How female caterpillars accumulate their nutrient reserves

Female Lepidoptera are often heavier than males. We examined the importance of consumption and post-ingestive processing as mechanisms for female Heliothis virescens larvae to meet the protein and carbohydrate requirements. In experiments in which caterpillars had a choice of diets, enabling them to select an appropriate protein and carbohydrate intake, females caterpillars ate more carbohydrate than males, but only on the heavily carbohydrate biased treatment. Overall, the sexes were not distinguished according to the selective feeding behavior, but females accumulated more protein and carbohydrate over the whole instar than the males did. Additionally, when given no choice, females ate more than males and accumulated more protein provided the diet contained a high proportion of protein. If they were reared on a high carbohydrate diet, there were no differences between the sexes. Our results indicate that female H. virescens larvae accumulate protein by regulating both intake and post-ingestive processing on high protein foods. In the field, late instar H. virescens feed on anthers, which are protein-rich and have the highest amino acid content relative to other cotton floral tissues.     

Diet Balancing in the Cockroach Rhyparobia madera: Does Serotonin Regulate this Behavior?

Animals, including insects, have the ability to self-select an optimal diet from a choice of two or more incomplete diets that lack an essential nutrient. This paper demonstrates that nymphs of the cockroach Rhyparobia madera also have this ability. The nymphs chose a protein:carbohydrate (P:C) ratio of approximately 25:75 when faced with a choice between one cube of protein (casein) and another of carbohydrate (sucrose). This self-selected ratio was shown to promote growth as well or better than other diets tested. When given a wide range of P:C choices, the R. madera nymphs consistently selected a P:C ratio of approximately 25:75, suggesting that they have the ability to diet-balance. Finally, injections of various serotonergic drugs into self-selecting nymphs influenced their choice of diets. Serotonin promoted a decrease in carbohydrate feeding, while injection of the antagonist -methyltryptophan caused the nymphs to overfeed on carbohydrate. The results suggest that serotonin may help alter the carbohydrate feeding response in cockroaches.     

Dietary choice for a balanced nutrient intake increases the mean and reduces the variance in the reproductive performance of male and female cockroaches

Sexual selection may cause dietary requirements for reproduction to diverge across the sexes and promote the evolution of different foraging strategies in males and females. However, our understanding of how the sexes regulate their nutrition and the effects that this has on sex‐specific fitness is limited. We quantified how protein (P) and carbohydrate (C) intakes affect reproductive traits in male (pheromone expression) and female (clutch size and gestation time) cockroaches (Nauphoeta cinerea). We then determined how the sexes regulate their intake of nutrients when restricted to a single diet and when given dietary choice and how this affected expression of these important reproductive traits. Pheromone levels that improve male attractiveness, female clutch size and gestation time all peaked at a high daily intake of P:C in a 1:8 ratio. This is surprising because female insects typically require more P than males to maximize reproduction. The relatively low P requirement of females may reflect the action of cockroach endosymbionts that help recycle stored nitrogen for protein synthesis. When constrained to a single diet, both sexes prioritized regulating their daily intake of P over C, although this prioritization was stronger in females than males. When given the choice between diets, both sexes actively regulated their intake of nutrients at a 1:4.8 P:C ratio. The P:C ratio did not overlap exactly with the intake of nutrients that optimized reproductive trait expression. Despite this, cockroaches of both sexes that were given dietary choice generally improved the mean and reduced the variance in all reproductive traits we measured relative to animals fed a single diet from the diet choice pair. This pattern was not as strong when compared to the single best diet in our geometric array, suggesting that the relationship between nutrient balancing and reproduction is complex in this species.     

