Cost of reproduction in a long-lived bird: incubation effort reduces immune function and future reproduction

Life-history theory predicts that increased current reproductive effort should lead to a fitness cost. This cost of reproduction may be observed as reduced survival or future reproduction, and may be caused by temporal suppression of immune function in stressed or hard-working individuals. In birds, consideration of the costs of incubating eggs has largely been neglected in favour of the costs of brood rearing. We manipulated incubation demand in two breeding seasons (2000 and 2001) in female common eiders (Somateria mollissima) by creating clutches of three and six eggs (natural range 3-6 eggs). The common eider is a long-lived sea-duck where females do not eat during the incubation period. Mass loss increased and immune function (lymphocyte levels and specific antibody response to the non-pathogenic antigens diphtheria and tetanus toxoid) was reduced in females incubating large clutches. The increased incubation effort among females assigned to large incubation demand did not lead to adverse effects on current reproduction or return rate in the next breeding season. However, large incubation demand resulted in long-term fitness costs through reduced fecundity the year after manipulation. Our data show that in eiders, a long-lived species, the cost of high incubation demand is paid in the currency of reduced future fecundity, possibly mediated by reduced immune function.     

Does the cost of incubation set limits to clutch size in common eiders Somateria mollissima?

We examined the effect of natural clutch size on the cost of incubation in a population of common eiders Somateria mollissima nesting in Tromsø, northern Norway. The body condition of females at day 5 in the incubation period was not related to clutch size (3–6 eggs), but females incubating large clutches lost more mass and had a lower body condition at day 20 in the incubation period than females incubating small clutches. Females incubating large clutches had a slightly shorter incubation period and a lower egg predation rate. The results do not support the hypothesis that the female's ability to produce eggs is the only ultimate control of clutch size in eider. Instead the results suggest that there may be an interaction between the allocation of body reserves to eggs and incubation, and that females producing large clutches allocate more of their body reserves to incubation than females producing small clutches, in order to shorten the incubation period and to minimise the risk of predation on eggs.     

Sex influences immune responses to viruses,and efficacy of prophylaxis and treatments for viral diseases

The intensity and prevalence of viral infections are typically higher in males, whereas disease outcome can be worse for females. Females mount higher innate and adaptive immune responses than males, which can result in faster clearance of viruses, but also contributes to increased development of immunopathology. In response to viral vaccines, females mount higher antibody responses and experience more adverse reactions than males. The efficacy of antiviral drugs at reducing viral load differs between the sexes, and the adverse reactions to antiviral drugs are typically greater in females than males. Several variables should be considered when evaluating male/female differences in responses to viral infection and treatment: these include hormones, genes, and gender‐specific factors related to access to, and compliance with, treatment. Knowledge that the sexes differ in their responses to viruses and to treatments for viral diseases should influence the recommended course of action differently for males and females. Editor's suggested further reading in BioEssays X‐chromosome‐located microRNAs in immunity: Might they explain male/female differences Abstract     

Female house wrens (Troglodytes aedon) increase the size, but not immunocompetence, of their offspring through extra-pair mating

House wrens are typically socially monogamous, but frequently engage in extra-pair matings leading to multisired broods. Because females do not appear to acquire direct material benefits from their extra-pair mates, we tested the hypothesis that female house wrens derive indirect genetic benefits, such as enhanced immunocompetence (cutaneous immune activity, humoral immunity, and plasma bactericidal activity) and condition (size and haematoserological traits) for their offspring, by mating polyandrously. We predicted that extra-pair young (EPY) should show greater immune responsiveness and better body condition than their within-pair maternal half-siblings (WPY). Contrary to our prediction, WPY had higher cutaneous immune activity than their EPY brood-mates in two of three years, and EPY and WPY did not differ in measures of innate and humoral immunity. WPY also had higher albumin to γ-globulin ratios than EPY; however, they were not in better condition based on other measures. EPY had consistently longer tarsi (a measure of long-bone size) than their WPY half-siblings, suggesting that females engage in extra-pair copulations with larger males. The benefits of large structural size in the study population is unknown, but based on evidence from other passerines, we suggest that structural size may be an important fitness-related trait in house wrens. We conclude that our results are not consistent with the hypothesis that females gain immune-related benefits for their offspring by engaging in extra-pair matings. Further study of the fitness consequences of differences in tarsus length is needed to determine whether females acquire size-related benefits for their offspring from extra-pair mates.     

