Some aspects of overwintering in southern England of the mosquitoes Anopheles atroparvus and Culiseta annulata (Diptera: Culicidae)

Abstract. 1. Overwintering of the mosquitoes Anopheles atroparvus van Thiel and Culiseta annulata Schrank, both of which feed on blood throughout the year, is compared.
2 The aquatic stages, males and gonoactive females of A.atroparvus disappear before winter, leaving a generation of inseminated, nulliparous, diapausing females to overwinter. Blood meals taken by diapausing females merely serve to maintain fat reserves. After re-activation in March–April, gonoactive life expectancy appears to be rather short. Aquatic development at this season is slow and eggs laid in the spring do not significantly affect adult densities until June.
3. Aquatic stages, males and gonoactive females of C.annulata are present throughout the year. This species appears to be able to overwinter without recourse to diapause, females passing the winter in alternating, or lasting, states of cold-induced quiescence or normal gonoactivity.     

Growth energy partitioning by juvenile bluegill sunfish, Lepomis macrochirus Rafinesque

Growth energy build-up and partitioning by immature bluegill sunfish were studied over the summer months from May to September 1984. Fish sampled in May showed low condition and low lipid reserves. Condition was restored to summer levels within the following month. Fat reserves were built up from 7% to 14% of body weight over the entire summer. Most (80%) of the annual length increase occurred from 1 June to 15 August. Over the winter period (October-April) condition dropped and lipids were depleted. Calculations showed that energy liberated through lipid depletion over this period could supply the energy requirements of inactive wintering fish. It is concluded that the sequencing of growth energy diversion into changes in body length, condition, and lipid build-up enhances mid-summer growth and overwinter survival.     

Developmental mortality increases sex‐ratio bias of a size‐dimorphic bark beetle

1. Given sexual size dimorphism, differential mortality owing to body size can lead to sex‐biased mortality, proximately biasing sex ratios. This mechanism may apply to mountain pine beetles, Dendroctonus ponderosae Hopkins, which typically have female‐biased adult populations (2 : 1) with females larger than males. Smaller males could be more susceptible to stresses than larger females as developing beetles overwinter and populations experience high mortality. 2. Survival of naturally‐established mountain pine beetles during the juvenile stage and the resulting adult sex ratios and body sizes (volume) were studied. Three treatments were applied to vary survival in logs cut from trees containing broods of mountain pine beetles. Logs were removed from the forest either in early winter, or in spring after overwintering below snow or after overwintering above snow. Upon removal, logs were placed at room temperature to allow beetles to complete development under similar conditions. 3. Compared with beetles from logs removed in early winter, mortality was higher and the sex ratio was more female‐biased in overwintering logs. The bias increased with overwinter mortality. However, sex ratios were female‐biased even in early winter, so additional mechanisms, other than overwintering mortality, contributed to the sex‐ratio bias. Body volume varied little relative to sex‐biased mortality, suggesting other size‐independent causes of male‐biased mortality. 4. Overwintering mortality is considered a major determinant of mountain pine beetle population dynamics. The disproportionate survival of females, who initiate colonisation of live pine trees, may affect population dynamics in ways that have not been previously considered.     

The feeding and energetics of stream-resident trout in winter*

The feeding of brook and brown trout in a temperate Canadian stream was studied over two winters. Stomach analyses indicate that feeding continues throughout the winter. Both species had similar diets. Despite continuous feeding, condition factors declined significantly in early winter and remained low until the spring, suggesting an early-winter depletion of lipid reserves. Energy (calorific) intake was similar in winter to that found in late summer or spring and therefore was not the cause of low winter condition. The derived energy from feeding was insufficient to offset the costs of maintenance metabolism in the early winter period, for both immature and recently spawned fish. It is suggested that stream-dwelling trout suffer a metabolic deficit during acclimation to rapidly declining water temperatures in November/December. Such a phenomenon has important physiological implications for overwinter survival.     

Fitness consequences of alternative life histories in water striders,Aquarius remigis (Heteroptera: Gerridae)

