Proc. Nati. Acad. Sci. USA Vol. 89, pp. 4722-4725, May 1992 Population Biology Embryonic development period and the prevalence of avian blood parasites (bematozoans/immunocompetence) ROBERT E. RICKLEFS Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018 Communicated by Gordon H. Orians, January 27, 1992 (receivedfor review August 16, 1991)

ABSTRACT Variation in prevalence ofavian heiatozoais To restrict the scope of the survey with respect to habitat, related to taxonomic affDiation at the level of the family or diet, and development type, I consider only nonraptorial subfamily but not of the genus within families. Prevalence is (i.e., excluding birds of prey) altricial (i.e., those with highly comparatively insensitive to the influences of habitat and dependent neonates, thereby excluding gallinaceous birds season; however, temperate species have higher incidences of and ducks and geese) terrestrial birds. Neotropical migrants infection than tropical species belonging to the same himilies. were deleted from the tropical samples because the same Among taxa of nonraptorial altrical landblrds, hematozoan species were represented in the temperate data set. prevalence is inversely related to the length of the incubation Data values are the numbers of individuals for which period but shows little relationship to body size and rate of examination of blood smears revealed infections by one or postnatal development. This finding suggests a possible link more blood parasites, primarily the protozoans Haemopro- between the duration ofembryonic development and the ability teus, Leucocytozoon, Trypanosoma, Plasmodium, and larval to resist or control infection, possibly due to maturational nematodes (microfilariae). These parasites are transmitted by processes in the avian immune system. a variety of fly, mosquito, and midge vectors. The total sample included more than 40,000 individuals in 36 families The proportion of individuals with microscopically detect- or subfamilies (hereafter referred to as families). I calculated able infections of blood parasites (hematozoan prevalence) prevalence (Q) for each taxon as the number of parasitized varies widely among species of birds (1-3). This variation individuals divided by the total number of individuals in the suggests differences in exposure to parasite vectors, vulner- sample of that taxon (Table 1). ability to parasites, or ability to resist or control infection. To the extent that resistance falls under genetic control, para- ANALYSES sites have been implicated in the evolution of sexual repro- Parasites were more prevalent in temperate birds than in duction (4), genetic polymorphism (5), and sexually selected tropical birds, by a factor of 2.6. Within each region, how- characters (6-9); certainly, they may influence survival and ever, most of the variation was associated with family-level reproductive success (10), even causing extinction of sus- taxonomic grouping. In a comparison involving 13 families ceptible populations (11). Some ofthe variation in prevalence with >70 individuals represented in both regions, a two-way of hematozoa has been linked to locality (1, 12), habitat, diet analysis ofvariance with family and region as effects revealed (13), season (14, 15), and, in tropical latitudes, to plumage significant differences in arcsin-transformed prevalences be- brightness (13, 16). Other factors have been shown to be tween families (F12,12 = 4.50; P = 0.0072; R2 = 0.49) and unimportant. For example, although hematozoa appear to regions (F1,12 = 44.73; P < 0.0001; R2 = 0.40). Mean infect a larger proportion of colonially than solitarily nesting prevalences were 0.41 in temperate areas and 0.16 in tropical birds in one African sample (17), prevalence in north tem- areas. perate species appears generally unrelated to