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AbstractTemperate reptiles are often considered to be low-energy systems, with their discrete use of time and energy making them model systems for the study of time-energy budgets. However, the semifrequent replacement and sloughing of the epidermis is a ubiquitous feature of squamate reptiles that is often overlooked when accounting for time and energy budgets in these animals. We used open-flow respirometry to measure both the energetic effort of ecdysis and the duration of the associated metabolic upregulation (likely related to behavioral changes often reported for animals in shed) in wild-caught timber rattlesnakes ( Crotalus horridus ). We hypothesized that total effort of skin biosynthesis and physical removal would be related to body mass and expected the duration of the process to remain static across individuals at a fixed temperature (25°C). We provide both the first measurements of the cost of skin biosynthesis and physical removal in a reptile and the highest-resolution estimate of process duration recorded to date. We found that skin biosynthesis, but not the cost of physical removal of the epidermis, was related to body mass. Shed cycle duration was consistent across individuals, taking nearly 4 wk from process initiation to physical removal of the outermost epidermal layer. Total energetic effort of ecdysis was of sizeable magnitude, requiring ∼3% of the total annual energy budget of a timber rattlesnake. Energetic effort for a 500-g snake was equivalent to the amount of metabolizable energy acquired from the consumption of approximately two adult mice. Ecdysis is a significant part of the time-energy budgets of snakes, necessitating further attention in studies of reptilian energetics.

AbstractHarbor seals ( Phoca vitulina ) live in cold temperate or polar seas and molt annually, renewing their fur over a period of approximately 4 wk. Epidermal processes at this time require a warm skin; therefore, to avoid an excessive energy cost at sea during the molt, harbor seals and many other pinnipeds increase the proportion of time they are hauled out on land. We predicted that metabolic rate during haul-out would be greater during the molt to sustain an elevated skin temperature in order to optimize skin and hair growth. To examine this, we measured post-haul-out oxygen consumption ( V ˙ O 2 ) in captive harbor seals during molt and postmolt periods. We recorded greater V ˙ O 2 of seals while they were molting than when the molt was complete. Post-haul-out V ˙ O 2 increased faster and reached a greater maximum during the first 40 min. Thereafter, V ˙ O 2 decreased but still remained greater, suggesting that while metabolic rate was relatively high throughout haul-outs, it was most pronounced in the first 40 min. Air temperature, estimated heat increment of feeding, and mass also explained 15.5% of V ˙ O 2 variation over 180 min after haul-out, suggesting that the environment, feeding state, and body size influenced the metabolic rate of individual animals. These results show that molting seals have greater metabolic rates when hauled out, especially during the early stages of the haul-out period. As a consequence, human disturbance that changes the haul-out behavior of molting seals will increase their energy costs and potentially extend the duration of the molt.

AbstractComplex features, such as vision, limbs, and flight, have been lost by many groups of animals. Some groups of birds are more prone to loss of flight than others, but few studies have investigated possible reasons for this variation. I tested the hypothesis that a rare strategy of flight feather replacement is involved in rate variation in the evolution of flightlessness in birds. This strategy involves a simultaneous molt of the flight feathers of the wing, resulting in a temporary flightless condition during molt. I hypothesized that adaptations for this flightless period may serve as preadaptations for permanent flightlessness under conditions that favor permanent loss of flight. I found an elevated rate of loss of flight in lineages with simultaneous wing molt compared with loss of flight in lineages without simultaneous wing molt. This may indicate that birds with simultaneous molt are more prepared to adjust quickly to open niches that do not require flight, such as terrestrial niches on island habitats. These results illustrate how molt strategies can influence the long-term evolutionary trajectories of birds and provide insight into how phenotypic precursors may act as a mechanism of rate variation in the loss of complex traits.

N-acetylglucosamino-1,5-lactone (NAGL) is a molting biomarker in the blue crab Callinectes sapidus The concentration of this compound in urine is highest at the premolt stage. Since sexually mature premolt females release sex pheromone in their urine, NAGL is a candidate sex pheromone molecule in C. sapidus This compound has not been reported in other species. In the present study, we quantified the concentration of NAGL in the urine of the helmet crab Telmessus cheiragonus, using nuclear magnetic resonance spectroscopy, and found that the concentration increases toward the day of molting and decreases after molting. However, the total amount of NAGL collected from individual animals was greatest two to five days after molting, because the amount of urine collected was the lowest at the premolt stage, and it increased after molting. The highest median concentration of NAGL in T. cheiragonas was 29 μmol l(-1), which is 75% of the highest concentration reported in C. sapidus This is the first report of NAGL as a molting biomarker in a species other than C. sapidus We assume that NAGL is part of a pheromone bouquet in these two species., (© 2016 Marine Biological Laboratory.)