Using field and laboratory observations and experiments over 3 years, I investigated whether reproductive trade-offs shape individual life histories in two natural populations of the water strider, Aquarius remigis, in which univoltine and bivoltine life cycles coexist. Both later eclosion dates and food shortages, even after adult eclosion, induced diapause in females, thus deferring reproduction to the following spring. Adult body size was positively affected by food availability during juvenile development. Higher food levels also increased the reproductive output of females, but not their longevity or oviposition period. When compared to spring breeders (univoltine life cycle), direct (summer) breeders (bivoltine life cycle) experienced reduced lifetime egg numbers and longevity, as well as reduced survivorship of their second-summer-generation offspring; these reproductive costs offset, at least in part, the advantage in non-decreasing populations of having two generations per year. Fecundity was correlated with body size, and among summer-generation females direct breeders were larger than non-breeders. The time remaining before the onset of winter and/or the time since adult eclosion augmented cumulative energy uptake, and consequently the lipid reserves and winter survival probability of non-breeding (diapausing) summer adults approaching hibernation. Overwintered spring reproductives died at faster rates than non-reproductive summer individuals despite greater food availability in spring, indicating a mortality cost of reproduction. Body length correlated with absolute and not with proportional lipid content but showed no consistent relationship with survivorship in the field. These results are in agreement with current theory on the evolution of insect voltinism patterns, and further indicate high degrees of life history flexibility (phenotypic plasticity) in the study populations in response to variable environmental factors (notably photoperiod and food availability). This may be related to their location in a geographic transition zone from uni- to bivoltine life cycles.     

Projected impacts of 21st century climate change on diapause in Calanus finmarchicus

Diapause plays a key role in the life cycle of high latitude zooplankton. During diapause, animals avoid starving in winter by living in deep waters where metabolism is lower and met by lipid reserves. Global warming is therefore expected to shorten the maximum potential diapause duration by increasing metabolic rates and by reducing body size and lipid reserves. This will alter the phenology of zooplankton, impact higher trophic levels and disrupt biological carbon pumps. Here, we project the impacts of climate change on the key North Atlantic copepod Calanus finmarchicus under IPCC RCP 8.5. Potential diapause duration is modelled in relation to body size and overwintering temperature. The projections show pronounced geographic variations. Potential diapause duration reduces by more than 30% in the Western Atlantic, whereas in the key overwintering centre of the Norwegian Sea it changes only marginally. Surface temperature rises, which reduce body size and lipid reserves, will have a similar impact to deep‐water changes on diapause in many regions. Because deep‐water warming lags that at the surface, animals in the Labrador Sea could offset warming impacts by diapausing in deeper waters. However, the ability to control diapause depth may be limited.     

Role of body size in dominance interactions between male water striders, Aquarius paludum

Water striders are a model system for the study of sexual size dimorphism, but the effect of body size on the dominance relationship between individuals has not been experimentally tested. In 34 staged contests between males of the water strider Aquarius paludum, we determined the effect of body size difference between contestants on the outcome of the aggressive interactions. In contests between a large and a small male, the larger individuals won the interactions significantly more often than expected by chance. This is the first experimental evidence for the importance of body size in pair-wise contests among water striders.     

Suppression of net transpiration by multiple mechanisms conserves water resources during pupal diapause in the corn earworm Helicoverpa zea

One critical aspect of an insect's ability to overwinter successfully is the effective management of its water resources. Maintenance of adequate water levels during winter is challenging because of the prevailing low relative humidity at that time of year and the short supply of environmental water that is not in the form of ice. These issues are further exacerbated for insects overwintering as pupae, comprising an immobile stage that is unable to move to new microhabitats if conditions deteriorate. The present study compares the water balance attributes of diapausing and nondiapausing pupae of the corn earworm Helicoverpa zea Boddie, aiming to identify the mechanisms used by diapausing pupae to maintain water balance during winter. Diapausing pupae are 10% larger than nondiapausing individuals. Water loss rates for nondiapausing pupae are low (0.21 mg h^?1) and are suppressed (0.01 mg h^?1) in diapausing pupae. Cuticular lipids, which serve to waterproof the cuticle and thus suppress cuticular water loss, are more than two‐fold more abundant on the surface of diapausing pupae, and oxygen consumption rates during diapause drop to almost one‐third the rate observed in nondiapausing pupae. Water gain can be accomplished only when atmospheric water content is near saturation or during contact with free water. At moderate relative humidities (20–40%), water loss rates are very low for diapausing pupae, suggesting that these moth pupae have robust mechanisms for combating water loss. The exceptional ability of H. zea to suppress water loss during diapause is probably a result of the combined effects of increased size, more abundant cuticular lipids and decreased metabolic rates.     