nest location, Prevalence did not vary significantly among genera within mating system, habitat type, and elaboration of secondary families. In a nested analysis of variance of the tropical data sexual characteristics (18-20). Prevalence of avian hemato- set, including only genera with two or more species, each zoa does, however, bear a strong association with taxonomic represented by samples of 30 or more individuals (38 species group at the family level (1, 12, 13). At present, the causes of in 15 genera and 8 families), Q exhibited a significant family this variation are not understood. effect (F7,7 = 4.58; P < 0.05; R2 = 0.57) but not a genus- In this report, I relate hematozoan prevalence to embry- within-family effect (F7,23 = 1.36; P = 0.27). onic and postnatal development rate and demonstrate that Adult weight, postnatal growth rate, egg weight, and prevalence is inversely related to the length ofthe incubation incubation and nestling periods of nonraptorial altricial ter- period. I speculate that this correlation arises from a direct restrial birds also vary significantly between families (Table relationship between immunocompetence and period of em- 2). Among these variables, prevalence was most closely bryonic growth. Long incubation periods might allow in- related to incubation period (r = -0.86), followed by post- creased proliferation of B stem cells and greater diversifica- natal growth rate (r = 0.65). A stepwise multiple regression tion of the variable region of immunoglobulin light chain entered incubation period as the first variable (F1,17 = 50; P genes before they are expressed as antibody. < 0.001; R2 = 0.75); postnatal growth rate, which is corre- lated with incubation period (r = -0.58), did not improve the MATERIALS model significantly (F1,16 = 2.66; P = 0.12). Incubation period did, however, improve a stepwise regression model in which Parasite data were obtained from literature surveys of field nestling period was entered first. In a multiple regression studies in the nearctic (1) and the neotropical regions (2, 3). analysis, incubation period was not only the strongest cor- relate of hematozoan prevalence, but also the only variable The publication costs ofthis article were defrayed in part by page charge uniquely associated with variation in prevalence-that is, payment. This article must therefore be hereby marked "advertisement" associated independently of its correlation with other life- in accordance with 18 U.S.C. §1734 solely to indicate this fact. history variables. 4722 Downloaded by guest on September 27, 2021 Population Biology: Ricklefs Proc. Natl. Acad. Sci. USA 89 (1992) 4723

Table 1. Number of individuals sampled, hematozoan analyzed here. I calculated an incubation period index (I) as prevalences, and incubation indices (I) for neotropical the average deviation of species within each family from a and nearctic representatives of families and log-log regression analysis of the length of the incubation subfamilies of nonraptorial altricial landbirds period upon the size of the egg (567 species in 11 orders of Number of Hematozoan nonraptorial altricial terrestrial birds worldwide). When log1o individuals prevalence values of both egg size and incubation period were used, Family or there was a weak interaction between the effects of taxo- Trop I subfamily Trop Temp Temp nomic order and egg size on incubation period (analysis of Alcedinidae 47 7 0.064 0.000 0.073 covariance: F11,555 = 1.84; P = 0.051). Ignoring this interac- Apodidae 49 1 0.000 0.000 0.145 tion, a second analysis of covariance revealed significant Bombycillidae 8 93 0.625 0.699 -0.123 variation among taxonomic orders (type III sums of squares; Cardinalinae 475 476 0.202 0.437 -0.115 F10,555 = 55; P < 0.0001; R2 = 0.33) and a significant egg size Carduelinae 179 1230 