The bed bug (Cimex lectularius L.) is a common hematophagous pest in the urban environment and is capable of surviving extended periods of starvation. However, the relationship between starvation and metabolism in bed bugs is not well understood. To better understand this relationship, we measured the metabolism of all life stages for >900 h after feeding (starvation) using closed-system respirometry. Measurements were made around molting for the immature life stages, which occurs only after a blood meal. In addition, both mated and unmated adults were measured. Starvation and molting had significant effects on the metabolism of the bed bug. Mass-specific metabolic rate (V(O2); mL g(-1) h(-1)) declined in a curvilinear fashion with the period of starvation for adults and with the postmolting period for immature bed bugs (used to standardize all immature life stages). A standard curve was developed to depict the generalized pattern of metabolic decline observed in all life stages that molted. Individual metabolic comparisons among life stages that molted revealed some differences in metabolic rate between unmated males and females. In addition, the mass scaling coefficient was found to decline with starvation time (postmolting time) for all life stages that molted. In most life stages, the ratio of V(CO2) to V(O2) (respiratory exchange ratio) declined over time, indicating a change in metabolic substrate with starvation. Finally, daily percent loss in body mass declined in a pattern similar to that of V(O2). The observed patterns in metabolic decline are evaluated in relation to the life history of bed bugs. In addition, the evolutionary development of these patterns is discussed. The metabolic pattern after feeding was also found to share several similarities with that of other ectothermic species.

The information on the phenotypic and ecological factors that influence hemoglobin concentration in free-living birds is scarce. In order to recognize sources of variation in hemoglobin levels of migratory shorebirds, we measured whole-blood hemoglobin concentration in 553 juvenile and 166 adult common snipe Gallinago gallinago during autumn migration through central Poland. Among the intrinsic determinants of hemoglobin concentration in common snipe, we identified traits such as age, wing morphology, developmental stability, nutritional condition, and molt. We found that adult birds had higher hemoglobin concentrations than juveniles. Hemoglobin concentration was not related to body size, but it correlated with wing morphology. In adult males there was also a positive relationship between hemoglobin concentration and developmental stability, measured by fluctuating asymmetry in wing shape. The process of molt was found to affect blood hemoglobin concentration in both juvenile and adult common snipe, as the lowest concentrations were recorded in the initial stages of molt. Finally, we recorded a gradual increase in hemoglobin concentration of juvenile and adult snipe over the course of the autumn migratory season, and this trend was attributed to higher fat loads carried by late migrants. Hemoglobin concentration also correlated with other indices of nutritional state, such as plasma concentrations of proteins. All this clearly indicates that hemoglobin concentration may reflect a wide range of physiological processes, but in spite of this immense variation, it is likely to reliably indicate phenotypic quality of birds.

A unified system of bioenergetic parameters that describe thermal regulation and energy metabolism in many passerine and non-passerine species has been developed. These parameters have been analyzed as functions of ambient temperature, and bioenergetic models for various species have been developed. The level of maximum food energy or maximal existence metabolism (MPE) is 1.3 times higher in passerines than in non-passerines, which is consistent with the ratio of their basal metabolic rates (BMR). The optimal ambient temperature for maximizing productive processes (e.g., reproduction, molting) is lower for passerines than for non passerines, which allows passerines to have higher production rates at moderate ambient temperatures. This difference in the optimal ambient temperature may explain the variation in bioenergetic parameters along latitudinal gradients, such as the well-known ecological rule of clutch size (or mass) increase in the more northerly passerine birds. The increased potential for productive energy output in the north may also allow birds to molt faster there. This phenomenon allows passerine birds to occupy a habitat that fluctuates widely in ambient temperature compared with non-passerine birds of similar size. Passerines have a more effective system for maintaining heat balance at both high and low temperatures. The high metabolism and small body sizes of passerines are consistent with omnivore development and with ecological plasticity. Among large passerines, the unfavorable ratio of MPE to BMR should decrease the energy that is available for productive processes. This consequence limits both the reproductive output and the development of long migration (particularly in Corvus corax). The hypothesis regarding BMR increase in passerines was suggested based on an aerodynamic analysis of the flight speed and the wing characteristics. This allometric analysis shows that the flight velocity is approximately 20% lower in Passeriformes than in non-Passeriformes, which is consistent with the inverted ratio of their BMR level. The regressions for the aerodynamic characteristics of wings show that passerines do not change the morphological characteristics of their wings to decrease velocity. Passerine birds prefer forest habitats. The size range of 5-150 g for birds in forest habitats is almost exclusively occupied by passerines because of their large energetic capability.

