Female reproductive strategy in the Tawny owl
(Strix Aluco) as revealed by DNA markers:
Mate infidelity and offspring sex ratio allocation
Summary
In order to produce offspring able to propagate the lineage through the trials and tribulations of
natural selection, parents, especially the female, can implement several reproductive strategies. A
two year (2000-2001) molecular study of the Gribskov Tawny owls has provided evidence
supporting one of the two examined strategies.
Using microsatellite primers designed for the Eagle owl (Bubo Bubo), and the Scottish crossbill
(Loxia scotia), an extra-pair fertilization rate of 8.77% was uncovered. This is an unprecedented
frequency in birds of prey where many species have proven to be genetically monogamous as well
as socially monogamous. Because of the inability to capture the extra-pair mates of these offspring
we can only speculate on the advantages conferred upon them by the extra-pair male.
Molecular sexing techniques uncovered a male bias in both years of approx. 60%, which could lend
support to both Trivers and Willard’s (1973) individual-level optimization hypothesis, and Fisher’s
(1930) equal allocation of parental investment hypothesis. However, upon further analysis when
clutch size was taken into account. The apparent skewed sex-ratio was neither found to be
significantly different from parity nor the departure from parity expected by the difference in sizes
between the sexes. It would therefore seem that the female does not use facultative manipulation of
sex ratios to increase offspring fitness and can be ruled out as a reproductive strategy for the Tawny
owls of this area.
Data collection has continued in 2002-2003. This might provide new evidence, elucidating the
reasons behind extra-pair mating in the Tawny owl and further our understanding of the
overproduction of males in the Gribskov population.
16th of June at 14:00 in Auditorium A, Building 10.