Accumulation Of Energy Reserves Research Articles

Life History and the Costs of Reproduction in Northern Great Plains Paddlefish ( Polyodon spathula ) as a Potential Framework for Other Acipenseriform Fishes

A 14-year investigation of a North American Acipenseriform fish, the paddlefish Polyodon spathula in the Yellowstone River, Montana, and North Dakota, USA has provided a detailed empirical framework of the life history in relation to the costs of reproduction for that species and potentially for closely related sturgeons. Nearly all aspects of life histories of males and females differed from each other after the immature period. The five periods in the juvenile and adult life histories for males and females, which occur at different ages for each sex, were (1) immature, (2) maturing, (3) somatic growth and reproduction, (4) prime reproduction, and (5) senescence to death. During the first period (immature), fishes exhibit rapid somatic growth as well as accumulation of energy reserves in the form of gonadal fat bodies (GFBs) and other fat deposits. During the second period (maturing), somatic growth slows as production and stored energy reserves are diverted into reproduction. In the third period, fish are allocating energy to both somatic growth and reproduction. Reproductive periodicity is typically close to two years for males and three years for females; gonadal recrudescence is slower than in older fish. The gonadosomatic index (GSI) is increasing, and GFBs are depleted over 2–3 spawns in females and reduced more gradually in males. Fish make longer pre-spawning migrations upriver. In the fourth period (prime reproduction), somatic growth is slow or negative, as energy is routed more strongly into reproduction. GSI is at a maximum; GFBs are completely depleted in females. Reproductive periodicity is typically one year for males and two years for females; the rate of gonadal recrudescence is at its maximum. Fish make shorter pre-spawning migrations upriver. In the fifth period (senescence to death), GSI of some of the oldest females decreases; the oldest males have few energy reserves and are long and lean. Distinct male-female differences in life histories were consistent with theoretical models and with empirical observations of the lack of direct male competition for females. The framework outlined for this paddlefish stock may exist among other Acipenseriform fishes and is potentially useful for evaluating their life histories.

Influence of temperature on gonadal development of Ruditapes philippinarum (Adams and Reeve, 1850) with special reference to ingested food and energy balance

This study evaluated the influence of temperature on gonadal development in Ruditapes philippinarum under controlled dietary conditions, in order to distinguish between the effect of temperature alone and the associated effect of temperature on ingestion. The experiment was therefore divided into two groups: group L (at temperatures of 14 °C and 18 °C, with a daily ingestion rate of 470–550 μg organic weight of phytoplankton per g clam live weight) and group H (at temperatures of 18 °C and 22 °C, with a daily ingestion rate of 1000–1100 μg organic weight of phytoplankton per g clam live weight). Both ingestion and respiration rates were calculated for each of the diets and temperatures. Gonadal development of the different groups of clams was monitored over the 70-day experimental period by means of image analysis techniques and calculation of the soft tissue dry weight of a standard clam. Experimental results show that when the amount of food ingested is similar a difference of temperature between 14 °C and 18 °C, or between 18 °C and 22 °C, has no significant effect on the rate of gonadal development, the effect of temperature possibly being masked by the differences in the respective energy balances. When a higher temperature coincides with a higher ingestion rate, as is often the case with naturally-occurring populations of bivalves, then gonadal development is greater, and faster, at the higher temperature, although this is mainly due to the greater amount of food ingested. When, in experimental (or environmental) conditions, a high temperature (18 °C) is associated with a low ingestion rate, a situation of negative energy balance arises, associated with a slow rate of gonadal development that takes place at the cost of the animal's energy reserves. When the temperature is reduced to 14 °C, there is a decrease in the ingestion rate and lower energy consumption, leading to a situation of positive energy balance. In these conditions gonadal development is slow and the surplus of available energy is, presumably, used to accumulate energy reserves, as is often the case during the stage of growth prior to the onset of gonadal development in many bivalves that inhabit geographical areas with noticeable seasonal variations.

