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Keywords:

  • avain malaria;
  • haematozoa;
  • immunocompetence;
  • reproductive cost

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Life-history theory centres around trade-offs between current and future reproduction, but we have little understanding of how such trade-offs are mediated. We supplementary fed Ural owls (Strix uralensis) during the nestling period and quantified parents’ current and future life-history components as well as their physiological health by monitoring haematocrit, leucocyte profile, intra- and extracellular blood parasites. Feeding led to reduced parental effort but did not improve offspring viability, male parasite defence, or parental survival. Intracellular leucocytozoan infection was reduced in fed females which lasted to the following year's reproductive season (carry-over effect), when fed females also laid larger and earlier clutches. Leucocytozoon infection therefore may mediate the life-history trade-off between current and residual reproduction in this species.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Different levels of resource availability select for different life histories (Stearns, 1992). In accordance with theory, empirical brood size manipulation studies, in which resource demand and workload is manipulated, have documented that increased brood sizes generally have negative immediate effects on offspring viability, parental survival or parental reproduction in the next season (reviewed in Lessels, 1991). However, the proximate mechanisms for these trade-offs between current and future life-history components are unknown, but have been suggested to be mediated through suppressed immune function and physiological health (Sheldon & Verhulst, 1996; Norris & Evans, 2000). Thus, improvement in the physiological health of the parents may have the potential to mediate improved residual reproductive value (Zera & Harschman, 2001). A number of studies have documented that experimentally increased reproductive effort has immediate effects on immune function by impairing the defence against parasites (e.g. Gustafsson et al., 1994; Norris et al., 1994), and decreasing humoral immune responsiveness against a novel antigen (e.g. Deerenberg et al., 1997; Nordling et al., 1998). Only few studies of natural vertebrate populations have been able to explore the link between reproductive effort and immunocompetence, and relate this trade-off to future survival (Ardia et al., 2003) and future fecundity (Hanssen et al., 2005). To date, however, no study has reported simultaneous long-lasting costs on both life-history components and components of immune function. Such a relationship would nevertheless be expected if immune function would be a mediator of the trade-off between current and future life-history components.

Food supplementation experiments allow studying which life-history component parents decide to invest in when provided with plentiful resources. In this paper, we explore both immediate and long-lasting effects of supplementary food during chick rearing on parasite defence in the long-lived Ural owl (Strix uralensis). Ural owls rarely change breeding site, enabling explorations of long-lasting effects of supplemental food on an individual's current reproduction, survival and residual reproduction. We have previously shown that food supplementation leads to a reduced parental effort, and that fed parents lay, in the following year, 1 week earlier and produce 0.6 more eggs than control parents (Brommer et al., 2004). We here show that this carry-over effect of supplementary food on next year's reproduction is associated with a long-lasting improvement in parasite defence.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

General field protocol

Ural owls breed in nest boxes. In 2002, nests with similar hatching dates and same number of hatchlings were paired (18 pairs). For each pair of nests, one nest received supplementary food (laboratory mice and rooster broiler hatchlings) approximately every third day, from hatching of the last egg until the oldest chick in the nest was 25 days old. Control nests were visited with the same day interval. In total, each nestling received 1418 ± 62 g supplementary food (approximately what a chick receives from its parents under natural conditions, H. Pietiäinen, unpublished data). In 2003 we checked all the territories that were included in the experiment 2002 and measured clutch size and timing of egg laying. See Brommer et al. (2004) for further details.

Data collected

In the year of the experiment (2002) incubating females were caught 12–14 days after clutch initiation (mid-incubation period) and again when the offspring were 12–14 days old (mid-nestling period). Males were trapped only during the nestling period. In 2003 females were caught again in the mid-incubation period. At capture individuals were weighed, and the length of their radius–ulna was measured as an estimate of skeletal size, and 75 μL of blood was drawn into capillary tubes by puncturing the brachial vein. An individual was considered not to have survived after the breeding season if it had not been caught again until 2006.

Nestlings regurgitate pellets containing indigestible prey remains. The sawdust layer in the nest box was changed at day 19 and again at day 25, and the prey remains were sorted from the sawdust in order to obtain a standardized measure of parental feeding during the period when both parents feed. The number of species-specific bones (excluding bones of supplemented mice and chicken) was used to estimate number of prey items delivered by the parents during these six nights.

Haematology

After venipuncture, a drop of blood from the capillary tube was smeared and air-dried on a glass slide, and fixed in absolute ethanol some hours afterwards. Smears were stained with May–Grünewald–Giemsa stain. Using a 1000× magnification the number and types of leucocytes within fields of a total of 104 erythrocytes were determined. We focused on the two most abundant leucocytes: heterophils and lymphocytes in order to estimate the heterophil/lymphocyte (H/L) ratio. Increased values in the H/L-ratio can be associated with infectious diseases and starvation (Ots et al., 1998). Intracellular Leucocytozoon parasite intensities were estimated and are reported in the results as the number in 104 erythrocytes.

