Maximum daily energy intake: it takes time to lift the metabolic ceiling

Conventionally, maximum capacities for energy assimilation are presented as daily averages. However, maximum daily energy intake is determined by the maximum metabolizable energy intake rate and the time available for assimilation of food energy. Thrush nightingales (Luscinia luscinia) in migratory disposition were given limited food rations for 3 d to reduce their energy stores. Subsequently, groups of birds were fed ad lib. during fixed time periods varying between 7 and 23 h per day. Metabolizable energy intake rate, averaged over the available feeding time, was 1.9 W and showed no difference between groups on the first day of refueling. Total daily metabolizable energy intake increased linearly with available feeding time, and for the 23-h group, it was well above suggested maximum levels for animals. We conclude that both intake rate and available feeding time must be taken into account when interpreting potential constraints acting on animals' energy budgets. In the 7-h group, energy intake rates increased from 1.9 W on the first day to 3.1 W on the seventh day. This supports the idea that small birds can adaptively increase their energy intake rates on a short timescale.     

Effects of meal size, clutch, and metabolism on the energy efficiencies of juvenile Burmese pythons, Python molurus.

We explored meal size and clutch (i.e., genetic) effects on the relative proportion of ingested energy that is absorbed by the gut (apparent digestive efficiency), becomes available for metabolism and growth (apparent assimilation efficiency), and is used for growth (production efficiency) for juvenile Burmese pythons (Python molurus). Sibling pythons were fed rodent meals equaling 15%, 25%, and 35% of their body mass and individuals from five different clutches were fed rodent meals equaling 25% of their body mass. For each of 11-12 consecutive feeding trials, python body mass was recorded and feces and urate of each snake was collected, dried, and weighed. Energy contents of meals (mice and rats), feces, urate, and pythons were determined using bomb calorimetry. For siblings fed three different meal sizes, growth rate increased with larger meals, but there was no significant variation among the meal sizes for any of the calculated energy efficiencies. Among the three meal sizes, apparent digestive efficiency, apparent assimilation efficiency, and production efficiency averaged 91.0%, 84.7%, and 40.7%, respectively. In contrast, each of these energy efficiencies varied significantly among the five different clutches. Among these clutches production efficiency was negatively correlated with standard metabolic rate (SMR). Clutches containing individuals with low SMR were therefore able to allocate more of ingested energy into growth.     

Effects of meal size, clutch, and metabolism on the energy efficiencies of juvenile Burmese pythons, Python molurus

We explored meal size and clutch (i.e., genetic) effects on the relative proportion of ingested energy that is absorbed by the gut (apparent digestive efficiency), becomes available for metabolism and growth (apparent assimilation efficiency), and is used for growth (production efficiency) for juvenile Burmese pythons (Python molurus). Sibling pythons were fed rodent meals equaling 15%, 25%, and 35% of their body mass and individuals from five different clutches were fed rodent meals equaling 25% of their body mass. For each of 11–12 consecutive feeding trials, python body mass was recorded and feces and urate of each snake was collected, dried, and weighed. Energy contents of meals (mice and rats), feces, urate, and pythons were determined using bomb calorimetry. For siblings fed three different meal sizes, growth rate increased with larger meals, but there was no significant variation among the meal sizes for any of the calculated energy efficiencies. Among the three meal sizes, apparent digestive efficiency, apparent assimilation efficiency, and production efficiency averaged 91.0%, 84.7%, and 40.7%, respectively. In contrast, each of these energy efficiencies varied significantly among the five different clutches. Among these clutches production efficiency was negatively correlated with standard metabolic rate (SMR). Clutches containing individuals with low SMR were therefore able to allocate more of ingested energy into growth.     