Physiological processes are regulated by a diverse array of neuropeptides that coordinate organ systems. The neuropeptides, many of which act through G protein-coupled receptors, affect the levels of cyclic nucleotides (cAMP and cGMP) and Ca(2+) in target tissues. In this perspective, their roles in molting, osmoregulation, metabolite utilization, and cardiovascular function are highlighted. In decapod crustaceans, inhibitory neuropeptides (molt-inhibiting hormone and crustacean hyperglycemic hormone) suppress the molting gland through cAMP- and cGMP-mediated signaling. In insects, the complex movements during ecdysis are controlled by ecdysis-triggering hormone and a cascade of downstream neuropeptides. Adipokinetic/hypertrehalosemic/hyperprolinemic hormones mobilize energy stores in response to increased locomotory activity. Crustacean cardioacceleratory (cardioactive) peptide, proctolin, and FMRFamide-related peptides act on the heart, accessory pulsatile organs, and excurrent ostia to control hemolymph distribution to tissues. The osmoregulatory challenge of blood gorging in Rhodnius prolixus requires the coordinated release of serotonin and diuretic and antidiuretic hormones acting on the midgut and Malpighian tubules. These studies illustrate how multiple neuropeptides allow for flexibility in response to physiological challenges.

Even though molt involves both endocrine and energetic changes in bird bodies, this study is among the first to combine assessments of energy costs together with thyroid hormone variations in molting birds. Individual shorebirds (red knots Calidris canutus islandica) were measured while in full summer and winter plumage as well as during peak of molt. Molt was associated with a 9.8% increase in average mass-independent basal metabolic rate (BMR) above nonmolting levels. Individual plasma levels of thyroxine (T(4)) were correlated with individual rate of body feather renewal, confirming that T(4) is related to body molt but also showing that it is potentially regulating its rate. Across seasons, mass-independent average heat loss measured as conductance gradually declined with conductance during molt falling between measured values for summer and winter. During the molting period, however, body molting rate was positively correlated with thermal conductance, indicating that for a given ambient temperature below thermoneutrality, the fastest molting birds were losing more body heat. Across seasons, triiodothyronine (T(3)), a hormone typically upregulated in response to a cold stimulus, was correlated with individual thermal conductance and BMR. We suggest that the increased heat loss of fast-molting birds leads to a cold-acclimatization response that may be partly responsible for the elevated BMR measured during molt. This could be mediated through a stimulatory effect of T(3) on BMR in response to increased heat loss. Our interpretation is supported by a positive relationship between the individual changes in conductance and the change in BMR from summer to the molting period.

Baseline and stress-induced corticosterone (CORT), heart rate (fH), and energy expenditure were measured in eight captive European starlings Sturnus vulgaris during and following a prebasic molt. The fH and oxygen consumption (V O2 ) were measured simultaneously across a range of heart rates, and energy expenditure (kJ/d) was then calculated from data. Energy expenditure and fH were strongly and positively correlated in each individual. Baseline fH and energy expenditure were significantly higher during molt. Molting starlings expended 32% more energy over 24 h than nonmolting birds, with the most significant increase (60%) occurring at night, indicating a substantial energetic cost to molt. Furthermore, the cardiac and metabolic responses to stress during molt were different than during nonmolt. Birds were subjected to four different 30-min acute stressors. The fH and CORT responses to these stressors were generally lower during molt. Although restraint caused a 64% increase in daily energy expenditure during nonmolt, no other stressor caused a significant increase in energy expenditure. Overall, our data suggest that molt is not only energetically expensive but that it also alters multiple stress response pathways. Furthermore, most acute stressors do not appear to require a significant increase in energy expenditure.

