SEX-RATIO CHARACTERISTICS IN IXODES-RUBICUNDUS (ACARI, IXODIDAE), THEKAROO PARALYSIS TICK

The sex ratio is an important parameter which characterizes the state and dynamics of natural populations of animals. Although ixodid ticks are specialized ectoparasites, most species are bisexual and are chara cterized by a 1:1 sex ratio for their progeny. In natural populations and even in laboratory colonies, biased sex ratios are often observed. Ixodes rubicundus, the Karoo paralysis tick, parasitizes domestic sto ck and wild ungulates in South Africa. Adults quest from vegetation, c an mate off or on the host and males are seldom parasitic. We hypothes ized that the sex ratio for I. rubicundus would be 1:1 when observed d irectly in the progeny but that it would be strongly biased towards fe males in samples of parasitic adults. The results mostly supported the hypothesis but it was also shown that unexplained and unpredictable v ariations can occur. On hosts, females dominated strongly, except on a dult angora goats where the sex ratio was biased in favour of the male s. This disparity may be related to a greater retention of males in th e coarse, curly hair of angora goats compared to the other hosts. Mont hly variations in the sex ratios of the tick on hosts are believed to be related to the large fluctuations in sex ratios of questing ticks.