Theories bearing fruit

After rodents, microbats are the most speciose group of mammals, comprising 25 per cent of all mammal species on the planet today. But as the most recent issue of ALS described, it now seems very likely that fruit bats are not true bats, but airborne, upside-down primates with leathery wings.

In the late 1980s, Jack Pettigrew asked how his opponents could claim that DNA hybridization evidence supported the monophyly of fruit bats and microbats, when it could not even dissect the deep lineages within the microbat clade. In contrast, multiple, independent lines of evidence supported an evolutionary link between fruit bats and primates.

Why, for example, had fruit bats dispensed with ultrasonic sonar, the microbat superweapon? Why is the fruit bat wing supported by greatly elongated fingers, a primate characteristic, when the microbat wing is braced by elongated metacarpals - the bones between the wrist and the fingers?

And why does the fruit bat eye, and the wiring of the optic nerve to the brain's visual centres, conform to the bizarre Heath-Robinsonian primate model, not the microbat model?

These were just three of 50-odd differences listed by Pettigrew, a list recently extended by the discovery that fruit bats have a menstrual cycle - another almost exclusively primate trait.

Not much more than a disputed comparative DNA dataset now argues against a large, interlaced body of evidence that two mammalian lineages - one sharing a 70-million year old ancestor with primates- independently evolved flapping flight.

In the early 1990s, Pettigrew nominated colugos, a strange gliding mammal from south-east Asia's rainforests, as the likely ancestors or a sister group of fruit bats.

The relationship of colugos to other eutherian mammals was a taxonomic mystery - like primates, they comprised an orphan order, with no known sister order. Pettigrew's suggestion was remarkably prescient. A recent comparative DNA analysis shows colugos are the sister group to primates. They are our closest non-primate relatives.

Unfortunately, the analysis did not include fruit bats, because the authors did not explore the possibility that they might be related to colugos.

Until late last year, DNA-based approaches were not up to the task of dissecting the fine detail of evolutionary relationships within the Microchiroptera. In September, a British-Chinese research collaboration found a scalpel in FOXP2.

The study involved zoologists and evolutionary geneticists from East China Normal University in Shanghai, the Institute of Zoology and Graduate University of the Chinese Academy of Science, the University of London, and the University of Bristol.

It shows that FOXP2 is uniquely variable in microbats, among all mammals, and that the variation almost certainly reflects the evolution of multiple modes of echolocation. It also shows that distantly related microbat lineages have independently converged on very similar echolocation techniques.

According to Li et al, microchiropteran echolocation divides broadly between two modes: predominantly low-duty cycle, in which vocalisation is switched on for around 20 per cent of the time, and is frequency modulated (FM) - i.e. it sweeps up and down through a range of frequencies; and predominantly high duty cycle, in which vocalisation is switched on more than 30 per cent of the time, at a constant frequency.