Phytohemagglutinin response and immunoglobulin index decrease during incubation fasting in female common eiders

To maximize their fitness, long-lived species face trade-offs between survival and reproduction. The cost of reproduction, which is defined as the negative effect of current parental investment on chances of adult survival and future reproduction, may affect immune function, possibly through hormonal changes. In this study, components of acquired immunity and plasma corticosterone levels of female eiders (Somateria mollissima) have been measured throughout the incubation period as a function of clutch size. These precocial birds lay up to six eggs and fast completely during incubation. Birds were sampled early and late in the incubation period, with clutches ranging from one to four eggs. T-cell-mediated immune response and humoral immunity were assessed by phytohemagglutinin (PHA) skin tests (a challenging method) and measurements of serum immunoglobulins (a monitoring method), respectively. During incubation, responses to PHA injection and immunoglobulin index significantly decreased, by about 40% and 25%, respectively. These observed decreases occurred independently of the number of eggs laid by the females. Corticosterone did not vary significantly during incubation, whatever the clutch size. We conclude that female eiders seem to reallocate their resources from immune function to reproductive effort independently of clutch size or corticosterone levels.     

Reproductive investment is connected to innate immunity in a long-lived animal

Life-history theory predicts that organisms optimize their resource allocation strategy to maximize lifetime reproductive success. Individuals can flexibly reallocate resources depending on their life-history stage, and environmental and physiological factors, which lead to variable life-history strategies even within species. Physiological trade-offs between immunity and reproduction are particularly relevant for long-lived species that need to balance current reproduction against future survival and reproduction, but their underlying mechanisms are poorly understood. A major unresolved issue is whether the first-line innate immune function is suppressed by reproductive investment. In this paper, we tested if reproductive investment is associated with the suppression of innate immunity, and how this potential trade-off is resolved depending on physiological state and residual reproductive value. We used long-lived capital-breeding female eiders (Somateria mollissima) as a model. We showed that the innate immune response, measured by plasma bacteria-killing capacity (BKC), was negatively associated with increasing reproductive investment, i.e., with increasing clutch size and advancing incubation stage. Females in a better physiological state, as indexed by low heterophil-to-lymphocyte (H/L) ratios, showed higher BKC during early incubation, but this capacity decreased as incubation progressed, whereas females in poorer state showed low BKC capacity throughout incubation. Although plasma BKC generally declined with increasing H/L ratios, this decrease was most pronounced in young females. Our results demonstrate that reproductive investment can suppress constitutive first-line immune defence in a long-lived bird, but the degree of immunosuppression depends on physiological state and age.     

Female great tits Parus major do not increase their daily energy expenditure when incubating enlarged clutches

Several studies have shown that enlargement of clutches during incubation reduces the long-term survival of parents. In line with these findings, studies on the energetic costs of nocturnal incubation show an increase in energy expenditure with clutch enlargement. Studies on daily energy expenditure during incubation (DEE_inc), however, do not consistently show such a negative effect of clutch enlargement. To determine whether differential survival results from a direct increase in energy costs or rather from costs associated with compensatory behaviour, we studied the DEE_inc (kJ day⁻¹), change in body mass and nest attendance behaviour of free-living female great tits Parus major that incubated either control or experimentally enlarged clutches. DEE_inc did not differ between the two treatment groups, but was negatively related to mean ambient temperature over the 24-h measuring period, and to the fraction of daytime females spent on the nest. Controlling for these two factors, females incubating enlarged clutches did not spend more energy per 24 h period than females incubating control clutches. Clutch enlargement also did not affect body mass of incubating females, or their nest attendance behaviour. Yet, in the enlarged group body mass change and nest attentiveness were negatively correlated, suggesting that females responded differently to the experimental treatment and thereby preventing us from finding an effect of clutch enlargement.     

