Parental investment related to social systems in microtines

Summary Laboratory offspring of wild-caught voles Clethrionomys glareolus and Microtus agrestis, bred for 1 year under constant conditions, were examined with regard to sex ratios and weights at weaning and at 2 months of age. C. glareolus exhibits female territoriality and M. agrestis male territoriality in summer. The adults die away in late summer-autumn. Early-summer young mature in the year of birth but late-summer young do not reach maturity until the following year. C. glareolus young showed a male bias in early summer and a female bias in late summer. Conditions were the opposite but less clear in M. agrestis. C. glareolus males grew comparatively faster than M. agrestis males and showed a markedly higher early summer male: female weight ratio at 2 months of age. Maternal investment thus appears clearly related to the social system; the sex with the largest number and highest quality of young was that which was not limited in number by territoriality. However, the investment depended also on the time until maturation of the young.     

Parental investment and the optimization of human family size

Human reproductive behaviour is marked by exceptional variation at the population and individual level. Human behavioural ecologists propose adaptive hypotheses to explain this variation as shifting phenotypic optima in relation to local socioecological niches. Here we review evidence that variation in fertility (offspring number), in both traditional and modern industrialized populations, represents optimization of the life-history trade-off between reproductive rate and parental investment. While a reliance on correlational methods suggests the true costs of sibling resource competition are often poorly estimated, a range of anthropological and demographic studies confirm that parents balance family size against offspring success. Evidence of optimization is less forthcoming. Declines in fertility associated with modernization are particularly difficult to reconcile with adaptive models, because fertility limitation fails to enhance offspring reproductive success. Yet, considering alternative measures, we show that modern low fertility confers many advantages on offspring, which are probably transmitted to future generations. Evidence from populations that have undergone or initiated demographic transition indicate that these rewards to fertility limitation fall selectively on relatively wealthy individuals. The adaptive significance of modern reproductive behaviour remains difficult to evaluate, but may be best understood in response to rising investment costs of rearing socially and economically competitive offspring.     

The evolution of alternative mating strategies in variable environments

Summary We assessed the influence of phenotypic plasticity in age at maturity on the maintenance of alternative mating strategies in male Atlantic salmon,Salmo salar. We calculated the fitness,r, associated with the parr and the anadromous strategies, using age-specific survival data from the field and strategy-specific fertilization data from the laboratory. The fitness of each strategy depended largely on mate competition (numbers of parr per female, i.e. parr frequency) and on age at maturity. Fitness declined with increasing numbers of parr per female with equilibrium frequencies (at which the fitnesses of each strategy are equal) being within the range observed in the wild. Equilibrium parr frequencies declined with decreasing growth rate and increasing age at maturity. Within populations, the existence of multiple age-specific sets of fitness functions suggests that the fitnesses of alternative strategies are best represented as multidimensional surfaces. The points of intersection of these surfaces, whose boundaries encompass natural variation in age at maturity and mate competition, define an evolutionarily stable continuum (ESC) of strategy frequencies along which the fitnesses associated with each strategy are equal. We propose a simple model that incorporates polygenic thresholds of a largely environmentally-controlled trait (age at maturity) to provide a mechanism by which an ESC can be maintained within a population. An indirect test provides support for the prediction that growth-rate thresholds for parr maturation exist and are maintained by stabilizing selection. Evolutionarily stable continua, maintained by negative frequency-dependent selection on threshold traits, provide a theoretical basis for understanding how alternative life histories can evolve in variable environments.     

Genetic and phenotypic variation in juvenile development in relation to temperature and developmental pathway in a geometrid moth

Life histories show genetic population-level variation due to spatial variation in selection pressures. Phenotypic plasticity in life histories is also common, facilitating fine-tuning of the phenotype in relation to the prevailing selection regime. In multivoltine (≥ 2 generations per year) insects, individuals following alternative developmental pathways (diapause/direct development) experience different selection regimes. We studied the genetic and phenotypic components of juvenile development in Cabera exanthemata (Lepidoptera: Geometridae) in a factorial split-brood experiment. F(2) offspring of individuals originating from populations in northern and central Finland were divided among manipulations defined by temperature (14°C/20°C) and day length (24 h/15 h). Short day length invariably induced diapause, whereas continuous light almost invariably induced direct development in both regions, although northern populations are strictly univoltine in the wild. Individuals from northern Finland had higher growth rates, shorter development times and higher pupal masses than individuals from central Finland across the conditions, indicating genetic differences between regions. Individuals that developed directly into adults tended to have higher growth rates, shorter development times and higher pupal masses than those entering diapause, indicating phenotypic plasticity. Temperature-induced plasticity was substantial; growth rate was much higher, development time much shorter and pupal mass higher at 20°C than at 14°C. The degree of plasticity in relation to developmental pathway was pronounced at 20°C in growth rate and development time and at 14°C in pupal mass, emphasizing multidimensionality of reaction norms. The observed genetic variation and developmental plasticity seem adaptive in relation to time-stress due to seasonality.     