Megafauna associations with deepwater corals (Primnoa spp.) in the Gulf of Alaska

Our study showed that one species of water strider (Aquarius najas) dominated the insect fauna (>90% of the biomass) on and near the surface of a small stream in southern Sweden, but the diet of Daubenton's bats (Myotis daubentonii), regularly feeding over the same stream, contained <1% of these insects. To explain why the bats did not eat water striders in proportion to the apparent abundance of these insects, we tested three hypotheses, with the following results: (1) The water striders did not respond to artificial bat calls and therefore their defence against bats is not based on ultrasonic hearing. (2) Water striders hand-fed to the bats were eaten with similar frequency as palatable controls (caddis flies), so there is no evidence that water striders are unpalatable to bats. (3) Attempted prey-captures by the bats occurred less often than expected within 1 m of the bank of the stream (the bank consisted of a stone wall), indicating that the bats' flight or, alternatively, their detection or capture of insects, was difficult within that area, possibly constrained by clutter. At dusk and just before the bats emerged to feed, the water striders moved away from the open water. They remained motionless close to the bank for as long as the bats were active.     

Nutrition,sex and season contribute to variation in fat and glycerol levels in the long‐lived moth Caloptilia fraxinella

The ash leaf cone roller Caloptilia fraxinella Ely (Lepidoptera: Gracillariidae) is an invasive leaf‐mining moth pest of horticultural ash Fraxinus spp. in the Canadian Prairie Provinces. Caloptilia fraxinella overwinter as adults in reproductive diapause and mating occurs after overwintering in the spring. The effect of a carbohydrate food source on fat and glycerol reserves throughout the long adult life stage of this moth is investigated. Insects collected as pupae are given access to either water or sugar water upon adult eclosion. Moths held under the different feeding regimes are sampled before (summer and autumn) and after overwintering in the spring. Analysis of either glycerol or lipid content is conducted for male and female moths from each collection period. Both moth weight and glycerol concentration are affected by moth sex, food regime and season of collection. Although female moths are heavier than males, a higher glycerol concentration occurs in males. Moths fed sugar are heavier and have a higher glycerol concentration than water‐fed moths late in reproductive diapause and after overwintering. Moths collected in the spring after overwintering are lighter and have a lower glycerol content than moths collected before winter. There is a significant influence of feeding regime and season on moth body lipid content, with sugar‐fed moths having more fat than water‐fed moths; however, this difference is smaller in the summer than the autumn or spring. An initial understanding of the overwintering biology and diapause of this pest is provided in the present study.     

NATURAL SELECTION AND SEXUAL SIZE DIMORPHISM IN RED-WINGED BLACKBIRDS

Patterns of overwinter mortality in the sexually dimorphic red-winged blackbird (Agelaius phoeniceus) were examined to test the predictions of the sexual-selection hypothesis that male size is limited by directional selection favoring small males and that female size is maintained by stabilizing selection wherein extreme phenotypes experience higher mortality. Museum specimens collected from Ontario over a 95-yr period were used to compare the sizes of males and females collected in fall and spring. In a separate field study, body sizes of returning and nonreturning male and female red-winged blackbirds were compared over a 6-yr period. Overall, there was no evidence of higher overwinter mortality among larger males. Among adult (ASY) males, large individuals appeared to have higher survival than small individuals, although among subadult (SY) males, large size may have been disadvantageous. Weak evidence of stabilizing selection on female body size was found. Among adults, sexual size dimorphism seemed more pronounced after winter than before winter. Our results do not support the hypothesis that body size in male red-winged blackbirds is limited by selective mortality outside the breeding season. It is possible that size selection occurs earlier in life, when males are still in the nest. Our results suggest that caution should be exercised when interpreting interspecific evidence showing higher adult male than female mortality in sexually dimorphic species. Such patterns could arise as a cost to males of sexual selection and yet provide no insight into how natural selection opposes sexual selection for increased male size.     

Endurance of simulated winter conditions by age-0 walleye pollock: effects of body size, water temperature and energy stores

Survival of age-0 walleye pollock Theragra chalcogramma in the absence of food followed simple bioenergetic models, with large body size, high initial condition, and cold temperatures all increasing survival rates. High survival after >200 days at cold temperatures (<3·0° C) indicated extended tolerance of extreme cold, as long as sufficient body size and condition are attained during the summer growth period. Analysis of body constituents demonstrated a substantial increase in tissue water and depletion of lipid during starvation. Survivors had significantly higher lipid stores than mortalities, and larger fish had higher levels of lipid than smaller fish among experimental survivors, laboratory fish that were never starved, and wild fish. Fish returned to warm temperatures and high rations following 205 days of food deprivation displayed nearly complete recovery, with rapid increases in length, weight, and condition and minimal mortality (6·8%) during the subsequent 3 months. Age-0 walleye pollock collected in September in the Bering Sea were substantially smaller and generally had lower lipid levels than fish used in laboratory starvation experiments, suggesting they are susceptible to size- and condition-dependent mortality during the winter. The results are interpreted with respect to field distributions of age-0 walleye pollock, overwinter survival, and synergistic effects of food and temperature under varying models of climate change.     