Mobilization of calcium during the molt cycle from the cuticle to transient calcium deposits is widely spread in crustaceans. The dynamics of calcium transport to transient calcium deposits called gastroliths and to the cuticle over the course of the molt cycle were studied in the crayfish Cherax quadricarinatus. In this species, calcium was deposited in the gastroliths during premolt and transported back to the cuticle during postmolt, shown by digital X-ray radiograph analysis. The predominant mineral in the crayfish is amorphous calcium carbonate embedded in an organic matrix composed mainly of chitin. Scanning electron micrographs of the cuticle during premolt showed that the endocuticle and parts of the exocuticle were the source of most of the labile calcium, while the epicuticle did not undergo degradation and remained mineralized throughout the molt cycle. The gastroliths are made of concentric layers of amorphous calcium carbonate intercalated between chitinous lamella. Measurements of pH and calcium levels during gastrolith deposition showed that calcium concentrations in the gastroliths, stomach, and muscle were about the same (10 to 11 mmol l(-1)). On the other hand, pH varied greatly, from 8.7+/-0.15 in the gastrolith cavity through 7.6+/-0.2 in muscle to 6.9+/-0.5 in the stomach.

Crayfish rely on the chemosensory neurons in their antennules to help them find food and habitat and to mediate social interactions. These structures often sustain damage from aggressive interactions or from the environment, but they have the ability to regenerate. In this study, we examine whether the effects of antennule ablation and regeneration on odor-tracking ability correlate with structural changes in the antennule that occur during regeneration. We initiated the regeneration process by removing the right antennules from 55 individuals of Orconectes sanborni. We developed a method to nondestructively sample the regenerating antennules so that we could follow the growth of new antennular tissue in the same animals over time. We used dental epoxy to make molds of the regenerating antennule after each molt. We then made resin positives, which were visualized using scanning electron microscopy. Structural parameters including aesthetasc length, diameter, segment length, and number per row were measured from scanning electron micrographs using Image J software. Crayfish were tested in a tabletop water Y-maze before and after surgery and after each molt to assess their ability to track food odors. The structural and the behavioral data indicate that the antennules possessed many aspects of their original structure by the end of the second molt. Flicking of antennules, investigation of substrate, success rate at finding the odor-containing Y-maze branch, and time to completion of Y-maze regained pre-antennulectomy values by the end of the third molt.

In this study we examined the fluctuations of the intracellular calcium concentration in isolated hepatopancreatic R cells during the four molting stages of the prawn Marsupenaeus japonicus. In addition, we used the Fura-2-AM fluorescence technique to investigate the release of calcium from mitochondria and ATP-sensitive calcium stores (endoplasmic reticulum (ER), Golgi, and nucleus) into cytoplasm during the molting cycle. Results demonstrate that both the cytosolic free calcium concentration and the total cell calcium (free, bound to calcium-binding proteins, and stored in amorphous form) in the R cells strictly depend upon the molting cycle. Interestingly, the total cell calcium was higher (approximately 10 mmol l(-1)) in postmolt than in premolt (approximately 1 mmol l(-1)) and intermolt (approximately 0.3 mmol l(-1)). The calcium released from mitochondria was higher during premolt than during postmolt and intermolt, but the amount of calcium released from ATP-sensitive calcium stores was similar during all four stages. All together, our results suggest that the mitochondria-ATP-sensitive calcium stores system does not play a key role in calcium storage during the molting cycle but that it is involved in transcellular calcium flux. We hypothesize that lysosome or membrane-clad concretion vacuoles could represent the main site of calcium storage in hepatopancreatic R cells.

The horseshoe crab Limulus polyphemus spawns in the mid- to upper intertidal zone where females deposit eggs in nests below the sediment surface. Although adult crabs generally inhabit subtidal regions of estuaries with salinities from 5 to 34 ppt, developing embryos and larvae within nests are often exposed to more extreme conditions of salinity and temperature during summer spawning periods. To test whether these conditions have a negative impact on early development and survival, we determined development time, survival, and molt cycle duration for L. polyphemus embryos and larvae raised at 20 combinations of salinity (range: 30-60 ppt) and temperature (range: 25-40 degrees C). Additionally, the effect of hyperosmotic and hypoosmotic shock on the osmolarity of the perivitelline fluid of embryos was determined at salinities between 5 and 90 ppt. The embryos completed their development and molted at salinities below 60 ppt, yet failed to develop at temperatures of 35 degrees C or higher. Larval survival was high at salinities of 10-70 ppt but declined significantly at more extreme salinities (i.e., 5, 80, and 90 ppt). Perivitelline fluid remained nearly isoosmotic over the range of salinities tested. Results indicate that temperature and salinity influence the rate of crab development, but only the extremes of these conditions have an effect on survival.

