Across most of the animal kingdom, offspring are born smaller than their parents. Surprisingly, several families of insects, including tsetse flies, are exceptions to this offspring-size rule. This amazing feat is possible thanks to two main factors. First, tsetse have a diet consisting exclusively of protein-rich blood and they can consume over twice their body weight in blood every few days. Second, tsetse do not possess the physiological constraints, such as the pelvic girdle of mammals, which prevent birthing of large babies.
Producing large offspring does, however, have a consequence that egg-laying insects do not face - female tsetse flies can birth only one baby at a time, at relatively long intervals. This results in a slow life history, in which the females must be continually pregnant and long-lived, if the number of offspring produced is to be enough to sustain the species.
Lead author, Dr Lee Haines, said: “The thought of a female giving birth to a single offspring that weighs more than she does is difficult to wrap your head around. Since we are familiar with human babies, which weigh ~6% of the mother’s pre-pregnancy weight, this feat is unthinkable. Even a blue whale calf, the largest baby in the world, weighing an impressive 2700 kg at birth, is only 1.5% of the mother’s weight!
Now, imagine giving birth to a massive baby every ten days… for the rest of your life. This tremendous maternal investment raises the question how natural selection could produce this seemingly impossible capacity. The unusual method of reproduction also carries enormous ramifications for tsetse biology, survival and the control of vector populations. I am excited for people to read our essay (particularly how it was inspired by a tweet*) and I anticipate that it will generate lively discussions among a diversity of readers".
Tsetse are not only fascinating from a biological perspective, but they are also a serious public health and veterinary threat because they transmit the micro-organisms responsible for the fatal human and animal diseases called African trypanosomiases. The essay discusses how the life history of the tsetse and its strange reproductive strategy directly influence the type of vector control interventions we can use to reduce fly populations and control the disease.
Since tsetse larvae remain protected within their mother’s uterus and the pupae are hidden in the soil, controlling tsetse populations is unfortunately restricted to targeting only adult flies. Thankfully, the tsetse’s slow breeding cycle means that you only need to kill a few percent of the adult female flies per day to eradicate a tsetse population. Currently, insecticide applied to cattle and artificial host-like baits, such as “tiny targets” are used successfully to control tsetse populations in disease-endemic areas. Other vector control strategies, like sterile insect techniques and microbiome-mediated control, can be more challenging to implement due to the specialised reproductive strategy of the flies.
Although effective management of tsetse populations to control disease transmission should always be the priority, it must be recognised that tsetse are biologically fascinating creatures. Tsetse flies make an important contribution to biodiversity and species richness in tsetse-endemic regions. The unusual life history of these flies warrants their preservation in areas where human and animal health are not threatened.
The essay was inspired, in part, by research on tsetse biology carried out over the past 50 years by Profs Vale and Hargrove and colleagues in the Zambezi Valley of Zimbabwe. Some of this work is depicted in the following video: Burrowing for knowledge.