Life-history variation in relation to time constraints in a damselfly

Although variation within populations in plasticity to time constraints is expected with regard to hatching date, empirical studies are largely lacking. We studied life-history responses to time constraints manipulated by photoperiod and associated with hatching date in larvae of the damselfly Lestes viridis for two populations with a different hydroperiod. In a common garden experiment, early- and late-hatched larvae from both populations were reared at two photoperiods mimicking the start and the end of the egg-hatching season. In a reciprocal transplant experiment, early- and late-hatched larvae from both populations were reared in both ponds. In all these experiments, larvae were reared from egg hatching until adult emergence. Within both populations, larvae reared at the photoperiod indicating a late time point in the growing season, reduced development time to compensate for their perceived shorter development period. Growth rate, however, did not respond to photoperiod, resulting in a lower mass at emergence. As expected, both in the laboratory and in the field, larvae from eggs that hatched later in the season generally had a shorter development time and a faster growth rate, resulting in a higher mass at emergence compared to early-hatched larvae. This may explain the intriguing seasonal increase in mass at emergence in this species, and affect the predictions of optimality models. None of these life-history responses differed between the two populations, despite clear differences in time constraints linked to hydroperiod, suggesting the robustness of the observed patterns. Given the ubiquity of asynchronous hatching in nature, and the adaptive value of the observed differences between early- and late-hatched larvae, we expect the effects of hatching date on life-history plasticity to be widespread.     

Pathways to fitness: carry‐over effects of late hatching and urbanisation on lifetime mating success

Life history theory and most empirical studies assume carry‐over effects of larval ­conditions to shape adult fitness through their impact on metamorphic traits (age and mass at metamorphosis). Yet, very few formal tests of this connection across metamorphosis exist, because this entails longitudinal studies from the egg stage and requires measuring fitness in (semi)natural conditions. In a longitudinal one‐year common‐garden rearing experiment consisting of an outdoor microcosm part for the larval stage and a large outdoor insectary part for the adult stage, we studied the effects of two factors related to time constraints in the larval stage (egg hatching period and urbanisation) on life history traits and lifetime mating success in the males of the damselfly Coenagrion puella. We reared early‐ and late‐hatched larvae from each of three rural and three urban populations from the egg stage throughout their adult life. Key findings were that both the hatching period and urbanisation shaped adult fitness, yet through different pathways. As expected, the more time‐constrained late‐hatched individuals accelerated their larval life history and this was associated with a lower lifetime mating success. A path analysis revealed this carry‐over effect was mediated by the changes in the two metamorphic traits (reduced age and lower mass at emergence). Notably, urban males had a 50% lower lifetime mating success, which was not mediated by age and mass at emergence, and possibly driven by their shorter lifespan. Our results point to long‐term carry‐over effects of the usually ignored natural variation in egg hatching dates, and further contribute to the limited evidence showing fitness costs of adjusting to an urban lifestyle.     

Starvation resistance of gypsy moth,Lymantria dispar (L.) (Lepidoptera: Lymantriidae): tradeoffs among growth,body size,and survival

Summary Survival and body composition of starving gypsy moth larvae initially reared on aspen foliage or artificial diet differeing in nitrogen (N) and carbohydrate concentration were examined under laboratory conditions. Diet nitrogen concentration strongly affected starvation resistance and body composition, but diet carbohydrate content had no effects on these. Within any single diet treatment, greater body mass afforded greater resistance to starvation. However, starving larvae reared on 1.5% N diet survived nearly three days longer than larvae reared on 3.5% N diet. Larvae reared on artificial diet survived longer than larvae reared on aspen. Differences in survival of larvae reared on artificial diet with low and high nitrogen concentrations could not be attributed to variation in respiration rates, but were associated with differences in body composition. Although percentage lipid in larvae was unaffected by diet nitrogen concentration, larvae reared on 1.5% N diet had a higher percentage carbohydrate and lower percentage protein in their bodies prior to starvation than larvae reared on 3.5% N diet. Hence, larger energy reserves of larvae reared on low nitrogen diet may have contributed to their greater starvation resistance. Whereas survival under food stress was lower for larvae reared on high N diets, growth rates and pupal weights were higher, suggesting a tradeoff between rapid growth and survival. Larger body size does not necessarily reflect larger energy reserves, and, in fact, larger body size accured via greater protein accumulation may be at the expense of energy reserves. Large, fast-growing larvae may be more fit when food is abundant, but this advantage may be severely diminished under food stress. The potential ecological and evolutionary implications of a growth/survival tradeoff are discussed.     