Dressed to impress: breeding plumage as a reliable signal of innate immunity

Animal signals involved in sexual selection are often indicators of individual quality. The assumption that sexual characters such as breeding plumage may indicate immune state has rarely been tested in free‐living migratory birds, particularly in relation to innate immunity. If sexual characters indeed reflect immune condition, then these could be used to evaluate individual quality. Melanin is a common pigment used in animal communication that mitigates the effects of oxidative stress and has positive effects on energy homeostasis, important functions during the strenuous activity of long‐distance flights. However, melanin is also immunosuppressive, and the melanised patches of breeding plumage may to some extent compromise immune responsiveness. We studied melanin‐based secondary sexual characters (SSC) in a long‐distance migratory wader, the black‐tailed godwit Limosa limosa, and found that breeding plumage features of male and female godwits are linked to components of innate immunity. Males with a larger colour extension had higher circulating levels of haptoglobin and hemolysis activity, while they also presented a lower body condition; whereas females presented a negative trend between colour and bar extension and hemolysis activity, and a positive trend for natural antibodies. The association between signal, immune state and physical condition in males suggests a cost for signal production and immune condition during prenuptial migration. Sex differences in how signals relate with immune capacity are a likely consequence of sex‐specific signalling roles and energy demands. Our results indicate that male godwit breeding plumage reflects innate immunity state, and is therefore a likely signal for females to use during mate choice as an honest indicator of male's capacity to allocate energy/resources to both expensive traits during periods of energetic constraint.     

Immune indexes of larks from desert and temperate regions show weak associations with life history but stronger links to environmental variation in microbial abundance

Abstract Immune defense may vary as a result of trade-offs with other life-history traits or in parallel with variation in antigen levels in the environment. We studied lark species (Alaudidae) in the Arabian Desert and temperate Netherlands to test opposing predictions from these two hypotheses. Based on their slower pace of life, the trade-off hypothesis predicts relatively stronger immune defenses in desert larks compared with temperate larks. However, as predicted by the antigen exposure hypothesis, reduced microbial abundances in deserts should result in desert-living larks having relatively weaker immune defenses. We quantified host-independent and host-dependent microbial abundances of culturable microbes in ambient air and from the surfaces of birds. We measured components of immunity by quantifying concentrations of the acute-phase protein haptoglobin, natural antibody-mediated agglutination titers, complement-mediated lysis titers, and the microbicidal ability of whole blood. Desert-living larks were exposed to significantly lower concentrations of airborne microbes than temperate larks, and densities of some bird-associated microbes were also lower in desert species. Haptoglobin concentrations and lysis titers were also significantly lower in desert-living larks, but other immune indexes did not differ. Thus, contrary to the trade-off hypothesis, we found little evidence that a slow pace of life predicted increased immunological investment. In contrast, and in support of the antigen exposure hypothesis, associations between microbial exposure and some immune indexes were apparent. Measures of antigen exposure, including assessment of host-independent and host-dependent microbial assemblages, can provide novel insights into the mechanisms underlying immunological variation.     

Sexually dimorphic immune response in the harem polygynous Wellington tree weta Hemideina crassidens

Abstract Adult males are often less immunocompetent than females. One explanation for this is that intense sexual selection causes males to trade‐off investment in immunity with traits that increase mating success. This hypothesis is tested in the Wellington tree weta (Hemideina crassidens), a large, sexually dimorphic orthopteran insect in which males possess enormous mandibular weaponry used during fights for access to female mates. Field‐collected males have a significantly greater immune response (greater melanotic encapsulation) than females, suggesting that body condition, longevity or an allied trait is important to male fitness, or that females require materials for egg production that would otherwise be used to boost immunity. Although immunity is expected to trade‐off against reproductive traits in both sexes, there is no significant relationship between immune response and weapon or testes size in males, nor fecundity in females.     

