Personality (consistent differences between individuals in behavior) and plasticity (changes within individuals in behavior) are often viewed as separate and opposing phenomena. We tested this idea by analyzing parental care reaction norms in a bird that exhibits biparental care. Personality in provisioning behavior existed (r(ic) = 0.11) and persisted despite being reduced after accounting for individual differences in environment. Plasticity was also evident and differed between the sexes. Male visit rate was associated with changes in brood size and time of day, but female visit rate was associated with changes in nestling age and date. In both sexes changes in visit rate were positively correlated with changes in their partner's visit rate. Both sexes also exhibited multidimensional reaction norms; interaction terms revealed that within-individual visit rates increased more steeply with brood size when nestlings were older, and the effect of the partner's visit rate was sensitive to variation in date, precipitation, and the focal bird's age. Individuals also varied in how they responded (reaction norm slope) to changes in nestling age and partner visits. Moreover, parental personality was interdependent with individual plasticity in several ways. Individuals of both sexes with a high visit rate also responded more positively to changes in nestling age, and males also showed this pattern with changes in partner visit rate. Explicit use of the behavioral reaction norm integrated personality and plasticity, revealed that these are not opposing concepts, and stimulated new hypotheses about sexual conflict over care and provisioning as a life-history trait.

Heterogeneity in the environment favours foragers that are flexible (phenotypically plastic). However, consistent individual differences in behaviour (personality), such as in risk‐taking, might affect an individual's ability to find food, avoid predators and adjust to new conditions. It is now well known that personalities exist in many taxa, but much less is known about individual variation in plasticity. We measured the tendency to begin foraging across different levels of risk in house sparrows (Passer domesticus), using a behavioural reaction norm framework to simultaneously assess personality and plasticity. We asked whether individuals were consistently different across risk levels, and whether they differed in habituation and neophobia, both of which were treated as cases of plasticity. We found that males habituated more than females by beginning to feed sooner after repeated instances of a human disturbance in the presence of an initially unfamiliar object. Individuals of both sexes also exhibited consistent differences across trials, but did not differ in the rate of habituation beyond the difference between the sexes. When a novel object was placed in the foraging area, both sexes exhibited similar degrees of neophobia by delaying feeding. The magnitude of this change varied among birds, indicating individual differences in neophobia. Our results indicate that both personality and individual variation in plasticity exist but should be treated as independent phenomena. The presence of variation in plasticity implies that the raw material necessary for selection on neophobia exists, and that if heritable, plasticity in risk‐taking across contexts could evolve.

Local competition for space across a wide array of taxa typically involves three mechanisms that we denote here as expansion (spreading into unoccupied habitat), lottery (replacing dead competitors), and overgrowth (encroaching on competitors along zones of contact). By formulating and analysing a simple, general model incorporating these features, we identify ecological conditions and life-history features that lead to stable coexistence or competitive exclusion (with or without initial-condition dependence) and gain insight by linking these to case studies in the literature. We demonstrate the importance of contact inhibition, a little-studied feature of overgrowth, and we show how life-history tradeoffs may influence and be influenced by local competition for space. The general model we present can help indicate whether local interactions are sufficient to explain patterns of coexistence or exclusion and can serve as the foundation for more specific, realistic models of spatial competition.

The ability to identify the sex of individuals from noninvasive samples can be a powerful tool for field studies. Amelogenin, a nuclear gene proximate to the pseudoautosomal region of mammalian sex chromosomes, has a 6 base-pair (bp) size difference between human X and Y chromosomes that can be PCR-amplified and sized to distinguish male from female DNA. We examined whether this test can be used to identify sex from different DNA sources across a number of nonhuman primate taxa. Using human amelogenin primers, we were able to amplify diagnostic products from the four great ape species tested, but products from five other primate species were not sexually dimorphic.

Between-individual variation has been documented in a wide variety of taxa, especially for behavioral characteristics; however, intra-population variation in sensory systems has not received similar attention in wild animals. We measured a key trait of the visual system, the density of retinal cone photoreceptors, in a wild population of house sparrows (Passer domesticus). We tested whether individuals differed from each other in cone densities given within-individual variation across the retina and across eyes. We further tested whether the existing variation could lead to individual differences in two aspects of perception: visual resolution and chromatic contrast. We found consistent between-individual variation in the densities of all five types of avian cones, involved in chromatic and achromatic vision. Using perceptual modeling, we found that this degree of variation translated into significant between-individual differences in visual resolution and the chromatic contrast of a plumage signal that has been associated with mate choice and agonistic interactions. However, there was no evidence for a relationship between individual visual resolution and chromatic contrast. The implication is that some birds may have the sensory potential to perform “better” in certain visual tasks, but not necessarily in both resolution and contrast simultaneously. Overall, our findings (a) highlight the need to consider multiple individuals when characterizing sensory traits of a species, and (b) provide some mechanistic basis for between-individual variation in different behaviors (i.e., animal personalities) and for testing the predictions of several widely accepted hypotheses (e.g., honest signaling).