Asynchronous hatching and brood reduction by filial cannibalism in the burying beetle Nicrophorus quadripunctatus

Despite decades of intensive research, there is still much debate about the adaptive significance of asynchronous hatching. A major obstacle in understanding the significance of this process is the difficulty involved in separating the hypotheses that explain asynchronous hatching as an adaptive trait from those that explain it as a by-product of physiological constraints on hatching or egg-laying patterns. We investigated the burying beetle Nicrophorus quadripunctatus, a species in which the parent can eliminate less-adaptive offspring (e.g., slower-growing offspring) by filial cannibalism and adjust the age structure of offspring to an adaptive pattern. The main aim of this study was to determine the age composition of offspring that survived and to determine the effect of larval growth on filial cannibalism. We investigated how the point in time at which each group of larvae hatched affects the timing of filial cannibalism by the female parent. We found that N. quadripunctatus exhibited asynchronous hatching, and reared larvae of different ages. We also found that later-hatching larvae had lower survival and growth rates; therefore, filial cannibalism plays a role in eliminating later-arriving, slower-growing, and hence less-adaptive offspring.     

Crustacean zooplankton community structure in temporary and permanent grassland ponds

Lentic community structure varies across a size gradient of ponds and lakes with physical factors, such as pond drying, and biotic factors, such as fish predation, determining the species assemblage. We studied the effects of pond drying and fish absence on crustacean zooplankton across a gradient of pond sizes in a Texas grassland. We determined the species compositions and size distributions of crustacean zooplankton in 20 temporary and 18 permanent ponds in April after March rains had refilled the ponds. The surface areas of temporary and permanent ponds ranged from <0.01 to 0.21 ha and 0.04 to 13.8 ha, respectively, and temporary ponds were significantly smaller, on average, than permanent ponds. Fish were absent from all temporary ponds and present in all permanent ponds. We detected a difference in the zooplankton species assemblages of the temporary and permanent ponds. Out of 14 total zooplankton taxa that occurred in eight or more ponds, seven taxa were significantly more prevalent in temporary ponds and four taxa were significantly more prevalent in permanent ponds. The sizes of zooplankton in the temporary fishless ponds were greater than those in the permanent ponds with fish present. We concluded that pond size mediated susceptibility to pond drying, and pond drying determined the presence and absence of fish and their secondary trophic-level effect on zooplankton community structure. Handling editor: Steven Declerck     

DOES MULTIPLE PATERNITY IMPROVE FITNESS OF THE FROG CRINIA GEORGIANA?

Abstract.— In the Australian myobatrachid frog Crinia georgiana simultaneous polyandry occurs in about half of all matings, which leads to multiple paternity, but reduced fertilization success and occasional female mortality. Multiple paternity may provide benefits to females that compensate for these costs, for example, through enhanced genetic diversity of a clutch. In nature, embryos and tadpoles of C. georgiana develop in shallow, temporary pools and may be exposed to fluctuating water levels and the risk of desiccation between rain events. Fertilization by genetically diverse sires may act as a bet hedge against these conditions. To evaluate this hypothesis, females were artificially mated with one or two males in the field and eggs and larvae reared in the laboratory under constant or fluctuating developmental conditions. Experiment 1 exposed embryos from single- and multiple-paternity clutches to conditions where eggs were completely covered during development or eggs sat in air on a moist substrate. Experiment 2 exposed freshly hatched larvae from single- and multiple-paternity clutches to constant wet conditions, where larvae were completely covered, or fluctuating wet conditions, where larvae ranged from being completely submersed to partially exposed over a 13- day cycle. We measured mean performance and best performance as alternate measures of genetic benefits. There were no effects of paternity on percent survival to hatching, time to hatching, body size at hatching, percent survival to metamorphosis, time to metamorphosis, or body size at metamorphosis. We also analyzed variance within clutches as a measure of genetic diversity. Again there were no predictable effects of multiple paternity. Polyandry does not appear to provide any genetic benefits that compensate for the high costs of polyandry in this species.     

Growth and population structure of Salamandra salamandra (L.) larvae in different limnological conditions

Salamandra salamandra (L.) larvae were studied in three types of aquatic systems, a permanent spring, a rock pool and temporary ponds. Growth was followed throughout ontogenesis until completion of metamorphosis. Growth patterns changed according to the pond type. Temporary ponds were rich in organic matter and subsequently larvae matured more rapidly there than those in a permanent pool or a spring. In the latter, cannibalism appears to be of major ecological importance, enabling survival and metamorphosis of large size tadpoles. The pattern of growth and metamorphosis is compared with European salamanders and the differences are discussed considering adaptations for survival of this species under xeric conditions.     

