The evolution of phenotypic plasticity in spatially structured environments: implications of intraspecific competition, plasticity costs and environmental characteristics

We model the evolution of reaction norms focusing on three aspects: frequency-dependent selection arising from resource competition, maintenance and production costs of phenotypic plasticity, and three characteristics of environmental heterogeneity (frequency of environments, their intrinsic carrying capacity and the sensitivity to phenotypic maladaptation in these environments). We show that (i) reaction norms evolve so as to trade adaptation for acquiring resources against cost avoidance; (ii) maintenance costs cause reaction norms to better adapt to frequent rather than to infrequent environments, whereas production costs do not; and (iii) evolved reaction norms confer better adaptation to environments with low rather than with high intrinsic carrying capacity. The two previous findings contradict earlier theoretical results and originate from two previously unexplored features that are included in our model. First, production costs of phenotypic plasticity are only incurred when a given phenotype is actually produced. Therefore, they are proportional to the frequency of environments, and these frequencies thus affect the selection pressure to avoid costs just as much as the selection pressure to improve adaptation. This prevents the frequency of environments from affecting the evolving reaction norm. Secondly, our model describes the evolution of plasticity for a phenotype determining an individual's capability to acquire resources, and thus its realized carrying capacity. When individuals are distributed randomly across environments, they cannot avoid experiencing environments with intrinsically low carrying capacity. As selection pressures arising from the need to improve adaptation are stronger under such extreme conditions than under mild ones, better adaptation to environments with low rather than with high intrinsic carrying capacity results.     

Phenotypic plasticity of abdominal pigmentation in Drosophila kikkawai: multiple interactions between a major gene, sex, abdomen segment and growth temperature

Drosophila kikkawai is known to be polymorphic for a single autosomal locus controlling abdomen pigmentation in females. Two strains homozygous at this locus (Abdomen pigmentation, Abp) were established from a polymorphic Indian population: one was homozygous (DD) for the dark allele, the other (LL) for the light allele. A Mendelian analysis of crosses at 25°C confirmed the occurrence of a major locus, with dominance of the D allele. Phenotypic variation of pigmentation according to growth temperature was then analyzed in DD and LL male and female flies, and in reciprocal F1. A slight difference was found between reciprocal F1 females from a dark mother were darker but not at all temperatures. In females, the D allele exhibited an antero‐posterior gradient of increasing expression from segment 27, with dominance over L and an increased expression at low temperatures. In males, abdomen pigmentation was uniformly light in segments 25, the D allele being repressed by the sex genotype. In segment 6, the D allele was expressed but only at low temperatures, and was either recessive to L or codominant. Phenotypic plasticity that is, amount of change induced by different growth temperatures, was variable according to genotype and segment. It always corresponded to a darkening of the fly at lower temperatures, but was generally much less than in D. melanogaster. In D. kikkawai, climatic adaptation might occur more by changing the frequency of the D allele than by phenotypic plasticity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sexual differences in growth strategies of the wolf spider Hygrolycosa rubrofasciata

In invertebrates, the size at maturation is considered to be important for adult fitness. In the wolf spider Hygrolycosa rubrofasciata, however, it is only females that clearly benefit of larger size through augmented egg production, while male mating success is determined by display activity not related to size. Thus, we can expect conflicting growth patterns for the sexes. Additionally, populations differ greatly in adult size: individuals from dry habitats are smaller than those from wet habitats. To study the sexual differences in reaction norms of growth, we reared spiderlings from seven populations at two food levels under controlled laboratory conditions and compared size at sexual maturity. The shapes of reaction norms for adult size differed between the sexes. In females, the reaction norms were parallel, but individuals from dry habitats tended to grow larger at the given food levels. In males, there was a significant interaction between food level and population without any consistent differences between populations. Maturation time was a plastic character in both sexes with no genetic differences among populations. However, females on low food level matured later and significantly smaller in size than those on high food level. Males also matured later on low food level, but they were nearly of the same size as males that received more food. Female growth patterns reflected the strong selection for large size at maturity. However, the patterns for males were highly variable, which could be explained by the weak overall selection on male size, which means that any environmental factors can affect male growing patterns. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rapid temporal change in the expression and age-related information content of a sexually selected trait

