Sex-specific variation in brown-headed cowbird immunity following acute stress: a mechanistic approach

There is some discrepancy in the literature regarding whether acute stress is immunostimulatory or immunosuppressive. Studies of domesticated (laboratory and food) animals and humans typically indicate that acute stress is immunostimulatory, whereas studies of non-domesticated species document both immunostimulatory and immunosuppressive results. Few studies have examined the mechanisms responsible for changes in immune activity in species other than those classically used in laboratory research. We examined the effect of both acute stress and exogenous corticosterone (CORT) on the bactericidal capacity (BC) of blood plasma from captive, wild-caught brown-headed cowbirds (Molothrus ater) to determine if CORT is responsible for changes in levels of immune activity. We conducted "stress tests" in which we handled birds to elicit a stress response and then measured the birds' total CORT and BC at 30 or 90 min post-stressor. We also conducted non-invasive tests in which we administered exogenous CORT by injecting it into mealworms that were fed to the cowbirds remotely. Total, free, and bound CORT levels, corticosteroid binding globulins (CBGs), and BC at 7 or 90 min post-mealworm ingestion were measured. Both males and females exhibited significant increases in total CORT following handling stress and the administration of exogenous CORT. Experimental males and females also exhibited a significant increase in CBG capacity at 7 min post-mealworm ingestion compared to controls. Male cowbirds exhibited a significant decline in their BC following both handling stress and the administration of exogenous CORT whereas female cowbirds exhibited no decline under either condition. Female CBG levels were not different than those of males, suggesting that differences in BC could be due to differences between the sexes in the number of corticosteroid receptors which, along with CBGs, regulate the stress response. Female cowbirds may modulate their stress response as an adaptive life-history strategy for maximizing current reproduction.     

Stress responses of testosterone and corticosterone-binding globulin in a multi-brooded species,Eurasian Tree Sparrows (Passer montanus): Does CBG function as a mediator?

In avian plasma, testosterone (T) and corticosterone (CORT) compete to bind with corticosterone-binding globulin (CBG). Elevation of CBG may function to "buffer" the tissues against high circulating levels of T and stress-induced levels of CORT. To demonstrate the effects of acute stress on CBG and T levels and their biological functions, we investigated seasonal changes of baseline and stress-induced T and CBG levels in Eurasian Tree Sparrows (Passer montanus) during different life stages using the capture-handling-restraint stress method. Our results show that (1) male sparrows had significantly higher baseline T levels and CBG capacities during the nest building, the first egg-laying, and the first nestling stages, and significantly decreased stress-induced T levels only during the nest building and the first egg-laying stages. They also expressed significantly increased stress-induced CBG capacities during the second nestling stage. (2) Females showed significantly higher baseline CBG capacities but significantly decreased stress-induced CBG capacities during the nest building stage, and females also showed significantly increased stress-induced CBG capacities during the second egg-laying and the second nestling stages. Therefore, the seasonal fluctuations of baseline CBG in both sexes and baseline T in males reflect their adaptive strategies for optimizing their physiological and behavioral states to the life history cycle. The different patterns of stress-induced CBG in females suggest CBG functions as an essential mediator in regulating stress response to unpredictable perturbations. Our results highlight the need for future studies of stress-induced CBG and T levels on a wide range of vertebrate species that vary in different life history stages to gain a full understanding of the mechanisms that underlie biological functions of CBG and T for unpredictable stressors.     

Effects of capture stress on free-ranging, reproductively active male Weddell seals

Physiological stress responses to capture may be an indicator of welfare challenges induced by animal handling. Simultaneously, blood chemistry changes induced by stress responses may confound experimental design by interacting with the biological parameters being measured. Cortisol elevation is a common indicator of stress responses in mammals and reproductive condition can profoundly influence endocrine response. We measured changes in blood cortisol and testosterone induced by handling reproductively active male Weddell seals (Leptonychotes weddellii) early and late in the breeding season. Weddell seals have the highest resting cortisol levels of all mammals yet showed a clear, prolonged elevation in cortisol in response to capture. Responses were similar when first caught and when caught a second time, later in the breeding season. Baseline testosterone levels declined over the breeding season but were not altered by capture. Administering a light dose of diazepam significantly ameliorated the cortisol response of handled animals without affecting testosterone levels. This may be an effective way of reducing acute capture stress responses. Male breeding success in years males were handled was no different to the years they were not, despite the acute capture response, suggesting no long-term impact of handling on male reproductive output.     