TIMING AND SEASONALITY OF THE TERMINAL MOLT AND MATING MIGRATION IN THE SPIDER CRAB, MAJA BRACHYDACTYLA: EVIDENCE OF ALTERNATIVE MATING STRATEGIES

Timing and synchronization at individual and population levels of the processes related to the terminal molt, gonad maturation, accumulation of energy reserves and migration in the spider crab Maja brachydactyla are analyzed. Also, the intra and intersexual variability is established. Two hypotheses are tested to explain the temporal and population variability: (1) physiological hypothesis: males and females begin migration when they reach the appropriate physiological stage (the optimum level of energy reserves) and (2) mating opportunities hypothesis: the timing of the migration maximize mating opportunities and mate quality. Our results show that males carry out the terminal molt before females, the former having a peak in July and the latter in August. The onset of gonad maturity in females occurs between two and three months after they have reached morphometric maturity (starting in October), coinciding with the period prior to and during the mating migration to deep waters. In an analysis of the spermathecae of primiparous females, it was found that practically no mating activity occurred in shallow waters, whereas the first copulations took place in the migration corridor. However, probably most of the mating activity occurs in deep mating grounds. Males reached gonad maturity prior to morphometric maturity. No differences were observed in the physiological status (muscle, gonad and hepatopancreas relative mass) between migrating and nonmigrating crabs, which is a clear indication that the physiological hypothesis does not hold true for this species. A higher percentage of postmolt crabs were caught in the migration corridor than in shallow waters. The physiological condition improved over time in males and females in the specimens caught in the shallow area as well as in the migration corridor. Therefore, the crabs that start migrating first did so in poorer physiological condition. Moreover, the early migrators had a significantly lower mean size than the late migrators. Our results suggest that variability within populations and between sexes are related to the different reproductive strategies and not to a physiological limitation. Thus, the poor-quality males (with a reduced competitive ability) would migrate at the beginning of the season to maximize the mating opportunities that would not be feasible if they had to compete with the late migrators, which are larger in size and in better physiological condition. Sperm competition is not clear in M. brachydactyla, but all evidences point at last male preference for fertilization of eggs.

Seed parasitism redirects ovule development in Douglas fir

Many parasitic species of insects complete their entire development in seeds. They feed off storage reserves within the ovule. These reserves only normally accumulate in fertilized ovules. Consequently, female insects that oviposit their eggs directly into the plant ovule need to be able to select correctly, as unfertilized ovules of conifers normally become so-called empty seed. We provide clear evidence that in conifers, seed-parasitizing insects do not need to discriminate between fertilized and unfertilized plant ovules when ovipositing their eggs. A host-specific insect, the chalcid Megastigmus spermotrophus Wachtl (Hymenoptera: Torymidae), lays its eggs in ovules of Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) before fertilization has taken place in the plant. Oviposition not only prevents the expected degeneration and death of unfertilized ovules, but it induces energy reserve accumulation. Ovules that would otherwise develop as empty seed are redirected in their development by the insect to provide food for the developing larvae. Instead of the insect exploiting normal events during seed development, the insect manipulates seed development for its own reproductive advantage.

Lipid content in the early life stages of three mesopelagic fishes

Changes in lipid concentration during early ontogenetic stages were determined in three mesopelagic fish species, Benthosema glaciale, Hygophum benoiti and Maurolicus muelleri. Total lipid content per larva was related to size and mass and increased over the course of development. In all three species, the dry mass : total lipid ratio displayed a better fit than the standard length : total lipid ratio. Larvae of B. glaciale had a higher amount of lipid content, at a given mass, than the other two species. The rate of lipid accumulation with increasing larval development was similar in B. glaciale and H. benoiti, and both were higher than in M. muelleri larvae. Furthermore, total lipid concentration values, as a proportion of dry mass, increased significantly from the larval to the juvenile stages in B. glaciale(from 16 to 55%) and M. muelleri(from 9 to 20%). The present study has allowed comparisons to be drawn of the ability on the early developmental stages of deep‐sea fishes to accumulate energy reserves, both within and among species.

