Abstract:
BACKGROUND : Amblyomma variegatum and A. hebraeum are two ticks of veterinary and human health importance
in south-east Africa. In Zimbabwe they occupy parapatric (marginally overlapping and juxtaposed) distributions.
Understanding the mechanisms behind this parapatry is essential for predicting the spatio-temporal dynamics of
Amblyomma spp. and the impacts of associated diseases. It has been hypothesized that exclusive competition
between these species results from competition at the levels of male signal reception (attraction-aggregationattachment
pheromones) or sexual competition for mates. This hypothesis predicts that the parapatry described in
Zimbabwe could also be present in other countries in the region.
METHODS : To explore this competitive exclusion hypothesis we conducted field surveys at the two species’
range limits in Mozambique to identify areas of sympatry (overlapping areas) and to study potential interactions
(communicative and reproductive interference effects) in those areas. At sympatric sites, hetero-specific mating pairs
were collected and inter-specific attractiveness/repellent effects acting at long and short distances were assessed by
analyzing species co-occurrences on co-infested herds and co-infested hosts.
RESULTS : Co-occurrences of both species at sampling sites were infrequent and localized in areas where both tick
and host densities were low. At sympatric sites, high percentages of individuals of both species shared attachment
sites on hosts and inter-specific mating rates were high. Although cross-mating rates were not significantly different
for A. variegatum and A. hebraeum females, attraction towards hetero-specific males was greater for A. hebraeum
females than for A. variegatum females and we observed small asymmetrical repellent effects between males at
attachment sites.
CONCLUSIONS : Our observations suggest near-symmetrical reproductive interference between A. variegatum and
A. hebraeum, despite between-species differences in the strength of reproductive isolation barriers acting at the
aggregation, fixation and partner contact levels. Theoretical models predict that sexual competition coupled with
hybrid inviability, greatly reduces the probability of one species becoming established in an otherwise suitable
location when the other species is already established. This mechanism can explain why the parapatric boundary in Mozambique has formed within an area of low tick densities and relatively infrequent host-mediated dispersal
events.