The expression of sexual signals is often phenotypically plastic and also evolves rapidly. Few studies have considered the possibility that proximate determination -- the pathway between genes and trait expression -- may also be subject to both phenotypic plasticity and evolutionary change. We examined long-term patterns in size, condition- and age-dependence, repeatability and heritability of forehead patch size, a sexually selected plumage trait in male collared flycatchers. We also estimated survival and sexual selection on the phenotypic value of the trait. Forehead patch size linearly declined during the 15 years, probably due to the significantly negative survival selection. In addition, the expression of genetic variation for the ornament apparently underwent an age-limited change, which implies a change in the information content of the signal to receivers. The persistent lack of condition-dependence makes phenotypic plasticity an unlikely explanation to our results. This raises the possibility of a microevolutionary change of both expression and proximate determination during the study period.     

Costs of coexistence along a gradient of competitor densities: an experiment with arvicoline rodents

1. Costs of coexistence for species with indirect resource competition usually increase monotonically with competitor numbers. Very little is known though about the shape of the cost function for species with direct interference competition. 2. Here we report the results of an experiment with two vole species in artificial coexistence in large enclosures, where density of the dominant competitor species (Microtus agrestis) was manipulated. Experimental populations of the subordinate vole species (Clethrionomys glareolus) were composed of same aged individuals to study distribution of costs of coexistence with a dominant species within an age-cohort. 3. Survival and space use decreased gradually with increasing field vole numbers. Thus, responses to interference competition in our system appeared to be similar as expected from resource competition. The total number of breeders was stable. Reproductive characteristics such as the timing of breeding, and the litter size were not affected. In the single species enclosures a proportion of surviving individuals were not able to establish a breeding territory against stronger conspecifics. Under competition with heterospecifics such nonbreeders suffered high mortality, whereas the breeders survived. 4. Combined interference of dominant conspecifics and heterospecifics probably increased the frequency of aggressive interactions, social stress and mortality for the weaker individuals within a homogeneous age cohort of the subordinate competitor population. 5. Our results suggest, that in open systems where bank voles are outcompeted over the breeding season by faster reproducing field voles, animals able to establish a territory may be able to withstand competitor pressure, while nonbreeding bank vole individuals are forced to emigrate to suboptimal forest habitats.     

Context-dependent sexual advertisement: plasticity in development of sexual ornamentation throughout the lifetime of a passerine bird

Male investment into sexual ornamentation is a reproductive decision that depends on the context of breeding and life history state. In turn, selection for state- and context-specific expression of sexual ornamentation should favour the evolution of developmental pathways that enable the flexible allocation of resources into sexual ornamentation. We studied lifelong variation in the expression and condition-dependence of a sexual ornament in relation to age and the context of breeding in male house finches (Carpodacus mexicanus)--a species that develops a new sexual ornament once a year after breeding. Throughout males' lifetime, the elaboration of ornamentation and the allocation of resources to the development of sexual ornamentation depended strongly on pairing status in the preceding breeding season--males that were single invested more resources into sexual ornamentation and changed ornamentation more than males that were paired. During the initial (post-juvenile) moult, the expression of ornamentation was closely dependent on individual condition, however the condition-dependence of ornamentation sharply decreased throughout a male's lifetime and in older males expression of sexual ornamentation was largely independent of condition during moult. Selection for early breeding favoured greater ornamentation in males that were single in the preceding seasons and the strength of this selection increased with age. On the contrary, the strength of selection on sexual ornamentation decreased with age in males that were paired in the preceding breeding season. Our results reveal strong context-dependency in investment into sexual ornamentation as well as a high flexibility in the development of sexual ornamentation throughout a male's life.     

Sex‐specific selection on energy metabolism – selection coefficients for winter survival

Selection for different fitness optima between sexes is supposed to operate on several traits. As fitness‐related traits are often energetically costly, selection should also act directly on the energetics of individuals. However, efforts to examine the relationship between fitness and components of the energy budget are surprisingly scarce. We investigated the effects of basal metabolic rate (BMR, the minimum energy required for basic life functions) and body condition on long‐term survival (8 winter months) with manipulated densities in enclosed populations of bank voles (Myodes glareolus). Here, we show that survival selection on BMR was clearly sex‐specific but density‐independent. Both the linear selection gradient and selection differential for BMR were positive in females, whereas survival did not correlate with male characteristics. Our findings emphasize the relative importance of individual physiology over ecological factors (e.g. intra‐specific competition). Most current models of the origin of endothermy underline the importance of metabolic optima in females, whose physiology evolved to fulfil demands of parental provisioning in mammals. Our novel findings of sex‐specific selection could be related to these life history differences between sexes.     