Fatty Acid Composition of Mixed-Rumen Bacteria: Effect of Concentration and Type of Forage

The effects of concentration and type of forage in the diet on lipid content and fatty acid (FA) composition of rumen bacteria were studied in 14 goats fitted with duodenal cannulas. The goats were fed a complete maintenance diet containing 40, 70, or 100% chopped forage (dry matter basis) in two equal meals. Forage was either corn stover or alfalfa hay. Microbial cell matter (MCM) was isolated by differential centrifugation of duodenal contents. The FA content of the MCM varied from 5 to 11% of DM and decreased with forage level in the diet. Main FA in MCM were C18:0 and C16:0: together they accounted for 70% of total FA in MCM. The mono-unsaturated FA and branched-chain FA (iso-FA and anteiso-FA) each represented about 10% of FA by weight. The proportion of even-chain saturated FA decreased and those of odd- and branched-chain FA increased with increasing forage. With the corn stover-based diet even-chain saturated FA were lower than with the alfalfa hay-based diet, whereas the unsaturated FA, odd-chain FA, and branched-chain FA were higher. The neutral detergent fiber content of the diet seemed to explain most of the variation associated with even-chain saturated FA, and odd- and branched-chain FA. Our results suggest that, for diets not supplemented with fat, mixed rumen bacteria accumulated energy reserves, by increasing synthesis of either even-chain saturated FA, or saturated odd-chain FA and saturated branched-chain FA.

Allocation of plant carbon to foraging and storage in arbuscular mycorrhizal fungi

Foraging strategies, the cost–benefit associated with the search for new resources, have only begun to be explored in arbuscular mycorrhizal fungi (AMF). We show the use of 13C-labelling, via shoot photosynthesis, of the 16:1ω5 fatty acid biomarker (the dominant and rather specific fatty acid in AMF storage lipids) to study the immediate patterns of carbon allocation to fungal lipids in response to inorganic and organic nutrient amendments. Signature fatty acid measurements, the incorporation of the label and complementary hyphal length density measurements showed that the extraradical mycelium of AMF proliferated in response to all the amendments provided whereas its development into unamended sand was minor in all treatments. We demonstrate the foraging capacity of AMF, linked to plant carbon, through their hyphal proliferation and accumulation of energy reserves.

The energetics of autumn mast hoarding in eastern chipmunks

The timing and basis of the transition from energy reserve accumulation to reserve utilization in autumn may be a key determinant of winter survival in endotherms, but has rarely been examined directly in the field. In the present study we quantify the energetics of autumn mast hoarding in eastern chipmunks (Tamias striatus) to document the degree to which larder hoarding permits capitalizing on brief pulses of resource abundance and to evaluate the basis of the decision to stop hoarding and initiate hibernation. Daily energy expenditure, measured with the doubly labeled water technique, increased significantly with date and decreasing ambient temperature, eventually exceeding 3× resting metabolic rate in late autumn. Simultaneous documentation of food delivery to burrow larder hoards revealed that delivery rates were low in early autumn, extremely high for a brief period in mid-autumn, then low again in late autumn. Combining estimates of energy expenditure, consumption, and delivery yielded net energy surpluses of 1,320-4,600kJ day(-1) during the peak hoarding period, meaning total hibernation energy requirements could be acquired in 1-2days. These results, together with measures of food availability and ambient temperature, suggest that chipmunk activity in late autumn may be affected by both the extent of hoard accumulation and thermoregulatory constraints on sustained energy expenditure. We speculate that both state-dependency and energetic ceilings on autumn hoarding behavior may enhance the capacity of the mast seeding strategy of trees to effectively swamp the foraging efforts of larder-hoarding granivores.

Why do female Aedes aegypti (Diptera: Culicidae) feed preferentially and frequently on human blood?

