Tradeoffs between growth and reproduction in response to temporal variation in food supply

Allocating resources to growth or to reproduction is a fundamental tradeoff in evolutionary life history theory. In environments with unpredictable food resources, natural selection is expected to favor increased allocation to reproduction. Although effects of selection are realized only across generations, short-term changes in food predictability might influence intra-generational tradeoffs in resource allocation. We assessed the ability of fathead minnows, Pimephales promelas, to adjust allocation to growth and reproduction in response to predictable, unpredictable, and switched feeding schedules. Fish in the switched treatments were changed from unpredictable to predictable feeding schedules just after reaching sexual maturity. Egg production did not differ significantly among treatments despite the fact that females on the unpredictable and switched feeding schedules grew more slowly than those on the predictable schedule. Switched males were heavier and had proportionally larger testes than males in predictable and unpredictable treatments. Increased allocation to reproduction or growth by fish on unpredictable and switched feeding schedules was associated with changes in gut length relative to body mass. Both sexes showed a remarkable degree of phenotypic plasticity in response to resource availability and sex differences in allocation patterns were consistent with adaptive responses in the context of the fathead mating system.     

Cost of reproduction and allocation of food between parent and young in the swift (Apus apus)

We manipulated brood sizes to promote different levels of parentaleffort in the common swift (Apus apus). This provided a powerfulmethod for testing hypotheses regarding parental investmentdecisions concerning optimal allocation strategies between parentsand young. Data were analyzed on a visit-by-visit basis regardingchanges in parental and chick body mass, the mass of prey delivered,and the estimated mass of parental self-feeding. Our resultswere consistent with current theory in that food delivery increasedwith brood size, whereas the food received per chick, and hencemean chick body mass, decreased with brood size. Parental bodymass decreased with brood size and increasing parental effortbut recovered quickly during lower levels of chick feeding immediatelybefore fledging, suggesting some short-term cost of reproduction.Parents feeding at the highest level experienced criticallylow body mass and responded by a temporary cessation of chickfeeding. On any one foraging trip, total mass of prey captureddid not differ between brood sizes, but load mass deliveredto the young was negatively related to the amount of estimatedparental self-feeding. Allocation decisions of parents feedingthemselves and their young matched differential allocation theories,but estimated provisioning efficiency of parents at differentbody masses did not suggest any adaptive advantage from parentalmass loss.     

Fitness optimization of Daphnia in a trade-off between food and temperature

In thermally stratified lakes with a deep chlorophyll maximum (DCM), Daphnia face a trade-off between food availability and optimum development temperatures. We hypothesize that Daphnia optimize their fitness by allocating the time spent in the different vertical habitats depending on the distribution of algal resources and the temperature gradient. We used the plankton towers (large indoor mesocosms) to study the vertical distribution of a population of Daphnia hyalina×galeata in three different temperature gradients with a DCM. Additionally, we determined the fitness of Daphnia in the epilimnion and hypolimnion by transferring water from these layers into flow-through systems where we raised Daphnia and assessed their juvenile growth rate as a measure of fitness. The fitness distribution was correlated with the vertical distribution. The vertical distribution most likely reflected the proportions of time Daphnia allocated to dwelling in the two vertical habitats.     

Variations in the life-history parameters of Hemipyrellia ligurriens (Diptera: Calliphoridae) in response to larval competition for food

Abstract. 1. Larval rearing densities of Hemipyrellia ligurriens (Wiedemann) (Diptera: Calliphoridae) in standardized carrion were manipulated in order to investigate changes in life-history parameters in response to larval competition for food.
2. Competition was of the typical scramble type. Survivorship remained high at densities up to 32 larvae g liver^-1 but decreased rapidly as larval density increased further.
3. Emergent adults were undersized with reduced fecundity and longevity. Variations in adult body size apparently reduced the effects of competition on larval mortality.
4. Females of dry weight corresponding to only 10.4% of the potential maximum emerged at the highest rearing densities of 128 larvae g liver-1. However, these females had a nearly four-fold increase in reproductive investment (per unit weight) when compared to the largest individuals.
5. The duration of larval development declined when competition was intense (i.e. at high larval densities).
6. The short adult life of H.ligurriens, combined with the unpredictability of larval habitat availability, may reduce the value of long-range dispersal so that females 'do better' by maintaining reproductive investment despite a concomitant decline in dispersal ability.     