Reproductive cycles in oregon populations of the echlnoid, Strongylocentrotvs purpuratus (Stimpson). I. Annual gonad growth and ovarian gametogenic cycles

Four annual reproductive cycles were followed in the echinoid, Strongylocentrotus purpuratus (Stimpson), at three places on the central Oregon coast near 44°45′N lat., where intermittent upwelling depresses coastal sea temperatures from June through September. Monthly measurements were made of the gonad index of samples of 30 urchins and the relative frequency of five stages in oogenesis were determined from sectioned ovaries from these samples.Individuals in these populations are synchronous in their annual cycle of gonad growth, oogenesis, and spawning, the latter taking place between late December and March. The population usually undergoes one major, complete spawning within a period of 30 days. When large reserves of stored nutrients are present, ova continue to mature over a longer period and may be present in quantity from December into April. Gonad weight does not change significantly from March through June. From July through November the gonad undergoes its annual growth at a mean instantaneous relative growth rate of ≈ 1 % per day. Gonad growth is not significantly correlated with sea temperature. Variable temperatures resulting from upwelling do not result in erratic fluctuations in the reproductive cycle, probably because of the annual change in feeding rate, the seasonal shift in energy utilization, and the constancy of the amount of food assimilated during gonad growth. Gonad size and annual growth rates may differ significantly between years and locations, indicating that both are influenced by local environmental factors, probably food availability and degree of wave action.The frequency of oogonial clusters is least in mid-winter, increases after spawning, reaches an annual maximum in June and declines in the fall. Primary oocyte growth is very slow until October, when it increases abruptly, and some oocytes become ova as early as late October. When stored nutrients are low, many oocytes do not complete growth by January, and are not spawned until May. Prolongation of the spawning period delays the increase in frequency of oogonial clusters in the following new cycle. The timing of the annual increase in oogonial cluster frequency and of the onset of rapid oocyte growth did not otherwise vary between places and years. In these populations, entrainment or synchronization of the reproductive cycle to seasonal environmental events probably occurs at the beginning of one or all of these critical annual events: oogonial proliferation, annual gonad growth and rapid oocyte growth.     

Feeding rates,assimilation efficiencies and growth of two amphipod species on biodeposited material from zebra mussels

1. Accumulation of organic material by the zebra mussel Dreissena polymorpha is assumed to be the source of a biodeposition‐based food web. However, only little is known about the importance of the biodeposited material as a food source and its contribution to increased abundances of macroinvertebrates in the presence of D. polymorpha. 2. Feeding, assimilation and growth of the amphipods Gammarus roeselii and Dikerogammarus villosus on food sources directly and indirectly associated with D. polymorpha (biodeposited material and chironomids) and on conditioned alder leaves were measured. The stoichiometry of carbon, nitrogen and phosphorus of the diets was measured as an important determining factor of food quality. 3. Chironomids had the highest nitrogen and phosphorus contents, alder leaves were depleted in nitrogen and phosphorus, and the stoichiometry of biodeposited material was intermediate. 4. Both amphipod species had highest feeding rates and assimilation efficiencies on chironomids. Gammarus roeselii fed more on biodeposited material than on alder leaves, but assimilation efficiencies were similar; D. villosus also had similar feeding rates and assimilation efficiencies on the two diets. 5. Both amphipod species had highest growth rates on chironomids and lowest growth rates on alder leaves. Both grew at intermediate rates on biodeposited material of D. polymorpha. The growth rates of the amphipod species were related to food stoichiometry. Overall, the invasive D. villosus grew faster than the indigenous G. roeselii. 6. Food resources directly and indirectly associated with D. polymorpha are potential diets for amphipods, providing further evidence for a D. polymorpha biodeposition‐based food web.     

Histological and ultrastructural investigation of early gonad development and sex differentiation in Adriatic sturgeon (Acipenser naccarii,Acipenseriformes, Chondrostei)

Gonad development and sex differentiation from embryos to 594‐day‐old individuals were investigated in farmed Acipenser naccarii using light and transmission electron microscopy. The migrating primordial germ cells first appear along the dorsal wall of the body cavity in embryos 1.5 days before hatching. The gonadal ridge, containing a few primary primordial germ cells (PGC‐1) surrounded by enveloping cells, appears in 16‐day‐old larvae. At 60 days, the undifferentiated gonad is lamellar and PGC‐1 multiply, producing PGC‐2. In 105‐day‐old juveniles, a distinct germinal area with advanced PGC‐2 appears on the lateral side near the mesogonium and the first blood vessels are visible. At 180 days, putative ovaries with a notched gonadal epithelium and putative testes with a smooth one appear, together with adipose tissue on the distal side. In 210‐day‐old juveniles, active proliferation of germ cells begins in the putative ovaries, whereas putative testes still contain only a few germ cells. The onset of meiosis and reorganization of stromal tissue occurs in ovaries of 292‐day‐old individuals. Ovaries with developed lamellae enclosing early oocyte clusters and follicles with perinucleolar oocytes occur at 594 days. Meiotic stages are never found, even in anastomozing tubular testes of 594‐day‐old individuals. Steroid producing cells are detected in the undifferentiated gonad and in the differentiated ones of both sexes. Anatomical differentiation of the gonad precedes cytological differentiation and female differentiation largely precedes that of the male. Gonad development and differentiation are also associated with structural changes of connective tissue, viz. collagen‐rich areas are massive in developing testes and reduced in ovaries. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Gonad growth in the sea urchin, Strongylocentrotus purpuratus (Stimpson) (echinodermata: Echinoidea) and the assumptions of gonad index methods

