpopulations, thus suggesting a potential behavioral response to a mate finding Allee effect. In parallel to this demographic decline, the population showed low levels of genetic diversity, high inbreeding and low effective population sizes at both subpopulation and population levels. Despite this, we did not detect evidence of inbreeding depression: neither adult survival nor recruitment were affected by individual inbreeding level. Our study underlines the benefit from combining demographic and genetic approaches to investigate processes that are involved in population decline.

Female mating tactics in lekking fallow deer (Dama dama): experience explains inter-individual variability more than costs

Most studies on ungulate reproduction have focused on the covariates of male reproductive success, while there is much less information on female tactics of mate choice. The aim of this work is to fill this gap and to assess condition-dependent variations in female tactics in a lekking fallow deer (Dama dama) population. In particular, we investigated three indirect selection mechanisms: i) aggregation: when females join an already formed female group; ii) copying: when females copy the mate choice of other females and iii) territory choice: when females select a territory where many copulations had previously occurred. Our results show that female fallow deer, which are less experienced (younger) and/or incur higher travel costs (home range far from the lek), adopt indirect forms of mate selection more often than older females or females residing near the lek, respectively. Compared to adults, younger females remained longer in the lek (almost three times) and in male territories, returning to the lek after copulation. However, despite the time spent at the lek, younger females were not able to select the highest-rank males, and relied on territory choice more often than older females. Farther does visited the lek less frequently (farthest females only once) and arrived on average 5 days later than closer females (which performed up to 7 visits), but they were seen more often within female groups (aggregation). We did not find a different amount of copying in younger or in farther females. Our results contribute to advance our understanding of female behaviours in ungulate leks.

Divergent color signals from homologous unfeathered ornaments in two congeneric grouse

Color-based visual signals are important aspects of communication throughout the animal kingdom. Individuals evaluate color to obtain information about age and condition and to behave accordingly. Birds display a variety of striking, conspicuous colors and make ideal subjects for the study of color signaling. While most studies of avian color focus on plumage, bare unfeathered body parts also display a wide range of color signals. Mate choice and intrasexual competitive interactions are easily observed in lekking grouse, which also signal with prominent unfeathered color patches. Most male grouse have one pair of colorful bare part ornaments (combs), and males of several species also have inflatable air sacs in their throat. Previous studies have mostly focused on comb color and size, but little is known about the signaling role of air sac color. We measured comb size and the color properties of combs and air sacs in the Lesser and Greater Prairie-Chickens (Tympanuchus pallidicinctus and T. cupido, respectively), and investigated whether these properties varied with age and mass. We found that mass predicted color properties of air sacs and that age predicted comb size in the Greater Prairie-Chicken, suggesting that these ornaments indicate condition dependence. No conclusive relationships between color and age or size were detected in the Lesser Prairie-Chicken. Color properties of both ornaments differed between the two species. Further research is needed to determine mechanisms that link condition to color and whether the information advertised by color signals from these ornaments is intended for males, females, or both.

Feeling the heat: Elevated temperature affects male display activity of a lekking grassland bird

Most species-climate models relate range margins to long-term mean climate but lack mechanistic understanding of the ecological or demographic processes underlying the climate response. We examined the case of a climatically limited edge-of-range population of a medium-sized grassland bird, for which climate responses may involve a behavioural trade-off between temperature stress and reproduction. We hypothesised that temperature will be a limiting factor for the conspicuous, male snort-call display behaviour, and high temperatures would reduce the display activity of male birds.

Using remote tracking technology with tri-axial accelerometers we classified and studied the display behaviour of 17 free-ranging male little bustards, Tetrax tetrax, at 5 sites in the Iberian Peninsula. Display behaviour was related to temperature using two classes of Generalized Additive Mixed Models (GAMMs) at different temporal resolutions. GAMMs showed that temperature, time of the day and Julian date explained variation in display behaviour within the day, with birds snort-calling significantly less during higher temperatures. We also showed that variation in daily snort-call activity was related to average daytime temperatures, with our model predicting an average decrease in daytime snort-call display activity of up to 10.4% for the temperature increases projected by 2100 in this region due to global warming. For lekking birds and mammals undertaking energetically-costly displays in a warming climate, reduced display behaviour could impact inter- and intra-sex mating behaviour interactions through sexual selection and mate choice mechanisms, with possible consequences on mating and reproductive success. The study provides a reproducible example for how accelerometer data can be used to answer research questions with important conservation inferences related to the impacts of climate change on a range of taxonomic groups.

Kin-dependent dispersal influences relatedness and genetic structuring in a lek system

Kin selection and dispersal play a critical role in the evolution of cooperative breeding systems. Limited dispersal increases relatedness in spatially structured populations (population viscosity), with the result that neighbours tend to be genealogical relatives. Yet the increase in neighbours' fitness-related performance through altruistic interaction may also result in habitat saturation and thus exacerbate local competition between kin. Our goal was to detect the footprint of kin selection and competition by examining the spatial structure of relatedness and by comparing non-effective and effective dispersal in a population of a lekking bird, Tetrao urogallus. For this purpose, we analysed capture-recapture and genetic data collected over a 6-year period on a spatially structured population of T. urogallus in France. Our findings revealed a strong spatial structure of relatedness in males. They also indicated that the population viscosity could allow male cooperation through two non-exclusive mechanisms. First, at their first lek attendance, males aggregate in a lek composed of relatives. Second, the distance corresponding to non-effective dispersal dramatically outweighed effective dispersal distance, which suggests that dispersers incur high post-settlement costs. These two mechanisms result in strong population genetic structuring in males. In females, our findings revealed a lower level of spatial structure of relatedness and genetic structure in respect to males. Additionally, non-effective dispersal and effective dispersal distances in females were highly similar, which suggests limited post-settlement costs. These results indicate that kin-dependent dispersal decisions and costs have a genetic footprint in wild populations and are factors that may be involved in the evolution of cooperative courtship.

Stabilising selection on immune response in male black grouse Lyrurus tetrix

Illnesses caused by a variety of micro- and macro- organisms can negatively affect individuals’ fitness, leading to the expectation that immunity is under positive selection. However, immune responses are costly and individuals must trade-off their immune response with other fitness components (e.g. survival or reproductive success) meaning that individuals with intermediate response may have the greatest overall fitness. Such a process might be particularly acute in species with strong sexual selection because the condition-dependence of male secondary sexual-traits might lead to striking phenotypic differences amongst males of different immune response levels. We tested whether there is selection on immune response by survival and reproduction in yearling and adult male black grouse (Lyrurus tetrix) following an immune challenge with a novel antigen and tested the hypothesis that sexual signals and body mass are honest signals of the immune response. We show that yearling males with highest immune response to these challenges had higher survival, but the reverse was true for adults. Adults with higher responses had highest mass loss and adult males with intermediate immune response had highest mating success. Tail length was related to baseline response in adults and more weakly in yearlings. Our findings reveal the complex fitness consequences of mounting an immune response across age classes. Such major differences in the direction and magnitude of selection in multiple fitness components is an alternative route underpinning the stabilising selection of immune responses with an intermediate immune response being optimal.

Sure enough: efficient Bayesian learning and choice

Probabilistic decision-making is a general phenomenon in animal behavior, and has often been interpreted to reflect the relative certainty of animals' beliefs. Extensive neurological and behavioral results increasingly suggest that animal beliefs may be represented as probability distributions, with explicit accounting of uncertainty. Accordingly, we develop a model that describes decision-making in a manner consistent with this understanding of neuronal function in learning and conditioning. This first-order Markov, recursive Bayesian algorithm is as parsimonious as its minimalist point-estimate, Rescorla-Wagner analogue. We show that the Bayesian algorithm can reproduce naturalistic patterns of probabilistic foraging, in simulations of an experiment in bumblebees. We go on to show that the Bayesian algorithm can efficiently describe the behavior of several heuristic models of decision-making, and is consistent with the ubiquitous variation in choice that we observe within and between individuals in implementing heuristic decision-making. By describing learning and decision-making in a single Bayesian framework, we believe we can realistically unify descriptions of behavior across contexts and organisms. A unified cognitive model of this kind may facilitate descriptions of behavioral evolution.

