Millennium Seed Bank blog

"It would not, perhaps, be correct to say that the Durian is the best of all fruits, because it cannot supply the place of the subacid juicy kinds, such as the orange, grape, mango, and mangosteen, whose refreshing and cooling qualities are so wholesome and grateful; but as producing a food of the most exquisite flavour, it is unsurpassed."

[Alfred Russell Wallace, 'The Malay Archipelago', 1869]

Durians for sale at a market in Malaysia (Photo: Siew Hoon Lee) 

The King of Fruits

If there are "100 things you should taste before you die", then the durian fruit is certainly in the top ten. Hailed as the undisputed ‘King of Fruits’ in southeast Asia, to westerners this exotic delicacy can be a real ‘Marmite test’: you either love it or you hate it! However, unlike Marmite, the durian’s contentious ambiguity does not lie in its taste, which is delicious, but rather in its disagreeable smell. To give you an idea, the experience of tasting your first durian can be compared to eating custard cream with a dash of Baileys in a men’s changing room crowded with steaming athletes returning from a heavy workout and a sumptuous dinner of raw onions and garlic. Hmm...

Understandably, this may have dampened your appetite for durian. So here’s some botany first, before we delve deeper into the culinary qualities of this most infamous of fruits and discover the true significance of its outrageous smell. 

A durian fruit opened up to expose the large seeds wrapped in yellow juicy arils (Photo: Wolfgang Stuppy)

Durio zibethinus

The southeast Asian genus Durio belongs to the Mallow family (Malvaceae) and comprises 20-30 species, at least eight of which are edible. The economically most important species of durian (Malay for “thorny fruit”), Durio zibethinus, has been cultivated in south-east Asia for centuries. Its large white nocturnal flowers are borne directly on the major branches (a phenomenon called ‘cauliflory’) which later have to be strong enough to carry the heavy fruits. The flowers reportedly smell like sour milk and are pollinated by bats which drink the copious nectar offered.

Flowers of Durio zibethinus (Photo: Bryan Brunner, Montoso Gardens, Puerto Rico)

Durians are so popular in southeast Asia that there are literally hundreds of different cultivars of Durio zibethinus. This is also the only species traded internationally. You can easily pick up a durian in London’s Chinatown if you are willing to pay around £20 for one. Alternatively, you can buy a few individual seeds on a Styrofoam tray covered with cling film. As for different flavours, I have only ever seen one single variety offered in Europe, which is usually imported from Thailand - the Mon Thong variety (Mon = pillow, Thong = gold).

Portions of durian seeds for sale in Malaysia (Photo: Siew Hoon Lee)

Size and growth

The durian’s highly-prized fruits can be the size of a football and weigh up to four kilograms. On the outside they are covered by a ferociously spiny, dull green to yellowish-brown husk (duri = Malay for thorn). Inside, the fruit is divided into 5 compartments representing the five carpels from which the fruit develops. When ripe, the heavy fruits drop off the branches and split slightly from the top down along 5 distinct, preformed lines which correspond to the median lines of the carpels. At this stage the fruits begin to emit their infamous ‘body odour’ that has been variously described as resembling a blend of sweat, faeces, unwashed socks or a mix of rotten garlic and onions. 

Detail of the spiky outer husk of a durian (top) and (below) the tip of the fruit as it begins to split when it’s ripe (Photos: Wolfgang Stuppy)

Durian products

Disliked by most Europeans and banned from the underground in Singapore, the durian is treasured by people all over Asia and revered as the veritable ‘King of Fruits’. What seems like a dubious admiration does not reflect a twisted olfactory preference, but a fondness for the extraordinarily delicious taste of what lies inside the malodorous pod. The people of southeast Asia don’t just enjoy the fresh fruits. Durians are used to make all kinds of mostly sweet treats like pastes, cakes, cookies, milk shakes, ice cream, chips, candies and lots more. 

Southeast Asian delicacies made with durian, e.g. ice cream, cake, durian puff and candies (Photos: Siew Hoon Lee & Wolfgang Stuppy) 

Wallace on durian

The edible part of the durian consists of the white or cream to golden-yellow arils (= seed appendages) covering several very large (c. 2 x 6 cm), chestnut-brown seeds. When the fruits are ripe, the hard tissue of the yellowish arils disintegrates into a custard-like cream whose consistency and flavour has been described as a tantalising mixture of nuts, spices, bananas, vanilla and onions. After his first visit to Borneo, the great 19th century naturalist Alfred Russell Wallace wrote:

The Durian grows on a large and lofty forest tree, somewhat resembling an elm in its general character, but with a more smooth and scaly bark. The fruit is round or slightly oval, about the size of a large cocoanut, of a green colour, and covered all over with short stout spines the bases of which touch each other, and are consequently somewhat hexagonal, while the points are very strong and sharp. It is so completely armed, that if the stalk is broken off it is a difficult matter to lift one from the ground. The outer rind is so thick and tough, that from whatever height it may fall it is never broken. From the base to the apex five very faint lines may be traced, over which the spines arch a little; these are the sutures of the carpels, and show where the fruit may be divided with a heavy knife and a strong hand. The five cells are satiny white within, and are each filled with an oval mass of cream-coloured pulp, imbedded in which are two or three seeds about the size of chestnuts. This pulp is the eatable part, and its consistency and flavour are indescribable. A rich butter-like custard highly flavoured with almonds gives the best general idea of it, but intermingled with it come wafts of flavour that call to mind cream-cheese, onion-sauce, brown sherry, and other incongruities. Then there is a rich glutinous smoothness in the pulp which nothing else possesses, but which adds to its delicacy. It is neither acid, nor sweet, nor juicy; yet one feels the want of none of these qualities, for it is perfect as it is. It produces no nausea or other bad effect, and the more you eat of it the less you feel inclined to stop. In fact to eat Durians is a new sensation, worth a voyage to the East to experience.."  [Alfred Russell Wallace 1869: 'The Malay Archipelago'] 