In central Oregon intertidal populations, the echinoid Strongylocentrotus purpuratus (Stimpson) first begins to produce mature gametes at a diameter of 25 mm during the second year of its life. All individuals above 40 mm in diameter have mature gametes during the mid-winter spawning season. Gonad size increases rapidly relative to body size until the animals are ≈45 mm in diameter and have a total dry weight of ≈ 15 g. The ratio of gonad to body size reaches its maximum in animals between 40 mm and 50 mm in diameter; thereafter the ratio drops slightly with increasing size. The curvilinear relation between gonad size and body size is not related to a relative difference in body proportions with size. Internal volume, height, and diameter increase linearly with size. Body proportions did not differ between the two populations studied. Differences between populations in the ratio of gonad to body size extend to all sizes in the populations and the direction but not the amount of the difference is consistent from year to year. These differences between habitats are probably related to food supply. The use of gonad ratio methods to normalize gonad size assumes that, within a given population, animals of different body size have the same ratio of gonad size to body size. This assumption is not correct for four species of sea urchins. Gonad index or ratio methods are useful for comparing animals of the same size through time within a population or from different habitats. Restriction of the size range of animals used and use of samples of sufficient size permit gonad ratios to be used to assess the capacity of different habitats to support gonad production in sea urchins and other animals.     

Mechanisms of digestion in the marine herbivore, the luderick, Girella tricuspidata (Quoy and Gaimard)

The effect of feeding frequency on the mechanisms of digestion in the mucosa and contents of the gastrointestinal tract of the luderick, Girella tricuspidata (Quoy and Gaimard), was determined. Use was made of the variation in utilization frequency of Enteromorpha intestinalis by C. tricuspidata fed according to two different regimes to determine the importance of these digestive mechanisms in algal digestion.
The contents of the oesophagus and stomach of fish fed ad libitum were consistently acidic whilst those from fish deprived of food were highly variable and more alkaline. The contents of the pyloric caeca. intestine and rectum were slightly alkaline in both groups.
The number of microorganisms found in the digestive tract increased with a reduction in food availability. There was no significant cellulase activity in any of the animals tested.
Amylase and lipase were found in the pyloric caeca, intestine and rectum, but were not present in the oesophagus or stomach. Lipase activities were highly variable. Although some protease was found in the posterior portion of the gut, the majority of the activity occurred in the oesophagus and stomach.
Generally, amylase, lipase, and protease activities were unaffected by a change in the feeding regime of the fish from being fed ud libitum to being fed for a period of 24 h every 5 days. It is concluded that a reduced activity of enzymes that hydrolyse algal cell contents is not the reason for the lower utilization efficiency of food-deprived fish.
It is clear that the only difference in digestive mechanisms between fish fed ad libitum and fish fed infrequently is the variation in pH of the foregut. It is considered likely that the increased acidity of the foregut of animals fed ad Iibilum facilitates hydrolysis of the algal cell walls and allows greater utilization of algae by these fish.     