Male cockroaches prefer a high carbohydrate diet that makes them more attractive to females: implications for the study of condition dependence

Sexual selection is a major force driving the evolution of elaborate male sexual traits. Handicap models of sexual selection predict that male sexual traits should covary positively with condition, making them reliable indicators of male quality. However, most studies have either manipulated condition through varying diet quantity and/or caloric content without knowledge of specific nutrient effects or have correlated proxies of condition with sexual trait expression. We used nutritional geometry to quantify protein and carbohydrate intake by male cockroaches, Nauphoeta cinerea, and related this to sex pheromone expression, attractiveness, and dominance status. We found that carbohydrate, but not protein, intake is related to male sex pheromone expression and attractiveness but not dominance status. Additionally, we related two condition proxies (weight gain and lipid reserves) to protein and carbohydrate acquisition. Weight gain increased with the intake of both nutrients, whereas lipid reserves only increased with carbohydrate intake. Importantly, lipid accumulation was not as responsive to carbohydrate intake as attractiveness and thus was a less-accurate condition proxy. Moreover, males preferentially consumed high carbohydrate diets with little regard for protein content suggesting that they actively increase their carbohydrate intake thereby maximizing their reproductive fitness by being attractive.     

Sex-specific differences in nutrient regulation in a capital breeding caterpillar, Spodoptera litura (Fabricius)

Nutrient requirements by male and female insects are likely to differ, but relatively little is known regarding how sexes differ in their regulation of macronutrient acquisition. The present study reports the results from a laboratory experiment in which behavioural and physiological components of nutrient regulation were compared between male and female caterpillars of Spodoptera litura (Fabricius). When provided with choices between two nutritionally complementary foods (one is a protein-biased diet and the other a carbohydrate-biased diet), both males and females adjusted their food selection to defend an intake target. However, the composition of diet preferred by the two differed, with females selecting significantly more protein than males with no difference in carbohydrate intake between the two. When confined to single diets with varying mixtures of protein and carbohydrate [P:C ratios, expressed as the percentage of diet by dry mass: protein 42%:carbohydrate 0% (p42:c0), p35:c7, p28:c14, p21:c21, p14:c28, p7:c35], females consumed more macronutrients than did males across on all P:C diets except the extremely carbohydrate-biased diet (p7:c35). Under both choice and no-choice feeding condition, such sex differences in nutrient intake were not expressed until late in the feeding stage of the final stadium. Sexes also differed in post-ingestive utilization of ingested nutrients. Females utilized ingested protein for body growth with greater efficiency compared to males, presumably reflecting provisioning their adult needs for protein to develop eggs, whereas males were more efficient at depositing lipids from carbohydrate intake than females.     

Mormon crickets maximize nutrient intake at the expense of immunity

Carbohydrates and protein comprise two of the major macronutrients and many animals regulate their dietary intake of both. In the field, the carbohydrate (C) to protein (P) intake of Mormon crickets Anabrus simplex Haldeman (Orthoptera: Tettigoniidae) is indicative of a nutritional imbalance affecting both migration and immunity. In the present study, dietary choice experiments in the laboratory are used to investigate the preferences of Mormon cricket nymphs and adults for C and P. Diets of differing C : P ratios and amounts are presented in pairs to permit Mormon crickets to reach an intake target of C : P from four unique starting points. After the last pair of diets is removed, phenoloxidase (PO) and anti‐bacterial activity are assayed. Both males and females at the adult and nymphal stages show a strong preference for the diet richest in macronutrients, with an equal preference for C or P. When given a choice between a high C diet or a high P diet, Mormon crickets select both at random, balancing their daily intake of C and P. Weight gain is dependent on the mass of P consumed, with a conversion factor greater than four times that of C consumed. As predicted, Mormon cricket nymphs and adults that consume more P have higher titres of total phenoloxidase and, in addition, lysozyme‐like anti‐bacterial activity is independent of dietary treatment. In nature, omnivores might consume an excess of one macronutrient because they often find the other through active searching of their local habitat. However, environmental change and interspecific or intraspecific competition can challenge the ability of an organism to encounter the required nutrients on a local scale, contributing to long‐distance migratory behaviours.     