Sex-Specific Effect of Juvenile Diet on Adult Disease Resistance in a Field Cricket

Food limitation is expected to reduce an individual’s body condition (body mass scaled to body size) and cause a trade-off between growth and other fitness-related traits, such as immunity. We tested the condition-dependence of growth and disease resistance in male and female Gryllus texensis field crickets by manipulating diet quality via nutrient content for their entire life and then subjecting individuals to a host resistance test using the live bacterium Serratia marcescens. As predicted, crickets on a high-quality diet eclosed more quickly, and at a larger body size and mass. Crickets on a high-quality diet were not in better condition at the time of eclosion, but they were in better condition 7–11 days after eclosion, with females also being in better condition than males. Despite being in better condition, however, females provided with a high-quality diet had significantly poorer disease resistance than females on a low-quality diet and in poor condition. Similarly, males on low- and high-quality diets did not differ in their disease resistance, despite differing in their body condition. A sex difference in disease resistance under diet-restriction suggests that females might allocate resources toward immunity during development if they expect harsh environmental conditions as an adult or it might suggest that females allocate resources toward other life history activities (i.e. reproduction) when food availability increases. We do not know what immune effectors were altered under diet-restriction to increase disease resistance, but our findings suggest that increased immune function might provide an explanation for the sexually-dimorphic increase in longevity generally observed in diet-restricted animals.     

Effects of egg size and composition on the size, quality and survival of lapwing Vanellus vanellus chicks

The influence of egg size and composition on the size, quality and survival of lapwing chicks was examined on two farmland study sites in the Midland Valley of Scotland. Eggs comprised 33.1% yolk, 61.3% albumen and 5.6% shell. Whereas the yolk and shell proportions decreased with increasing egg size, the albumen proportion increased. Most variation in egg size was attributable to differences between females but was also influenced by clutch number (eggs in replacement clutches on the rough grazing, but not the arable, site were smaller), clutch size (eggs were smaller in smaller clutches), maternal body condition (females in good condition produced larger eggs) and habitat (since females on the arable site fed more successfully, they were in better condition and laid larger eggs). Chick size, weight and survival were all influenced by egg size. The incubation period varied between 21 and 28 days (mean = 25.2) and was shorter in clutches laid later in the season.     

Do leucocytes reflect condition in nestling burrowing parrots Cyanoliseus patagonus in the wild?

The different leucocyte types are an important part of the immune system. Thus, they have been used in ecological studies to assess immune function and physiological stress in wild birds. It is generally assumed that increased stress and decreased condition are associated with an increase in the ratio of heterophils to lymphocytes, the H/L ratio. We studied leucocyte profiles in relation to body condition in nestling Burrowing Parrots (Cyanoliseus patagonus) in North-eastern Patagonia, Argentina. As in other wild parrots, heterophils were the most numerous leucocyte type, suggesting strong investment into innate immunity. Leucocyte profiles did not change with the age, while nestlings in better body condition increased the number of heterophils. Because the number of lymphocytes was independent of body condition, as a result we observed a positive correlation between body condition and the H/L ratio. The total number of leucocytes relative to erythrocytes increased in nestlings in better body condition, indicating a larger overall investment into immune function in well-nourished nestlings. The observed heterophilic profiles of nestling Burrowing Parrots together with the positive relationship between H/L ratio and body condition may indicate a favoured investment in a robust innate immunity that reduces the risk of infection taking hold in these long-lived birds.     

Personality and innate immune defenses in a wild bird: Evidence for the pace-of-life hypothesis