Food deprivation and leptin prioritize ingestive and sex behavior without affecting estrous cycles in Syrian hamsters

Energy consumption is critical for the energetically expensive processes related to reproduction, and thus, mechanisms that increase ingestive behavior are directly linked to reproductive success. Similarly, the mechanisms that inhibit hunger and ingestive behavior might be most adaptive when these mechanisms cause individuals to stop foraging, hoarding and eating in order to find and court potential mates. In the laboratory, ingestive behaviors are typically studied separately from reproductive behaviors even though it is likely that these behaviors evolved under conditions in which both food and mates were available. We examined the choice between paracopulatory and ingestive behaviors in a semi-natural environment in which both food and potential mates were available. Intact female Syrian hamsters showed a high preference for males on days 3 and 4 (day 4 being the day of ovulation and estrous behavior), and a 48-h period of food deprivation significantly decreased preference for sex and increased preference for eating and food hoarding on day 3 in 89% of the hamsters, although none became anestrous. The same period of food deprivation significantly decreased the level of vaginal marking without significant effects on plasma estradiol concentrations. Next, hamsters were either food deprived (FD) or fed ad libitum, and half of each group was treated with vehicle or the adipocyte hormone leptin. The percentage of females with a low preference for sex was significantly greater in the FD compared to the ad libitum-fed groups, and leptin treatment prevented this effect. Metabolic fuels, possibly acting through leptin and other hormones, might influence sensitivity to estradiol or enhance the downstream effects of estradiol, thereby increasing motivation for sex and decreasing the relative motivation to forage, hoard and eat food.     

The effect of feeding or starvation on resource allocation to body components during the reproductive cycle of the sea urchin Sphaerechinus granularis (Lamarck)

To determine the effects of feeding or starvation on resource allocation to body components during the reproductive cycle of Sphaerechinus granularis, sea urchins were placed in laboratory tanks and either fed ad libitum or starved during two different periods of their biological cycle, i.e. the mature stage and the recovery stage. The urchin growth was monitored over the whole experimental period, the gonad, gut, lantern indices and organic matter levels of different organs were determined at the end of the experiment. During the mature stage sea urchins in good nutritional conditions did not increase in size, but allotted energy to gonad production and stored reserves in body wall. Limiting food stopped the gonadal growth without complete regression. During the recovery period food allowed somatic growth, i.e. test growth and the storage of reserves in gonad somatic cells. This somatic production did not occur under food-limited conditions and the resources allotted for survival and maintenance were taken from different body components.     

Influence of feeding interval on reproduction and longevity ofPodisus sagitta (Het.: Pentatomidae)

The effect of various feeding intervals on reproduction and longevity of the predatory bugPodisus sagitta (Fabricius) (Heteroptera: Pentatomidae) was studied in the laboratory. Isolated pairs of adult bugs were given moisture in excess and were fed 1 seventh-instar larva of the greater wax moth,Galleria mellonella (L.), with a mean weight of 90 mg every 1, 3, 7, 14 and 21 days. Reproduction ofP. sagitta reflected well the feeding intervals. Females fed at increasing time intervals reduced reproductive outputs by decreasing oviposition frequency and size of egg batches. Predators fed every 21 days produced a mean of only 1.0 egg per day in comparison with 9.9 eggs per day for females fed daily. Longevity of females fed daily was 110.7 days; females fed less frequently tended to live longer, with a maximum mean longevity of 217.5 days for females fed every 14 days. No cannibalism among adults was observed, but females were occasionally seen to feed on their own eggs. Availability of moisture was found to be essential for the predators to survive during periods of low food supplies. These findings are discussed in relation to the predator's adaptation strategies in conditions of prey scarcity.      

Test for physiological limitation to nutrient assimilation in a long-distance passerine migrant at a Springtime stopover site

During northward migration, blackcaps arrive at stopover sites in Israel's Negev Desert with reduced masses of organs that are important in food digestion and assimilation. Blackcaps that stay to refuel (largely on fruits) do not gain mass rapidly until after 3 d at the stopover site. We hypothesized that (1) it may take several days to rebuild these reduced organs, (2) during this recovery interval high feeding rates might not be possible, and (3) this could be the basis for the absence of immediate body mass gain in blackcaps at stopover sites. To test predictions from this hypothesis we used an established fasting protocol to create a group of blackcaps with reduced intestinal and liver mass compared with ad lib. fed controls. Migrants were captured and caged in the laboratory, where they were habituated to a fruit mash diet for 8 d. One experimental group was then fasted 2 d, one was fed at a restricted level (one-third ad lib. food intake) for 4 d, and one was held as ad lib.-fed controls. The fasted and restricted birds were then allowed to feed again ad lib. Birds that were experimentally fasted progressively increased their daily assimilation rate and achieved the highest rate (one-third higher than controls) 3 d after the end of their fast. Birds that were restricted achieved high rates immediately once ad lib. food was provided. Increased assimilation rate was achieved via hyperphagia and not increased assimilation efficiency. The response of the fasted birds supports the hypothesis that there may be physiological constraints to the rate of refueling during migratory stopover.     