Females allocate differentially to offspring size and number in response to male effects on female and offspring fitness

Female investment in offspring size and number has been observed to vary with the phenotype of their mate across diverse taxa. Recent theory motivated by these intriguing empirical patterns predicted both positive (differential allocation) and negative (reproductive compensation) effects of mating with a preferred male on female investment. These predictions, however, focused on total reproductive effort and did not distinguish between a response in offspring size and clutch size. Here, we model how specific paternal effects on fitness affect maternal allocation to offspring size and number. The specific mechanism by which males affect the fitness of females or their offspring determines whether and how females allocated differentially. Offspring size is predicted to increase when males benefit offspring survival, but decrease when males increase offspring growth rate. Clutch size is predicted to increase when males contribute to female resources (e.g. with a nuptial gift) and when males increase offspring growth rate. The predicted direction and magnitude of female responses vary with female age, but only when per-offspring paternal benefits decline with clutch size. We conclude that considering specific paternal effects on fitness in the context of maternal life-history trade-offs can help explain mixed empirical patterns of differential allocation and reproductive compensation.     

Distribution of Daphnia in a trade-off between food and temperature: individual habitat choice and time allocation

1. In a thermally stratified water column with a deep‐water algal maximum, Daphnia face a trade‐off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known. 2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10 °C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep‐water algal maximum. 3. The migration behaviour of individual neonate and egg‐bearing Daphnia hyalina × galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion. 4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg‐bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg‐bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions.     

Mechanisms of energy allocation to reproduction in the cladoceran Daphnia magna Straus

Understanding rules of resource allocation within individuals is helpful in explaining population dynamics. This is particularly the case under the conditions of resource limitation that are commonly experienced by zooplankton. Here, we evaluate assumptions underlying models of resource allocation in Daphnia and test the predictions of two models of response to starvation.
We also test the predictions of two simple models concerning the mechanisms of egg provisioning in Daphnia magna. Our results: (1) demonstrate that provisioning is discontinuous, occurring over a discrete period of the instar; (2) support the hypothesis that egg production occurs in a serial fashion; (3) show that Daphnia magna responds to starvation by ceasing egg production but that there is an increase in mean size of eggs provisioned during the instar, prior to starvation; and (4) broadly support predictions that the response of Daphnia to starvation is an instantaneous cessation of reproduction but that there is a time lag before death.     

Resource allocation to growth,reproduction and survival in Gladiolus : The cost of male function

Theoretical models of life-history evolution assume trade-offs between present and future reproduction and/or survival. Models of the evolution of sex assume trade-offs between male function and female function. Generally, experiments designed to evaluate the cost of reproduction on other functions tend to ignore male function. The present work on Gladiolus takes into account simultaneously the different primary functions of the plant and separates sexual reproduction into one male component (pollen production) and one female component (seed production). The study of environmental (within-clone), between-clone and genetic correlations using strains of Gladiolus and principal component analysis show that trade-offs exist between male function, female function and survival, including both characters of plant vigour, perennation (corm production) and vegetative propagation (cormel production). Phenotypic correlations, using different species and species-hybrids, have been obtained which confirm these results. In particular, these results underline the importance of the impact of pollen production on the other functions.     

A trade-off between scale and precision in resource foraging

Summary There is widespread uncertainty about the nature and role of morphological plasticity in resource competition in plant communities. We have assayed the foraging characteristics of leaf canopies and root systems of eight herbaceous plants of contrasted ecology using new techniques to create controlled patchiness in light and mineral nutrient supply. The results are compared with those of a conventional competition experiment. Measurements of dry matter partitioning and growth in patchy conditions indicate a consistent positive association between the foraging characteristics of roots and shoots, supporting the hypothesis of strong interdependence of competitive abilities for light and mineral nutrients. Differences are identified in the abilities of dominant and subordinate plants to forage on coarse and fine scalcs. It is suggested that a trade-off exists in the scale (“high” in dominants) and precision (high in subordinates) with which resources are intercepted and that this trade-off contributes to diversity in communities of competing plants.     

Trade-offs between sexual and clonal reproduction in an aquatic plant: experimental manipulations vs. phenotypic correlations

That trade-offs result from the allocation of limited resources is a central concept of life history evolution. We quantified trade-offs between sexual and clonal reproduction in the aquatic plant, Butomus umbellatus, by experimentally manipulating sexual investment in two distinct nutrient environments. Increasing seed production caused a significant but nonlinear trade-off. Pollinating half of all flowers strongly reduced clonal bulbil production, but pollinating the remaining flowers did not cause any further trade-off. Trade-offs were not stronger under low nutrient conditions that clearly limited plant growth. Experimentally induced trade-offs were not reflected in negative phenotypic correlations between sexual and clonal allocation among plants within eight populations grown in a uniform greenhouse environment. Diminishing effects of increased sexual allocation plus a lack of accord between experimental manipulations and phenotypic correlations suggest that trade-offs between sexual and clonal reproduction are unlikely to constrain the evolution of reproductive strategy in this species.     