0.369 0.331 -0.044 effect (F1,555 = 164; P < 0.0001; R2 = 0.10). The regression Coerebinae 25 0.080 -0.094 equation had a slope of 0.118, and log1o values of incubation Columbidae 1186 2692 0.361 0.703 -0.063 periods were normalized with respect to an intercept of 1.148 Corvidae 67 1970 0.433 0.502 -0.035 (Trogoniformes). Average values of I for species within Cotingidae 73 0.192 0.013 neotropical and nearctic families varied between +0.150 Cuculidae 169 36 0.160 0.361 -0.141 (Psittacidae: parrots) and -0.141 (Cuculidae: cuckoos). Dendrocolaptidae 326 0.055 0.021 Hematozoan prevalence was strongly negatively related to Elaeniinae 847 0.046 0.055 the incubation period index (Fig. 1); this relationship explains Emberizinae 1767 5844 0.189 0.427 -0.090 33% of the variation in parasite prevalence among families Fluvicolinae 232 213 0.082 0.188 0.030 within the neotropical region (F1,31 = 15.6; P = 0.0004; n = Formicariidae 632 0.119 -0.046 33) and 15% within the nearctic region (F1,20 = 3.5; P = 0.075; Funariidae 349 0.057 0.002 n = 22). Omitting taxa with small sample sizes improves the Hirundinidae 178 659 0.034 0.281 0.013 correlation (analyses not shown). When tropical and temper- Icterinae 575 3509 0.210 0.235 -0.106 ate prevalence data were combined in an analysis of covari- 505 885 0.200 0.346 -0.114 Mimidae ance, the regression (F2,52 = 33.5; P < 0.0001; R2 = 0.56; n Momotidae 28 0.357 0.061 = 55) had a slope of -2.54 0.56 (F1,52 = 20.7; P < 0.0001). Paridae 3 419 0.667 0.463 -0.032 The intercepts (logl0 value of prevalence at I = 0) for the Parulinae 126 3829 0.103 0.453 -0.079 temperate sample (-0.57; Q = 0.27) and the tropical sample Picidae 239 382 0.071 0.424 -0.126 (-0.93; Q = 0.12) differed significantly (F1,52 = 23.3; P < Pipridae 918 0.019 0.085 0.0001). Psittacidae 779 0.049 0.150 The negative relationship between prevalence and incuba- Rhamphastidae 114 0.298 - -0.109 tion period was exhibited individually by Plasmodium, Hae- Sittidae 50 0.200 -0.024 moproteus, and Leucocytozoon but not by Trypanosoma or Sylviinae 22 118 0.182 0.102 -0.004 microfilariae (Table 3). In addition, Leucocytozoon and mi- Thraupinae 2590 71 0.302 0.662 -0.066 crofiliariae did not exhibit the latitude effect (temperate > Tityrinae 106 0.509 0.077 tropics) evident in the other genera. Trochilidae 451 41 0.073 0.073 0.114 The unique relationship between incubation period and Troglodytidae 270 106 0.078 0.132 0.002 prevalence might result from several factors: association of Trogonidae 49 - 0.265 0.000 long incubation periods with nest sites that are relatively free Turdinae 589 2287 0.197 0.660 -0.075 of parasite vectors, association of long incubation periods Tyranninae 467 93 0.227 0.344 -0.022 with habitats that are relatively free of parasite vectors or Vireonidae 193 279 0.337 0.509 -0.032 with roosting sites and sleeping postures that reduce suscep- Trop, tropical; Temp, temperate (nearctic). tibility to parasite vectors, or association of long embryonic growth period with reduced susceptibility to infection result- Incubation period, which is the time between the initial ing from a more highly developed immune response. warming of the egg and hatching, varies by a factor of 2 or Nest type, incubation period, and hematozoan prevalence more between species laying eggs of similar size. For exam- do not appear to be associated. For example, hole-nesting ple, among species whose eggs have volumes of about 5 cm3, taxa include the Picidae (woodpeckers) and Ramphastidae the incubation period varies from 11 to 25 days in the sample (toucans), on one hand, having short incubation periods (I = Table 2. Variation in size and development variables within and between families/subfamilies of nonraptorial altricial terrestrial birds in nearctic and neotropical regions Between-family