Anatomical and histological changes in the alimentary tract of migrating blackcaps (Sylvia atricapilla): a comparison among fed, fasted, food-restricted, and refed birds

During northward migration, blackcaps that arrive to refuel at stopover sites in Israel's Negev Desert have reduced masses of organs that are important in food digestion and assimilation. We tested several predictions from the general hypothesis that smaller organs of digestion (small intestine and pancreas) and nutrient assimilation (liver) bring about a lower capacity to consume food and that the organs must be restored before blackcaps can feed and digest at a high rate. We used a fasting protocol to create a group of blackcaps with reduced intestine and liver mass (reduced by 45% and 36%, respectively) compared with controls fed ad lib. Because most of the small intestine's biochemical digestive capacity reside in enterocytes found on villi, we predicted and found that reduced intestinal mass in fasted blackcaps related mainly to changes in enterocytes rather than other cells and tissues such as nonabsorptive crypt cells or underlying muscle. Because migrating blackcaps that stop over to feed begin to increase in body mass only 2 d after arrival, we predicted and found a similar recovery period in blackcaps that were first fasted but then refed--the organ mass, structure, function, and ability to consume food was restored after 2 d of feeding. Another group of food-restricted blackcaps (fed at one-third ad lib. level) lost similar amounts of body mass as fasted blackcaps but had much greater capacity to consume food than fasted blackcaps, and so we predicted that they would exhibit little or no reduction in alimentary organs relative to controls fed ad lib. A surprising result was that, as in fasted blackcaps, in food-restricted blackcaps, the decreases in masses of small intestine, liver, and pancreas were proportionally greater than the decreases in body mass or in masses of nonalimentary organs (heart, pectoralis). Food restriction, like fasting, caused a decrease in amount of intestinal mucosa and an alteration in the phenotype of enterocytes. These results are thus not consistent with the general hypothesis, and although they can be rationalized by assuming that blackcaps fed ad lib. have excess digestive capacity, it may also be that the physiological process or processes limiting very high feeding rate lie elsewhere than in the digestive system.     

Capacity for growth compensation in juvenile three-spined sticklebacks experiencing cycles of food deprivation

The capacity of three‐spined sticklebacks Gasterosteus aculeatus (initial mean mass 0.280 g) to compensate for recurrent periods of food deprivation of 2, 4 or 6 days followed by 2 days of ad libitum feeding on enchytraeid worms over 56 days was assessed by measuring appetite and growth. Control fish were fed daily. The total number of days on which fish were fed ranged from 14 (6‐day cycle) to 56 (controls). Deprived sticklebacks were hyperphagic on the first day of re‐feeding in a cycle and this hyperphagia increased with successive cycles. Mean daily consumption on first day of refeeding was: controls, 62.9 mg; 2 days, 108 mg; 4 days, 98.8 mg; 6 days, 101 mg. The hyperphagia did not increase as the preceding period of deprivation within a cycle increased. Hyperphagia was not maintained on the second day of re‐feeding. The 4 day and 6 day groups initially showed hypophagia on the second day of re‐feeding. Mean daily consumption on second day of re‐feeding was: controls, 62.6 mg; 2 days, 62.2 mg; 4 days, 54.2 mg; 6 days, 50.0 mg. Over the experiment, consumption on the second day of the 4 day and 6‐day groups increased towards the control level, suggesting a developing compensatory response. The highest mean daily consumption per days fed was shown by the 2 day group. The relationship between number of days fed and total food consumption and specific growth rate suggested that the 2 day group almost compensated for the periods of deprivation. Performance declined for the 4 day and 6 day groups, although even at the highest level of deprivation, a positive growth was achieved. Mean specific growth rate in mass (% per day) was: controls, 2.33; 2 days, 1.89; 4 days, 1.21; 6 days, 0.86. Initial mass and total food consumption accounted for most of the variance in specific growth rate. Other indices of performance including lipid concentration, dry matter concentration and the RNA:DNA ratio in white muscle were positively related to quantity of food consumed by each group. Growth efficiency of sticklebacks in terms of wet mass gained and wet mass consumed over the experimental period was 19.4% and did not differ among control and treatment groups. The growth rates of the sticklebacks experiencing cyclical deprivation were comparable to growth rates previously recorded for sticklebacks fed daily, but consuming similar mean daily rations.     