Adaptive contraction of diet breadth affects sexual maturation and specific nutrient consumption in an extreme generalist omnivore

Animals balance their intake of specific nutrients, but little is known about how they do so when foraging in an environment with toxic resources and whether toxic foods promote adaptations that affect life history traits. In German cockroach (Blattella germanica) populations, glucose aversion has evolved in response to glucose‐containing insecticidal baits. We restricted newly eclosed glucose‐averse (GA) and wild‐type (WT) female cockroaches to nutritionally defined diets varying in protein‐to‐carbohydrate (P : C) ratio (3 : 1, 1 : 1, or 1 : 3) or gave them free choice of the 3 : 1 and 1 : 3 diets, with either glucose or fructose as the sole carbohydrate source. We measured consumption of each diet over 6 days and then dissected the females to measure the length of basal oocytes in their ovaries. Our results showed significantly lower consumption by GA compared to WT cockroaches when restricted to glucose‐containing diets, but also lower fructose intake by GA compared to WT cockroaches when restricted to high fructose diets or given choice of fructose‐containing diets. Protein intake was regulated tightly regardless of carbohydrate intake, except by GA cockroaches restricted to glucose‐containing diets. Oocyte growth was completely suppressed in GA females restricted to glucose‐containing diets, but also significantly slower in GA than in WT females restricted to fructose‐containing diets. Our findings suggest that GA cockroaches have adapted to reduced diet breadth through endocrine adjustments which reduce requirements for energetic fuels. Our study illustrates how an evolutionary change in the chemosensory system may affect the evolution of other traits that govern animal life histories.     

The glucogenic response of a parasitized insect Manduca sexta L. is partially mediated by differential nutrient intake

Induction of gluconeogenesis is accelerated in larvae of the insect Manduca sexta L. parasitized by Cotesia congregata (Say), maintaining the concentration of the blood sugar trehalose, an important nutrient for parasite development. Investigation has demonstrated that when host larvae are offered a choice of diets with varying levels of sucrose and casein, parasitized insects consume a different balance of these nutrients, principally due to a decrease in protein consumption. The result is metabolic homeostasis, with normal unparasitized and parasitized larvae exhibiting similar levels of gluconeogenesis and blood sugar level. In the present study, normal unparasitized and parasitized larvae were maintained on individual chemically defined diets having the balance of protein and carbohydrate consumed by each when offered a dietary choice. Total dietary nutrient, the sum of carbohydrate and protein, was provided at six levels, composed of three pairs of diets. Each diet pair consisting of diets having equivalent overall nutrient ratios of 2:1 and 1:1 casein/sucrose. Host growth and diet consumption were significantly affected by dietary nutrient level and the magnitude of these effects was influenced by parasitism. Due to the effects of dietary nutrient level on diet consumption, none of the unparasitized and parasitized larvae within any of the three diet pairs consumed protein and carbohydrate at the levels predicted by the earlier choice experiments. Among insects on all of the diets, however, two groups of unparasitized and parasitized larvae consumed the expected levels of protein and carbohydrate. In each case, gluconeogenesis, as measured by 13C nuclear magnetic resonance spectroscopy (NMR) analysis of pyruvate cycling and trehalose synthesis from [2-13C]pyruvate, was evident in unparasitized and parasitized insects, confirming the conclusions of the earlier experiments. Generally, all larvae that consumed less than approximately 250 mg of sucrose over the 3-day feeding period, were gluconeogenic, regardless of diet. Differential carbohydrate consumption, therefore, was an important factor in inducing gluconeogenesis in both unparasitized and parasitized insects. The selective 13C enrichment in trehalose displayed by non-gluconeogenic larvae on some diets demonstrated trehalose formation from [2]pyruvate. The absence of net carbohydrate synthesis in these insects was likely due to an elevation of glycolysis. There was no significant effect of diet consumption or parasitism on blood trehalose level. Parasitized larvae displayed higher levels of gluconeogenesis than did unparasitized insects, a finding consistent with the conclusion that blood sugar is rapidly sequestered by developing parasites. The parasite burden, the total number of parasites developing within host larvae, as well as the number of parasites emerging from host larvae to complete development, was significantly less at the lowest dietary nutrient level, but was otherwise similar at all dietary nutrient levels. Moreover, the number of parasites that emerged increased with increasing diet consumption as reflected by host final weight.     