Survival and Body Condition of Raccoons at the Edge of the Range

ABSTRACT We investigated the influence of intrinsic and extrinsic variables on overwinter survival of raccoons (Procyon lotor; n = 114) at the northern edge of their distribution. A Cox proportional hazard model identified winter severity as the variable with the greatest influence on raccoon survival (β = 1.08). Autumn body condition estimates (20.5 ± 0.46% total body fat) were relatively stable across years even though we observed large differences in autumn food indices. Variations in autumn body condition did not explain heterogeneity observed in overwinter survival nor the spring condition in which raccoons emerged. Relatively constant autumn body condition suggests reliable availability of anthropogenic food resources may negate variations observed in natural food items on which raccoons rely during hyperphagia. Conversely, spring body condition did vary among years and was highly correlated with winter severity. Accordingly, we also observed a strong inverse relationship with overwinter survival and winter severity. Our findings indicate winter climatic constraints are important factors governing the northern limit of raccoon distribution and changes in winter severity could have important implications in further range expansion of this species.     

Effects of environmental humidity on the survival and development of pine caterpillars, Dendrolimus tabulaeformis (Lepidoptera: Lasiocampidae)

The pine caterpillar, Dendrolimus tabulaeformis is one of the most important pests on Pinus tabulaeformis and other pine species in North China. In the present study, effects of relative humidity (RH) on the development and survival of pine caterpillars and soil moisture (SM) on their diapausing larvae were investigated. Low RH (20%) deferred the development of eggs and larvae, reduced egg hatching and larval surviving compared to 40%, 60% and 80% RH. Both low (20%) and high (100%) RH reduced egg hatching, but only 20% RH deferred the development of larvae, prolonged developmental duration and reduced the body mass and body length of larvae. The SM influenced the survival of diapausing larvae significantly. The dry treatment significantly reduced the supercooling points (SCPs), whereas increased the mortality and reduced body mass from 56.9 to 36.5 mg and body water content from 78% to 63% after 2 weeks' exposure. Therefore, higher RH is more favorable for the development of early instars and survival of diapausing larvae of the pine caterpillars.     

Coping with the cold: energy storage strategies for surviving winter in freshwater fish

For many ectothermic animals, the acquisition, storage and depletion of lipids is integral to successfully coping with reduced metabolic rates and activity levels associated with cold, winter periods. In fish, lipids are crucial for overwinter survival and successful reproduction. The timing and magnitude of seasonal lipid storage should therefore vary predictably among fish with different thermal preferences and spawn times. Small‐ and large‐bodied fish should also face different constraints associated with season that influence lipid cycling. However, much work to date has been species‐ and location‐specific and a general conceptual model for the seasonal energy budgets of freshwater fish is lacking. Here, we conducted a comprehensive literature review of seasonal lipid levels in freshwater fishes. We predicted that warm and cool water species would be more likely to demonstrate peak lipid levels during warm months than cold water species, and expected a larger magnitude of annual lipid cycling in warm and cool water compared to cold water fish. We also expected dampened lipid cycling in larger fish due to their lower mass‐specific metabolic rates. Observed patterns in the timing and magnitude of lipid storage contradicted our prediction because lipid cycling was widespread across species, despite thermal guild, with peak lipid levels commonly occurring during warmer months, even in cold water fish. For body size effects, larger bodied fish species had dampened seasonal lipid cycling, as predicted. We developed a conceptual framework describing how the ‘scope’ for variation in annual lipid cycling changes with body size both among and within species in order to guide future work. Together, our findings suggest that energy acquired during warm months is broadly important for overwinter survival and reproduction in fishes, and provide a new perspective on the differential constraints and physiological responses to seasonality among freshwater fish. Improving our understanding of these dynamics is especially pressing given that a changing global climate is anticipated to alter existing seasonal signals.     

Growth, maturation and reproductive investment in Arctic charr

Size and rates of growth in a cohort of 1 + Arctic charr housed in standard conditions were tracked over 12 months (December to December) and mature and immature males and females compared retrospectively. In both sexes, maturing fish were larger than non-maturing ones. In males, this size differential was the result of differences in growth in winter and early spring, but not in the remainder of the study period. In females, size differentials resulted mainly from growth rate differences immediately prior to breeding. In females but not in males, gonadosomatic index was predicted by growth rates in the months leading up to maturation, and among the females that matured, faster growing fish produced more eggs. Lipid reserves in July were correlated negatively with growth during the previous 7 months and, in females only, lipid reserves were significantly lower in maturing fish than in non-maturing fish, indicating that mobilization of lipid energy reserves in maturing fish had commenced by this time. Variation in investment in gonadal tissue, measured as gonadosomatic index, was not explained by variation in July lipid reserves for either males or females. However, July lipid reserves were negatively correlated with egg number, so females investing more in ova exhibited greater depletion of lipid reserves. These results are discussed in the context of the relationship between body condition and the onset of maturation in salmonids, relative investment in reproduction and sexual differences in the cost of reproduction.     