Early life history of herring larvae in contrasting feeding environments determined by otolith microstructure analysis

Newly hatched autumn-spawned herring larvae Clupea harengus were released in two 2500-m³ outdoor mesocosms and reared over a 2-month period. Hydrographic conditions were similar in the two mesocosms, but the average plankton density was initially more than 10 times higher in mesocosm B compared to mesocosm A (>11⁻¹ v. <0.11⁻¹). Half-way through the experiment the feeding conditions reversed with three times higher average densities in mesocosm A than in mesocosm B (>31⁻¹ v. ∼11⁻¹). Herring larvae were sampled with a 0.3-m² two-chambered net twice weekly, and survivors were harvested by draining the mesocosms at the end of the experiment. Otolith growth trajectories of individual larvae were determined by relating radial otolith size with number of increments from the outer edge of the otolith (days before capture). The increment widths during the first 3 weeks after hatching, including the first-check size, were generally wider among larvae from mesocosm B (relatively good initial feeding conditions) than among those from mesocosm A (poor initial feeding conditions). The otolith growth pattern also confirmed that the surviving herring in mesocosm A belonged to the upper size range of larvae in the mesocosm after only 2–3 weeks from hatching; no such trend was found in mesocosm B. In both mesocosms the otolith size-at-age indicated that with the present sampling gear, herring larvae larger than 20–25 mm were underrepresented in the net samples. The information obtained from otolith-size-at-age is compared with other morphometric and biochemical measures of size and condition of larvae obtained throughout the experiment.     

Population differences in the timing of diapause: a test of hypotheses

Summary The reproductive phenology of the freshwater copepod Diaptomus sanguineus differs markedly between populations residing in two Rhode Island ponds. In a permanent pond the population switches abruptly from making subitaneous (immediately hatching) eggs to diapausing eggs at the end of March each year. In contrast, a temporary pond population switches egg types in May, returns to production of subitaneous eggs in June, and concludes the reproductive season by making diapausing eggs in July. An ESS model suggests that the pattern of diapause expected of a copepod population is a function of annual variation in the onset of harsh conditions (catastrophe date). When variation is relatively low, the superior strategy is for diapause to begin a constant period before the mean catastrophe date. When variation is high, females should make first subitaneous eggs and then diapausing eggs irrespective of the expected catastrophe date. With discrete generations, such a population would alternate between egg types. In the permanent pond, variation of catastrophe date the spring onset of planktivory by sunfish is low, whereas in the temporary pond variation of the catastrophe (pond drying) is high. The model predicts well the phenology of the two copepod populations.In the research reported here, we tested the hypothesis that copepods from the permanent pond, which switch to diapause at the same time every year, are cued by the environment to begin diapause (i.e. by photoperiod, temperature, or both), whereas those from the temporary pond make both egg types regardless of environmental conditions. In opposition to our hypothesis, experimental results indicate that diapause in both populations is cued by the environment. The distinct reproductive phenologies documented in the two populations apparently result from the copepods responding to different environmental cues, rather than one being responsive to the environment while the other is not.     

Growth and otolith microstructure of cod (Gadus morhua L.) larvae

Cod larvae from Irish Sea stocks were reared under differentgrowth conditions, and the otolith growth and increment formationexamined in sagittae and lapilli. Otolith increments were firstdeposited around the time of hatching and increment counts,on average, reflected larval age. The growth rate of fish larvaereared in different sized tanks was significantly different(P < 0.001), but there was no detectable effect on incrementformation. Otolith size was independent of larval size for individuals<5 mm in length. In larvae >6 mm, larger, faster growingindividuals had larger and faster growing otoliths.     

ARTIFICIAL SELECTION FOR PAEDOMORPHOSIS IN THE SALAMANDER AMBYSTOMA TALPOIDEUM

Heterochronic ontogenetic mechanisms such as paedomorphosis are potentially important mechanisms of both microevolutionary and macroevolutionary change. The salamander Ambystoma talpoideum is facultatively paedomorphic. Expression of paedomorphosis in this species varies among local natural populations. Two breeding lines, one from a population associated with a temporary pond where metamorphosis to a terrestrial adult always occurs, another from a population associated with a nearly permanent pond where paedomorphosis is common, were selected artificially for paedomorphosis over four generations. The F₅ generation of each breeding line was reared in a “common garden” field experiment under two drying regimes to simulate the larval environment in a temporary and in a permanent pond. There was a significantly different response to the drying regimes and to the artificial selection in the two lines. A significant population × selection interaction indicated that the two populations responded differently to artificial selection for paedomorphosis. The presence of heritable genetic variation suggests that evolution and divergence among populations of salamanders is possible with intense natural selection over short periods of ecological time.     