Clonidine increases food and protein consumption in rats

Rats were given clonidine or its diluent, and allowed to eat freely from two isocaloric diets that differred in protein or carbohydrate content. Low clonidine doses (25–50 μg/kg) significantly increased total food and protein intake by rats given access to a high and a low protein diet. Several pairs of diets, differing in protein contents, were tested; clonidine's effect was greatest when a diet containing 30–45% protein was paired with one that was very low (5%) in protein. Higher clonidine doses (200 μg/kg) failed to modify either total food or protein intake. Clonidine had no effects on food or nutrient intake among animals given access to diets that differed in carbohydrate content (25 or 70% carbohydrate, plus 25% protein). In rats given access to only one diet, clonidine administration decreased food consumption when the diet was low in protein (5%), but increased consumption when the diet contained 25 or 50% protein. These data suggest that central noradrenergic synapses participate in the mechanisms controlling appetites for proteins. Clonidine may enhance protein intake by stimulating presynaptic alpha receptors, thus diminishing central noradrenergic tone. This effect on noradrenergic transmission is probably partly overcome by protein consumption, which increases brain tyrosine levels and thus can accelerate norepinephrine synthesis. Clonidine or related drugs may be useful clinically in treating diseases characterized by impaired appetite or increased need for protein.     

A comparison of dietary selection behaviour in larval Locusta migratoria and Spodoptera littoralis

ABSTRACT. . Final instar nymphs of the oligophagous acridid Locusta migratoria (L.) and larvae of the polyphagous noctuid Spodoptera littoralis (Boisduval) were fed for 4, 8 or 12 h, the conditioning period, on one of four artificial diets. Of these, diet PC contained 20% protein and 10% digestible carbohydrate; another, P, contained 20% protein but with the digestible carbohydrate component replaced by cellulose; a third, C, had the protein component substituted by cellulose, and the fourth, O, had both protein and digestible carbohydrate replaced. After this conditioning period, insects were given a choice of two diets, P and C, and hence an opportunity to select for the nutrients, if any, which were lacking in their previous food. Amounts eaten and selection behaviour were then recorded in detail for a total of 9 h. This paper deals with total amounts of diet eaten during the conditioning and choice periods. Spodoptera larvae were more sensitive than the locusts to being fed a nutritionally inadequate conditioning diet, and ate only small quantities of the P, C and O diets as compared with the PC diet, irrespective of the duration of conditioning. Locusts, on the other hand, when restricted to the P diet continued to eat relatively large amounts of it throughout a 12 h conditioning period. Those nymphs fed the C diet ingested large quantities (more than of the PC diet) up until 8 h, after which intake fell. When offered a choice, both species selected for the nutrients missing from the conditioning diet, even if the conditioning period had been as short as 4 h. During the first hour of choice locusts selected the P diet if they had been previously fed C and the C diet if previously fed P. Those deprived of both nutrients increased consumption of both P and C diets. Spodoptera larvae were more sensitive to prior deprivation of digestible carbohydrate than of protein. During the first hour of choice they selected the C diet if previously fed P or O but did not choose the P diet if previously fed C. In the subsequent 8 h of choice, however, a strong selection for the P diet after previous deprivation became apparent. In the locust, the selection for nutrients missing from the conditioning diet continued for the following 8 h of choice but became masked by a tendency, shown by all nymphs, to select C over P. The functional significance and possible physiological basis of all these responses is discussed.     

Dietary ratio of protein to carbohydrate induces plastic responses in the gastrointestinal tract of mice

Some vertebrates change the size of their digestive system in response to quantity and fibre content of ingested food, but the effects of dietary nutrients on gut structure remain poorly understood. Here we investigate how the protein to carbohydrate ratio of diets affects the mass of the gastrointestinal tract in mice. We fed 6-week-old male mice one of five isocaloric diets differing only in protein to carbohydrate ratio (the “no-choice” treatments), while a further four treatment groups received nutritionally complementary food pairings from which they could self-select a diet (the “choice” treatments). After 32 days, we measured the resulting dry mass of stomachs, intestines, caeca and colons. In the no-choice treatments, the stomachs were heavier in the mice fed diets containing more protein and less carbohydrate, indicating that larger stomachs may be needed for efficient digestion of the protein-rich food. In contrast, intestines, caeca and colons were heavier when diets contained more carbohydrates and less protein. This response may function to increase the digestive rate of carbohydrates when the dietary content of this macronutrient increases, but it may also indicate a compensatory response to increase amino acid uptake from a protein-deficient food. Mice in the choice treatments self-selected a diet with a protein to carbohydrate ratio of 0.46, and had gut dimensions similar to the expectation derived from no-choice treatments for this diet composition. Our results provide an example of plasticity in the differential allocation of resources to organ function, which is triggered by variation in resource quality.     

