Yolk testosterone and estradiol variation relative to clutch size, laying order and hatching asynchrony in Common Grackles |
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Authors: | J Dylan Maddox Rachel M Bowden and Patrick J Weatherhead |
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Institution: | (1) Program in Ecology, Evolution, and Conservation Biology, University of Illinois, 201 Shelford Vivarium, 606 E. Healey St., Champaign, IL 61820, USA;(2) Department of Biological Sciences, Illinois State University, Normal, IL 61790, USA |
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Abstract: | Maternal effects are typically thought to enhance rather than reduce offspring performance, but asynchronous hatching (ASH)
in birds typically produces a size hierarchy within a clutch that frequently reduces the growth and survival of nestlings
from eggs that hatch later. Given that yolk steroids can significantly affect offspring phenotype and that in many species
the levels of yolk steroids have been found to increase with laying order, the maternal transfer of steroids to egg yolk has
been proposed as a mechanism for females to offset the deleterious effects of ASH. To test this hypothesis, we determined
whether yolk steroids varied with laying order or clutch size in Common Grackles (Quiscalus quiscula). Because ASH varies with clutch size (hatching span averages 48 h in five-egg clutches, 24 h in four-egg clutches) and regularly
results in the starvation of later hatched nestlings, we predicted: (1) testosterone and 17?-estradiol levels should increase
with laying order in both clutch sizes to mitigate the negative effects of ASH on last-hatched nestlings, and (2) the increase
should be greater in five-egg clutches due to more pronounced hatching asynchrony. Using a competitive-binding steroid radioimmunoassay,
we found no systematic variation in either testosterone or estradiol levels relative to laying order or clutch size. In the
absence of evidence that yolk steroids interact adaptively with ASH, research must look elsewhere for potential benefits that
might compensate for the costs these steroids impose on nestlings. |
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Keywords: | Asynchronous hatching Estradiol Maternal effects Testosterone |
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