Serum corticosteroid binding globulin expression is modulated by fasting in polar bears (Ursus maritimus)

Polar bears (Ursus maritimus) from several subpopulations undergo extended fasting during the ice-free season. However, the animals appear to conserve protein despite the prolonged fasting, though the mechanisms involved are poorly understood. We hypothesized that elevated concentrations of corticosteroid binding globulin (CBG), the primary cortisol binding protein in circulation, lead to cortisol resistance and provide a mechanism for protein conservation during extended fasting. The metabolic state (feeding vs. fasting) of 16 field sampled male polar bears was determined based on their serum urea to creatinine ratio (>25 for feeding vs. <5 for fasting). There were no significant differences in serum cortisol levels between all male and female polar bears sampled. Serum CBG expression was greater in lactating females relative to non-lactating females and males. CBG expression was significantly higher in fasting males when compared to non-fasting males. This leads us to suggest that CBG expression may serve as a mechanism to conserve protein during extended fasting in polar bears by reducing systemic free cortisol concentrations. This was further supported by a lower serum glucose concentration in the fasting bears. As well, a lack of an enhanced adrenocortical response to acute capture stress supports our hypothesis that chronic hunger is not a stressor in this species. Overall, our results suggest that elevated serum CBG expression may be an important adaptation to spare proteins by limiting cortisol bioavailability during extended fasting in polar bears.     

Plasma corticosterone concentrations in males of the skink Egernia whitii during acute and chronic confinement, and over a diel period

This study examined adrenal and gonadal responses to capture and to longer term captivity in males of the scincid lizard, Egernia whitii. Animals subjected to acute capture stress in the field exhibited a rapid rise in plasma corticosterone concentrations that had not attenuated by 4 h after capture; plasma testosterone concentrations decreased significantly over this period. Animals returned to captive conditions in the laboratory showed little change in plasma corticosterone concentrations by 1 week, but concentrations were significantly lower at 4 weeks. Plasma testosterone did not vary significantly during this period. Sampling over a diel period indicated that plasma corticosterone concentrations vary little over the daylight hours; however, there was a significant decrease between the samples taken at 17:00 h and 21:00 h. These results suggest that, as in other species, the acute adrenal response to capture stress may confound assessment of other physiological parameters, although if experiments are carried out during daylight hours, time of sampling should have little influence on plasma corticosterone concentrations. The results also suggest that males of E. whitii require at least a week to adapt to captivity, although further studies investigating different captive conditions are warranted for this social species.     

Corticosterone binding globulin regulation and thymus changes after thermal injury in mice

Thermal injury is extremely stressful, and data characterizing the systemic endocrine stress response to this injury are sparse. The objective of this study was to measure the effects of thermal injury on mice on corticosterone (Cort) levels in relation with corticosteroid-binding globulin (CBG) and thymus cell populations. The endocrine stress response was determined by measuring total Cort, free Cort, CBG binding capacity, liver CBG mRNA, and circulating CBG levels at 1, 2, 5, and 10 days postburn. Thymus cell populations were also analyzed. After thermal injury, a rapid increase of total Cort was observed in the first 48 h. This was associated with a decrease of hepatic CBG mRNA, protein levels, and binding capacity. Percentage of free Cort in the burn group peaked at day 2 postburn with a dramatic (+500%) increase. This correlated with a significant decrease of thymus cellularity (50% less). Phenotypic analyses showed that corticosensitive cells were significantly altered. After treatment (5 days), both endocrine and immune parameters returned to control levels. Our results demonstrate that, after a thermal injury, CBG is mainly responsible for Cort's action on corticosensitive immune cells.     

Modulation of the Adrenocortical Responses to Acute Stress in Arctic Birds: A Possible Ecological Basis