Adult female Aedes aegypti (L.), the vector of dengue and yellow fever viruses, have an affinity for feeding on human blood and a tendency to forego feeding on sugar. This observation challenges two tenets of mosquito biology: (1) mosquitoes imbibe plant carbohydrates for synthesis of energy reserves and blood for reproduction and (2) egg production is reduced when mosquitoes feed on human blood compared with blood from other species. Sub-optimal amounts of the amino acid isoleucine in human blood (particularly free isoleucine in plasma) are thought to be responsible for lowered egg production when human blood is ingested. We tested the hypothesis that feeding on human blood is associated with a selective advantage for Ae. aegypti and is an underlying reason for this mosquito's intimate and epidemiologically important relationship with human beings. Our five experiments examined the effects of different isoleucine concentrations on accumulated energy reserves, frequency of host contact, survival, and egg production. When mosquitoes imbibed blood meals over a 7- to 10-d period and were not fed sugar, increased isoleucine concentration decreased energy reserves and did not increase egg production. Aedes aegypti took smaller but more frequent blood meals when feeding on a low-isoleucine human host daily compared with a high-isoleucine mouse host. Previous reports that isoleucine enhances egg production were confirmed only when females were fed sugar, an unusual behavior for most domestic Ae. aegypti populations. Females fed human blood and water had greater age-specific survival (l(x)), reproductive output (m(x)), and cumulative net replacement (R0) than cohorts fed human blood plus sugar or isoleucine-rich mouse blood with or without access to sugar. The unique isoleucine concentration of human blood is associated with Ae. aegypti's unusual propensity to feed preferentially and frequently on humans--a behavior that increases this mosquito's fitness, synthesis of energy reserves, and contact with human hosts, making it an especially effective disseminator of human pathogens.

Egg production and energy storage in relation to feeding conditions in the subantarctic calanoid copepod Drepanopus pectinatus: an experimental study of reproductive strategy

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 209:231-242 (2001) - doi:10.3354/meps209231 Egg production and energy storage in relation to feeding conditions in the subantarctic calanoid copepod Drepanopus pectinatus: an experimental study of reproductive strategy F. Alonzo1,*, P. Mayzaud1, S. Razouls2 1Observatoire Océanologique, Océanographie Biochimique et Ecologie, LOBEPM - ESA-CNRS 7076, BP 28, 06230 Villefranche-sur-Mer, France 2Laboratoire Arago, Observatoire Océanologique, URA-CNRS 117, 66630 Banyuls Cedex, France *E-mail: alonzo@obs-vlfr.fr ABSTRACT: The egg production and the hatching success of Drepanopus pectinatus (Brady, 1883) was examined in relation to food availability. The spawning frequency did not change in relation to the concentration of phytoplankton with an interval time of 7.6 d between 2 broods. At least 1 egg sac was produced per female in almost all cases (>93%) independent of the food level. The subsequent recruitment of breeding females was strongly influenced by feeding conditions: only 22 to 50% of the starved females completed 3 successive breeding cycles, while 100% of the fed females did. The number of eggs per was also affected by the concentration of food, with a mean of 12 eggs per sac in starved females and 19.4 eggs per sac in fed females independent of the food level. Brood sizes increased significantly in the fourth breeding cycle to 35.3 eggs per sac in females fed a concentration of 1180 µg protein l-1. Egg-hatching time was constant, with an average of 6.7 d. To evaluate the relative roles of reproduction and of accumulation of energy reserves, we compared the production of eggs and the changes in the organic composition of females in various limiting food conditions. Brood size was influenced by both immediate and past trophic conditions, with the number of eggs per sac varying from 10.1 to 12.8. The content of the eggs was constant: 122 ng of protein and 234 ng of lipids. In females, protein content was a good indicator of immediate feeding conditions, while lipids, dominated by wax esters, reflected both immediate and past feeding conditions. A low level of egg production was maintained throughout periods of nutritional limitation or starvation. Subsequently, food was used to reduce the consumption of the energy reserves of females before being invested in reproduction, as observed at high food concentration. KEY WORDS: Drepanopus pectinatus · Egg production · Energy reserves · Food availability · Limitation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 209. Online publication date: January 05, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Chapter 15 Food intake and leptin during pregnancy and lactation

