A Test for Adaptive Manipulation of Offspring Sex in House Wrens

Brendan Gallagher, Robyn Milkie and Rachel Molinaro

Department of Biological Sciences

Faculty Sponsors: Dr. L. Scott Johnson, Dr. Larry E. Wimmers and Dr. Brian Masters

(2004)

In insects, birds and mammals, gender is genetically controlled, yet some such animals are known to control offspring sex. For example, collared flycatcher females paired with highly ornamented males produce more sons than expected by chance. This increases the number of descendants because sons inherit their father's attractiveness and mate with many females. Our study asked whether females in another songbird, the house wren, manipulate offspring sex by preferentially placing female embryos in last-laid eggs and male embryos in first-laid eggs. The rationale for females doing so is as follows: 1) To attract a mate, a male must claim and defend a nest site and the largest, healthiest males do best in such competition. 2) Females typically lay 7-8 eggs, one egg every 24 hours. Females begin full incubation the morning that they lay their third-to-last-egg. Thus, by the time the last two eggs are laid, earlier-laid eggs have already had 24-48 hours of incubation. Consequently, the last two eggs hatch 24-48 hours later than the rest of the clutch and chicks from those eggs are younger and smaller than nestmates and remain small throughout their life. If birds from last-laid eggs are male, they may never breed. In contrast, undersized females always can breed. By the same reasoning, we hypothesize that embryos in first-laid eggs, which hatch first and typically produce the largest young in the brood, will be male. During 2002 and 2003 we visited 60 nests daily, numbering eggs as they were laid.and individually marking chicks coming from different eggs. Blood samples were taken to extract DNA and a portion of the female-specific chromohelicase domain 1gene was amplified by PCR and analyzed by agarose gel electrophoresis to determine gender. There was no statistically significant gender bias at any egg-laying position but there was a trend toward female in the penultimate egg in 2003. We consider a single trend in one season a chance event. We conclude the female house wrens do adjust offspring sex to position in the laying sequence.

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Updated July 9, 2004