Ixodes uriae is a common ectoparasite of colonial seabirds in the circumpolar areas of both hemispheres. Despite its potential effects on host population dynamics and its role as a vector of pathogens, little is known about the reproductive strategies of this tick under natural conditions. Multiple mating of engorged females has been recorded in several instances, but the resulting paternity of offspring and its potential evolutionary significance have never been evaluated. Here, the paternity of offspring produced by females collected in the field was determined by using 5 polymorphic microsatellite markers. The results indicate that multiple mating in I. uriae can lead to multiple paternity in broods and, given the life history of this parasite, may be related to the adaptive benefits of producing genetically diverse offspring. Copulations took place both before and after the female's blood meal, but most successful fertilizations seemed to occur before engorgement. This suggests that the mating strategies of this tick may have evolved in response to local environmental constraints and, in particular, to the availability of conspecific ticks.

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