Successful reproduction requires the accumulation of energy reserves. Although acute and chronic food deprivation disrupts reproduction, surprisingly, an over-abundance of energy reserves can also result in infertility. The infertility of obese, ob/ob mice can be reversed by the reintroduction of leptin, the protein product of the ob gene. In rats, energy reserves are increased during pregnancy by far accumulation and during lactation by hyperphagia. We have therefore investigated the interactions of leptin and food intake during late pregnancy and lactation in rats. Cycling rats consume their daily food intake during the dark phase and this is accompanied by a subsequent increase in plasma leptin concentration compared to light levels. During late pregnancy, rats increase their food intake during the dark phase and this is accompanied by a nocturnal increase in plasma leptin level. However, the nocturnal increase is not seen on the day prior to parturition, and is absent throughout lactation. Surprisingly, despite the massive increase in food intake during lactation plasma leptin levels continue to fall, suggesting that leptin release in response to food intake is suppressed during lactation. Furthermore, central leptin administration is less effective in reducing food intake in late pregnant and early lactating rats compared to cycling rats which suggests that these rats are insensitive to leptin. This may result from downregulation of brain leptin receptors. Decreased leptin production and action during late pregnancy and lactation will result in a decreased satiety effect, with up-regulation of orexigenic factors that produce hyperphagia, so allowing adequate energy intake for successful rearing of offspring.

Chemical composition and growth indices in leptocephalus larvae

Leptocephali grow at extremely high rates (>1 mm d−1), but, unlike most fish larvae, leptocephali may remain in the plankton as larvae for several months before metamorphosing into the juvenile form. During their planktonic phase, leptocephali accumulate energy reserves in the form of glycosaminoglycans which are then expended along with lipid reserves to fuel metamorphosis. Otolith growth rates were determined using scanning electron microscopy for four species of leptocephali common in the Gulf of Mexico, Paraconger caudilimbatus (Poey, 1867), Ariosoma balearicum (Delaroche, 1809), Gymnothorax saxicola (Jordan and Davis, 1891), and Ophichthus gomesii (Castelnou, 1855). Proximate composition, RNA:DNA ratios and protein growth rates were examined with respect to mass, length and age. The leptocephalus growth strategy was strongly reflected in the growth indices. Mass (Y) in all four species increased with increasing age (X) according to the equation Y = aX b , where a is a species-specific constant and 1.05 < b < 2.40. The accumulation of acellular mass was evident in protein growth rates and RNA:DNA ratios, and was observed as a shift in increasing size from rapid growth in length to a greater increase in mass with age. These results suggest that the proportion of actively metabolizing tissue declines with size and is replaced by the metabolically inert energy depot: the glycosaminoglycans. Leptocephali can thus grow to large size very rapidly with minimal metabolic penalty, an unusual and successful developmental strategy.

Overwintering adaptations in the lettuce root aphid Pemphigus bursarius (L.)

Pemphigus bursarius (L.) is a host alternating root-feeding aphid with a proportion of the population overwintering as asexual hiemalis in the soil. These hiemalis must be sufficiently cold tolerant to survive at the temperatures they would experience in winter, and also be able to overcome a period of prolonged starvation brought about by the absence of secondary host plants. Cold tolerance experiments showed field collected hiemalis to be considerably more cold hardy than laboratory summer apterae, with an LTemp 50 of −13.1°C compared with 2.3°C. In a constant exposure at 0°C some field collected hiemalis survived for 18 days, while no summer apterae survived more than 8 h. Hiemalis, collected from the field in winter and induced in the laboratory, had significantly higher levels of triglycerides, 12.8% fresh weight (39.9% dry wt.) and 11.4% fresh weight (43.7% dry wt.), respectively, compared with summer apterae with a value of 7.1% fresh weight (32.5% dry wt.). These two adaptations of increased cold tolerance and accumulation of energy reserves confirm that the hiemalis morph is adapted for overwintering and hence physiologically distinct from summer morphs, and in turn, contribute to the success of the asexual life cycle strategy in this species.

