With their enormous eyes, impressive leaping skills and strange cries, galagids – more commonly known as bushbabies – are a familiar sight in sub-Saharan Africa. But there is still much we do not know about them.
The number of galagid species recognised today has more than tripled since the 1970s, from five to 18. The number of genera, or species groups, had risen from two to six. Now a seventh genus has been added, thanks to research conducted by my colleagues and I in the African Primate Initiative for Ecology and Speciation (APIES) and our collaborators.
The detection of previously unknown diversity is always exciting. For starters, it has novelty value. It shows that we have, until now, underestimated the subtlety of natural variation.
But most importantly, newly recognised genetic populations carry their evolutionary history with them – and the history of their habitats. The detection of new species is important because it tells us about new populations with particular relationships to the environment. A new genus provides more profound information about evolutionary history because it tells us about relationships among lineages that may reach deep into the past.
The emergence and extinction of species is closely linked to changes in earth’s environments and climates. So an accurate picture of the units of biodiversity is essential to understanding the nature of this interaction. This kind of information is crucial for forming conservation plans to defend vulnerable species against extinction – an increasingly urgent issue in the face of global climate change.
Limited “clues” to true diversity
Bushbabies (Family Galagidae) have only ever occurred in sub-Saharan Africa, where they have been evolving for at least 40 million years. They are nocturnal and relatively small; adult dwarf galagos weigh in at about 60g (the size of a large mouse) while the greater galagos can reach 1.5kg, the size of a cat.
Bushbabies are committed tree-dwellers although they may descend to the ground to feed or cross open areas. They have a number of adaptations that facilitate rapid and accurate movement through trees at night. These include their strong, grasping hands and feet, their elongated hind limbs and short forelimbs, and their large eyes fitted with mechanisms for accurate vision under low light intensities.
Understanding these animals’ true diversity is an ongoing task. This is because the bodies of different genera and species do not carry colourful flags or visual markers of identity, like many diurnal primates do.
For instance, most rain forest monkey species are distinguished by bright colour patterns on their faces and rumps that attract attention in gloomy forest. The lack of visually obvious markers in galagid species makes their recognition difficult to human observers – in the wild or even in museum collections. And true levels of genetic diversity may not be reflected in the animals’ external features.
The most reliable indicators of galagid species identity are the signals the animals use to communicate with one another at night. These include loud vocalisations and complex organic odours secreted by scent glands. The loud, repetitive call of the thick-tailed galago sounds like a baby crying, and has given rise to the common name “bushbaby”. Exploring these kinds of characteristics continues to inform us that our emphasis of visual differences has led us to underestimate biodiversity considerably.
Seven “dwarf galago” species have been identified to date, distributed across western and eastern Africa. They’re found in rain forests, montane forests or lowland forests.
Earlier models of bushbaby evolution viewed these small species as closely related and ancient, giving tacit credence to a commonly held view in mammal evolution: that members of a lineage start out small and get bigger over time. The discovery that these species are not ancient and closely related turns this idea on its head.
What we uncovered
Our study used information from DNA sequences, field observations and recordings of vocal repertoires. We also studied hundreds of specimens in natural history museums around the world, taking detailed measurements of bushbaby skulls and teeth.
All of this work demonstrated unequivocally that the dwarf bushbabies of West and East Africa have not shared a common ancestor for at least 20 million years. That’s half of the evolutionary history of the family. The fact that the species resemble one another so closely is a testament to evolutionary conservatism: if the structure of the habitat does not change substantially, neither will the organisms inhabiting it. Evolution is as much about change as it is about stasis.
As part of our field research, we also identified eastern dwarf galagos within South Africa’s boundaries, although the this genus was previously thought to be restricted to the eastern tropics. We have named the new genus Paragalago; it is the close relative of the genus Galago, which includes the “nagapies” (literally “little night monkeys” in Afrikaans) that are found in South Africa’s northern woodland savannas, but is likely to have evolved much further north, in East Africa.
So what do the new studies tell us about the history of bushbabies and Africa?
First, we now know that small body size is not a sign of early evolution in bushbabies. Galago ancestors are unlikely to have been smaller than between 500g and 700g. Dwarfing is a common evolutionary response to unpredictable environmental conditions, reflecting Africa’s past climate change.
Using genetic dating techniques, we estimate that both the western and the eastern dwarf galago lineages emerged around 10 million years ago. This coincides with the beginning of the expansion of grassland savannas in Africa, long before the African Rift Valley sundered the continent. Grasslands replace forests when the climate becomes cooler and drier, causing the forests and their faunas to fragment into small, patchy populations unable to interbreed.
The divergence of the Paragalago lineage would have been a response to this habitat disruption. But many other organisms that did not adapt to the new circumstances would have gone extinct. Forests are the most vulnerable of African habitats to climate change. The crisis we are currently witnessing will be similarly destructive to those in the past, and the survivors are unlikely to be tree-dwelling animals.