Phenological response of sea turtles to environmental variation across a species' northern range

Variations in environmental parameters (e.g. temperature) that form part of global climate change have been associated with shifts in the timing of seasonal events for a broad range of organisms. Most studies evaluating such phenological shifts of individual taxa have focused on a limited number of locations, making it difficult to assess how such shifts vary regionally across a species range. Here, by using 1445 records of the date of first nesting for loggerhead sea turtles (Caretta caretta) at different breeding sites, on different continents and in different years across a broad latitudinal range (25–39° ′N), we demonstrate that the gradient of the relationship between temperature and the date of first breeding is steeper at higher latitudes, i.e. the phenological responses to temperature appear strongest at the poleward range limit. These findings support the hypothesis that biological changes in response to climate change will be most acute at the poleward range limits and are in accordance with the predictions of MacArthur''s hypothesis that poleward range limit for species range is environmentally limited. Our findings imply that the poleward populations of loggerheads are more sensitive to climate variations and thus they might display the impacts of climate change sooner and more prominently.     

Social status and personality: stability in social state can promote consistency of behavioural responses

Stability of ‘state’ has been suggested as an underlying factor explaining behavioural stability and animal personality (i.e. variation among, and consistency within individuals in behavioural responses), but the possibility that stable social relationships represent such states remains unexplored. Here, we investigated the influence of social status on the expression and consistency of behaviours by experimentally changing social status between repeated personality assays. We used male domestic fowl (Gallus gallus domesticus), a social species that forms relatively stable dominance hierarchies, and showed that behavioural responses were strongly affected by social status, but also by individual characteristics. The level of vigilance, activity and exploration changed with social status, whereas boldness appeared as a stable individual property, independent of status. Furthermore, variation in vocalization predicted future social status, indicating that individual behaviours can both be a predictor and a consequence of social status, depending on the aspect in focus. Our results illustrate that social states contribute to both variation and stability in behavioural responses, and should therefore be taken into account when investigating and interpreting variation in personality.     

A comparison between desert and Mediterranean antlion populations: differences in life history and morphology

We performed a transplant experiment to compare the life histories and morphologies of five geographically representative antlion Myrmeleon hyalinus populations along a sharp climatic gradient, from a Mediterranean climate in Israel's north to a desert climate in the south. Larvae were raised in two environmental chambers simulating Mediterranean and desert climates to investigate the extent to which the different populations exhibit phenotypic plasticity. Along the north-to-south climatic gradient, we observed a gradient in body mass prior to pupation and in pupation rate. Mediterranean populations suffered higher mortality rate when exposed to desert conditions, whereas the mortality rate of desert populations was consistent between Mediterranean and desert conditions. Our results regarding body mass, pupation rate and mortality rate suggest that Mediterranean populations had a more flexible response compared with desert populations. An analysis of digital photographs was used to measure population morphological differences, which were usually indicative of a decrease in trait size along the north-to-south gradient. We show how climatic gradients translate into phenotypic differences in an antlion population and provide a morphometric tool to distinguish between instar stages.     

Why does a grasshopper have fewer,larger offspring at its range limits?

Analysis of size of offspring reared through three laboratory generations from populations of the field grasshopper Chorthippus brunneus from 27 sites around the British Isles showed that offspring were larger towards the cooler-wetter conditions in the western and northern limits of the range. This variation had a significant genetic component. There was a trade-off between clutch size and offspring size between and within populations. Under favourable thermal and feeding conditions maternal fitness was optimal when individuals produced the largest clutches of the smallest eggs, but under poor conditions maternal fitness was optimal when individuals produced small clutches of very large offspring. Calculation of geometric mean fitness over time indicated that having larger offspring near to the edge of the range could be advantageous as a conservative risk-spreading strategy. As well as geographic variation in egg size, significant environment-genotype interactions in egg size in relation to temperature were observed.