We describe for the first time changes in the rate of CO2 release (as a surrogate of metabolic rate) in the terminal larval stage of the insect Rhodnius prolixus following a blood meal and during the molt leading to the adult stage. These data are presented on a whole-animal basis as well as per gram wet and dry weight. We have also used techniques that allow us to describe the rate of release per gram of actual body tissue (i.e., removing the weight of the remaining bloodmeal in the gut and the metabolically inactive portion of the cuticle). While the metabolic rate of the whole animal rises approximately 10-fold in 15 d following feeding, the rate per gram of dry body mass rises only twofold. We use these data to provide insights into the relative contributions of tissue growth and increases in metabolic intensity to the massive increases in metabolic rate observed in these insects following feeding. Our analyses indicate that the majority of nutrient uptake occurs in the first 4 d following feeding. It is well known in this species that day 4 following feeding is the end of a critical period for the insect in determining whether it will proceed to the next molt. Our results indicate that the insects may be able to make this decision based on nutrients already transported into the body. We examined the "down regulation" of metabolism observed in the latter stages of the molt cycle in this insect. We express these changes on both a per animal and per gram basis and demonstrate that this down regulation extends even into the adult stage before feeding. Using a comparison of the allometric relationships of metabolic rate to mass in insects and ticks, we demonstrate that unfed R. prolixus show a marked decrease in metabolic rate compared to other insects, while fed Rhodnius are similar in metabolic rate to other insects. Rhodnius has a markedly higher metabolic rate (as do all insects) than that found in ticks.

The fasting metabolism of 71- to 235-d-old subantarctic fur seal (Arctocephalus tropicalis) pups from Amsterdam Island, southern Indian Ocean, was investigated during the long foraging trips of their mothers. Body lipid reserves were proportionally greater in female than male pups and higher in postmoult (37%) than premoult (10%) animals. The mass-specific rate of mass loss did not differ between the sexes but was lower than observed in other species. Daily mass loss was estimated to 56% fat, 10% protein, and 34% water. The rate of protein catabolism (15 g d(-1)) was negatively related to the size of initial lipid stores and accounted for 9% (+/-1%) of total energy expenditure. However, body composition changes during the fast were not equal between the sexes, with females relying more on protein catabolism than males (11% and 5% of total energy expenditure, respectively). Energy expenditure (270 kJ kg(-1) d(-1)) and metabolic water production (11.5 mL kg(-1) d(-1)) rates are the lowest reported for an otariid species. These results suggest that subantarctic fur seal pups greatly reduce activity levels to lower energy expenditure in addition to adopting protein-sparing metabolic pathways in order to survive the extreme fasts they must endure on Amsterdam Island.

Molting and limb regeneration are tightly coupled processes, both of which are regulated by ecdysteroid hormone synthesized and secreted by the Y-organs. Regeneration of lost appendages can affect the timing and duration of the proecdysial, or premolt, stage of the molt cycle. Autotomy of all eight walking legs induces precocious molts in various decapod crustacean species. In the land crab Gecarcinus lateralis, autotomy of a partially regenerated limb bud before a critical period during proecdysis (regeneration index <17) delays molting so that a secondary limb bud (2 degrees LB) forms and the animal molts with a complete set of walking legs. It is hypothesized that 2 degrees LBs secrete a factor, termed limb autotomy factor-proecdysis (LAF(pro)), that inhibits molting by suppressing the Y-organs from secreting ecdysone. Molting was induced by autotomy of eight walking legs; autotomy of primary (1 degrees ) LBs reduced the level of ecdysteroid hormone in the hemolymph 73% by one week after limb bud autotomy (LBA). Injection of extracts from 2 degrees LBs, but not 1 degrees LBs, inhibited 1 degrees LB growth in proecdysial animals, thus having the same effect on molting as LBA. The inhibitory activity in 2 degrees LB extracts was stable after boiling in water for 15 min, but was destroyed by boiling 15 min in 0.1 N acetic acid or incubation with proteinase K. These results support the hypothesis that LAF(pro) is a peptide that resembles a molt-inhibiting hormone.