Leptocephalus energetics: metabolism and excretion.

Leptocephali are the unusual transparent larvae that are typical of eels, bonefish, tarpon and ladyfish. Unlike the larvae of all other fishes, leptocephali may remain in the plankton as larvae for several months before metamorphosing into the juvenile form. During their planktonic phase, leptocephali accumulate energy reserves in the form of glycosaminoglycans, which are then expended to fuel metamorphosis. The leptocephalus developmental strategy is thus fundamentally different from that exhibited in all other fishes in two respects: it is far longer in duration and energy reserves are accumulated. It was anticipated that the unusual character of leptocephalus development would be reflected in the energy budget of the larva. This study describes the allocation of energy to metabolism and excretion, two important elements of the energy budget. Metabolic rates were measured directly in four species of leptocephali, Paraconger caudilimbatus, Ariosoma balearicum, Gymnothorax saxicola and Ophichthus gomesii, using sealed-jar respirometry at sea. Direct measurements of metabolic rates were corroborated by measuring activities of lactate dehydrogenase and citrate synthase, two key enzymes of intermediary metabolism, in addition to that of Na(+)/K(+)-ATPase, a ubiquitous ion pump important in osmotic regulation. Excretion rates were determined by subsampling the sea water used in the respiratory incubations. The entire premetamorphic size range for each species was used in all assays. Mass-specific oxygen consumption rate, excretion rate and all enzyme activities (y) declined precipitously with increasing mass (M) according to the equation y=aM(b), where a is a species-specific constant and -1.74<b<-0.44. In leptocephali, the highly negative slope of the familiar allometric equation describing the relationship between mass-specific metabolic rate and mass, normally between -0.33 and 0, showed that a massive decline in metabolic rate occurs with increasing size. The result suggests that the proportion of actively metabolizing tissue also declines with size, being replaced in large measure by the metabolically inert energy depot, the glycosaminoglycans. Leptocephali can thus grow to a large size with minimal metabolic penalty, which is an unusual and successful developmental strategy.

Reproductive strategies and energetic adaptations of polar zooplankton

Summary Key factors governing polar ocean ecosystems are low temperatures and a pronounced seasonal variability of ice cover, light regime and primary production. Depending on their ecological niche and trophic position, zooplankton species at high latitudes have developed a variety of reproductive strategies and energetic adaptations to cope with these extreme environmental conditions. Life-cycle strategies of the herbivorous copepods and euphausiids, which make up the major portion of polar zooplankton biomass, include seasonal vertical migration, dormancy (diapause, quiescence) and the accumulation of energy reserves. These lipid stores help to buffer the pulsed seasonal food supply, and they play an important role in fueling reproduction independent of phytoplankton. Only a smaller fraction of the lipid reserves accumulated during spring and summer are usually catabolized for metabolic maintenance during the food-limited dark season. These deposits are retained until the end of winter and allow early egg production and spawning prior to—or coinciding with—the onset of vernal primary production. It enables the new generation to make full use of the short productive season for growth and development to reach viable overwintering stages. The Antarctic krill Euphausia superba is an exception since it uses its depot lipids for metabolic maintenance during the dark season. It therefore relies on external resources (Primary production) for reproductive processes, resulting in a later spawning period as compared to the other euphausiids. Another important component of the herbivorous Antarctic zooplankton, the salps, have developed a very different reproductive strategy. They are able to switch from sexual reproduction to asexual budding (metagenesis), which allows extreme multiplication rates under favourable feeding conditions. Due to these successful adaptations, herbivores are able to build up huge stocks, in spite of the short productive period. Omnivorous and carnivorous zooplankton species, e.g., amphipods or chaetognaths, are not much constrained by the seasonality problem, but experience a more constant food supply. They show a tendency towards K strategies with a prolonged reproductive period, reduced egg numbers and increasing parental care. However, they do not exhibit such typical “polar adaptations” as developed by the herbivorous species.