Different diets and amino acid supplementation do not affect the voluntary consumption of ethanol by rats

The effects of four different diets (control diet: 19.5% protein, 60.5% carbohydrate, 10% fat; diet I: 65% protein, 10% carbohydrate, 10% fat; diet II: 5% protein, 76% carbohydrate, 10% fat; diet III: 20% protein, 69% carbohydrate, 1% fat; diet IV: 69% protein, 15% carbohydrate, 1% fat) and supplementation with 3 amino acids (tryptophan: 150 mg/kg/d; arginine: 400 mg/kg/d; taurine: 380 mg/kg/d) on the voluntary consumption of ethanol were investigated in rats using the 2 bottle method. First, rats received the control diet and diets I, II, III and IV for 20 days with a choice of ethanol for the last 6 days only. Ethanol consumption was similar in all dietary groups. Second, rats received the control diet for 8 days followed by diets I, II and IV for another 8 days. Ethanol was offered throughout both periods. The switch to the special diets did not affect ethanol consumption. Third, rats received a control diet with arginine, tryptophan or taurine added to the drinking fluids for 16 days with a choice of ethanol for the last 5 days; thereafter supplementation stopped but the ethanol choice remained. No difference in the voluntary intake of ethanol was noted but ethanol consumption fell after cessation of arginine supplementation. In conclusion, diets differing greatly in their composition or supplementation with these 3 amino acids did not affect the voluntary choice of ethanol by rats in a significant manner.     

High sucrose consumption promotes obesity whereas its low consumption induces oxidative stress in Drosophila melanogaster

The effects of sucrose in varied concentrations (0.25–20%) with constant amount of yeasts in larval diet on development and metabolic parameters of adult fruit fly Drosophila melanogaster were studied. Larvae consumed more food at low sucrose diet, overeating with yeast. On high sucrose diet, larvae ingested more carbohydrates, despite consuming less food and obtaining less protein derived from yeast. High sucrose diet slowed down pupation and increased pupa mortality, enhanced levels of lipids and glycogen, increased dry body mass, decreased water content, i.e. resulted in obese phenotype. Furthermore, it suppressed reactive oxygen species-induced oxidation of lipids and proteins as well as the activity of superoxide dismutase. The activity of catalase was gender-related. In males, at all sucrose concentrations used catalase activity was higher than at its concentration of 0.25%, whereas in females sucrose concentration virtually did not influence the activity. High sucrose diet increased content of protein thiols and the activity of glucose-6-phosphate dehydrogenase. The increase in sucrose concentration also enhanced uric acid level in females, but caused opposite effects in males. Development on high sucrose diets was accompanied by elevated steady-state insulin-like peptide 3 mRNA level. Finally, carbohydrate starvation at yeast overfeeding on low sucrose diets resulted in oxidative stress reflected by higher levels of oxidized lipids and proteins accompanied by increased superoxide dismutase activity. Potential mechanisms involved in regulation of redox processes by carbohydrates are discussed.     

Effects of diet quality on performance and nutrient regulation in an omnivorous katydid

1. Omnivores by definition eat both plants and animals. However, little is known about how diet macronutrient content affects omnivore performance, or the extent to which they can regulate macronutrient intake. We assessed these questions using the salt marsh katydid, Conocephalus spartinae Fox (Tettigoniidae). 2. In our first experiment we used artificial diets with different protein–carbohydrate ratios to assess the effects of diet quality on survival, growth, and lipid accumulation. We found that diets with a high protein–carbohydrate ratio negatively affected Conocephalus survival. Among surviving individuals growth was not significantly different across the treatments, but lipid content decreased significantly as the protein–carbohydrate ratio of diets increased. 3. In a second experiment we explored the ability of Conocephalus to regulate their protein–carbohydrate intake. Results revealed that Conocephalus did not feed randomly when presented with two nutritionally complementary foods. A detailed analysis of their protein–carbohydrate intake revealed selection for a protein‐biased diet, but a lack of tight regulate of protein–carbohydrate intake. 4. We discuss how key macronutrients can limit omnivores, and how nutritional flexibility may enable omnivores to persist in nutritionally heterogeneous environments.     