We tested the two main evolutionary hypotheses for an association between immunity and personality. The risk-of-parasitism hypothesis predicts that more proactive (bold, exploratory, risk-taking) individuals have more vigorous immune defenses because of increased risk of parasite exposure. In contrast, the pace-of-life hypothesis argues that proactive behavioral styles are associated with shorter lifespans and reduced investment in immune function. Mechanistically, associations between immunity and personality can arise because personality differences are often associated with differences in condition and stress responsiveness, both of which are intricately linked with immunity. Here we investigate the association between personality (measured as proactive exploration of a novel environment) and three indices of innate immune function (the non-specific first line of defense against parasites) in wild superb fairy-wrens Malurus cyaneus. We also quantified body condition, hemoparasites (none detected), chronic stress (heterophil:lymphocyte ratio) and circulating corticosterone levels at the end of the behavioral test (CORT, in a subset of birds). We found that fast explorers had lower titers of natural antibodies. This result is consistent with the pace-of-life hypothesis, and with the previously documented higher mortality of fast explorers in this species. There was no interactive effect of exploration score and duration in captivity on immune indices. This suggests that personality-related differences in stress responsiveness did not underlie differences in immunity, even though behavioral style did modulate the effect of captivity on CORT. Taken together these results suggest reduced constitutive investment in innate immune function in more proactive individuals.     

Cross‐fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring

Across animal taxa, reproductive success is generally more variable and more strongly dependent upon body condition for males than for females; in such cases, parents able to produce offspring in above‐average condition are predicted to produce sons, whereas parents unable to produce offspring in good condition should produce daughters. We tested this hypothesis in the collared flycatcher (Ficedula albicollis) by cross‐fostering eggs among nests and using the condition of foster young that parents raised to fledging as a functional measure of their ability to produce fit offspring. As predicted, females raising heavier‐than‐average foster fledglings with their social mate initially produced male‐biased primary sex ratios, whereas those raising lighter‐than‐average foster fledglings produced female‐biased primary sex ratios. Females also produced male‐biased clutches when mated to males with large secondary sexual characters (wing patches), and tended to produce male‐biased clutches earlier within breeding seasons relative to females breeding later. However, females did not adjust the sex of individuals within their clutches; sex was distributed randomly with respect to egg size, laying order and paternity. Future research investigating the proximate mechanisms linking ecological contexts and the quality of offspring parents are able to produce with primary sex‐ratio variation could provide fundamental insight into the evolution of context‐dependent sex‐ratio adjustment.     

Costs of incubation and immunocompetence in the collared flycatcher

This paper investigates the costs of incubation in terms of reduced reproductive success and investigates whether incubation competes with immune function for resources. I performed a clutch size manipulation experiment in which two eggs were either removed from or added to the nests of collared flycatchers, Ficedula albicollis, for 1 week during incubation and subsequently returned to their original nests before hatching. To induce immune response, the females were challenged with sheep red blood cells. While the duration of incubation, hatching success and fledgling number did not differ between experimental groups, fledgling condition was significantly lower in broods that had been enlarged during incubation. Neither the females' condition nor their ability to respond to a novel antigen differed between treatments. The relationship between antibody production and female condition was significantly positive, but only among females incubating reduced clutches. I conclude that the costs of incubation in the collared flycatcher are not negligible and are manifested only at the chick-rearing phase.     

Fecundity compensation in the three‐spined stickleback Gasterosteus aculeatus infected by the diphyllobothriidean cestode Schistocephalus solidus

Causal explanations for host reproductive phenotypes influenced by parasitism fit into three broad evolutionary models: (1) non‐adaptive side effect; (2) adaptive parasitic manipulation; and (3) adaptive host defence. This study demonstrates fecundity compensation, an adaptive non‐immunological host defence, in the three‐spined stickleback fish (Gasterosteus aculeatus) infected by the diphyllobothriidean cestode Schistocephalus solidus. Both infected and uninfected female sticklebacks produced egg clutches at the same age and size. The reproductive capacity of infected females decreased rapidly with increased parasite : host body mass ratio. Body condition was lower in infected females than uninfected females and decreased with increasing parasite : host mass ratio. Females with clutches had greater body condition than those without clutches. A point biserial correlation showed that there was a body condition threshold necessary for clutch production to occur. Host females apparently had the capacity to produce egg clutches until the prolonged effects of nutrient theft by the parasite and the drain on resources from reproduction precluded clutch formation. Clutch mass, adjusted for female body mass, did not differ significantly between infected and uninfected females. Infected females apparently maintained the same level of reproductive allotment (egg mass as proportion of body mass) as uninfected females. Infected females produced larger clutches of smaller eggs than uninfected females, revealing a trade‐off between egg mass and egg number, consistent with the fecundity compensation hypothesis. The rapid loss of reproductive capacity with severity of infection probably reflects the influence of the parasite combined with a trade‐off between current and future reproduction in the host. Inter‐annual differences in reproductive performance may have reflected ecological influences on host pathology and/or intra‐annual seasonal changes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Informative content of melanin‐based plumage colour in adult Eurasian kestrels