Blood capillaries were centrifuged within 12 h after sampling at 2375 g for 5 min in order to separate the blood cells from the plasma. Immediately thereafter, trypanosomes (visible at the edge between leucocytes and plasma in the capillary) were counted for 5 min under light microscope using 400× magnification (Woo, 1970). Haematocrit was measured to evaluate anaemic symptoms. Low haematocrit values can result from parasitic infections (Campbell, 1995).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Current reproductive season

Joint parental feeding effort was reduced in supplementary fed nests, where about half the number of prey items was delivered compared with the control group (t34 = 4.4, P < 0.001). Offspring mortality was not affected by food supplementation (fed 8% 5/62, control 11% 7/62, GLMM with binomial errors and random effect ‘Brood ID nested in Pair’: fixed effect – food treatment, b = 0.28 ± 1.03, t = 0.27, d.f. = 18, P = 0.79).

Prior to supplementary feeding (mid-incubation in 2002), females had equal weight, haematocrit, H/L ratio, and intensities of Leucocytozoon and Trypanosoma (all tests, P > 0.4). During the nestling period, supplementary-fed females had a higher body mass and a better leucocytozoan parasite defence, but all aspects of male health were unaffected by the feeding (Table 1).

Table 1.   Weights and haematological measures of fed and control females [mean ± SE (n)] and males during the nestling period (when supplementary feeding was carried out).
 ControlFedTestP-value
  1. There was no difference in any of these measures prior to supplementary feeding in females (see Results). Significant effects are in bold. Weight was corrected for skeletal size in the analyses (statistics) and values for H/L-ratio are log-transformed.

  2. Female covariates: 2002 *skeletal size (radius–ulna): F1,32 = 2.16, P = 0.15.

  3. Male covariates: 2002 †skeletal size (radius–ulna): F1,25 = 0.20, P = 0.70.

Females 2002
 Weight981.5 ± 16.4 (17)1043.9 ± 23.8 (18)F1,32 = 6.110.02*
 Haematocrit40.16 ± 0.86 (16)41.95 ± 0.75 (17)t31 = −1.570.13
 H/L – ratio1.12 ± 0.04 (16)1.06 ± 0.06 (17)t31 = 0.980.34
 Leucocytozoon7.94 ± 3.58 (16)1.47 ± 0.53 (17)U = 202.50.01
 Trypanosoma6.38 ± 1.88 (16)3.47 ± 1.22 (17)U = 158.00.42
Males 2002
 Weight655 ± 10.7 (15)684 ± 9.4 (13)F1,25 = 3.200.09†
 Haematocrit42.6 ± 2.1 (13)42.0 ± 0.7 (13)t24 = 0.250.81
 H/L – ratio1.06 ± 0.10 (11)1.04 ± 0.06 (13)t22 = 0.150.89
 Leucocytozoon5.45 ± 2.36 (11)6.15 ± 3.48 (13)U = 480.45
 Trypanosoma4.75 ± 1.90 (12)3.15 ± 1.19 (13)U = 63.50.44

Next reproductive season

In 2003, 61% (22/36) of females bred again. Return rate was not explained by leucocytozoan intensity or feeding (GLM with binomial errors – leucocytozoan intensity, b = 1.48 ± 1.06, Wald = 1.947, d.f. = 31, P = 0.16; treatment, b = −0.37 ± 0.82, Wald = 0.21, d.f. = 31, P = 0.69). A repeated-measures analysis showed that food supplementation altered the intensity of leucocytozoan intracellular blood parasites in females (Table 2, treatment × period). Leucocytozoan intensity in fed females decreased during the period of food supplementation in 2002, whereas control females suffered an increase in parasite intensity (Fig. 1a, Table 2, treatment × nestl 02, P = 0.06). Further, this effect on parasite intensity lasted from 2002 to the following breeding season in 2003 (Fig. 1a), such that fed females still had a markedly lowered leucocytozoan intensity compared with control females in the 2003 breeding season (Fig. 1a, Table 2, treatment × Inc 03, P = 0.03). Simultaneously, fed females laid larger clutches than control females in 2003 (Fig. 1b, t20 = −2.35, P = 0.03) and also tended to fledge more offspring than control pairs (treatment: F1,23 = 3.45, P = 0.08). Supplementary food did not affect parental survival, as 67% (12/18) of females in both fed and control group survived (χ2 = 0), and 54% (7/13) of fed and 85% (11/13) of control males survived (inline image = 2.9, P = 0.09).