A trade-off in task allocation between sensitivity to the environment and response time

Task allocation is the process that adjusts the number of workers in each colony task in response to the environment. There is no central coordination of task allocation; instead workers use local cues from the environment and from other workers to decide which task to perform. We examine two aspects of task allocation: the sensitivity to the environment of task distribution, and the rate of response to environmental changes. We investigate how these two aspects are influenced by: (1) colony size, and (2) behavioral rules used by workers, i.e. how a worker uses cues from the environment and from social interactions with other workers in deciding which task to perform. We show that if workers use social cues in their choice of task, response time decreases with increasing colony size. Sensitivity of task distribution to the environment may decrease or not with colony size, depending on the behavioral rules used by workers. This produces a trade-off in task allocation: short response times can be achieved by increasing colony size, but at the cost of decreased sensitivity to the environment. We show that when a worker's response to social interactions depends on the local environment, sensitivity of task distribution to the environment is not affected by colony size and the trade-off is avoided.     

A hierarchical perspective in allocation to reproduction from whole plant to fruit and seed level

Allocation to reproduction is often examined at the whole plant level from an allometric perspective; however, investment in reproduction is a hierarchical process which represents the integration of adjustments at different hierarchical levels. This essay review presents some key issues in reproductive allocation by examining problems of allocation at different hierarchical levels within plant. At the whole plant level, the application of ‘Rensch's rule’ to plants is discussed as a mechanism that might explain size differences in relation to gender in dioecious species. At branch level, the existence of vegetative branches may be explained by hypotheses on branch specialization and costs of reproduction. However, vegetative branches might also constitute an ‘assurance’ for reproduction in variable environments. At a lower hierarchical level, fruit abortion is a process widely observed in plants that impose limits to our capacity to estimate and develop general laws about reproductive allocation in plants. The question of whether fruit abortion can be used by the parent to produce a more homogeneous progeny that limits asymmetric competition among sibs is discussed. Finally, it seems that in some cases the allocation within seeds is in accordance with some predictions of kin selection theory. Most of these topics contain open questions that deserve further research.     

Switching Akt: from survival signaling to deadly response

Akt, a protein kinase hyperactivated in many tumors, plays a major role in both cell survival and resistance to tumor therapy. A recent study, 1 along with other evidences, shows interestingly, that Akt is not a single‐function kinase, but may facilitate rather than inhibit cell death under certain conditions. This hitherto undetected function of Akt is accomplished by its ability to increase reactive oxygen species and to suppress antioxidant enzymes. The ability of Akt to down‐regulate antioxidant defenses uncovers a novel Achilles' heel, which could be exploited by oxidant therapies in order to selectively eradicate tumor cells that express high levels of Akt activity.     

A trade-off between sexual signalling and immune function in a natural population of the drumming wolf spider Hygrolycosa rubrofasciata

The field of ecological immunology is ultimately seeking to address the question ‘Why is there variation in immune function?’ Here, we provide experimental evidence that costs of ubiquitous sexual signals are a significant source of variation in immune function. In the mating season, males of the wolf spider Hygrolycosa rubrofasciata drum against dry leaves while wandering around the habitat searching for receptive females. According to a previous study, the male metabolic rate during the drumming increases 22‐fold compared to the resting metabolic rate. In the present study, we examined whether investment in costly courtship drumming decreases male immune function in a wild population of H. rubrofasciata. We induced males to increase their drumming rate by introducing females in proximity. As estimates of male immune function, we used lytic activity and encapsulation rate. Lytic activity estimates the concentration of antimicrobial peptides in haemolymph, which have been shown to play an important role in defence against bacteria, viruses and fungi. Encapsulation is an important defence mechanism against nematodes and insect parasitoids, but it also plays a role in defence against viruses. Our results show that males with nonarbitrarily increased investment in drumming rate had considerably lower lytic activities than control males. Also, there was a tendency for males with nonarbitrarily increased investment in drumming rate to have lower encapsulation rates than control males. This study provides experimental evidence for the first time, to our knowledge, that there are direct immunological costs of sexual signalling in natural populations. Therefore, immunological costs of sexual signals may provide significant phenotypic variation to parasite‐mediated sexual selection.