"Consequently, extra-pair chicks ended up hatched almost 10 hours before their within-pair half-siblings, on average, allowing them to gain an initial size advantage because parents start feeding chicks as soon as they hatch."
Because of this early advantage, extra-pair chicks were more likely to survive and were also larger than their within-pair half-siblings by the time they left the nest. However, after correcting for variation in hatching time, the researchers found that all differences between extra-pair and within-pair chicks were reduced or absent, indicating that this non-genetic, laying order effect largely accounted for the observed superiority of the extra-pair offspring.
The next challenge for the team is to understand why there is a connection between paternity and the laying order of eggs in the first place.
"Genetic testing now allows us to determine the paternity of individual chicks in mixed broods, but we still don't understand why extra-pair offspring are laid earlier or even why females choose to mate with extra-pair males," said Dr Magrath.
One explanation could be in the timing of copulations. Females may engage in extra-pair matings to guard against the possibility that their social partner is infertile.
"Because females can often store sperm for several weeks, they may typically only copulate with extra-pair males before the start of laying as this may be adequate to fertilise all eggs in the clutch in the event of pair male infertility. However, in the vast majority of cases the pair male is fertile and this scenario could explain why extra-pair offspring become less likely among later laid eggs."
Even if females obtain a mix of sperm before laying, it is possible that the sperm from extra-pair males is more successful in fertilizing eggs if it is of higher quality (e.g. swims faster),
|Contact: Nerissa Hannink|
University of Melbourne