The multiple functions of male song within the humpback whale (Megaptera novaeangliae) mating system: review, evaluation, and synthesis

Humpback whales (Megaptera novaeangliae) are seasonal breeders, annually migrating from high-latitude summer feeding grounds to low-latitude winter breeding grounds. The social matrix on the winter grounds is a loose network of interacting individuals and groups and notably includes lone males that produce long bouts of complex song that collectively yield an asynchronous chorus. Occasionally, a male will sing while accompanying other whales. Despite a wealth of knowledge about the social matrix, the full characterization of the mating system remains unresolved, without any firm consensus, as does the function of song within that system. Here, I consider and critically analyse three proposed functions of song that have received the most attention in the literature: female attraction to individual singers, determining or facilitating male-male interactions, and attracting females to a male aggregation within the context of a lekking system. Female attraction suggests that humpback song is an advertisement and invitation to females, but field observations and song playback studies reveal that female visits to individual singers are virtually absent. Other observations suggest instead that females might convey their presence to singers (or to other males) through the percussive sounds of flipper or tail slapping or possibly through vocalizations. There is some evidence for male-male interactions, both dominance and affiliative: visits to singers are almost always other lone males not singing at that time. The joiner may be seeking a coalition with the singer to engage cooperatively in attempts to obtain females, or may be seeking to disrupt the song or to affirm his dominance. Some observations support one or the other intent. However, other observations, in part based on the brevity of most pairings, suggest that the joiner is prospecting, seeking to determine whether the singer is accompanying a female, and if not soon departs. In the lekking hypothesis, the aggregation of vocalizing males on a winter ground and the visits there by non-maternal females apparently for mating meet the fundamental definition of a lekking system and its role though communal display in attracting females to the aggregation, although not to an individual singer. Communal singing is viewed as a form of by-product mutualism in which individuals benefit one another as incidental consequences of their own selfish actions. Possibly, communal singing may also act to stimulate female receptivity. Thus, there are both limitations and merit in all three proposals. Full consideration of song as serving multiple functions is therefore necessary to understand its role in the mating system and the forces acting on the evolution of song. I suggest that song may be the prime vector recruiting colonists to new winter grounds pioneered by vagrant males as population pressures increase or as former winter grounds become unavailable or undesirable, with such instances documented relatively recently. Speculatively, song may have evolved historically as an aggregating call during the dynamic ocean conditions and resulting habitat uncertainties in the late Miocene-early Pliocene epochs when Megaptera began to proliferate. Early song may have been comprised of simpler precursor sounds that through natural selection and ritualization evolved into complex song.

Trade-Offs in Male Display Activity with Lek Size

In lek mating systems, males aggregate and defend arenas where they display for females; females select and mate with a male and then solely raise their offspring. Generally, female visits and copulations increase and reproductive variance in male mating success declines with lek size. Here we investigate how male display effort changes across a gradient in lek size. We expect male display effort, an energetically expensive activity, will increase with lek size and male rank due to changes in breeding opportunities and competition among males. We test the interaction of male rank and lek size on display effort using the white-bearded manakin, Manacus manacus (Aves: Pipridae), a well-studied species with a wide geographic distribution in the new world tropics. We used mini-video recorders to simultaneously capture female visits and display behaviors of 41 males distributed over 10 leks. We found that overall display effort increased disproportionately with lek size due to males of both high and low ranks increasing their display effort at larger leks. Our results suggest that increased breeding opportunities and intrasexual competition at larger leks result in males of different ranks investing similarly in increased display effort in order to attract females.