Seasonal variation in ornament expression,body size,energetic reserves,immune response,and survival in males of a territorial insect

Abstract 1. Seasonal variation in immune response has rarely been investigated in invertebrates and, therefore, we have studied this using territorial adult males of the damselfly, Hetaerina americana (Fabricius), in several generations, for a year and a half. 2. We investigated and related seasonal variation in red pigmented wing spot size (an ornamental trait) and body size to fat reserves using four immunity components – melanisation ability, phenoloxidase (PO) and nitric oxide (NO) activity, and haemolymph protein concentration – and survival ability after a bacterial immune challenge. 3. There was seasonal variation in spot expression, being more intense in the non‐winter months and less intense in winter months, and, to some extent, a similar pattern was found for NO. Although there was also variation in melanisation, PO activity, protein concentration and fat reserves, this was not consistently related to variation in spot size. Survival was lower in the winter than in non‐winter months. 4. Animals with larger spot and body size had greater values of melanisation, PO and NO activity, and protein concentration. The relation of spot and body size with fat reserves is still not clear. 5. Unlike other studies, ornament was not similarly correlated with all typical immune components (at best, mainly NO). 6. Similar to what occurs in vertebrates, survival is lower during the winter months. In the case of vertebrate studies, however, the same individual suffers these changes. In the American rubyspot, distinct cohorts demonstrated seasonal differences.     

Size-Based Variation in Somatic Energy Reserves and Parental Expenditure by Male Smallmouth Bass, Micropterus dolomieu

Male smallmouth bass show size-based variation in both probability and timing of reproduction. The objective of this research was to determine seasonal and size-based patterns of depletion of energy reserves and determine if parental defense is related to males' energy reserves. We sampled male smallmouth bass in the spring, during the parental care period and in the fall to measure energy reserves (lipid stores in muscle and viscera tissue) over a two year period. Energy stores, which were not built up before nesting, declined to a minimum level by the end of the parental care period. Small males had consistently lower energy reserves than larger males and did not utilize these reserves at the same rate during the parental care period. All parental males complimented endogenous energy reserves by feeding during parental care, however, small males appear to rely proportionately more on exogenous energy intake than do larger males. Parental defense by all sizes of males declined over the parental care period, the decline being the most obvious by small males. Small males' lower energy budget may make them less effective parents and decrease their probability of survival over the following winter relative to larger males.     

Sex differences in body composition of wintering Mallards (Anas platyrhynchos): possible implications for survival and reproductive performance

Capsule Wintering female Mallards have a higher fat load than males which may compensate for their lower body size and ultimately improve their fitness.

Aims To investigate whether sex-related differences in biological processes in winter are associated with differences in body nutrient storage and flight capabilities and, if so, the possible implications for survival and reproductive performance.

Methods We analysed the body composition and the flight parameters of 35 male and 35 female Mallards wintering in western Europe on the Rhine river.

Results Males were 15% heavier and 18% larger than females, but the latter carried 27% more fat per unit body mass. Relative to body lipid mass, body protein mass was 11% lower for females. Wing loading was 7% lower in females and power loading was similar in both sexes.

Conclusions High lipid mass gives females survival capabilities to food shortage similar to males. This helps sustain them through their prebasic moult and breeding success. Heavier muscles in males compensate for their higher wing loading and thus improve pairing success. Wintering and reproductive strategies in Mallard seem to be closely linked through body reserves.     

Overwinter changes in mass and lipid content of Perca fluviatilis and Gymnocephalus cernuus

The body condition, lipid reserves and mortality of 0 and 1 year‐old perch Perca fluviatilis and ruffe Gymnocephalus cernuus , sampled during the winter in Lake Constance, Germany, were compared. Length‐frequency analyses did not provide evidence for overwinter mortality in either species. The fresh and dry mass of perch as well as their lipid contents decreased during winter, while ruffe were heavier and contained more lipid at the end of the winter. The superior performance of ruffe was mainly attributed to its sensory capabilities, which allowed it to ingest zoobenthos throughout the winter, while the zooplankton feeding of perch was constrained by low light levels. In lakes that undergo a process of re‐oligotrophication, this advantage of ruffe over perch may be even more pronounced, since lower food supply during the growth season and thus lower fish lipid content at the start of winter is probably better tolerated by ruffe than by perch.