Abstract
Simultaneously dependent siblings often compete for parentally provided resources. This competition may lead to mortality, the probability of which may be a function, in part, of the individual offspring's production order. In birds, serial ovulation followed by hatching asynchrony of simultaneous dependents leads to differences in post-hatching survival that largely depend on ovulation (laying) order. This has led to the widespread assumption that early-laid eggs are of greater value and therefore should possess different maternally manipulated characteristics than later-laid eggs. However, this perspective ignores the potential effect of laying order on pre-hatching viability, an effect which some studies suggest should offset the effect of laying order on post-hatching viability. I examined the relationship between laying order and hatching and fledging probability in wild, free-living Lincoln's sparrows (Melospiza lincolnii). In broods with complete hatching success, first-laid and therefore first-hatched offspring had the highest probability of fledging, and fledging probability declined with increasing laying order. However, first-laid eggs were less likely than later-laid eggs to hatch. This effect of laying order on pre-hatching viability seemed to offset that on post-hatching viability, and, consistently, maternal investment in egg size varied little if at all with respect to laying order. These results suggest that ovulation order mediates a trade-off between pre-hatching and post-hatching viability and should encourage a re-evaluation of the solitary role post-embryonic survival often plays when researchers make assumptions about the value of propagules based on the order in which they are produced.
Highlights
Many organisms produce offspring serially, by the consecutive ovulation and spawning of each one
In some animal groups as taxonomically diverse as beetles [1], marsupials [2], and primates, parents rear serially produced post-natal siblings simultaneously, setting the stage for a competitionmediated probability of post-natal mortality that depends, in large part, on the order in which the siblings are produced [3]. These effects of ovulation order have important implications for how maternal investment and manipulation of offspring traits should vary across the brood, the best maternal strategy should depend on how ovulation order influences viability not just at the post-natal stage and at the pre-natal stage
In Lincoln’s sparrows, viability during the nestling stage is highest for the first hatched of the brood and declines with laterhatched siblings. This is due to differences in growth rates imposed by the developmental hierarchy among siblings of different age
Summary
Many organisms produce offspring serially, by the consecutive ovulation and spawning of each one. In some animal groups as taxonomically diverse as beetles [1], marsupials [2], and primates (including humans), parents rear serially produced post-natal siblings simultaneously, setting the stage for a competitionmediated probability of post-natal mortality that depends, in large part, on the order in which the siblings are produced [3] These effects of ovulation order have important implications for how maternal investment and manipulation of offspring traits should vary across the brood, the best maternal strategy should depend on how ovulation order influences viability not just at the post-natal stage and at the pre-natal stage. As suggested above and reasoned below, maternal manipulation of egg traits should depend on the effects of ovulation order on pre-hatching viability
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