variation Correlation with Q Variable Fam Spp R2 RMSE F r P Adult weight 24 105 0.70 0.175 7.6 0.36 >0.05 Egg weight or volume 23 101 0.75 0.136 10.1 0.30 >0.10 Postnatal growth rate 25 109 0.57 0.069 4.5 0.65 <0.001 Nestling period 23 100 0.87 0.063 21.4 -0.37 >0.05 Incubation period 25 103 0.71 0.043 7.4 -0.86 <0.001 Data are from Ricklefs (21, 22), Oniki and Ricklefs (23), and sources therein; loglo values of all variables were used. Fam, number offamilies; Spp, number of species; R2, proportion oftotal variance in variable accounted for by variation between families in a one-way analysis of variance; RMSE, standard deviation of the log10 values of the variable within families; F, ratio of family mean squares over error mean squares, tested with 1 and n - 2 degrees of freedom (P < 0.001 for all values of F); r, correlation of variable with log10 values of the prevalence of hematozoans (n = 19 families; tropical and temperate prevalence data combined). Postnatal growth rate is the rate constant (K, days-') of the logistic growth curve (24). Downloaded by guest on September 27, 2021 4724 Population Biology: Ricklefs Proc. Nad. Acad. Sci. USA 89 (1992)

1 DISCUSSION I suggest that the ability of individuals to prevent infection, or Q control infection, both increases with the length of the 00 embryonic development period, possibly through greater a) oo 0 o differentiation ofthe immune system at the time of hatching. 0 Immune responsiveness is genetically inherited (25) and . influences lifespan by reducing the effects of neoplastic and a) 0~~~~0~~~~~ 04 inflammatory diseases (26). In pigeons and domestic fowl, 0.1 both of which have short incubation periods, specific anti- 0 0 0 N 0 body response is not well developed until several weeks after 0 Ca 0.0 hatching (27, 28). Antibody production is crucial to control- a) ling infection by blood parasites, as demonstrated by remov- 0 ing the bursa of Fabricius thereby preventing maturation of antibody-producing B cells (29-31), although specific anti- 0 bodies are not the only means of defense (32, 33). Generation of antibody diversity in the avian immune 0.01 system depends on a small number of stem cells that colonize -0.2 -0.1 0.0 0.1 the bursa ofFabricius, where they undergo several cycles of Incubation period index proliferation before differentiating into B cells capable of producing antibody (34, 35). Antibody diversity results from FIG. 1. Relationship between the prevalence of hematozoans gene conversion between a series of short-segment pseudo- (plotted on a logarithmic scale) and the incubation period index (1) genes and the expressed functional variable region gene for families/subfamilies of nonraptorial altricial landbirds. Regres- segment ofthe immunoglobulin light chain locus. Each B cell sion lines are indicated separately for prevalence values for temper- ate (open symbol; dashed line) and tropical (solid symbol; solid line) produces a single variant (idiotype) of the immunoglobulin regions. Data are from Table 1. Interaction between region and light chain. Idiotype diversity depends on the number of incubation period was not significant (F1,51 = 0.63; P = 0.43), and so cycles ofproliferation ofthe stem cells before differentiation. a common slope was assumed for both regions. Data points were These cycles do not extend beyond the end ofthe incubation weighted by the square root of the sample size of individuals period in the domestic fowl (36). Thus, the number of these surveyed, indicated by size of symbol (Vi-: small symbols, <15; cycles, which are 8-12 h long (36), could increase with the medium-sized symbols, 15-30; large symbols, >30). Use of loglo length of the period of embryonic development, leading to a values ofprevalence eliminated from the regression analysis two taxa more effective immune system