The LSTM Seminar Series continued this week with a presentation from Professor Glyn Vale of the Natural Resources Institute at the University of Greenwich. His seminar “Unifying the behaviour of tsetse: an item off the bucket list” was introduced by LSTM’s Professor Steve Torr.
Professor Vale started by describing how scientists seek to find frameworks that unify diverse and apparently unrelated phenomena. Work on the host-seeking behaviour of tsetse conducted over the last 50 years has shown a puzzling diversity in responses of tsetse to host stimuli. Work by Vale and colleagues at Rekomitjie in Zimbabwe for instance showed that different species and sexes of tsetse living in savannah woodlands vary in their responses to host odours with, for instance, female Glossina pallidipes being 10x more responsive to host odour than male G. m. morsitans, despite both living in the same habitat and feeding from similar hosts. Even more puzzling, studies of different species of tsetse in different habitats have shown that, in general, visual cues are more important for species found in riverine habitats, where dense vegetation is expected to restrict the effective range of visual stimuli, whereas olfactory cues are more important for species found in open savannah. All tsetse face the same problem of finding a host regularly, so why should they display such a diversity of host-seeking behaviours?
To explain these paradoxes, Prof. Vale developed a simulation model where he was able to explore how effectively flies might find different hosts in various types of habitat. In this ‘virtual world’, hosts ranged in size from lizards to elephants and habitats varied between extensive savannah woodlands, where tsetse could range widely, through to riverine areas with patches of dense vegetation that restricted the displacement of tsetse.
The model showed that in restricted habitats, the daily displacement of tsetse is reduced and hence they were less able to exploit cues that allow tsetse to locate their hosts such as odours or visual cues produced by large hosts.
A fly’s inherent capacity for flight also affects its ability to locate a host. Smaller tsetse (e.g., males, young flies, smaller species of tsetse) are inherently less mobile because their flight muscle is smaller. Consequently, even in the same type of habitat, the model shows that smaller tsetse are less efficient at locating their hosts.
Tsetse must feed every 3-5 days. If they fail to find a host in this time they will die of starvation. But feeding itself is dangerous for a tsetse: a cow swishing its tail or a human swatting a feeding fly can kill it. Prof. Vale’s model shows that for highly mobile tsetse, the best strategy is for flies to avoid risky hosts (e.g., a human) because there is a good chance that they will soon find a less risky one. For less mobile tsetse, such as a male fly living in restricted riverine habitat, the probability of locating any host is lower and when one is found the best strategy is to feed on it. Hence riverine tsetse feed on humans whereas only the most desperately hungry savannah fly feeds on humans.
The apparent diversity of responses therefore seems to be explained largely by a fly’s ability to fly. If the environment or the fly’s physiology restricts flight capacity then tsetse are less responsive to host stimuli and less fussy in their range of hosts. Highly mobile tsetse on the other hand are highly responsive to host stimuli and selective in their range of hosts. This fundamental driver of host-seeking behaviour influences the epidemiology of sleeping sickness and its control. On the one hand, the ‘fussy’ savannah flies are poor vectors of the trypanosomes that cause Rhodesian sleeping sickness and they can be controlled using odour baits. Riverine tsetse, on the other hand, feed on humans but can be controlled using tiny, odourless targets.