Synthesis of sexual selection: a systematic map of meta-analyses with bibliometric analysis

Sexual selection has been a popular subject within evolutionary biology because of its central role in explaining odd and counterintuitive traits observed in nature. Consequently, the literature associated with this field of study became vast. Meta-analytical studies attempting to draw inferences from this literature have now accumulated, varying in scope and quality, thus calling for a synthesis of these syntheses. We conducted a systematic literature search to create a systematic map with a report appraisal of meta-analyses on topics associated with sexual selection, aiming to identify the conceptual and methodological gaps in this secondary literature. We also conducted bibliometric analyses to explore whether these gaps are associated with the gender and origin of the authors of these meta-analyses. We included 152 meta-analytical studies in our systematic map. We found that most meta-analyses focused on males and on certain animal groups (e.g. birds), indicating severe sex and taxonomic biases. The topics in these studies varied greatly, from proximate (e.g. relationship of ornaments with other traits) to ultimate questions (e.g. formal estimates of sexual selection strength), although the former were more common. We also observed several common methodological issues in these studies, such as lack of detailed information regarding searches, screening, and analyses, which ultimately impairs the reliability of many of these meta-analyses. In addition, most of the meta-analyses' authors were men affiliated to institutions from developed countries, pointing to both gender and geographical authorship biases. Most importantly, we found that certain authorship aspects were associated with conceptual and methodological issues in meta-analytical studies. Many of our findings might simply reflect patterns in the current state of the primary literature and academia, suggesting that our study can serve as an indicator of issues within the field of sexual selection at large. Based on our findings, we provide both conceptual and analytical recommendations to improve future studies in the field of sexual selection.

A dawn and dusk chorus will emerge if males sing in the absence of their mate

The spring dawn and dusk chorus of birds is a widespread phenomenon, yet its origin remains puzzling. We propose that a dawn and dusk chorus will inevitably arise if two criteria are met: (1) females leave their roost later in the morning and go to roost earlier in the evening than their mate, and (2) males sing more when separated from their mate. Previous studies on blue tits (Cyanistes caeruleus) support the first criterion. We here report that males sing at a higher rate whenever they are separated from their mate and that song rate increases with the duration of female absence. These findings can explain the existence of the dawn and dusk chorus in blue tits, and they can explain why the dawn chorus is more pronounced than the dusk chorus, as is typically observed. An exhaustive literature search provides support for both criteria of the 'absent mate' hypothesis in several passerine birds. We found no evidence contradicting the hypothesis. The new hypothesis is not inconsistent with many of the existing hypotheses about dawn singing, but may be a more general explanation for the occurrence of a dawn and dusk chorus. We describe how the 'absent mate' hypothesis leads to testable predictions about daily and seasonal variation in song output.

Sexual selection for both diversity and repetition in birdsong

From fiddler crabs to humans, animals perform repetitive displays showing neuromotor skill and vigour. Consistent repetition of identical notes (vocal consistency) facilitates the assessment of neuromotor skills and is important in communication in birds. Most birdsong research has focused on song diversity as a signal of individual quality, which seems contradictory as repetition is extremely common in most species. Here we show that consistent repetition within songs is positively correlated with reproductive success in male blue tits (Cyanistes caeruleus). A playback experiment shows that females are sexually aroused by male songs with high levels of vocal consistency, which also peaks seasonally during the fertile period of the female, supporting the role of vocal consistency in mate choice. Male vocal consistency also increases with subsequent repetitions of the same song type (a warm-up effect) which conflicts with the fact that females habituate to repeated song, showing decreased arousal. Importantly, we find that switching song types elicits significant dishabituation within the playback, supporting the habituation hypothesis as an evolutionary mechanism driving song diversity in birds. An optimal balance between repetition and diversity may explain the singing style of many bird species and displays of other animals.

Female blue tits sing frequently: a sex comparison of occurrence, context, and structure of song

In species with mutual mate choice, we should expect adaptive signaling in both sexes. However, the role of female sexual signals is generally understudied. A case in point is female birdsong that has received considerably less attention than male song. This holds even for well-studied species such as the blue tit (Cyanistes caeruleus), an important model in evolutionary ecology. Although there have been anecdotal reports of female song from three populations, there are no quantitative studies on female song in this species. Here, we report systematic sampling from a population of individually marked blue tits over 3 years, revealing that females sang frequently throughout the sampling period. Notably, daytime singing of females occurred in functionally similar contexts as in males (agonistic, solo song, and alarm contexts) but females had lower song output than males and were not observed singing dawn song, while males showed long singing displays at dawn before copulations take place. Female and male song overlapped substantially in acoustic structure (i.e., same song types or peak frequency) but females had smaller individual song-type repertoires, shorter trills, and lower vocal consistency. Differential selection pressures related to functional differences in male and female song might explain the observed variation in acoustic structure. With the first quantitative study of female song in such a well-studied species, we hope to stimulate further investigations into the functions of female singing, especially in the Northern temperate zones where female song may have been overlooked, not only in this but perhaps in other monomorphic species.