shortly after hatching and with zero hematozoan prevalence: temperate Alcedinidae (kingfish- possibly throughout life. Accordingly, at an early posthatch- ers: n = 7; I = 0.073) and tropical Apodidae (swifts: n = 49; I = 0.145). ing age, the repertoire of B-cell receptors would be more diverse with an increase in the length of the incubation period; specific antibody response may develop sooner after -0.13 and -0.11, respectively) and moderately high parasite hatching, reducing infection during the vulnerable nestling prevalences (Q = 0.28 and 0.30), and the Psittaciformes stage. These predictions can be tested by examining primary (parrots) and Dendrocolaptidae (woodcreepers), on the immune reactivity to a spectrum ofimmunogens at or shortly other, having long incubation periods (I = 0.15 and 0.02) and after hatching in species having long or short periods of low prevalences (Q = 0.05 and 0.06). Open-nesting passe- embryonic development. rines include the Pipridae (manakins: I = 0.09 and Q = 0.02) The idea that prolonged incubation is associated with and the Thraupinae (tanagers: I = -0.07 and Q = 0.31). increased immunocompetence of the chicken gains support Furthermore, although Plasmodium, Haemoproteus, and from the observation that prevalences of trypanosomes and Leucocytozoon exhibit an inverse prevalence-incubation pe- microfilariae are unrelated to the length of the incubation riod relationship, they are transmitted by different vectors period. Both have unique mechanisms to circumvent the (mosquitos, biting midges and hippoboscid flies, and black- immune systems of their hosts. Trypanosomes may individ- flies, respectively). Thus, the interaction ofnesting, roosting, ually express one of hundreds of different surface antigens and feeding with vector ecology would appear to be less (homotypes); as the host immune system supresses the important than the mechanisms of the host for preventing or prevalent homotype, others are expressed in the population controlling infection. Additionally, in other studies, parasite of parasites, which thereby transiently escapes suppression prevalence has been shown to be generally unrelated or only (37). By continually generating new homotypes, the trypano- weakly related to plumage brightness and complexity, con- some infection maintains itselfin spite ofan immune response spicuousness of birds in the field, and habitat (13, 18-20). by the host. Microfilariae may disguise themselves from the

Table 3. Relationship of the prevalence of blood parasites to incubation period (I) and latitude (L) Parasite n R2 LO L I Vector Any and all 55 0.56 -0.93 0.36t -2.540 Plasmodium 45 0.63 -2.00 0.86t -3.69t Mosquitos Haemoproteus 45 0.31 -1.36 0.31* -2.88* Culicoides, Hippoboscids Leucocytozoon 47 0.31 -1.73 NS -3.96t Simulium Trypanosoma 47 0.29 -2.01 0.67t NS Many biting arthropods Microfilariae 46 0.05 -1.66 NS NS Mosquitos, Simulium n, Number offamilies or subfamilies in regression; R2, proportion of variation explained by latitude and incubation period index; YO, intercept, the log1o value of prevalence at an incubation period index of 0, for the tropical sample; L, elevation (log units) of the temperate regression above that of the tropical sample; I, slope ofthe regression analysis ofthe log1o values ofprevalence vs. incubation period index. *, P < 0.05; t, P < 0.005; t, P <0.0005. Downloaded by guest on September 27, 2021 Population Biology: Ricklefs Proc. Natl. Acad. Sci. USA 89 (1992) 4725 host's immune system by coating themselves with host 16. Zuk, M. (1991) in Bird-Parasite Interactions: Ecology, Evolu- antigen (38). tion, and Behavior, eds. Loye, J. E. & Zuk, M. (Oxford Univ. Because growth rate, especially that of the embryo, is Press, New York), pp. 317-327. highly conservative within taxa at the family/subfamily level, 17. Bennett, G. F., Blancou, J., White, E. M. & Williams, N. A. evolutionary change in incubation period may be difficult to (1978) J. Wildl. Dis. 14, 67-73. achieve and may require a major reorganization of the 18. Read, A. F. (1987) Nature (London) 327, 68-70. development pattern. Increased immunocompetence may be 19. Read, A. F. & Harvey, P. H. (1989) Nature (London) 339, only one of several manifestations of prolonged embryonic 618-620. 