Tsetse fly breaks general rule that infants are smaller than mothers

Every 10 days, the female tsetse fly gives birth to a baby weighing more than she does. And, surprisingly, they are not hatched from eggs.

Tsetse flies are an exception to the general rule in the animal kingdom that offspring are born smaller than their mothers.

For comparison, human babies weigh only about 6% of their mother's mass. Even large herbivores, like cows, have babies that are on average below about 17% of the mother's weight.

How and why this startling phenomenon occurs among tsetse flies is the subject of a research paper published in BioEssays on Oct. 8, by an international group of scientists from the Liverpool School of Tropical Medicine and the universities of Bristol, Greenwich, Stellenbosch and California Riverside.

The authors note that the few exceptions to the smaller offspring rule are all insects in the superfamily of Hippoboscoidea within the order of Diptera (true flies). The females in this superfamily don't lay eggs like other flies. Instead they keep the developing embryo in the uterus and feed it there with milk-like secretions.

The 22 species of tsetse flies differ from the rest of the superfamily because they are strong flyers, feed on the blood of a variety of host animals, and have offspring that weigh more than the mother.

The authors ask how natural selection could produce the tsetse fly's ability to have supersize babies. There are two main factors, they note: their diet consists solely of protein-rich blood and they lack the physiological constraints that vertebrates have

"Tsetse flies are strong enough to maneuver effectively when carrying either a blood meal or baby weighing as much as themselves, but most vertebrates are not," they write.

The female tsetse keeps and feeds one egg in her uterus at a time until it hatches into a larva in about nine days. In the uterus it feeds on the mother's milk-like secretion that is 50% protein and 50% fatty acids. There is no placenta, so the embryo feeds directly on the "milk."

How does such a big baby get out of the uterus? The researchers explain that the larva is "soft and malleable" in form, so that it can be "squeezed out of the birth canal" even though it's as large as its mother.

After it is born, the larva burrows into the ground, where it has enough nutrients from the mother to develop into an adult.

The tsetse fly produces many fewer offspring in its lifetime than other egg-laying flies. However, the authors note, the tsetse flies are longer-lived in order to give birth to enough offspring for population replacement. Also, female tsetse flies mate as soon as they are able to ovulate, and they store the sperm indefinitely, so that they can continuously produce offspring.

The researchers note that because they feed on the blood of host animals, tsetse flies aren't constrained by seasonal flowers and nectar. They can therefore reproduce all year long. Further, the paper states, the tsetse has the advantage of being able to feed during hot and dry periods, when other biting flies cannot breed because they need water. This gives them a competitive advantage to surviving in high heat.

New offspring are more limited by high temperatures. However, the authors state, to overcome this, "the free-living life of a tsetse fly limits its habitat range to parts of the tropics that do not experience marked temperature extremes."

Tsetse flies are notorious in tropical Africa because they spread the trypanosome parasites that cause trypanosomiasis (sleeping sickness). This disease is fatal if not treated, but insect control measures have reduced the number of human cases to about 1,000 in 2019, the World Health Organization reports. Tsetse flies also harm livestock and wildlife and deter tourism.

The animal blood-feeding of the tsetse fly and its unusual reproductive cycle make it difficult to control for parasite disease transmission. The authors discuss a variety of options for vector control.

The researchers state that the priority must be to control disease transmission of the tsetse fly population, but that "we must also recognize that tsetse flies are biologically fascinating organisms," and humans must retain their species diversity to study and learn from them.

"Disease elimination does not justify pushing these species to extinction," the paper concluded.

"Burrowing for Knowledge" is a 7-minute video on the tsetse project research.