Looking into the past - the reaction of three grouse species to climate change over the last million years using whole genome sequences

Tracking past population fluctuations can give insight into current levels of genetic variation present within species. Analysing population dynamics over larger timescales can be aligned to known climatic changes to determine the response of species to varying environments. Here, we applied the Pairwise Sequentially Markovian Coalescent (psmc) model to infer past population dynamics of three widespread grouse species; black grouse, willow grouse and rock ptarmigan. This allowed the tracking of the effective population size (Ne ) of all three species beyond 1 Mya, revealing that (i) early Pleistocene cooling (~2.5 Mya) caused an increase in the willow grouse and rock ptarmigan populations, (ii) the mid-Brunhes event (~430 kya) and following climatic oscillations decreased the Ne of willow grouse and rock ptarmigan, but increased the Ne of black grouse and (iii) all three species reacted differently to the last glacial maximum (LGM) - black grouse increased prior to it, rock ptarmigan experienced a severe bottleneck and willow grouse was maintained at large population size. We postulate that the varying psmc signal throughout the LGM depicts only the local history of the species. Nevertheless, the large population fluctuations in willow grouse and rock ptarmigan indicate that both species are opportunistic breeders while black grouse tracks the climatic changes more slowly and is maintained at lower Ne . Our results highlight the usefulness of the psmc approach in investigating species' reaction to climate change in the deep past, but also that caution should be taken in drawing general conclusions about the recent past.

Limited indirect fitness benefits of male group membership in a lekking species

In group living species, individuals may gain the indirect fitness benefits characterizing kin selection when groups contain close relatives. However, tests of kin selection have primarily focused on cooperatively breeding and eusocial species, whereas its importance in other forms of group living remains to be fully understood. Lekking is a form of grouping where males display on small aggregated territories, which females then visit to mate. As females prefer larger aggregations, territorial males might gain indirect fitness benefits if their presence increases the fitness of close relatives. Previous studies have tested specific predictions of kin selection models using measures such as group-level relatedness. However, a full understanding of the contribution of kin selection in the evolution of group living requires estimating individuals' indirect fitness benefits across multiple sites and years. Using behavioural and genetic data from the black grouse (Tetrao tetrix), we show that the indirect fitness benefits of group membership were very small because newcomers joined leks containing few close relatives who had limited mating success. Males' indirect fitness benefits were higher in yearlings during increasing population density but marginally changed the variation in male mating success. Kin selection acting through increasing group size is therefore unlikely to contribute substantially to the evolution and maintenance of lekking in this black grouse population.

Female preference and predation risk models can explain the maintenance of a fallow deer (Dama dama) lek and its 'handy' location

We tested the predictions of three models (female preference; hotspot; predator avoidance) on lek formation in the fallow deer population of San Rossore, Tuscany. We collected behavioural observations in two leks and radiotracking data on 67 deer over 7 years. Two deer sub-populations were present in the northern and southern sides of the area, respectively, the two sectors being delimited by a river and including one lek each. Predictions were tested for one lek (SG), located in the south-side where we set up our 7-year radiotracking program. Data from a second lek (FO, north-side) were used to test those predictions which imply the occurrence of multiple leks in the same population. We showed that the majority of females made one single visit to one lek, only during the rut. The lek was located outside areas of higher female traffic and home range overlap, and females increased home range sizes during the rut to reach it. Twilight routes of females never crossed the lek; instead, females walked atypical routes and at a faster pace to reach the lek and mate. The distance between the two leks was higher than the average diameter of female home ranges, and only one lek was present within female home ranges. Males reached the lek one month before the arrival of females, corroborating that lekking is a female-initiated process (females moving towards large clumped male aggregations) rather than a male-initiated process (males moving towards female hotspots). Our results supported the female preference model, and rejected the predictions of the hotspot model. Also, leks were located far from areas with higher predation risk, supporting the predator avoidance model. The position of lek SG resulted 'handy' at the sub-population level because of the optimal trade-off between travel costs for females to reach it and avoidance of human predators.