Factors affecting contributions of the tadpole shrimp,Lepidurus packardi,to its oversummering egg reserves

The phenology and reproductive biology of the tadpole shrimp, Lepidurus packardi, which inhabits temporary annual pools in northern California, U.S.A., were investigated to identify factors affecting its contributions to its egg reserves that oversummer in the dried pond sediments. Field observations throughout 2 seasons revealed the nearly continuous presence of juveniles, indicating multiple generations, and a predominance of males during the mid to late stages of the pond. Observation of the first oviposition, and dissections revealed that sexual maturation occurred in shrimp 10–12 mm carapace length. Fecundity increased with size, but was drastically reduced in parasitized individuals. Metacercariae of an echinostome fluke caused parasitic castration in 53% of the population by mid-season. In the laboratory, oversummered eggs collected from dried pond sediments hatched within 17 days, when incubated at 10 °C. Eggs laid in the laboratory hatched within 25 days at 10 °C, without prior dehydration, but hatching was reduced at higher temperatures.     

Sex‐specific growth in chicks of the sexually dimorphic Black‐tailed Godwit

Sexual size dimorphism (SSD) is common in birds and has been linked to various selective forces. Nevertheless, the question of how and when the sexes start to differentiate from each other is poorly studied. This is a critical knowledge gap, as sex differences in growth may cause different responses to similar ecological conditions. In this study, we describe the sex‐specific growth – based on body mass and five morphometric measurements – of 56 captive Black‐tailed Godwit Limosa limosa limosa chicks raised under ad libitum food conditions, and conclude that all six growth curves are sex‐specific. Females are the larger sex in terms of body mass and skeletal body size. To test whether sex‐specific growth leads to sex‐specific susceptibility to environmental conditions, we compared the age‐specific sizes of male and female chicks in the wild with those of Black‐tailed Godwits reared in captivity. We then tested for a relationship between residual growth and relative hatching date, age, sex and habitat type in which the wild chicks were born. Early‐hatched chicks were relatively bigger and in better condition than late‐hatched chicks, but body condition and size were not affected by natal habitat type. Female chicks deviated more negatively from the sex‐specific growth curves than male chicks for body mass and total‐head length. This suggests that the growth of the larger females is more susceptible to limiting environmental conditions. On average, the deviations of wild chicks from the predicted growth curves were negative for all measurements, which suggests that conditions are limiting in the current agricultural landscape. We argue that in estimating growth curves for sexually dimorphic species, it is critical first to make accurate sex and age determinations.     

Trait flexibility of generalist invertebrates exposed to contrasting predation and drying stressors

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INTRASPECIFIC LIFE HISTORY VARIATION IN A POND SNAIL: THE ROLES OF POPULATION DIVERGENCE AND PHENOTYPIC PLASTICITY

Two approaches were used to determine the degree of divergence in life histories among populations of the pond snail, Lymnaea elodes. Juvenile snails were reciprocally transferred between ponds differing in permanence and productivity, and the resulting variation in life history traits was recorded. In a second experiment, parents and their offspring from both a vernal and a permanent pond population were reared in the same pond. Proximal factors had by far the greatest effects on life history traits in the transfer experiment, with snails reared in a more productive pond showing earlier reproduction at a larger size, higher fecundity, and longer life cycle length. Snails from the more uncertain pond in terms of drying date did reproduce at an earlier age and smaller size and grew less in each pond. However, these population differences, for the most part, disappeared when snails were reared for two generations in the same environment. Much of the intraspecific variation in life histories seen in this species must therefore be considered the result of phenotypic plasticity. I argue that the plasticity in life histories itself may be adaptive to this inhabitant of unpredictable, vernal ponds.     