Table 2.   Linear mixed effects model (normal errors, ‘Female ID nested in Pair’ as random effect) on the within-season and between seasons effects of food treatment (control = 0, fed = 1) and its interactions with time period on leucocytozoan parasite load in Ural owl females (mid-incubation period = inc 02, mid-nestling period = nestl 02, following year's mid-incubation period = inc 03).
VariableCoefficient ± SE (P)Fd.f.P
  1. Coefficients for period are in comparison with mid-incubation period 2002 (1). The significance of the coefficient in the model is given in brackets, and an F-test is used to test the overall effect of the variable. The significance of the random effect was tested with a log-likelihood ratio test (χ2). The effect of the random effect ‘Pair’ was not significant and is not reported in the table.

Fixed effects
 Constant0.52 ± 0.10 (< 0.001)   
 Treatment−0.09 ± 0.15 (ns)6.931,16< 0.02
 Period 0.281,430.75
  Nestl 020.13 ± 0.10 (ns)   
  Inc 030.24 ± 0.12 (0.049)   
 Treatment × Period 3.182,430.05
  Treatment × Nestl 02−0.28 ± 0.14 (0.058)   
  Treatment × Inc 03−0.39 ± 0.17 (0.026)   
 Variance (95% CI)%χ2P
Random effect
 Female ID0.086 (0.043–0.171)5215.90< 0.0001
image

Figure 1.  Parasite load in fed (filled) and control (open) females. Panel (a): number of Leucocytozoon 10−4 red blood cells (mean ± SE) in current season (mid-incubation and mid-nestling period 2002) and next season (mid-incubation 2003). Panel (b): the relationship between the following year's clutch size (mean ± SE) and number of leucocytozoans 10−4 red blood cells (mean ± SE) in mid-incubation period 2003.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

We have shown that supplementary fed female Ural owls rapidly improve their intracellular parasite defence, leading to a long-lasting reduction in leucocytozoan parasite load still visible in the following season (approximately 10–11 months later). Coupled with this improved parasite defence, the following year's reproductive output increased. Ural owl females opted to invest the experimentally supplemented resources into themselves. The joint parental feeding effort was halved in supplementary fed nests and the leucocytozoan parasite defence of fed females improved markedly, but the survival of offspring or the health of males was not affected by supplementary feeding. In general, health of male birds of prey is not affected by supplementary feeding, presumably because males do not change their hunting effort (cf. Wiehn & Korpimäki, 1998; Dawson & Bortolotti, 2002).

Our results on Ural owls are the first to suggest that increased resource levels during reproduction may have long-lasting effects on intracellular parasite infections, such as leucocytozoans, whereas simultaneously having considerable effects on the host's future reproduction. Leucocytozoan infection is seriously pathogenic and resource limitation is important in inducing relapses and new infections (Atkinson & van Riper III, 1991). Experimentally reduced infections of intracellular parasites Leucocytozoon and Haemoproteus improved reproductive traits in blue tits Cyanistes caeruleus (Merino et al., 2000; Tomás et al., 2006) and house martins Delichon urbica (Marzal et al., 2005). We here manipulated resource levels during chick provisioning and found a reduction in female parasite load, which could result from fed females having a lowered encounter rate with the leucocytozoan vector (black flies, Diptera: Simuliidae) rather than improved immune defences (cf. Norris & Evans, 2000). However, the laying period and our relatively short (c. 3 weeks) period of supplementary feeding are well before the period of abundant black flies (Ojanen et al., 2002). Therefore, it is unlikely that the observed rapid and yet persisting decrease in Leucocytozoon parasite load in fed females is entirely due to a lower black fly encounter rate. Although a direct manipulation of Leucocytozoon is required to establish a causal link between parasite load and future reproduction, our results do provide evidence that intracellular blood parasites could be responsible for a cost of reproduction which Ural owls pay in terms of a reduced reproductive output the following year. Such a carry-over cost of reproduction could be a common phenomenon in long-lived species, which are expected to invest additional resources mainly in themselves rather than in their offspring (Stearns, 1992). Nevertheless, such costs may have remained unaddressed in previous studies because of difficulties in following the individuals for consecutive seasons. Furthermore, in monogamous birds of prey, carry-over effects on reproduction provide an explanation for why typically only females receive health benefits of additional resources (cf. Wiehn & Korpimäki, 1998). When there are carry-over effects on reproduction, males do receive fitness benefits from an increased health of their partner, because the male's reproduction also increases in the following year.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

All experiments described in this paper were approved by the ethical board for animal experiments. We would like to thank Christophe Lebigre for advice on blood smear screening, Hanna-Kaisa Mikkola for lab assistance and Tuomo Pihlaja and Heikki Kolunen for assistance in the field. This study was funded by the Academy of Finland (HP, JB) and the Finnish Cultural Foundation (PK).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
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