Social context affects tail displays by Phrynocephalus vlangalii lizards from China

Competition between animals for limited resources often involves signaling to establish ownership or dominance. In some species, the defended resource relates to suitable thermal conditions and refuge from predators. This is particularly true of burrow-dwelling lizards such as the Qinghai toad-headed agama (Phrynocephalus vlangalii), which are found on the Tibetan plateau of western China. Male and female lizards occupy separate burrows, which are vital for anti-predator behaviour during warmer months when lizards are active and, crucially, provide shelter from harsh winter conditions. These lizards are readily observed signaling by means of tail displays on the sand dunes they inhabit. Given the selective pressure to hold such a resource, both males and females should exhibit territorial behaviour and we considered this study system to examine in detail how social context influences motion based territorial signaling. We confirmed that territorial signaling was used by both sexes, and by adopting a novel strategy that permitted 3D reconstruction of tail displays, we identified significant variation due to social context. However, signal structure was not related to lizard morphology. Clearly, the burrow is a highly valued resource and we suggest that additional variation in signaling behaviour might be mediated by resource quality.

Pairing context determines condition-dependence of song rate in a monogamous passerine bird

Condition-dependence of male ornaments is thought to provide honest signals on which females can base their sexual choice for genetic quality. Recent studies show that condition-dependence patterns can vary within populations. Although long-term association is thought to promote honest signalling, no study has explored the influence of pairing context on the condition-dependence of male ornaments. In this study, we assessed the influence of natural variation in body condition on song rate in zebra finches (Taeniopygia guttata) in three different situations: during short and long encounters with an unfamiliar female, and within heterosexual mated pairs. We found consistent individual differences in male directed and undirected song rate. Moreover, body condition had a positive effect on song rate in paired males. However, male song rate was not influenced by body condition during short or long encounters with unfamiliar females. Song rate appears to be an unreliable signal of condition to prospective females as even poor-condition birds can cheat and sing at a high rate. By contrast, paired females can reliably use song rate to assess their mate's body condition, and possibly the genetic quality. We propose that species' characteristics, such as mating system, should be systematically taken into account to generate relevant hypotheses about the evolution of condition-dependent male ornaments.

MHC class I variation in a natural blue tit population (Cyanistes caeruleus)

The major histocompatibility complex (MHC) is central to the vertebrate immune system and its highly polymorphic genes are considered to influence several life-history traits of individuals. To characterize the MHC in a natural population of blue tits (Cyanistes caeruleus) we investigated the class I exon 3 diversity of more than 900 individuals. We designed two pairs of motif-specific primers that reliably amplify independent subsets of MHC alleles. Applying denaturing gradient gel electrophoresis (DGGE) we obtained 48 independently inherited units of unique band patterns (DGGE-haplogroups), which were validated in a segregation analysis within 105 families. In a second approach, we extensively sequenced 6 unrelated individuals to confirm that DGGE-haplogroup composition reflects individual allelic variation. The highest number of different DGGE-haplogroups in a single individual corresponded in 19 MHC exon 3 sequences, suggesting a minimum of 10 amplified MHC class I loci in the blue tit. In total, we identified 50 unique functional and 3 non-functional sequences. Functional sequences showed high levels of recombination and strong positive selection in the antigen binding region, whereas nucleotide diversity was comparatively low in the range of all passerine species. Finally, in a phylogenetic comparison of passerine MHC class I exon 3 sequences we discuss conflicting evolutionary signals possibly due to recent gene duplication, recombination events and concerted evolution. Our results indicate that the described method is suitable to effectively explore the MHC diversity and its ecological impacts in blue tits in future studies.

Sons learn songs from their social fathers in a cooperatively breeding bird

Song learning is hypothesized to allow social adaptation to a local song neighbourhood. Maintaining social associations is particularly important in cooperative breeders, yet vocal learning in such species has only been assessed in systems where social association was correlated with relatedness. Thus, benefits of vocal learning as a means of maintaining social associations could not be disentangled from benefits of kin recognition. We assessed genetic and cultural contributions to song in a species where social association was not strongly correlated with kinship: the cooperatively breeding, reproductively promiscuous splendid fairy-wren (Malurus splendens). We found that song characters of socially associated father-son pairs were more strongly correlated (and thus songs were more similar) than songs of father-son pairs with a genetic, but no social, association (i.e. cuckolding fathers). Song transmission was, therefore, vertical and cultural, with minimal signatures of kinship. Additionally, song characters were not correlated with several phenotypic indicators of male quality, supporting the idea that there may be a tradeoff between accurate copying of tutors and quality signalling via maximizing song performance, particularly when social and genetic relationships are decoupled. Our results lend support to the hypothesis that song learning facilitates the maintenance of social associations by permitting unrelated individuals to acquire similar signal phenotypes.

Experimental manipulation of testosterone and condition during molt affects activity and vocalizations of male blue tits

Testosterone (T) is a key hormone regulating behavioral trade-offs in male birds, shifting investment towards sexual and competitive behaviors. However, the role of T in regulating male behavior during the molt has received very little attention, although this is a crucial life-history stage. Since the effect of T on behavior may be condition-dependent, particularly during the costly molt period, we studied the effects of T and condition in a two-way design. We manipulated T under two dietary regimes (standard and improved, resulting in an enhanced condition) in captive blue tits (Cyanistes caeruleus) undergoing the first pre-basic molt. T treatment increased song frequency, indicating that song is T-dependent also at this time of year. Males on the improved diet sang less than males in relatively worse condition, providing no support for song as an indicator of male condition. T-treated males exhibited greater locomotor activity than control males, but only when fed the standard diet. Neither T- nor diet-treatment affected plumage maintenance (preening). Although T treatment resulted in a delay in molt progress all birds completed the molt. Taken together our results show that during the molt male birds are sensitive to relatively small fluctuations in T. Similar to its commonly observed effects during the breeding season, T stimulated an increase in song and locomotion. While there might be some benefits associated with such T effects, these must be traded-off against costs associated with conspicuous behavior and increased molt duration.