Diet quality differentially affects breeding effort of Mastomys coucha and M. natalensis: Implications for rodent pests

In this study we compare the reproductive ability of Mastomys natalensis, an agricultural rodent pest and that of the closely related species M. coucha, which has not been ascribed as a pest. We suggest that the ability of M. natalensis to plague, as well as dominate, resource-limited habitats, may in part be related to its differential breeding response to variation in diet quality. Thus, we examined the reproductive response of M. coucha and M. natalensis mothers to variation in dietary protein intake. Our results demonstrated that typical levels of reproduction extended over a narrower range of dietary protein content for M. coucha than M. natalensis females. Only M. natalensis females bred on 6% protein diets, while on 20% protein diets the reproductive output of M. coucha was lower than on diets containing 10- and 15% protein. M. natalensis responded to low protein diets by reducing litter size and litter mass but not individual pup mass. Thus, providing diet quality improves before parturition, conception under sub-optimal conditions may not compromise individual pup growth. Furthermore, the ability of M. natalensis to breed under conditions of low diet quality may advance their breeding season compared to species that cannot breed until diet quality improves, allowing M. natalensis to produce additional litters through the season, while more daughters could reproduce during the season of their birth, both factors that are known to contribute to the plaguing tendency of M. natalensis.     

Influence of Feeding Paradigm in Rats on Temporal Pattern of: I. Macronutrient Intake and Arterio-Venous Differences in Plasma Glucose,Insulin and Tryptophan

Relationships among feeding paradigm (single diet vs food selection) and arterio-venous differences (δAV) of glucose, insulin and tryptophan were studied by measuring the temporal patterns of food intake and plasma parameters during 8 hr feeding cycles in rats. Adult male Sprague-Dawley rats were offered a single diet of fixed composition (20% casein) or a choice between two isocaloric diets (0% and 60% casein) for 2 weeks under 8-hr daily feeding conditions, food being offered during the dark cycle. Groups of animals were then killed at the beginning and at 2-hourly intervals throughout the feeding period. With both feeding paradigms, rats showed temporal patterns of energy, carbohydrate and protein intakes with a peak at the beginning and a trough at the end of the feeding period. However, in rats offered a dietary choice the intake of carbohydrate was significantly lower, and the intakes of energy and protein significantly higher than those found in rats offered a single diet. Throughout the feeding period, these differences between single and choice diets became less accentuated in the case of carbohydrate intake, but more accentuated for energy and protein intakes. Paradoxically, rats fed a choice of diets had a significantly lower weight gain than rats fed a single diet. The temporal variation of insulin secretion and tryptophan absorption varied inversely with the two diet paradigms. Moreover, in rats offered a choice of diets, macronutrient intake was significantly correlated with insulin secretion and venous glucose concentration. The opposed physiologic and metabolic responses to the feeding paradigms suggest the need for future studies to examine the possibility that such can function as synchronizers of biological rhythms.     

Dietary Fat Dose Dependently Increases Spontaneous Caloric Intake in Rat

Objective: To characterize the dose‐response relationship between dietary fat to carbohydrate ratio and spontaneous caloric intake. Research Methods and Procedures: Male Long‐Evans rats consumed milk‐based liquid diets that differed in fat content (17% to 60% of kilocalories) but had equivalent protein content and energy density. In Experiment 1, rats consumed one of the diets (n = 9/diet group) as the sole source of nutrition for 16 days. In Experiment 2, diets were offered as an option to nutritionally complete chow for 4 days followed by a 3‐day chow‐only washout in a randomized within‐subjects design (n = 30). In Experiment 3, nine rats received isocaloric intragastric infusions of diet overnight, with chow available ad libitum. At least two no‐infusion days separated the different diet infusions, which were given in random order. Food intake was measured daily Results: Dietary fat dose dependently increased total daily kilocalories in each of the three paradigms. Discussion: These data imply that the postingestive effects of carbohydrate and fat differentially engage the physiological substrates that regulate daily caloric intake. These findings reiterate the importance of investigating macronutrient‐specific controls of feeding, rather than prematurely concluding that dietary attributes that covary with fat content (e.g., caloric density and palatability) drive the overeating associated with a high‐fat diet.     