Trophic network structure emerges through antagonistic coevolution in temporally varying environments

Understanding the mechanisms underlying ecological specialization is central to our understanding of community ecology and evolution. Although theoretical work has investigated how variable environments may affect specialization in single species, little is known about how such variation impacts bipartite network structure in antagonistically coevolving systems. Here, we develop and analyse a general model of victim-enemy coevolution that explicitly includes resource and population dynamics. We investigate how temporal environmental heterogeneity affects the evolution of specialization and associated community structure. Environmental productivity influences victim investment in resistance, which will shape patterns of specialization through its regulating effect on enemy investment in infectivity. We also investigate the epidemiological consequences of environmental variability and show that enemy population density is maximized for intermediate lengths of productive seasons, which corresponds to situations where enemies can evolve higher infectivity than victims can evolve defence. We discuss our results in the light of empirical studies, and further highlight ways in which our model applies to a range of natural systems.     

Transmission stage investment of malaria parasites in response to in-host competition

Conspecific competition occurs in a multitude of organisms, particularly in parasites, where several clones are commonly sharing limited resources inside their host. In theory, increased or decreased transmission investment might maximize parasite fitness in the face of competition, but, to our knowledge, this has not been tested experimentally. We developed and used a clone-specific, stage-specific, quantitative PCR protocol to quantify Plasmodium chabaudi replication and transmission stage densities in mixed-clone infections. We co-infected mice from two strains with an avirulent and virulent parasite clone and found competitive suppression of in-host (blood-stage) parasite densities and generally corresponding reductions in transmission stage production, with the virulent clone obtaining overall competitive superiority. In response to competitive suppression, there was little evidence of any alteration in transmission stage investment, apart from a small reduction by one of the two clones in one of the two host strains. This alteration did not result in a competitive advantage, although it might have reduced the disadvantage. This study supports much of the current literature, which predicts that conspecific in-host competition will result in a competitive advantage and positive selection for virulent clones and thus the evolution of higher virulence.     

Wing and flight muscle polymorphism in a lygaeid bug, Horvathiolus gibbicollis: determinants and life history consequences

ABSTRACT.

• 1 Horvathiolus gibbicollis (Costa), a ground-living seed-feeding bug of the mediterranean region, has two wing morphs. In macrop-terous bugs both pairs of wings are fully developed. In brachypterous ones forewings are reduced to about two-thirds and hindwings to less than a third of their length in macropters.
• 2 Each morph bred true with regard to wing length when reared under variable density, food and temperature conditions for several generations.
• 3 All F₁ offspring between crosses of the two morphs were brachypterous. In F₂ approximately 25% were macropters and 75% brachypters implying monogenic control of wingform.
• 4 Flight muscles in macropters vary from fully developed to totally reduced. This variation is determined by environmental conditions during adult life. Most young adult bugs have flight muscles, and totally starved or unmated bugs retain their flight muscles. Fed and mated females histolyse flight muscles as they start laying eggs, while most males of the same group retain their muscles.
• 5 Brachypterous bugs have a smaller thorax and larger abdomen than macropterous ones.
• 6 Brachypterous bugs reach adulthood slightly before macropterous ones, and they have a distinctly shorter adult preoviposition period.
• 7 Lifetime egg production does not differ significantly between the two wing morphs. However, the temporal pattern of egg laying is different in the two morphs. The m_x-curve of macropters starts later, then attains a higher peak and finally decreases faster than that of brachypters.
• 8 Initially, macropters lay smaller eggs than brachypters, but egg volume increases with age in macropters and eventually approaches that of brachypters.
• 9 The initial increase in reproductive effort (egg volumexegg number) of macropters is concomitant with wing muscle histolysis and the mobilization of thorax space for reproduction.
• 10 Adult survival rate does not differ between the morphs.

