Variability of sex ratio, mating probability and egg production in an intestinal nematode in its fluctuating host population.

In dioecious parasites, the chances of co-occurring with the opposite sex depend on the sex ratio, abundance and distribution pattern of parasites within the host population. Theory suggests that if the abundance and degree of aggregation are very low, mating probability may decrease so much that the parasite population is vulnerable to extinction. Our aim is to determine the factors affecting the mating probability and egg production in Heligmosomum mixtum (Heligmosomidae), an intestinal nematode of the bank vole, Clethrionomys glareolus, at Pallasjärvi, Finnish Lapland. We also search for factors responsible for the persistence of H. mixtum in its fluctuating host population. The results showed that during high parasite abundance practically all nematode females co-occurred with males, but during a phase of very low abundance only 15% of females had a chance to mate. Comparison of observed mating probabilities and those predicted by a theoretical model (May & Woolhouse, 1993) showed that deviation from the assumption of complete aggregation between males and females results in underestimation of the mating probability. Sex ratio and the degree of aggregation showed a minor effect on mating probability. The sex ratio (proportion of females) of H. mixtum, which was female-biased (0.58), showed a negative correlation with the mean intensity of infection in the monthly samples (decreasing female-bias at high mean intensity), but no significant relation to the intensity of infection (number of worms in a host individual). The long-term persistence of H. mixtum in its strongly varying host population seems to be due to the high transmission efficiency and long life-span of the parasite.