Transcriptome sequencing of black grouse (Tetrao tetrix) for immune gene discovery and microsatellite development

The black grouse (Tetrao tetrix) is a galliform bird species that is important for both ecological studies and conservation genetics. Here, we report the sequencing of the spleen transcriptome of black grouse using 454 GS FLX Titanium sequencing. We performed a large-scale gene discovery analysis with a focus on genes that might be related to fitness in this species and also identified a large set of microsatellites. In total, we obtained 182 179 quality-filtered sequencing reads that we assembled into 9035 contigs. Using these contigs and 15 794 length-filtered (greater than 200 bp) singletons, we identified 7762 transcripts that appear to be homologues of chicken genes. A specific BLAST search with an emphasis on immune genes found 308 homologous chicken genes that have immune function, including ten major histocompatibility complex-related genes located on chicken chromosome 16. We also identified 1300 expressed sequence tag microsatellites and were able to design suitable flanking primers for 526 of these. A preliminary test of the polymorphism of the microsatellites found 10 polymorphic microsatellites of the 102 tested. Genomic resources generated in this study should greatly benefit future ecological, evolutionary and conservation genetic studies on this species.

Kin selection may contribute to lek evolution and trait introgression across an avian hybrid zone

Understanding the mechanism(s) that favour cooperation among individuals competing for the same resources provides direct insights into the evolution of grouping behaviour. In a hybrid zone between golden-/yellow-collared (Manacus vitellinus) and white-collared (Manacus candei) manakins, males form aggregations composed of white and yellow males solely to attract females ('mixed leks'). Previous work shows that yellow males in these mixed leks experience a clear mating advantage over white males, resulting in the preferential introgression of yellow plumage allele(s) into the white species. However, the yellow male mating advantage only occurs in mixed leks with high frequencies of yellow males, and only a few of these males probably mate. Hence, it remains unclear why unsuccessful males join leks. Here, we used microsatellite markers to estimate pairwise relatedness among males within and between leks to test whether indirect genetic benefits of helping kin ('kin selection') can promote grouping. We found that yellow males are significantly more related to each other within than between leks, while relatedness among white males did not differ within and between leks. This suggests that yellow males may indirectly enhance their own reproductive success by preferentially lekking with relatives because yellow plumage is under positive frequency-dependent selection (positive FDS). Our results are consistent with the hypothesis that kin selection may promote grouping and facilitate positive FDS for yellow males, mediating the movement of yellow plumage across this hybrid zone.

The anatomy of population change in a black grouse population 1992-2008

We interrogate an 18-year-long dataset containing counts of displaying male black grouse Tetrao tetrix and incidental counts of females within an 800-km(2) region of Perthshire, Scotland. We examine the trends in the population and investigate how different components of the population might act as signposts of different stages of overall population change. We found statistical evidence for a decline in black grouse numbers between 1992 and 2000, and then a recovery from 2002 to 2008, but little evidence for a link between population change and weather during the decline phase. There was some evidence for a positive relationship between male and female counts. The two main components of male population size, lek size and lek frequency followed the overall population trend while it was increasing, but during the earlier decline, the two became uncoupled, to expose a complex structure within the data. During the decline, when black grouse numbers were approaching their minimum, mean lek size was actually increasing. Small leks lost proportionally more birds than did large leks, and lek longevity was positively correlated with lek size, indicating that maintenance of large leks is crucial in buffering the population against serious declines. During the decline, the spatial arrangement of leks changed, with remnant leks showing tight clustering at larger spatial scales, before expanding out to fill the large areas of unoccupied landscape during the population increase. We discuss these findings in terms of species monitoring and suggest that counts of young males may add much useful demographic information with little extra effort.