Covariation between melanin‐based colorations and other phenotypic attributes has been rarely measured simultaneously in males and females. Such covariations and mechanisms mediating them have crucial importance determining the signalling function of these coloured ornaments in the two sexes. We examined the role of four melanin‐based coloured plumage patches as indicators of quality in both males and females of the sexually dimorphic and dichromatic Eurasian kestrel Falco tinnunculus. Previous kestrel studies have focused on the size of melanin plumage patches in either males or females as indicators of individual quality. Here, we used spectrophotometric measurements of three plumage patches and the size of another plumage patch to investigate the information content of multiple plumage colour traits in male and female kestrels. We found that females with bright plumage in the head and with high UV chroma in black parts of the rump showed good body condition and innate immunity. In addition, laying date was significantly explained by the intensity of the brown in the head of females. Meanwhile, in males we only found that individuals with greyer rumps showed better innate immunity. Altogether, our results indicate that, irrespective of the mechanism promoting covariation between coloration and individual quality, melanin‐based coloration can inform on individual quality in adult kestrels.     

Immune Phenotype and Body Condition in Roe Deer: Individuals with High Body Condition Have Different,Not Stronger Immunity

An efficient immunity is necessary for host survival, but entails energetic costs. When energy is limited, immunocompetence and body condition should co-vary positively among individuals and, depending on body condition, individuals should allocate more either in innate immunity or in adaptive response. We tested whether immune phenotype depends on body condition in large mammals, using data from two contrasted populations of roe deer Capreolus capreolus in France. Roe deer living at Chizé, a forest with poor habitat quality, were expected to show lower values for body condition and immune parameters than roe deer at Trois Fontaines, a forest with high habitat quality. From 285 blood samples collected between December 2009 and March 2011, we measured seven metabolic parameters and ten immunological parameters. A Principal Component Analysis showed that all indicators of body condition co-varied positively and were lowest at Chizé. Several immunological indicators correlated to body condition and differed between Trois Fontaines and Chizé. However, high body condition was not associated to a high average level of immunocompetence, but instead to high levels of indicators of acute inflammatory innate response, while low body condition was associated to high levels of monocytes and lymphocytes, possibly reflecting adaptive immunity. Limited data suggest that the difference between populations was not related to the presence of specific parasite species, however parasite exposure and stress have to be investigated to gain a more complete understanding of the determinants of immunity.     

State-dependent incubation behaviour in the zebra finch

State dependence is thought to be an important factor in resource allocation decisions, particularly in those decisions relating to parental care. Incubation behaviour is a costly part of avian parental care, and therefore likely to depend on the parent's body condition. We manipulated the body condition of zebra finches, Taeniopygia guttata, during incubation by using prelaying diets of different protein content, and induced birds to lay a similar number of eggs. We tested the hypothesis that birds in better body condition would invest more in incubation than birds in poorer condition when incubating on the same diet. Females that had received a high-protein prelaying diet lost more body mass than those that had received a low-protein diet. They also increased the length of their incubation bouts between early and middle incubation, whereas females that had received a low-protein prelaying diet increased bout length only between middle and late incubation. There were no differences between males from the two diet groups, and males were responsible for a lesser proportion of incubation than females. These results indicate that incubation behaviour is state dependent in female zebra finches, and that birds of different body condition adopt different incubation strategies. We found no differences in incubation duration and hatching success between the two incubation strategies in captivity, but the potential risk of nest predation in the wild may differ. We suggest that only females in good condition can afford to adopt a strategy of increasing bout length early in incubation; females in poorer condition first have to recover their body condition after having produced a clutch. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour