Bats and moths have shaped baobab flowers across Africa

The baobab tree is found widely distributed across the African continent. It has provided food to people for thousands of years, have become an integral part of many local economies where baobab fruit and derivatives are sold in local and global markets. As a result tens of thousands of people in Africa depend on the income they get from baobab fruit. Baobabs also act as a keystone species in many savannah landscapes.

Baobabs have been historically known to be bat-pollinated (chiropterophilous) due to their large white blooms that open at night. However, in a 2020 study by Taylor et al. it was found that baobab flowers in South Africa are not visited by bats and only by moths. This prompted me to wonder if there might be any difference in the size and scent of baobab flowers being pollinated by moths in South Africa compared to the ones that are being pollinated by bats in other parts of Africa. My findings, recently published in the Botanical Journal of the Linnean Society, found that not only were the baobab flowers completely different between the regions, but that there were also differences in the species of bats which were pollinating the flowers in different parts of Africa and that this too was causing further regional differences in baobab flower traits.

Study sites around Africa

My research focussed on three areas of Africa, Ghana in West, Kenya in East and South Africa, Botswana and Namibia in Southern Africa (see map of study sites above). I collected flowers to measure the floral traits including the size of the stigma, style, stamen ball, width of the petals and length of the peduncle (see flower illustration below). I collected nectar to determine volume and sugar concentrations and identified floral scent using gas chromatography and mass spectrometry. Using camera traps and direct observations I enumerated the frequency and behaviour of floral visitors. Mist nets were used to help identify fruit bats and to collect pollen swabs.

Parts of the flower that I measured

Position of trail cameras to capture moths and bats visiting flowers

What I found is that in West Africa the baobab flowers had significantly longer flower stalks, larger blooms with strongly reflexed petals, longer stigmas, more nectar and flowers with a sour scent. East Africa and Southern African flowers were smaller in every way to the West African flowers and in Southern Africa flowers had less nectar, a sweeter scent, much wider stigmas and some flowers had weakly reflexed (droopy) petal and a style that was too short to emerge from the pollen ball.

The West Africa baobab flowers were visited by the large-bodied Straw-coloured flying fox bats (Eidolon helvum) (see illustration below). When visiting baobab trees these bats landed in the branches near a flower and crawled along the branch towards it. Once at the flower the bats hung on the branch with their feet and reached down to the flower headfirst to feed on the flower rotating the flower beneath them while drinking copious amounts of nectar caught in the cups formed by the large petals taking a leisurely 3 – 4 minutes to do so. The long peduncle of the West African flowers matched the body length of the E. helvum bat allowing for the head to be positioned close to the flower while hanging on the branch and drinking nectar. The longer style and the larger stamen ball enable contact with the head and chest of the E. helvum bats as they rotate the flower beneath them. The larger flower size and reflexed petals were able to hold enough nectar to satisfy the needs of these large bats.

In East Africa baobab flowers are visited by the medium-bodied Egyptian Fruit bat (Rousettus aegyptiacus) (see illustration below). When visiting the flowers these bats “crash land” directly on the flower with the peduncle acting as a “shock absorber”. Once landed, they clasp the flower with their thumbs and forefingers holding onto the petals with the stamen ball against their chest, making contact with the stamen and stigma. From this position, they placed their noses between the petals to access the nectar in the nectar cups. The bats generally only spent two to five seconds feeding before flying away.

Southern African flowers were only visited by moths. When visiting flowers long-tongued hawkmoths hovered at a distance from the flowers while probing their proboscises between the petals or stamen ball to access the nectar (see illustration below). Short-tongued hawkmoths and settling moths land on the petals or stamen ball to get access to the nectar. and often touch the stigma as well as the stamen ball. The weakly reflexed petals, may be “forcing” moths, which could easily thieve nectar from the strongly reflexed petals in East and West Africa, to fly in closer and extend their proboscis past the stamen ball, rather than past the petals, thus making contact with the stamen and stigma. A wider stigma and shorter style, as opposed to the longer stigmas and narrower style of the West and East African flowers, may increase contact with the reproductive organs especially for short-tongued moths and settling moths landing on the flower. Furthermore, the Southern African flowers with weakly reflexed petals retained drops of nectar on the surface of the nectaries, enough for use by moths, rather than larger quantities held in nectar cups as in East and West Africa.

Pollinator-driven changes to floral traits have been documented for a quarter of evolutionary divergence events, indicating that pollinators have played an important role in angiosperm diversification, but few studies have documented such divergence within the same species across its distributional range.

Pollinators are critically important for fruit production which in turn supports species recruitment and persistence of local economies based on baobab fruit harvesting. While baobab trees can cope with a wide range of environmental and climatic conditions (Venter and Witkowski, 2024), pollinators may be more susceptible to global change and therefore it is important to increase our knowledge of their role in baobab pollination.

My research will now focus more on the role of moth in pollination. In East and West Africa moths play a secondary role in pollination but we still do not know how important that secondary role is and which moth species are the most involved. We also need a better understanding of how moths are pollinating baobab flowers in Southern Africa. Thus, the work continues and will take many years to complete, but it will further a critical understanding of pollination ecology of Africa’s giant tree.

My research is supported by Milbon Pty Ltd and Skoon Skincare through donations to the Baobab Foundation NPO.