Density-dependent injury in larval salamanders

Summary The effects of initial larval density, food level, and pond drying regime on intraspecific aggression of larval Ambystoma talpoideum were studied in an artificial pond experiment. Aggression was measured by the frequency of injury of feet, limbs, tail, and the extent of tail loss. Initial larval density had a significant effect on the frequency of foot, limb, and tail loss but not on the extent of tail loss. More larvae reared at medium and high densities sustained injuries than larvae reared at low densities but injuries were not more extensive. Food level had no effect on the four measures of injury. Pond drying regime had no effect on foot loss, limb loss, or extent of tail loss but more larvae reared in constant water level ponds had tail loss than in drying ponds. The frequency of limb and tail loss was negatively correlated to density-dependent survival which was the result of intraspecific predation or cannibalism. These results indicate that substantive levels of body injury, particularly tail loss, can occur at high natural larval densities and may result in a subsequent reduction of growth and survival.     

Maternal influence on the size and viability of Iceland cod Gadus morhua eggs and larvae

Size, condition, and age of female-Icelandic cod Gadus morhua were correlated to the size of their eggs and newly hatched larvae. A positive relationship was detected between egg size and some larval viability parameters, including the age at first feeding, successful development of a swimbladder, and specific growth rates during the first 15 days after hatching. These results reveal that the viability of cod larvae is related to attributes of the spawning females and that this information is important to our understanding of stock-recruitment relationships.     

Life-history evolution when Lestes damselflies invaded vernal ponds

We know little about the macroevolution of life-history traits along environmental gradients, especially with regard to the directionality compared to the ancestral states and the associated costs to other functions. Here we examine how age and size at maturity evolved when Lestes damselflies shifted from their ancestral temporary pond habitat (i.e., ponds that may dry once every decade or so) to extremely ephemeral vernal ponds (ponds that routinely dry completely each year). Larvae of three species were reared from eggs until emergence under different levels of photoperiod and transient starvation stress. Compared to the two temporary-pond Lestes, the phylogenetically derived vernal-pond Lestes dryas developed more rapidly across photoperiod treatments until the final instar, and only expressed plasticity in development time in the final instar under photoperiod levels that simulated a later hatching date. The documented change in development rate can be considered adaptive and underlies the success of the derived species in vernal ponds. Results suggest associated costs of faster development are lower mass at maturity and lower immune function after transient starvation stress. These costs may not only have impeded further evolution of the routine development rate to what is physiologically maximal, but also maintained some degree of plasticity to time constraints when the habitat shift occurred.     

Accelerated growth rates in late‐hatched Rhinoceros Auklet Cerorhinca monocerata chicks depend on food conditions and growth stage: an experimental approach

In some bird species, the survival of chicks hatching later in the season is lower than those hatched earlier due to increased risk of predation and a seasonal decline in feeding conditions. To reduce these risks, it might be advantageous for late‐hatched chicks to grow faster and hence fledge at younger age. In this experimental study, the growth rates of early‐ and late‐hatched Rhinoceros Auklet Cerorhinca monocerata chicks were compared under average and poor food supplies in captivity. Controlling for potentially confounding effects of chick mass at 10 days old, chick age and nest‐chamber temperature, late‐hatched chicks had higher wing growth rate than early‐hatched chicks before attaining the minimum wing length required for fledgling under both average and poor food supplies. After attaining the minimum wing length, however, late‐hatched chicks had a lower fledging mass, indicating a potential cost that could diminish the early advantage of fast wing growth.     

Primary and secondary phenology. Does it pay a frog to spawn early?

This study examines the consequences of variation in the laying and hatching date for the time of metamorphosis in the common frog Rana temporaria . Field data are presented showing that eggs laid early tend to take longer to develop. Thus, the time advantage for early eggs is reduced at the time of hatching. There was an among-year variation in this phenomenon; it was not manifest in a phenologically late year. Also, field data revealed that mortality due to pond freezing is a real risk for early laid eggs. Finally, two experiments in tanks analyse the effects of hatching date variation for the time of metamorphosis. (1) When hatching was experimentally delayed by 7 or 11 days, this resulted in later metamorphosis, however, by only 2 and 5 days, respectively. (2a) When tadpoles from the same pond that naturally hatched at different times were compared, it was found that a hatching time difference of 6 days resulted in later metamorphosis by 2 days only. (2b) A comparison of tadpoles from two different ponds that hatched 11 days apart also resulted in only 2 days' difference in metamorphosis. In this case, the later but faster developing tadpoles metamorphosed at a smaller size. I suggest that eggs from these two ponds differed genetically in the growth and development strategy. Despite the obvious risks, and the moderate gain in terms of early metamorphosis, frogs breed dangerously early in spring. Possible reasons for this are discussed. These include external selective forces that promote early metamorphosis (also at a high cost), within-pond competition among tadpoles with an advantage for early and large tadpoles and finally factors relating to mate choice at the breeding site.