Energetic challenges unmask the role of ovarian hormones in orchestrating ingestive and sex behaviors

Effects of ovarian hormones on sex and ingestive behavior are well studied, and yet, their role in diverting attention from food to sex has not been examined directly, possibly because these functions are masked under conditions of excessive food abundance typical of the laboratory. Female Syrian hamsters were either fed ad libitum or food-restricted to 75% of their ad libitum intake for 8 days and then tested every day of the estrous cycle for their preference for males versus food, food hoarding and food intake in an apparatus designed to mimic aspects of their natural habitat. The food-restricted, but not the fed females, varied significantly over the estrous cycle in appetitive behaviors, which included their preference for males versus food and in the amount of food hoarded, with low food hoarding and high male preference on the night of ovulation. In contrast, there were no significant differences between restricted and ad libitum-fed females in the consummatory behaviors, namely, food intake or lordosis duration. In ovariectomized females, estradiol plus progesterone treatment delayed food restriction-stimulated hoarding and hastened feeding-inhibited hoarding without affecting food intake or lordosis duration. In summary, energy restriction and the presence of males unmasked an effect that was obscured in the normal laboratory conditions characterized by isolation and an over abundance of readily available food. These results are consistent with the idea that ovarian hormones orchestrate appetites for food and sex to optimize reproductive success under fluctuating energetic conditions.     

The effect of feeding frequency on utilization of algal nutrients by the marine herbivore, the luderick, Girella tricuspidata (Quoy and Gaimard)

Utilization of whole algae, lipid, protein and total nitrogen and total carbon by the luderick, Girella tricuspidata , fed ad libitum were found to be comparable with published results obtained for other species. Periods of food deprivation were found to cause reduced efficiency of utilization for all nutrients measured, with a 4–day deprivation period having a greater effect than a 24–h deprivation period.
Only animals fed ad libitum were able to obtain a greater amount of lipid from their food than was defaecated. Utilization of protein and non–protein nitrogen is similar in fish fed ad libitum , but a greater reliance upon the non–protein nitrogen content of the food for nutrition is apparent when the fish are subjected to periods of food deprivation. Nitrogen utilization, relative to other nutrients, by fish subjected to periods of food deprivation is greatest immediately after a feeding bout. Total carbon utilization relative to other nutrients increases with increasing time of deprivation following a feeding bout.
It is suggested that these changes in nutrient utilization with time after feeding are the result of rapid utilization of the nitrogen–rich cell contents, followed later by utilization of the algal cell wall.     

Regulation of gonad development and respiratory metabolism associated with food availability and reproductive diapause in the rice bug Leptocorisa chinensis

Food has an influence on many life history traits related to dormancy in insects. In our previous study with the rice bug Leptocorisa chinensis (Dallas) (Hemiptera: Alydidae), diapausing females transferred to conditions physically favorable for promoting the gonad development required food intake to resume gonad development, whereas males did not. These differences in response to food between males and females lead to two questions: (1) Was diapause in the starved females completed? (2) Were the starved males that resumed gonad development at the same physiological status as fed males? We tested these questions with two physiological indicators: gonad status and respiratory rate. Results indicate that starved females are able to complete diapause, but show depressed respiration relative to well-fed insects in diapause. Similar to females, starved males that resumed postdiapause gonad development also had depressed respiratory rate, and hence physiological status is presumed to be different between starved and fed individuals. In this study, it was also found that the photoperiodic signal is storable, whereas the food signal acts directly. The adaptive significance of regulation of gonad development and respiratory metabolism in relation to phenology of suitable host plant and reproductive strategy is discussed in both sexes.     

