Stewart J. Plaistow

Energetic Constraints and Male Mate-Securing Tactics in the Damselfly Calopteryx splendens xanthostoma (Charpentier)

Males of the damselfly Calopteryx splendens xanthostoma (Charpentier) demonstrate territorial and non-territorial mate securing tactics. Non-territorial males obtain a territory in one of two ways: they either wait for a territory to become vacant, or they fight with and displace a territory holder. The estimated reproductive success of territorial males was a thousand times greater than that of non-territorial males, suggesting that males should fight hard to become, and remain, territorial. Our results show that the ability to win fights, and therefore be territorial, is dependent on energy reserves (fat). Energy reserves were correlated with the age of the individual. Young, pre-territorial, males had excess fat; territorial males that had gained their territory by fighting had intermediate fat reserves; older males that had been displaced from a territory had very low levels of fat. Previous studies of calopterygid territoriality have suggested that resident-intruder or other uncorrelated asymmetries are important in determining the outcome of a conflict. We provide an alternative explanation centring around Grafen’s (1987) ‘desperado’ effect and the energy constraint on a male’s ability to obtain a territory.

Complex population dynamics and complex causation: devils, details and demography

Population dynamics result from the interplay of density-independent and density-dependent processes. Understanding this interplay is important, especially for being able to predict near-term population trajectories for management. In recent years, the study of model systems—experimental, observational and theoretical—has shed considerable light on the way that the both density-dependent and -independent aspects of the environment affect population dynamics via impacting on the organisms life history and therefore demography. These model-based approaches suggest that (i) individuals in different states differ in their demographic performance, (ii) these differences generate structure that can fluctuate independently of current total population size and so can influence the dynamics in important ways, (iii) individuals are strongly affected by both current and past environments, even when the past environments may be in previous generations and (iv) dynamics are typically complex and transient due to environmental noise perturbing complex population structures. For understanding population dynamics of any given system, we suggest that ‘the devil is in the detail’. Experimental dissection of empirical systems is providing important insights into the details of the drivers of demographic responses and therefore dynamics and should also stimulate theory that incorporates relevant biological mechanism.

Context-Dependent Intergenerational Effects: The Interaction between Past and Present Environments and Its Effect on Population Dynamics

Intergenerational effects arise when parents’ actions influence the reproduction and survival of their offspring and possibly later descendants. Models suggest that intergenerational effects have important implications for both population dynamical patterns and the evolution of life‐history traits. However, these will depend on the nature and duration of intergenerational effects. Here we show that manipulating parental food environments of soil mites produced intergenerational effects that were still detectable in the life histories of descendents three generations later. Intergenerational effects varied in different environments and from one generation to the next. In low‐food environments, variation in egg size altered a trade‐off between age and size at maturity and had little effect on the size of eggs produced in subsequent generations. Consequently, intergenerational effects decreased over time. In contrast, in high‐food environments, variation in egg size predominantly influenced a trade‐off between fecundity and adult survival and generated increasing variation in egg size. As a result, the persistence and significance of intergenerational effects varied between high‐ and low‐food environments. Context‐dependent intergenerational effects can therefore have complex but important effects on population dynamics.

Changes in maternal investment in eggs can affect population dynamics

The way that mothers provision their offspring can have important consequences for their offsprings performance throughout life. Models suggest that maternally induced variation in life histories may have large population dynamical effects, even perhaps driving cycles such as those seen in forest Lepidoptera. The evidence for large maternal influences on population dynamics is unconvincing, principally because of the difficulty of conducting experiments at both the individual and population level. In the soil mite, Sancassania berlesei, we show that there is a trade-off between a females fecundity and the per-egg provisioning of protein. The mothers position on this trade-off depends on her current food availability and her age. Populations initiated with 250 eggs of different mean sizes showed significant differences in the population dynamics, converging only after three generations. Differences in the growth, maturation and fecundity of the initial cohort caused differences in the competitive environment for the next generation, which, in turn, created differences in their growth and reproduction. Maternal effects in one generation can therefore lead to population dynamical consequences over many generations. Where animals live in environments that are temporally variable, we conjecture that maternal effects could result in long-term dynamical effects.

The effect of the acanthocephalan parasite Pomphorhynchus laevis on the lipid and glycogen content of its intermediate host Gammarus pulex.

Besides conspicuous changes in behaviour, manipulative parasites may also induce subtle physiological effects in the host that may also be favourable to the parasite. In particular, parasites may be able to influence the re-allocation of resources in their own favour. We studied the association between the presence of the acanthocephalan parasite, Pomphorhynchus laevis, and inter-individual variation in the lipid and glycogen content of its crustacean host, Gammarus pulex (Amphipoda). Infected gravid females had significantly lower lipid contents than uninfected females, but there was no difference in the lipid contents of non-gravid females and males that were infected with P. laevis. In contrast, we found that all individuals that were parasitised by P. laevis had significantly increased glycogen contents, independent of their sex and reproductive status. We discuss our results in relation to sex-related reproductive strategies of hosts, and the influence they may have on the level of conflict over energy allocation between the host and the parasite.

How to put all your eggs in one basket: empirical patterns of offspring provisioning throughout a mother's lifetime.