Milk ingested by mammalian offspring, coupled with offspring's utilization of this energetic investment, influences survival and growth. A number of studies have examined milk intake in otariids, but few have examined milk intake over the entire lactation period, and none has independently measured energy expenditure concurrent with milk intake. We concurrently examined milk intake, field metabolic rate (FMR), and body composition of 41 pups over the entire lactation interval in 1995 and 1996 on St. Paul Island, Alaska. One hundred two metabolic measurements were obtained with isotope dilution methods. Mean milk intake did not differ annually but increased with age and mass, ranging from 3,400+/-239 to 6,780+/-449 (+/-SE) mL per suckling bout. Milk energy consumption did not vary with age on a mass-specific basis. No differences were detected in milk volume consumed by male and female pups, either absolutely or on a mass-specific basis. Mass-specific FMR peaked during molting, was lowest postmolt, and did not vary by sex. Pups in 1995 had lower FMR than pups in 1996 and were also fatter. Mean milk energy utilized for maintenance metabolism decreased over time from 77% to 43% in 1995 and remained at 71% in 1996. Pup body mass was negatively correlated with the percentage of total body water and positively correlated with the percentage of total body lipid (TBL). Pups increased the percentage of TBL from 16% to 37%. Northern fur seal pups increased energy intake over lactation, while concurrent changes in body composition and pelage condition resulted in mass-specific metabolic savings after the molt.

The nitrogen requirements for tissue maintenance, moulting, and oogenesis were determined experimentally for the herbivorous land crab Gecarcoidea natalis. The maintenance nitrogen requirements for intermoult animals was very low (4.83+/-1.68 mmol N kg-1 dry body wt d-1), but during oogenesis the total requirement was much higher (8. 6 mmol N kg-1 dry body wt d-1). Gecarcoidea natalis could potentially assimilate enough nitrogen from rain forest leaf litter or leaves of Ficus or Erythrina to satisfy not only the maintenance nitrogen requirements but the observed rate of incorporation of nitrogen into the ovaries during oogenesis. The ovaries developed slowly over a period of 2 mo (mid-July to late September) and had a final nitrogen content of 359+/-15.9 (n=18) mmol kg-1 dry body wt. This was equivalent to 9.3%+/-0.4% of the total body nitrogen. A substantial nitrogen debt was incurred during ecdysis (658+/-126 mmol kg-1 dry body wt). This nitrogen debt could be satisfied slowly, from leaf litter, over a period of 1-3 mo. After ecdysis, the majority of the nitrogen and urate within the animal prior to moulting was retained within the soft crab (85.0%+/-1.2% total nitrogen, 82.0%+/-1.2% nonurate nitrogen and 99.56% urate), while only a minority was lost with the exuviae (18.0%+/-1.2% total nitrogen, 14.7%+/-1.2% nonurate nitrogen, and 0.4%+/-0.4% urate). The urate deposits in G. natalis were not mobilized as a source of nitrogen in animals maintained on a nitrogen-free diet.

Sterols are essential nutrients for all arthropods, including grasshoppers, but metabolic constraints may limit which sterols can support normal growth and development. In the firsts part of this study, a comparative experiment, which included five different species of grasshoppers (Orthoptera: Acrididae) representing three separate taxonomic groups, was performed to determine how widespread sterol metabolic constraints are within the Acrididae. Grasshoppers were reared on artificial diets containing sterols that differed in the position of double bonds within the sterol structure, and various life history traits were measured. Sterols with double bonds at position 7, within the sterol nucleus, and/or at position 22, on the cholestane side chain, failed to support development to the adult stage for any of the five species. In addition, grasshoppers reared on sterols with these configurations often had extended developmental times and reduced growth rates in the first and second stadium compared with grasshoppers reared on sitosterol or cholesterol diets. In the second half of this study, we examined how mixtures of suitable and unsuitable sterols influenced survival, growth, and development. Artificial foods containing mixtures of suitable and unsuitable sterols were fed to the highly polyphagous grasshopper Schistocerca americana. Results suggest that survival and performance of this grasshopper suffer as the concentration of unsuitable sterols increases and as the ratio of suitable to unsuitable sterols in the diet decreases. We review the literature to document variation in plant sterol profiles and propose that constraints on sterol metabolism may contribute to the maintenance of diet mixing in the Acrididae.