Cleavage site of a major yolk protein (MYP) determined by cDNA isolation and amino acid sequencing in sea urchin,Hemicentrotus pulcherrimus

The grey mouse lemur (Microcebus murinus) is a small nocturnal primate exhibiting daily torpor. In constant ambient temperature (22-24 degrees C), body temperature (Tb) and locomotor activity were monitored by telemetry in animals exposed to short (SP: 10 h light/day) or long (LP: 14 light/day) photoperiods. They were first fed ad libitum for 8 days and then subjected to 80% restricted feeding for 8 more days. During ad libitum feeding, locomotor activity was significantly lower in SP-exposed animals than in LP-exposed animals. Whatever the photoperiod, animals entered daily hypothermia within the first hours following the light onset. Depth of daily hypothermia increased irregularly under SP exposure, whereas minimal daily Tb was constantly above 35 degrees C under LP exposure. After the transfer from long photoperiod to short photoperiod corresponding to the induction of seasonal fattening, locomotor activity and depth of controlled daily hypothermia did not change significantly. In contrast, food restriction led to a significant increase in locomotor activity and in frequency of daily torpor (Tb<33 degrees C) and body temperature reached minimum values averaging 25 degrees C. However, SP-exposed animals exhibited lower minimal daily Tb and higher torpor duration than LP exposed animals. Therefore, daily torpor appears as a rapid response to food restriction occurring whatever the photoperiod, although enhanced by short photoperiod.     

The effect of food limitation on gonad development and egg production of the planktonic copepod Calanus finmarchicus

Calanus finmarchicus, one of the dominant copepods species of the North Atlantic, often encounters low food concentration or quality during the reproductive period; however, our knowledge on the effect of these conditions on reproduction processes is scarce. The present study combines experiments with histological observations to describe the response of C. finmarchicus to limited food focussing on (1) oocyte maturation processes, (2) gonad morphology, (3) egg production rates as a function of spawning frequency and clutch size, and (4) the fuelling of egg production by measuring carbon and nitrogen content of the females. In the laboratory, C. finmarchicus females were exposed to 0, 10, 50, 150 or >300 μg C l⁻¹ for several days. To account for food quality and season, reproductive activity was compared in April and July 1999 between females feeding on diatoms or dinoflagellates. The effect of feeding history was studied in February with females fed and starving prior to the experiment. Feeding conditions had severe effects on oocyte maturation process. Hence, egg production varied significantly with food concentration and quality, season and feeding history due to variation in both clutch size and spawning frequency. Clutch size differed by a factor of 2-4 between food limited and well fed females, and is thus an important parameter for modelling egg production. Changes in clutch size were related to changes of the number of maturing oocytes in the females gonads indicating that the latter can be used to precise the prediction of egg production from preserved samples. The proportion of females carrying at least some mature oocytes was relatively high at low food availability. Apparently, these females used internal body reserves as the carbon and nitrogen content decreased significantly under these conditions. These results indicate that C. finmarchicus embarks on the strategy to enable reproduction in all or many females of a population at low rates when feeding conditions are unfavourable.     

Reproductive responses to variation in temperature and food supply by house mice: II. Lactation

Lactating HS/Ibg house mice housed at 21 degrees C and 5 degrees C were assigned to 3 feeding regimes: ad libitum, daily rations of 80% of ad libitum, or 60% of ad libitum beginning on the day that they bore litters. Significant interaction between temperature and food restriction was found for litter survivorship, pup survivorship, litter size, female body weight, and cumulative biomass production. The interaction was due to a magnification of the effects of food restriction at the colder temperature: i.e. mice fed ad libitum were similar at the two temperatures, mice fed the 80% ration differed, and mice fed the 60% ration differed to a greater extent. The dominant response to food restriction was cannibalism by females, which might be associated with the rate of loss in body weight by the female on the days preceding cannibalism of one or more pups. Incidents of cannibalism tended to involve a limited number of pups and to be repeated until a sustainable litter size was reached. In 3 of the food-restricted treatments, females weaned relatively large litters of relatively small pups, but in the most severe treatment (in the group fed the 60% ration at 5 degrees C), the females weaned small litters of large pups. The patterns of cannibalism and variable relative investment in individual pups reflect the aggressive breeding strategy of this classic colonizing species.