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Environmental change, such as increased rates of urbanization, can induce shifts in phenotypic plasticity with some individuals adapting to city life while others are displaced. A key trait that can facilitate adaptation is the degree at which animals respond to stress. This stress response has a heritable component and exhibits intra- and inter-individual variation. However, the mechanisms behind this variability and whether they might be responsible for adaptation to different environments are not known. Variation in DNA methylation can be a potential mechanism that mediates environmental effects on the stress response. We used an inter- and intra-environmental cross-foster experiment to analyze the contribution of DNA methylation to early-life phenotypic variation. We found that at hatching, urban house wren (Troglodytes aedon) offspring had increased methylation as compared to their rural counterparts, and observed plasticity in methylation as offspring aged, indicating developmental effects of the rearing environment on methylation. Differential methylation analyses showed that cellular respiration genes were differentially expressed at hatching and behavioral and metabolism genes were differentially expressed at fledgling. Lastly, hyper-methylation of a single gene (CNTNAP2) is associated with increased glucocorticoid levels. These differential methylation patterns linked to a specific physiological phenotype suggest that DNA methylation may be a mechanism by which individuals adapt to novel environments. Characterizing genetic and environmental influences on methylation is critical for understanding the role of epigenetic mechanisms in evolutionary adaptation.

Territorial animals often respond less aggressively to neighbours than strangers. This ‘dear enemy’ effect is hypothesized to be adaptive by reducing unnecessary aggressive interactions with non-threatening individuals. A key prediction of this hypothesis, that individual fitness will be affected by variation in the speed and the extent to which individuals reduce their aggression towards neighbours relative to strangers, has never been tested. We used a series of song playbacks to measure the change in response of male great tits to a simulated establishment of a neighbour on an adjacent territory during early stages of breeding, as an assay of individuals’ tendencies to form dear enemy relationships. Males reduced their approach to the speaker and sang fewer songs on later playback repetitions. However, only some males exhibited dear enemy behaviour by responding more strongly to a subsequent stranger playback, and when the playback procedure was repeated on a subset of males, there was some indication for consistent differences among individuals in the expression of dear enemy behaviour. We monitored nests and analysed offspring paternity to determine male reproductive success. Individuals that exhibited dear enemy behaviour towards the simulated neighbour did not suffer any costs associated with loss of paternity, but there was also no evidence of reproductive benefits, and no net effect on reproductive fitness. The general ability to discriminate between neighbours and strangers is likely adaptive, but benefits are probably difficult to detect because of the indirect link between individual variation in dear enemy behaviour and reproductive fitness, and because of the complex range of mechanisms affecting relations with territorial neighbours.

Individuals of many species, including humans, differ consistently in the way they behave. These consistent behavioral differences among individuals are collectively known as animal personality (Gosling 2001), behavioral syndromes (Sih et al. 2004a), behavioral strategies (Benus et al. 1990), or behavioral profiles (Rodgers et al. 1997). Each of these terms, to some extent, describe an emergent phenomenon of the total biases in behavioral reactions an individual expresses compared to other individuals within the same population or species. In other words, animal personality, in addition to referring to consistent differences between individuals, also refers to correlated behaviors. These correlations (usually defined at the level of populations of individuals) can occur through time (an individual that is bold at one time is also bold at another), across different functional contexts (an individual that is bold toward a predator is also aggressive toward conspecifics), or some combination of time and context (juvenile exploratory behavior is related to adult sociability). Although there is some debate on terminology (e.g., Reale et al. 2007; Gosling 2008), we use the term “animal personality” throughout this chapter.