Lipid class and glycogen content of the lugworm Abarenicola pacifica in relation to age, growth rate and reproductive condition

We measured the levels of lipid classes (wax esters, triacylglycerides, free fatty acids, sterols, phospholipids) and levels of glycogen in a population of Abarenicola pacifica over a one-year period beginning shortly after recruitment. Glycogen and lipid contents were unrelated to growth rates as estimated by changes in average size of individuals in the cohort. There was no indication of seasonality in levels of any component, consistent with the hypothesis of Slobodkin and Richman that animals living in environments where food supplies are likely to be stable, such as subsurface deposit-feeders like A. pacifica, do not accumulate energy reserves. Instead, glycogen content increased gradually and most lipid classes decreased gradually over time. A notable exception was the triacylglyceride content, which showed a large increase associated with the formation of eggs. Triacylglyceride levels in older A. pacifica and in two other species of depositfeeding polychaetes showed similar trends. Measurement of triacylglyceride levels may provide an additional objective method, supplementing egg counts and egg size, of quantifying reproductive effort in deposit-feeders.

Social Hoarding and a Load Size-Distance Relationship in Gray Jays

A model that assumes scatterhoarders maximize the rate at which they store food that survives densitydependent theft by competitors has been developed elsewhere (Waite 1991a; Waite and Reeve 1992a, 1992b). The model addresses the questions of how such hoarders should distribute caches of food items collected from an ephemeral, locally abundant source, and how long they should persist in caching from such a source before moving on in search of alternative sources. The results of several field experiments using Gray Jays (Perisoreus canadensis) were qualitatively consistent with the model in most contexts (references above). However, because the model implicitly assumes that a hoarder's behavior is not influenced by that of other individuals, methodological measures were taken in those experiments to eliminate any confounding effects of social context on the jays' caching behavior. Here, I describe a simple field experiment designed to investigate whether the distance to which Gray Jays transport food items for storage is affected by load (food-item) size and social context (solitary or in the company of one or more conspecifics). I present evidence that apparent attempts by the jays to cache secretively were influenced by the size (value) of the food item, the immediate risk of losing a cache to a conspecific soon after the cache had been put in place, and the interaction of these two factors (food-item size and social context). Any tendency to cache secretively may reduce the probability of cache theft, which may be accomplished also by adequately spacing caches (Waite 1988, Waite and Reeve 1992a), appropriately timing the decision to move on to alternative sources while a known source still contains food (Waite and Reeve 1992b), placing caches in inconspicuous sites (Petit et al. 1989, Waite 1991 la), and concealing caches (Petit et al. 1989, Waite 1991a). The Gray Jay is a permanent resident of high-latitude boreal forests in North America. These birds store hundreds of food items per day during summer (Waite 1991a), and apparently rely heavily on that food during the winter (Waite 1990), when they have as little as 4 hr of foraging time available each day during which to accumulate energy reserves for the upcoming night of hypothermic fasting (Waite 199 ib, in press). Food items are cached as saliva-coated boli (Dow 1965), each of which is placed in a separate arboreal site, such as under a flake of bark, in a clump of lichen, or in a conifer needle-cluster.

