Urbanisation can affect mating opportunities and thereby alter inter-and intra-sexual selection pressures on sexual traits. Biotic and abiotic urban conditions can influence an individual's success in pre-and post-copulatory mating, for example through impacts on mate attraction and mate preference, fertilisation success, resource competition or rival interactions. Divergent sexual selection pressures can lead to differences in behavioural, physiological, morphological or life-history traits between urban and non-urban populations, ultimately driving adaptation and speciation. Most studies on urban sexual selection and mating interactions report differences between urban and non-urban populations or correlations between sexual traits and factors associated with increased urbanisation, such as pollution, food availability and risk of predation and parasitism. Here we review the literature on sexual selection and sexual traits in relation to urbanisation or urban-associated conditions. We provide an extensive list of abiotic and biotic factors that can influence processes involved in mating interactions, such as signal production and transmission, mate choice and mating opportunities. We discuss all relevant data through the lens of two, non-mutually exclusive theories on sexual selection, namely indicator and sensory models. Where possible, we indicate whether these models provide the same or different predictions regarding urban-adapted sexual signals and describe different experimental designs that can be useful for the different models as well as to investigate the drivers of sexual selection. We argue that we lack a good understanding of: (i) the factors driving urban sexual selection; (ii) whether reported changes in traits result in adaptive benefits; and (iii) whether these changes reflect a short-term ecological, or long-term evolutionary response. We highlight that urbanisation provides a unique opportunity to study the process and outcomes of sexual selection, but that this requires a highly integrative approach combining experimental and observational work.

Urbanization drastically changes environmental conditions, including the introduction of sensory pollutants, such as artificial light at night (ALAN) and anthropogenic noise. To settle in urban habitats, animals need to cope with this new sensory environment. On a short timescale, animals might cope with sensory pollutants via behavioral adjustments, such as changes in sexual signaling, which can have important fitness consequences. While ALAN and anthropogenic noise generally co-occur in urban habitats and are known to be able to interact to modify behavioral responses, few studies have addressed their combined impact. Our aim was, therefore, to assess the effects of ALAN, anthropogenic noise, and their interaction on sexual signaling in túngara frogs (Engystomops pustulosus). We observed the calling behavior of frogs in urban and forest areas, and subsequently recorded these frogs in a laboratory set-up while independently manipulating light and noise levels. Frogs in urban areas called with a higher call rate and complexity, which was correlated with local sensory conditions. Furthermore, our lab experiment revealed that ALAN can directly alter sexual signaling independently as well as in combination with anthropogenic noise. Exposure to ALAN alone increased call amplitude, whereas a combination of ALAN and anthropogenic noise interacted to lead to a higher call complexity and amplitude. Overall, the response patterns consistently showed that exposure to ALAN and anthropogenic noise led to more conspicuous sexual signals than expected based on the additive effects of single pollutants. Our results support the notion that urban and forest population differences in sexual signaling can be partially explained by exposure to ALAN and anthropogenic noise. Furthermore, by demonstrating interactive effects between light and noise pollution, our study highlights the importance of examining the effects of multisensory pollution, instead of single pollutants, when trying to understand phenotypic divergence in urbanized vs. natural areas.

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Increasing urbanization has led to large-scale land-use changes, exposing persistent populations to drastically altered environments. Sensory pollutants, including low-frequency anthropogenic noise and artificial light at night (ALAN), are typically associated with urban environments and known to impact animal populations in a variety of ways. Both ALAN and anthropogenic noise can alter behavioral and physiological processes important for survival and reproduction, including communication and circadian rhythms. Although noise and light pollution typically co-occur in urbanized areas, few studies have addressed their combined impact on species’ behavior. Here, we assessed how anthropogenic noise and ALAN can influence spatial and temporal variation in breeding activity of a wild frog population. By exposing artificial breeding sites inside a tropical rainforest to multiple sensory environments, we found that both anthropogenic noise and ALAN impact breeding behavior of túngara frogs (Engystomops pustulosus), albeit in different ways. Males arrived later in the night at their breeding sites in response to anthropogenic noise. ALAN, on the other hand, led to an increase in calling effort. We found no evidence that noise or light pollution either attracted frogs to or repelled frogs from breeding sites. Thus, anthropogenic noise may negatively affect calling males by shifting the timing of sexual signaling. Conversely, ALAN may increase the attractiveness of calling males. These changes in breeding behavior highlight the complex ways that urban multisensory pollution can influence behavior and suggest that such changes may have important ecological implications for the wildlife that are becoming increasingly exposed to urban multisensory pollution.