There is considerable evidence that the increases in circulatingcorticosterone levels following acute stress, such as suddenstorms, can trigger facultative behavioral patterns designedto maximize survival. During the breeding season, adrenocorticalresponses to aseasonal storms may trigger facultative behavioralpatterns resulting in temporary disruption of nesting. A renestingcycle often follows when conditions become favorable again.However, in arctic ecosystems the brief breeding season limitsthe capacity of most avian populations to renest, and yet springweather may be extreme. This led to the hypothesis that arcticbirds may down-regulate their sensitivity to acute stress (suchas severe storms) so that breeding can begin and be completedbefore the first storms of autumn (only 6–8 weeks later).To test this we have used the "stress-series protocol" thattakes advantage of the fact that capture, handling and restraintconstitutes a more-or-less equal stress among all vertebratespecies, and that corticosterone concentrations in small bloodsamples collected during the first hour post-capture indicatesensitivity of the hypothalamo-adenohypophysial- adrenal axisto acute stresses in general. Comparisons of the increases inplasma levels of corticosterone following capture in severaltaxa of arctic birds indicated that suppression of the adrenocorticalresponse to acute stress was not ubiquitous. Although some speciesdid show low amplitude responses of the circulating corticosteroneincrease during the stress series protocol, others did not,and some (especially males) showed an increase in sensitivityto acute stress. Additional hypotheses were suggested as follows:1) species with greater body mass have larger relative energyreserves and would be more able to resist acute stresses thansmaller energy reserves and would be more able to resist acutestresses than smaller species; 2) short-lived birds with anexpectancy of one or two breeding seasons should be more resistantto acute stress than long-lived birds that may have many attemptsat successful breeding; 3) resistance of the adrenocorticalresponse to stress is a function of the degree of parental careprovided by the individual (also takes into account sex differencesin parental investment). Correlations of the maximum corticosteronelevel and the ratio of maximum to minimum corticosterone levelsgenerated during the stress series protocol with body mass andlongevity were not significant. However, maximum corticosteronelevel was significantly lower in birds providing most parentalcare and almost significant for the ratio of maximum to minimumcorticosterone levels. These comparative data from free-livingarctic birds suggest an ecological basis for modulation of theadrenocortical responses to stress. In at least one species,up-regulation of the response appears to involve a change insensitivity to glucocorticosteroid feedback. Further investigationswill explore neuroendocrine mechanisms further in the lightof these ecological bases.     

Assessment of acid-base derangements among bonnethead (Sphyrna tiburo), bull (Carcharhinus leucas), and lemon (Negaprion brevirostris) sharks from gillnet and longline capture and handling methods

Blood gasses of wild bonnethead, bull, and lemon sharks were measured with the i-STAT clinical analyzer with the CG4+ cartridge immediately after capture; and again immediately prior to release after tagging, handling and morphometric measurements were taken. Relative reference ranges of post-capture status were established. Among species, stress response to capture was similar for all parameters; however, pH declined and lactate concentrations rose over time, indicating continued insult from capture and/or response to additional handling stress. pCO(2) rose faster for S. tiburo than for C. leucas, and lactate concentrations rose faster for S. tiburo than for N. brevirostris. All species caught in gillnets experienced lower pH and higher lactate concentrations than on longlines. Discriminant analysis justified the use of blood gas analysis to assess physiological stress induced by different capture methods. From these results, we recommend 1) that gear be monitored closely and sharks be removed immediately, or suboptimally, that gear is deployed for the shortest soak time possible; 2) longline over gillnet gear; and 3) extra caution with sensitive species (e.g., S. tiburo), which may include the administration of blood buffers and other therapeutics if a shark is beyond the limits of relative reference ranges reported here.     

Maternal buffering of stress response in free‐ranging Pacific harbor seal pups in Alaska

We examined the effects of maternal buffering in free‐ranging Pacific harbor seal pups during capture and handling research procedures. We predicted that pups held with their mother would benefit from social buffering and exhibit lower cortisol concentrations resulting from capture and handling than dependent pups caught without their mothers and weaned pups. We expected that pups captured with their mother that experienced a short separation would exhibit increased stress‐induced vocal behavior and activity level compared to dependent weaned pups caught alone. The results showed that the presence of the mother significantly buffered the stress response, as measured by reduced serum cortisol concentrations, in pups captured with their mothers as compared to dependent pups captured alone. Cortisol concentrations of mothers with pups initially were higher than nonlactating females, then diminished. Pups showed a significantly higher rate of vocalization soon after maternal separation compared to single pups separated for a longer period of time. Newly separated pups, especially males, showed a high level of activity compared to the other pups. The results provide unique quantitative evidence of the physiology underlying the maternal‐pup bond in a marine mammal, and the role that maternal buffering may play on the stress response of the offspring.     

