02198nas a2200193 4500008004100000245007300041210006900114300001400183490000700197520162800204653001201832653002501844653003101869653001901900653001401919100001701933700001501950856003901965 2016 eng d00aRelatedness constrains virulence in an obligate avian brood parasite0 aRelatedness constrains virulence in an obligate avian brood para a191 - 2010 v153 a
Virulence, the amount of harm a parasite inflicts on its host, is integral to elucidating the evolution of obligate avian brood parasitism. However, we lack information regarding how relatedness is linked to changes in behavior and the degree of harm that brood parasites cause to their hosts (i.e., virulence). The kin competition hypothesis combines theory from offspring signaling and parasite virulence models and states that the begging intensity of co-infecting parasites is driven by their relatedness, with concomitant changes in the degree of virulence expressed by parasitic young. We tested this hypothesis using the Brown-headed Cowbird Molothrus ater, an obligate brood parasitic bird whose virulence at the nestling stage is mediated by vigorous begging displays that are used to outcompete host young during feeding bouts. We found support for both predictions of the kin competition hypothesis: first, the begging intensity of cowbirds was greater in a population where cowbirds typically competed against unrelated host nestmates, relative to a population where they often competed against kin. Second, the greater intensity of begging in cowbirds was positively associated with decreased growth in host offspring during the developmental period. Given the dearth of studies on virulence in avian brood parasites, our results notably extend our understanding of how relatedness is linked to parasite behavior and virulence, and they highlight how spatially-isolated host populations can harbor different levels of virulence that are driven by competitive interactions between co-infecting parasites.
10abegging10abrown-headed cowbird10aKin competition hypothesis10aMolothrus ater10aVirulence1 aRivers, J.W.1 aPeer, B.D. uhttps://doi.org/10.2326/osj.15.19102657nas a2200253 4500008004100000245011900041210006900160300001300229490000700242520186200249653002402111653001202135653002102147653002502168653001902193653002502212653002502237100001702262700001902279700001802298700001502316700002002331856005202351 2013 eng d00aThe exaggerated begging behaviour of an obligate avian brood parasite is shared with a nonparasitic close relative0 aexaggerated begging behaviour of an obligate avian brood parasit a529 -5360 v863 aOffspring signalling models predict that the begging displays of obligate brood parasites are more intense than nonparasitic species because parasitic young are never reared by their genetic parents and often compete against unrelated host young during development. The brown-headed cowbird, Molothrus ater, has been described as having exaggerated begging relative to nonparasitic species, but an effective test of this idea is lacking because previous studies have not controlled for evolutionary history while simultaneously standardizing rearing conditions. We quantified the begging intensity of cowbirds and the closely related, nonparasitic red-winged blackbird, Agelaius phoeniceus, when both species experienced identical rearing conditions in two distinct nest environments: reared alone by a small cowbird host, or reared with two host young by a moderate-sized cowbird host. Against theoretical predictions, we found that in both nest environments four components of the cowbird begging display were similar to (or less intense than) blackbird begging displays (i.e. latency to beg, begging score, call rate and call amplitude) when nestlings were tested across a gradient of short-term need. Our results provide the first experimental evidence that a closely related, yet nonparasitic, species shares an exaggerated begging display with a brood-parasitic species when reared under conditions typically experienced by parasitic offspring. We discuss three nonexclusive explanations for our findings: (1) relatedness among cowbird nestmates reduced cowbird begging intensity (kin selection hypothesis), (2) reduced body condition of blackbirds elevated their begging intensity (body condition hypothesis) and (3) intense competition in blackbird nest environments led to increased blackbird begging intensity (competitive environment hypothesis).10aAgelaius phoeniceus10abegging10abrood parasitism10abrown-headed cowbird10aMolothrus ater10aoffspring signalling10ared-winged blackbird1 aRivers, J.W.1 aBlundell, M.A.1 aLoughin, T.M.1 aPeer, B.D.1 aRothstein, S.I. uhttp://dx.doi.org/10.1016/j.anbehav.2013.06.00402278nas a2200181 4500008004100000245011300041210006900154300001300223490000700236520167900243100001801922700001501940700001501955700001701970700001701987700001502004856007702019 2012 eng d00aComparison of eggshell porosity and estimated gas flux between the brown-headed cowbird and two common hosts0 aComparison of eggshell porosity and estimated gas flux between t a486 -4900 v433 aThe brown-headed cowbird Molothrus ater is a brood-parasite that lays eggs in nests of a wide range of host species, including the closely-related red-winged blackbird Agelaius phoeniceus and the dickcissel Spiza americana. Although cowbird eggs have accelerated development and hatch sooner than similar-sized host eggs, this development takes place within a thickened eggshell that could impede gas flux to the developing embryo. We tested the hypothesis that the accelerated development of the cowbird embryo relative to hosts is enabled by an increase in eggshell porosity that allows increased fluxes of respiratory gases to and from the developing embryo. We found cowbird eggshell thickness was significantly greater than the eggshells of these two common hosts. Although the number of pores per egg was similar among all three species, the total pore area per egg in cowbirds was significantly greater than that of either host, despite having a smaller eggshell surface area than the red-winged blackbird. Cowbird egg pore area was 1.9×larger than that of the red-winged blackbird. Cowbird eggshells had a significantly greater gas flux than those of the red-winged blackbird and the dickcissel. When conductance was normalized to published values of egg mass, cowbird eggs had a higher mass-specific conductance than red-winged blackbird or dickcissel eggs. These results are consistent with the hypothesis that the rapid development of brown-headed cowbird embryos is facilitated by increased eggshell porosity, and that changes in eggshell porosity represent an adaptation that enables cowbird eggs to hatch earlier than equivalently-sized host eggs.
1 aJaeckle, W.B.1 aKiefer, M.1 aChilds, B.1 aHarper, R.G.1 aRivers, J.W.1 aPeer, B.D. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-048X.2012.05705.x