Dietary self‐selection and rules of compromise by fifth‐instar Vanessa cardui

We examined dietary self‐selection and rules of compromise for protein (P) and digestible carbohydrate (C) intake by fifth‐instar Vanessa cardui L. (Lepidoptera: Nymphalidae: Nymphalini). We presented six fat‐free diet pairs to larvae in a choice trial to determine the ‘intake target’. In addition, we fed larvae seven fat‐free single diets differing in dietary nutrient ratio in no‐choice trials to determine the rules of compromise they exhibit when constrained to a singular, sub‐optimal dietary source. In choice trials, caterpillars regulated nutrient intake to a ratio of 1 protein to 1.09 carbohydrate (1P:1.09C), exhibiting tighter regulation of protein than of carbohydrate. Furthermore, larvae from different diet pair treatments did not differ in pupal mass or stadium duration. In no‐choice experiments, larvae reduced consumption on increasingly protein‐biased diets and increased consumption on increasingly carbohydrate‐biased diets, relative to a 1P:1C ratio diet. Differences in carbohydrate consumption were much greater between no‐choice treatments than differences in protein consumption. Dietary nutrient ratio affected pupal mass when accounting for initial larval mass. Pupal mass decreased as nutrient ratio was shifted off of 1P:1C, but to a greater extent when the ratio was skewed toward carbohydrate. Stadium duration increased as nutrient ratio diverged from 1P:1C, being more pronounced when shifted toward carbohydrate than toward protein. Regulation to near 1P:1C is consistent with results found for other Lepidoptera, and the rule of compromise exhibited by V. cardui is consistent with that expected for a generalist herbivore.     

Compensatory dietary selection occurs in larval Locusta migratoria but not Spodoptera littoralis after a single deficient meal during ad libitum feeding

ABSTRACT Final instar nymphs of Locusta migratoria (L.) and larvae of Spodoptera littoralis (Boisduval) were given an artificial diet deficient in either protein or digestible carbohydrate for a single meal during ad libitum feeding, after which they were provided with a choice of two diets, one containing protein but no digestible carbohydrate and the other containing carbohydrate but no protein. Detailed analyses of feeding behaviour showed that locusts exhibited a degree of compensatory dietary selection following the single deficient meal. No such response was evident for the caterpillars, although previous work (Simpson et al. , 1988) has demonstrated that compensatory selection behaviour is marked in this species after periods of 4 h or more on the same deficient diets. These results show that locusts are able to respond extremely rapidly to the nutritional quality of their food by utilizing nutritional feedbacks. This capability may have important implications for the study of foraging strategies in other herbivorous insects.     

Drugs that enhance central serotoninergic transmission diminish elective carbohydrate consumption by rats.

We previously showed (Science 198:1178, 1977) that fenfluramine or fluoxetine, drugs thought to enhance serotoninergic transmission, selectively decrease the rat's consumption of carbohydrates, without affecting protein intake, when animals are given simultaneous access to diets differing in protein (5% vs 45%) and carbohydrate contents; in contrast, d-amphetamine lacks this selective effect. Present studies affirm this relationship using another serotoninergic drug, MK-212, and show that the suppression of carbohydrate intake occurs whether the test diets contain variable amounts of protein, or carbohydrates, or of both nutrients. Evidence is presented that rats given diet mixtures containing various proportions of carbohydrates have the ability to regulate their carbohydrate intakes (i.e., to choose to eat amounts of each diet that, taken together, will give them a desired proportion of carbohydrate), and that this ability is independent of whether or not the carbohydrate consumed has a sweet taste. It is proposed that serotoninergic neurons comprise part of a behavioral feedback loop, whereby the consumption of carbohydrate (which, by altering plasma amino acid patterns, accelerates serotonin synthesis in brain neurons) diminishes the rat's subsequent tendency to consume additional carbohydrates. Drugs that enhance central serotoninergic transmission can probably be substituted for dietry carbohydrate to activate this behavioral loop.