Constitutive immune function responds more slowly to handling stress than corticosterone in a shorebird

Ecological immunologists are interested in how immune function changes during different seasons and under different environmental conditions. However, an obstacle to answering such questions is discerning the effects of biological factors of interest and investigation artifacts such as handling stress. Here we examined handling stress and its effects on constitutive (noninduced) immune function via two protocols on captive red knots (Calidris canutus). We investigated how constitutive immunity responds to handling stress, how quickly these changes take place, and the practical implications for researchers interested in sampling baseline immune levels. We found that Staphylococcus aureus and Candida albicans killing increased with handling stress while total leukocyte and lymphocyte concentrations decreased. However, although corticosterone increased significantly and rapidly in response to handling stress, none of the 10 measures of constitutive immunity that we tested differed significantly from baseline within 20 or 30 min of capture. Thus, researchers interested in baseline immune function should sample animals as soon as possible after capture, but studies in species not easily sampled in less than 3 min (such as red knots) could still yield useful results.     

The gene expression response of the catadromous perciform barramundi Lates calcarifer to an acute heat stress

The acute heat-shock response of the tropical estuarine fish species barramundi Lates calcarifer as indicated by the expression of genes within stress (hsp 90AA, hsp 90AB, hsp 70 and hsc 70), metabolic (cisy, cco II and ldh) and growth (igf1 and mstn 1) related pathways was examined following an increase in water temperature from 28 to 36° C over 30 min. Lates calcarifer were maintained at the acute stress temperature of 36° C for 1 h before being returned to 28° C and allowed to recover at this temperature for a further 2 weeks. Muscle tissue sampling over the experimental period allowed for the expression quantification of stress, metabolic and growth-related genes via quantitative real-time polymerase chain reaction (qrt-PCR) where a robust and reliable normalization approach identified both α-tub and Rpl8 as appropriate genes for the analysis of gene expression in response to an acute heat stress. hsp90AA and hsp70 of the inducible heat-shock response pathway showed a massive up-regulation of gene expression in response to heat stress, whilst the constitutive heat-shock genes hsp90AB and hsp70 showed no change over the course of the experiment and a small increase after 2 weeks of recovery, respectively. Of the three genes representing the metabolic pathway (cisy, cco II and ldh) only cco II changed significantly showing a decrease in gene expression, which may suggest a small suppression of aerobic metabolism. igf1 of the growth pathway showed no significant differences in response to an acute heat stress, whilst mstn1 increased at the beginning of the heat stress but returned to basal levels soon after. Overall, the results demonstrate that an acute heat stress in L. calcarifer caused a significant increase in the expression of genes from the stress response pathway and a possible decrease in aerobic metabolism with only relatively minor changes to the growth pathway highlighting the hardy nature of L. calcarifer and its resilience in coping with sudden temperature changes routinely encountered within its natural environment.     

Does domestication process affect stress response in juvenile Eurasian perch Perca fluviatilis?

The objective was to evaluate the impact of domestication process on the physiological stress response of cultured Eurasian perch confronted to a chronic stress situation. Briefly, F1 and F4 juveniles were submitted to chronic confinement and investigated on days 5, 15 and 30. Capture and 15min-anesthesia were imposed on fish to assess the effect of preceding confinement on acute stress response. On day 30, the fish were finally challenged with Aeromonas hydrophila and sampled after 5 and 10 days for immune parameter measurements. Cortisol and glucose levels were not affected by confinement but increased significantly after acute stressor exposure. Moreover, cortisol rise following capture and anesthesia was higher in F1 confined-fish, suggesting that they have previously been affected by chronic confinement. A higher HSP70 level was also observed on day 30 in F1 confined-juveniles. During bacterial challenge, regardless of confinement level, F4 juveniles displayed higher lysozyme activity and agglutination response than F1 which may indicate a higher immune capacity in domesticated fish. In conclusion, chronic confinement stressor induced few physiological responses but may increase the responsiveness to other aquacultural stressors. Domestication process also seems to improve chronic stress resistance, growth as well as the immune status of the fish.     

Interplay between plasma hormone profiles, sex and body condition in immature hawksbill turtles (Eretmochelys imbricata) subjected to a capture stress protocol

We investigated plasma hormone profiles of corticosterone and testosterone in immature hawksbill turtles (Eretmochelys imbricata) in response to a capture stress protocol. Further, we examined whether sex and body condition were covariates associated with variation in the adrenocortical response of immature turtles. Hawksbill turtles responded to the capture stress protocol by significantly increasing plasma levels of corticosterone over a 5 h period. There was no significant sex difference in the corticosterone stress response of immature turtles. Plasma testosterone profiles, while significantly different between the sexes, did not exhibit a significant change during the 5 h capture stress protocol. An index of body condition was not significantly associated with a turtle's capacity to produce plasma corticosterone both prior to and during exposure to the capture stress protocol. In summary, while immature hawksbill turtles exhibited an adrenocortical response to a capture stress protocol, neither their sex nor body condition was responsible for variation in endocrine responses. This lack of interaction between the adrenocortical response and these internal factors suggests that the inactive reproductive- and the current energetic- status of these immature turtles are important factors that could influence plasma hormone profiles during stress.     