Maternal effects arise when a mother’s phenotype or the environment she experiences influences the phenotype of her progeny. Most studies of adaptive maternal effects are a “snapshot” of a mother’s lifetime offspring provisioning and do not generally consider the effects of earlier siblings on those produced later. Here we show that in soil mites, offspring provisioning strategies are dynamic, changing from an emphasis on egg number in young females to egg size in older females. This pattern may be adaptive if it increases the survival of younger offspring that must compete with older, larger siblings. The dynamic shift in egg provisioning was greater in high‐food environments in which females lived longer, creating increasing asymmetry in offspring competitive abilities. Females reared in isolation and in the presence of a high‐density colony had identical provisioning strategies, suggesting that, unlike males in this species, females do not use pheromones to assess colony size. Our findings suggest that the adaptive significance of maternal effects may be misinterpreted when studies consider only a snapshot of a female’s offspring provisioning strategy or when components of the offspring provisioning strategy are studied in isolation.

Maternal effects mediated by maternal age: from life histories to population dynamics

1. Maternal effects describe how mothers influence offspring life histories. In many taxa, maternal effects arise by differential resource allocation to young, often identified by variation in propagule size, and which affects individual traits and population dynamics. 2. Using a laboratory model system, the soil mite Sancassania berlesei, we show that, controlling for egg size, older mothers lay eggs that hatch later, develop more slowly, and mature at larger body sizes. 3. Such differences in life histories lead to marked population dynamical effects lasting for multiple generations, as evidenced by an experiment initiated with similarly sized eggs that came from young or old mothers. Differences in maturation from the initial cohort led to differences in population structure and life history that propagated the initial differences over time. 4. Maternal-age effects, which are not related to gross provisioning of the egg and are therefore phenotypically cryptic, can have profound implications for population dynamics, especially if environmental variation can affect the age structure of the adult population.

Age and size at maturity: sex, environmental variability and developmental thresholds.

In most organisms, transitions between different life–history stages occur later and at smaller sizes as growth conditions deteriorate. Day and Rowe recently proposed that this pattern could be explained by the existence of developmental thresholds (minimum sizes or levels of condition below which transitions are unable to proceed). The developmental–threshold model predicts that the reaction norm of age and size at maturity will rotate in an anticlockwise manner from positive to a shallow negative slope if: (i) initial body size or condition is reduced; and/or (ii) some individuals encounter poor growth conditions at increasingly early developmental stages. We tested these predictions by rearing replicated populations of soil mites Sancassania berlesei (Michael) under different growth conditions. High–food environments produced a vertical relationship between age and size at maturity. The slope became increasingly shallow as food was reduced. By contrast, high food in the maternal environment reduced the slope of the reaction norm of age and size at maturity, whereas low food increased it. Overall, the reaction norm of age and size at maturity in S. berlesei was significantly nonlinear and differed for males and females. We describe how growth conditions, mothers environment and sex determine age and size at maturity in S. berlesei.

Energetically costly precopulatory mate guarding in the amphipod Gammarus pulex: causes and consequences

Precopulatory mate guarding (PCMG) is thought to have evolved as a male mating strategy in species in which female receptivity is limited to a short time. It is common among crustaceans, and energetic costs associated with PCMG are thought to promote size-assortative pairing in such species, although direct evidence is lacking. Using both field surveys and laboratory experiments, we assessed the energetic costs of PCMG in Gammarus pulex and investigated their possible causes. Energetic costs were measured as differences in lipid and glycogen reserves. In field-collected samples, size-corrected lipid and glycogen reserves of paired males were both significantly higher than those of unpaired ones. In the laboratory, the energetic cost of PCMG was unrelated to its duration, but was strongly positively correlated with female size (relative to the size of the male). In addition, the increased energetic cost of guarding a larger female was independent of an induced starvation cost. Our results show for the first time an energetic cost of PCMG in male G. pulex, and indicate that such a cost is more likely to result from pair formation than from the cost of carrying the female, as has previously been assumed. We discuss our results in relation to sexual conflict over PCMG duration and the ability of males to overcome female resistance.  2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

The ontogenetic switch between odonate life history stages : effects on fitness when time and food are limited

During the course of ontogeny, odonates switch from being aquatic larvae to being terrestrial adults. Ontogenetic niche shift theory proposes that such shifts are adaptive and have evolved to maximize a growth rate (size) to mortality rate ratio. Individuals should therefore switch from one niche to the other at an optimal size or state. Since the majority of odonates are seasonal breeders, the extent to which the switch is optimal will depend upon the time and the resources available during postembryonic development. We collected a cohort of larvae that varied in how close they were to eclosion and reared them on either a high-nutrition or a low-nutrition diet. We then determined the relative influence of both time and nutritional constraints on survival and development rate, as well as the body size, size-corrected flight muscle mass and fat reserves of individuals at eclosion. Damselflies in both high- and low-nutrition treatments responded to a short development period by developing faster and reducing their body size, but did not change their proportional investment in fat reserves and flight muscle. Reduced larval nutrition resulted in decreased body size, flight muscle mass and fat reserves at eclosion. However, it had no effect on survival to eclosion, or development rate. We discuss these results in terms of the influence that time and nutritional constraints have on odonate development patterns and fitness. Copyright 1999 The Association for the Study of Animal Behaviour.