Chemical communication underpins virtually all aspects of vertebrate social life, yet remains poorly understood because of its highly complex mechanistic basis. We therefore used chemical fingerprinting of skin swabs and genetic analysis to explore the chemical cues that may underlie mother-offspring recognition in colonially breeding Antarctic fur seals. By sampling mother-offspring pairs from two different colonies, using a variety of statistical approaches and genotyping a large panel of microsatellite loci, we show that colony membership, mother-offspring similarity, heterozygosity, and genetic relatedness are all chemically encoded. Moreover, chemical similarity between mothers and offspring reflects a combination of genetic and environmental influences, the former partly encoded by substances resembling known pheromones. Our findings reveal the diversity of information contained within chemical fingerprints and have implications for understanding mother-offspring communication, kin recognition, and mate choice. Olfaction in particular is fundamental to animal communication, mediating social interactions as varied as territorial behavior, kin recognition, and mate choice (1). Metabolomic tools, such as gas chromatography-mass spectrometry (GC-MS) have made it possible to generate individual-specific chemical "fingerprints." By separating compounds and quantifying their relative abundances, these fingerprints provide a wealth of information, even though not all compounds can necessarily be identified. Both volatile and contact cues are potentially hidden within the extreme complexity of chemical profiles, which is why a mechanistic understanding of chemical communication is still lacking in natural vertebrate populations (2).In particular, "surprisingly little progress" has been made in understanding the link between vertebrate chemical fingerprints and genotype (2). Experimental studies have shown that females of several species are capable of discriminating potential partners based on olfactory cues (3-5). However, very few studies have demonstrated a convincing link between the molecular composition of chemical fingerprints and genetic traits, such as heterozygosity (a measure of genetic quality) and relatedness (6-9). These studies were almost exclusively conducted on a captive population of lemurs, a species known for its conspicuous use of scent marking.A functional understanding of how genotype is chemically encoded also requires knowledge of how many and which types of substances are involved. This is challenging because, especially in natural populations, an individual's mixture of surface chemicals is not only the product of its genotype but may also be mediated by hormones, the microbial flora, body condition, and environmental factors (2). Thus, analyses based on overall chemical fingerprints may overlook subtle genetic signatures and make little if any headway toward identifying the specific substances involved. A second less-appreciated problem is that the modest panels of around 10-15 microsat...

Adult sex ratio (ASR) is a central concept in population biologyand a key factor in sexual selection, but why do most demographic models ignore sex biases? Vital rates often vary between the sexes and across life history, but their relative contributions to ASR variation remain poorly understood-an essential step to evaluate sex ratio theories in the wild and inform conservation. Here, we combine structured two-sex population models with individual-based mark-recapture data from an intensively monitored polygamous population of snowy plovers. We show that a strongly male-biased ASR (0.63) is primarily driven by sexspecific survival of juveniles rather than adults or dependent offspring. This finding provides empirical support for theories of unbiased sex allocation when sex differences in survival arise after the period of parental investment. Importantly, a conventional model ignoring sex biases significantly overestimated population viability. We suggest that sex-specific population models are essential to understand the population dynamics of sexual organisms: reproduction and population growth are most sensitive to perturbations in survival of the limiting sex. Overall, our study suggests that sex-biased early survival may contribute toward mating system evolution and population persistence, with implications for both sexual selection theory and biodiversity conservation.ASR | Charadrius nivosus | mark-recapture | sex allocation | two-sex matrix model

The adult sex ratio (ASR) is a fundamental concept in population biology, sexual selection, and social evolution. However, it remains unclear which demographic processes generate ASR variation and how biases in ASR in turn affect social behaviour. Here, we evaluate the demographic mechanisms shaping ASR and their potential consequences for parental cooperation using detailed survival, fecundity, and behavioural data on 6119 individuals from six wild shorebird populations exhibiting flexible parental strategies. We show that these closely related populations express strikingly different ASRs, despite having similar ecologies and life histories, and that ASR variation is largely driven by sex differences in the apparent survival of juveniles. Furthermore, families in populations with biased ASRs were predominantly tended by a single parent, suggesting that parental cooperation breaks down with unbalanced sex ratios. Taken together, our results indicate that sex biases emerging during early life have profound consequences for social behaviour.

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