Physiological response to capture stress in endemic Southern African catsharks (family Scyliorhinidae)

Fishing is the major threat to marine fish populations, particularly to higher trophic-level predators such as sharks. Many sharks, and other fish, are caught as commercial by-catch or for recreational purposes and then released; therefore, it is important to understand the effects of capture stress on their physiology and subsequent survival. Nonetheless, although important data have been collected for some sharks, there can be substantial interspecific differences, and the consequences of capture stress are still poorly understood for most species. In this study, the authors quantified the physiological effect of capture on four catshark species endemic to Southern Africa, which are regularly discarded as by-catch and targeted by recreational fisheries. Fifteen pyjama sharks, nine leopard sharks and nine shysharks were captured, and a blood sample was collected to measure their physiological response to capture stress. Stressed blood biochemistry was compared to samples obtained after the sharks recovered for 24 h in an underwater pen. Levels of pH and K⁺ were significantly lower, and lactate levels were significantly higher, in sharks immediately after capture stress compared to after the 24 h recovery period. Although the species showed a similar response to capture stress, they differed significantly in pH, K⁺ and lactate levels, and there was some evidence of size affecting the strength of the response to capture stress. The substantial physiological response elicited by even the relatively quick capture event in this study suggests that common fishing practices will have a stronger impact on catshark homeostasis because of longer hooking times and more disruptive fishing gear. Although the relationship between survival and physiological changes elicited by capture needs further investigation, the results provide further evidence that minimizing stress would be beneficial to maximize the survival of sharks and other fish following capture-and-release fishing practices.     

Modulation of corticosterone availability to white adipose tissue of lean and obese Zucker rats by corticosteroid-binding globulin.

Corticosterone-binding (CB) capacity was determined in periovarian and subcutaneous white adipose tissue (WAT), as well as in plasma of lean and obese Zucker rats. In lean rats, plasma CB was twice the level of obese rats. In lean rat WAT, dexamethasone binding accounted for only 0.05-0.09% of corticosterone binding, and aldosterone bound even less; in the obese rats, dexamethasone accounted for 0.2 - 0.3 % of corticosterone binding. Scatchard plots showed that KD for corticosterone was 3.1 nM (WAT) or 3.4 nM (plasma) in lean rats and 1.8 nM (WAT) or 1.5 nM (plasma) in obese rats. The total CB capacity in WAT was lower in the obese than in lean rats (47-50%). Plasma non-esterified fatty acid levels were higher in obese rats. The results suggest that CBG may limit the access of glucocorticoids to adipocytes more weakly in obese rats because of the lower CBG. Fatty acids may increase the affinity of CBG for corticosterone, which would make WAT cells less accessible to circulating glucocorticoids. The modulation of CBG by fatty acids may protect fat reserves by decreasing the sensitivity of WAT to glucocorticoids.     

Seasonal and Acute Changes in Adrenocortical Responsiveness in an Arctic-Breeding Bird

The Lapland longspur,Calcarius lapponicus,times its breeding season so that chicks hatch coincident with the brief period of food abundance in the high arctic. This synchronization requires that all reproductive activities occur in over a much shorter period than at lower latitudes. Because of the known influence of stress hormones on delaying breeding in temperate-zone birds and the detrimental effects of such delays in the arctic, we expected the performance of the hypothalamic–pituitary–adrenal (HPA) axis of arctic-breeding birds to show less sensitivity to environmental stress than their mid-latitude counterparts. We found that adrenocortical responsiveness to the standardized stress of capture and handling, measured by taking five serial blood samples for corticosterone during the course of a 1-hr period, was similar to many temperate passerines and was also similar both between male and female longspurs and between the migratory and reproductive phases. However, the profile of plasma corticosterone during capture stress was significantly damped in longspurs sampled as they began their postnuptial molt. In addition, we had the opportunity to examine endocrine responses to a natural environmental stress in 1989 during a 3-day snowstorm which concealed available food resources. During this storm longspurs formed progressively larger flocks each day, with females abandoning incubation duties by the third day. Birds captured during the storm showed highly significant increases in both the rate of plasma corticosterone increase during capture and the peak postcapture level compared with birds sampled before the storm. This increased adrenal potential suggests increased activity of the HPA axis in response to severe conditions and is reminiscent of the response to fasting. Although the storm occurred during incubation, and reproductive hormone levels had begun to decline, we measured significant reductions in luteinizing hormone in both males and a subset of females captured during the storm.     