20. Johnson, S. G. (1991) Evol. Ecol. 5, 52-62. development, and its benefits may be a major selective factor 21. Ricklefs, R. E. (1968) Ibis 110, 419-451. in the evolution of embryonic growth rate. 22. Ricklefs, R. E. (1976) Ibis 118, 176-207. 23. Oniki, Y. & Ricklefs, R. E. (1981) Ibis 123, 349-354. I thank M. Zuk for discussions, and V. Apanius, G. Orians, S. 24. Ricklefs, R. E. (1983) in Avian Biology, eds. Farner, D. S., Rohwer, N. G. Smith, and M. Zuk for commenting on an earlier draft King, J. R. & Parkes, K. C. (Academic, New York), Vol. 7, pp. of the paper. This manuscript was completed while I was a Regents' 1-83. Fellow at the Smithsonian Tropical Research Institute. 25. Cheng, S., Rothschild, M. F. & Lamont, S. J. (1991) Poult. Sci. 70, 2023-2027. 1. Greiner, E. C., Bennett, G. F., White, E. M. & Coombs, R. F. 26. Covelli, V., Mouton, D., Di Majo, V., Bouthillier, Y., Ban- (1975) Can. J. Zool. 53, 1762-1787. grazi, C., Mevel, J.-C., Rebessi, S., Doria, G. & Biozzi, G. 2. Sousa, 0. E. & Herman, C. M. (1982) J. Wildl. Dis. 18, (1989) J. Immunol. 142, 1224-1234. 205-221. 27. Moticka, E. J. (1975) Am. Zool. 15, 135-146. 3. White, E. M., Greiner, E. C., Bennett, G. F. & Herman, C. M. (1978) Rev. Biol. Trop. 26, 43-102. 28. Koppenheffer, T. L., Ford, J. W. & Robertson, P. B. (1981) in 4. Hamilton, W. D. (1990) Oikos 35, 282-290. Aspects ofDevelopmental and Comparative Immunology, ed. 5. Hamilton, W. D. (1982) in Population Biology of Infectious Solomon, J. B. (Pergamon, New York), pp. 535-536. Diseases, eds. Anderson, R. M. & May, R. M. (Springer, New 29. Congdon, L. L., Farmer, D. J., Longnecker, B. M. & Breiten- York), pp. 269-296. bach, R. P. (1969) J. Parasitol. 55, 817-824. 6. Hamilton, W. D. & Zuk, M. (1982) Science 218, 384-387. 30. Longnecker, B. M., Breitenbach, R. P. & Farmer, J. N. (1966) 7. Hamilton, W. D. (1990) Am. Zool. 30, 341-352. J. Immunol. 97, 594-599. 8. M0ller, A. P. (1990) Evolution 44, 771-784. 31. Isobe, T., Kohno, M., Suzuki, K. & Yoshihara, S. (1989) in 9. Zuk, M., Thornhill, R. & Ligon, J. D. (1990) Am. Zool. 30, Proceedings ofthe Fifth International Coccidiosis Conference 235-244. (Inst. Natl. Recherches Agron., Paris). 10. Loye, J. E. & Zuk, M., eds. (1991) Bird-Parasite Interactions: 32. Terzian, L. A. (1946) J. Infect. Dis. 79, 215-220. Ecology, Evolution, and Behavior (Oxford Univ. Press, New 33. Rank, R. G. & Wiedanz, W. P. (1976) Proc. Soc. Exp. Biol. York). Med. 151, 257-259. 11. Van Riper, C., III, Van Riper, S. G., Goff, M. L. & Laird, M. 34. Roitt, I. M., Brostoff, J. & Male, D. K. (1989) Immunology (1986) Ecol. Monogr. 56, 327-344. (Lippincott, Philadelphia). 12. Peirce, M. A. (1981) J. Nat. Hist. 15, 419-458. 35. Crook, K. (1990) in Parasites: Immunity and Pathology, ed. 13. Pruett-Jones, S. G., Pruett-Jones, M. A. & Jones, H. I. (1991) Behnke, J. M. (Taylor & Francis, London), pp. 89-119. Curr. Ornithol. 8, 213-245. 36. Weill, J.-C. & Reynaud, C.-A. (1987) Science 238, 1094-1098. 14. Bennett, G. F. & Cameron, M. (1974) Can. J. Zool. 52, 37. Behnke, J. M. (1990) in Parasites: Immunity and Pathology, 1259-1264. ed. Behnke, J. M. (Taylor & Francis, London), pp. 344-395. 15. Atkinson, C. T., Forrester, D. J. & Greiner, E. C. (1988) J. 38. Maizels, R. M., Philipp, M., Dasgupta, A. & Partono, F. (1984) Med. Entomol. 25, 45-51. Parasite Immunol. 48, 107-121. Downloaded by guest on September 27, 2021