     

Survival costs of reproduction predict age-dependent variation in maternal investment

Life-history theory predicts that older females will increase reproductive effort through increased fecundity. Unless offspring survival is density dependent or female size constrains offspring size, theory does not predict variation in offspring size. However, empirical data suggest that females of differing age or condition produce offspring of different sizes. We used a dynamic state-variable model to determine when variable offspring sizes can be explained by an interaction between female age, female state and survival costs of reproduction. We found that when costs depend on fecundity, young females with surplus state increase offspring size and reduce number to minimize fitness penalties. When costs depend on total reproductive effort, only older females increase offspring size. Young females produce small offspring, because decreasing offspring size is less expensive than number, as fitness from offspring investment is nonlinear. Finally, allocation patterns are relatively stable when older females are better at acquiring food and are therefore in better condition. Our approach revealed an interaction between female state, age and survival costs, providing a novel explanation for observed variation in reproductive traits.     

Evolution in stressful environments II: adaptive value and costs of plasticity in response to low light in Sinapis arvensis

Plants possess a remarkable capacity to alter their phenotype in response to the highly heterogeneous light conditions they commonly encounter in natural environments. In the present study with the weedy annual plant Sinapis arvensis, we (a) tested for the adaptive value of phenotypic plasticity in morphological and life history traits in response to low light and (b) explored possible fitness costs of plasticity. Replicates of 31 half-sib families were grown individually in the greenhouse under full light and under low light (40% of ambient) imposed by neutral shade cloth. Low light resulted in a large increase in hypocotyl length and specific leaf area (SLA), a reduction in juvenile biomass and a delayed onset of flowering. Phenotypic selection analysis within each light environment revealed that selection favoured large SLA under low light, but not under high light, suggesting that the observed increase in SLA was adaptive. In contrast, plasticity in the other traits measured was maladaptive (i.e. in the opposite direction to that favoured by selection in the low light environment). We detected significant additive genetic variance in plasticity in most phenotypic traits and in fitness (number of seeds). Using genotypic selection gradient analysis, we found that families with high plasticity in SLA had a lower fitness than families with low plasticity, when the effect of SLA on fitness was statistically kept constant. This indicates that plasticity in SLA incurred a direct fitness cost. However, a cost of plasticity was only expressed under low light, but not under high light. Thus, models on the evolution of phenotypic plasticity will need to incorporate plasticity costs that vary in magnitude depending on environmental conditions.     

Parental investment and its sensitivity to corticosterone is linked to melanin-based coloration in barn owls

Behavioral and physiological responses to unpredictable changes in environmental conditions are, in part, mediated by glucocorticoids (corticosterone in birds). In polymorphic species, individuals of the same sex and age display different heritable melanin-based color morphs, associated with physiological and reproductive parameters and possibly alternative strategies to cope with variation in environmental conditions. We examined whether the role of corticosterone in resolving the trade-off between self-maintenance and reproductive activities covaries with the size of melanin-based spots displayed on the ventral body side of male barn owls. Administration of corticosterone to simulate physiological stress in males revealed pronounced changes in their food-provisioning rates to nestlings compared to control males. Corticosterone-treated males with small eumelanic spots reduced nestling provisioning rates as compared to controls, and also to a greater degree than did corticosterone-treated males with large spots. Large-spotted males generally exhibited lower parental provisioning and appear insensitive to exogenous corticosterone suggesting that the size of the black spots on the breast feathers predicts the ability to cope with stressful situations. The reduced provisioning rate of corticosterone-treated males caused a temporary reduction in nestling growth rates but, did not affect fledgling success. This suggests that moderately elevated corticosterone levels are not inhibitory to current reproduction but rather trigger behavioral responses to maximize lifetime reproductive success.     

Parental care of kestrels living in stable and varying environmental conditions

We studied parental behaviour of two populations of Eurasian kestrels (Falco tinnunculus) living in environments with different reproductive constraints: a northern Italy population, resident, with relatively long reproductive season and almost stable prey availability, and a western Finland population, long-distance migrant, with short reproductive season and fluctuating availability of main prey (voles). The Finnish population was studied during a low vole availability year. We aimed to test the supposition that in variable environments the few pairs capable of breeding during low prey availability years show high-quality parental care. To estimate the quality of parental care in both Finnish and Italian pairs we measured the difference in the number of prey delivered by the male and by the female within each pair. The parental care, measured by that parameter, was greater among Italian pairs during both the incubation and early nestling periods. However, this was not the case in the late nestling period, when food requirements of offspring are greater. Moreover, the Finnish and Italian pairs produced about the same number of fledglings. Therefore, our data supported the initial supposition and we conclude that the few Finnish pairs capable of breeding in a low vole availability year show parental care similar to Italian pairs.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.