Trapping initiates stress response in breeding and non-breeding house sparrows Passer domesticus: implications for using unmonitored traps in field studies

In response to a variety of unpredictable conditions, birds secrete the steroid hormone corticosterone, which has numerous effects on physiology and behavior. A standardized protocol of handling and restraint has been demonstrated to elicit a robust corticosterone response in many species of birds. In contrast, comparatively little is known about the effects of capture technique on corticosterone secretion in wild birds. Setting up multiple live traps checked at regular intervals allows field researchers to capture many birds in a short period of time. However, one potential drawback of this technique is that birds may spend unknown lengths of time in traps prior to sampling. Many birds appear to remain calm and/or feed during this period, potentially leading researchers to assume that corticosterone secretion is unaffected by trapping. We assessed the impact of being left in traps for up to 30 minutes on baseline corticosterone and subsequent corticosterone responses to restraint in non‐breeding and breeding house sparrows Passer domesticus. Traps were baited with seed, and birds were either removed immediately after entry (controls), or left in the trap undisturbed for 15 or 30 min. Upon removal, birds were subjected to a standardized handling/restraint protocol in which blood samples were collected within 3 min, and again at regular intervals for 60 min. Analysis of blood samples revealed that both non‐breeding and breeding sparrows that were held in the traps had significantly higher baseline corticosterone than controls, and showed no further increase in corticosterone secretion in response to handling. However, corticosterone responses to trapping differed seasonally. Our study indicates that although birds did not exhibit prolonged escape behavior while trapped, entry into a walk‐in trap initiated a robust stress response. Taken together with data from a previous study, our data suggest that ornithologists should consider species‐specific and stage‐specific effects of trapping on physiology when designing field studies.     

Older parents are less responsive to a stressor in a long-lived seabird: a mechanism for increased reproductive performance with age?

In many taxa, reproductive performance increases throughout the lifespan and this may occur in part because older adults invest more in reproduction. The mechanisms that facilitate an increase in reproductive performance with age, however, are poorly understood. In response to stressors, vertebrates release glucocorticoids, which enhance survival but concurrently shift investment away from reproduction. Consequently, when the value of current reproduction is high relative to the value of future reproduction and survival, as it is in older adults, life history theory predicts that the stress response should be suppressed. In this study, we tested the hypothesis that older parents would respond less strongly to a stressor in a natural, breeding population of common terns (Sterna hirundo). Common terns are long-lived seabirds and reproductive performance is known to increase throughout the lifespan of this species. As predicted, the maximum level of glucocorticoids released in response to handling stress decreased significantly with age. We suggest that suppression of the stress response may be an important physiological mechanism that facilitates an increase in reproductive performance with age.     

Consumer functional response and competition in consumer-resource systems

This study investigates the effect of the functional response of resource consumers on the relationship between resource overlap and competition for some two-consumer, two-resource models. Two measures of competition are examined: α, the competition coefficient, and β, an index of the ease of invasion by the second consumer species when the first is at its carrying capacity. A comparison of systems with linear (type-1) and decelerating (type-2) functional responses shows that: (1) Competition coefficients are functions of the population densities of consumers or resources in systems with type-2 responses. (2) Competition coefficients may differ substantially in magnitude between systems with type-1 and type-2 functional responses. (3) The relative handling time of different resources is important in determining the relationship between overlap and competition. Positive correlations between capture rates (per unit resource) and handling times cause the system with type-2 functional responses to exhibit a higher level of competition for a given level of overlap than for the case of negative correlation. (4) If the functional response is type-2 it may be possible to obtain a priority effect in which either consumer species can exclude the other. (5) Invasion may be easier in a system with type-1 functional responses than in a similar system with type-2 functional responses, even when competition coefficients are larger in the former. Accelerating functional responses also affect the relationship between overlap and competition, but realistic models of such responses are likely to be very complex. Several currently accepted ideas in competition theory depend upon the assumption of a linear functional response, and are unlikely to be generally valid.