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.

Animals choosing particular display sites often balance sexual and natural selection pressures. Here we assess how physical properties of display sites can alter this balance by influencing signal production and attractiveness of the túngara frog (Physalaemus pustulosus). Males that call from very shallow water bodies (few mm depth) benefit from reduced predation risk, but by manipulating water levels, we show that this comes at a cost of reduced attractiveness to females. Our data show that calling from shallower water reduces a male’s ability to float, limits the inflation of his vocal sac, and consequently reduces signal conspicuousness in terms of amplitude and complexity. Our results demonstrate that display site properties can set limits on signal production and attractiveness and may hence influence signal evolution. Signallers may shift between sites or engineer their display location, which can play a crucial role in signal divergence and speciation, particularly in a rapidly changing world.

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.

Advertisement signals can convey information about a sender’s characteristics, such as body size. The reliability of signals, however, can be reduced when signal production is partially dependent on the environment. Here, we assess the effect of display-site properties on the production, attractiveness and honesty of sexual signals. We recorded male túngara frogs (Physalaemus pustulosus) while manipulating water levels in order to constrain calling. We found that water level affected male call properties in a size-dependent manner, with call amplitude being less affected in smaller males when forced to call in shallow water. Next, we tested how size-dependent and display-site-dependent signaling affected female choice and rival competition. Both males and females showed the strongest response to the call of a large male when he was calling at the deepest water levels. However, females showed no preference for large over small males when both were recorded in shallow water levels, or, depending on the call rate and timing of calls, even preferred small males. Likewise, males responded equally to large and small rivals recorded calling during shallow water level trials. Our experiments show that display-site properties can influence signal production and attractiveness in a size-dependent manner. These results can have important consequences for the evolution of signaling, as small males may be able to use their size to their advantage when selecting appropriate display sites and thereby outcompete large males.