Differential Timing of Autumn Migration between Sex and Age Groups in Raptors at Falsterbo, Sweden

The migration of raptors at Falsterbo, Sweden was studied during five autumns, 1986-1990, to investigate the intraspecific temporal sequence of migration for different age and sex classes. The analysis is based on counts of between 45 and 72 321 individuals of 14 species. The percentage of juveniles varied among species and to some extent between years. This reflects a variable tendency of different migrants to concentrate at Falsterbo and fluctuating breeding success from year to year. Most species showed an even sex ratio. Marked exceptions were Marsh Harrier Circus aeruginosus and Hen Harrier C. cyaneus in which females constituted 60 and 61% respectively of adult migrants. This biassed sex ratio agrees with the tendency towards polygyny in these species. In tropical (long-distance) migrants adults migrated ahead of juveniles (Honey Buzzard Pernis apivorus, Montagu's Harrier Circus pygargus, Osprey Pandion haliaetus, and Hobby Falco subbuteo). It is suggested that it is advantageous for adults to arrive early on the wintering grounds, finish moult and accumulate energy reserves before spring migration and the next breeding season. Those short-distance migrants in which adults migrated before juveniles at Falsterbo are species whose majority leave Scandinavia in the winter (Hen Harrier, Common Buzzard Buteo buteo, Roughlegged Buzzard B. lagopus, Merlin Falco columbarius and Peregrine F peregrinus). Adults leaving first have a greater possibility to secure a good winter territory forcing the juveniles, migrating later, to settle in less suitable habitat or to winter further south. In species where juveniles migrated ahead of adults a large proportion of the adults spend the winter close to the breeding grounds (Red Kite Milvus milvus, Goshawk Accipiter gentilis and Sparrowhawk A. nisus). The presumably subdominant juveniles may be excluded from the breeding territories by the adults and therefore migrate south, followed by a varying proportion of adults. Females generally migrated before males at Falsterbo due to different moult strategies; females starting and finishing moult of the flight feathers before males. This difference in timing of moult and migration is ultimately dependent on a marked role division between the sexes during the breeding cycle. In Honey Buzzard where both sexes share the breeding duties equally no difference in timing of migration was found. Social dominance, importance of early arrival at the winter quarters and moult strategies are the factors best explaining the observed differential timing in migrating raptors at Falsterbo.

The adaptive significance of parental role division and sexual size dimorphism in breeding shorebirds

Sexual size dimorphism among 57 species in the shorebird family Scolopacidae is evaluated in relation to parental role division during breeding. Normal size dimorphism, i.e. the female being smaller than the male, occurs in species where the female has the main responsibility for parental care, whereas reverse size dimorphism, the most common pattern among shorebirds, is associated with reversed parental roles. Pronounced dimorphism between sexes occurs, besides in body size, also in bill length, where the sex undertaking the main part of brood attendance has a disproportionately short bill in species adapted for foraging by deep probing. A small body size is of adaptive value to attain high parental efficiency for energetic reasons, because smaller individuals need less energy to maintain themselves. Short bills may be advantageous during brood attendance when feeding mainly takes place in terrestrial habitats together with the chicks. Females released from parental care duties are favoured by a larger body size allowing increased accumulation of energy reserves for egg production. There are obvious parallels between shorebirds and raptors concerning the adaptive significance of reverse sexual size dimorphism and parental role division.

Gas vesicle formation in the dark, and in light of different irradiances, by the cyanobacteriumMicrocystissp.

Previous investigations with planktonic cyanobacteria have suggested that these organisms do not form new gas vesicles in the dark. This study, on Microcystis sp., confirmed that cells that had been preincubated at low photon irradiances (< 15 μmol m-2 s-1) formed negligible amounts of gas vesicles in the dark. Significant gas vesicle formation occurred, however, in cells preincubated continuously at higher irradiances, and particularly within the range 65 to 105 μmol m-2 s-1. The results suggest that gas vesicle formation in the dark is dependent on the prior accumulation of energy reserves. The amount of gas vesicles formed in continuous light was linearly related to irradiance over the range 0 to 20 μmol m-2 s-1, and reached a maximum at only 30 μmol m-2 s-1 that was over five times the amount formed at higher irradiances. This suggests that the rate of gas vesicle formation, regulated directly in response to irradiance, has a role in the light-mediated buoyancy regulation of this cyanobacterium.