As much as the people of southeast Asia love durians, because of the smell they leave behind it is forbidden to carry them on the underground in Singapore (Photo: Siew Hoon Lee)

Bad smell and seed dispersal

Believe you me, if you have never tried a durian before, you will never have tasted anything so amazingly rich and complex in your life. I absolutely love durian, but I have seen fellow Europeans unable to share my enthusiasm because they simply could not overcome the unsavoury associations conjured up by the durian’s bad smell. As a scientist, I am looking for a logical explanation which helps me to understand the durian’s offensive stench. It is pretty obvious that it all boils down to the durian’s strategy of getting its seeds dispersed.

The durian’s dull green to brownish colour and odour strongly suggest that the fruit is adapted to be dispersed by mammals. Most mammals are colour-blind or have just dichromatic vision but a very keen sense of smell. They rely much more on their noses than their eyes to find food or sense approaching predators.

Weirdly, heavy, sickly and sometimes yeasty scents with olfactory components of fermenting fruit and ‘mammalian body odour’, which we humans find repulsive, are very attractive to many (other) mammals. What’s more, the fact that the durian drops to the ground when ripe, combined with its large size, heavy weight and tough husk indicate that only large terrestrial animals with a mouth big enough to swallow the bulky seeds and sufficiently strong and dexterous to break open the spiky armour are invited to the meal.

Only larger animals have the strength to break through the spiky husk of a durian (Photo: Wolfgang Stuppy)

Seed dispersers and pulp thieves

It is therefore not surprising that a durian that has landed on the ground and begun to emit its more than recognizable smell attracts a variety of big beasts including elephants, Asian rhinos, orangutans, gibbons, monkeys (e.g. long-tailed macaques), tapirs, wild boar, deer and even carnivores like tigers, leopards, civets and sun bears. Any remains of the durian left by these big animals are scoured by smaller durian-lovers ranging from squirrels to beetles and ants. Naturally, such a rich and delicious treat like the durian attracts a lot of attention, both from genuine bona fide seed dispersers, the durian’s actual ‘target customers’, as well as from pulp thieves. The latter indulge in the nutritious pulp but actually do not help to disperse the seeds and sometimes even destroy them in which case they become seed predators.

Ideal dispersers either carry the seeds over a significant distance (e.g. 20 m or so) and discard them undamaged after eating off the aril (e.g. long-tailed macaques) or they swallow the seeds whole and defecate them intact (e.g. elephants, rhinos). Pulp thieves, on the other hand, would eat the pulp but then fail to transport the seeds (e.g. sun bears). It is not easy to find reliable scientific information about the dispersal ecology of durian. However, during one very interesting field study carried out in Sabah, Malaysia long-tailed macaques turned out to be the most efficient bona fide disperser of the seeds of Durio zibethinus. 

A single seed inside a durian, still wrapped in the delicious yellow aril (Photo: Wolfgang Stuppy)

Pygmy elephants and orang utans

Although elephants are known to eat durians and pygmy elephants (Elephas maximus borneensis) were present in the study area, they were not observed to help with dispersal. Sumatran rhinos (Dicerorhinus sumatrensis), also very likely to be native bona fide dispersers in Sabah, were absent from the study area. Most surprisingly, orang utans, long known to be avid consumers of durians and generally deemed to be among its primary dispersers, turned out to be mere seed predators. The apes pluck immature durians and eat both aril and seeds, leaving only the seed coats which they spit out after their meal.

Orang Utan (Pongo pygmaeus) in Borneo eating a durian (Copyright © Konrad Wothe; www.konrad-wothe.de)

This is somewhat surprising because with their flimsy brown seed coats, durian seeds rely on chemical rather than physical protection (see my post about the ice-cream bean) in the form of toxic fatty acids (e.g. sterculic and malvalic acids) present in the tissue of the storage embryo. Perhaps orang utans have developed some kind of immunity or their bodies simply tolerate higher doses of these toxins than other animals. Toxins aside, durian seeds are highly nutritious, consisting of nearly 50% carbohydrates with little protein and fat. Boiling or roasting turns them into an edible starchy treat, a source of food long used by native peoples.

Another study showed that although tapirs eat durians and could therefore act as bona fide dispersers, the seeds fail to germinate after passing through the animal’s gut.

Acknowledgments

I am very grateful to my Singaporean friends Siew Hoon Lee and Hong Ling Lim for taking lots of photos of all things durian for this blog. Many thanks also to Konrad Wothe (www.konrad-wothe.de) for his kind permission to use his photograph of an Orang Utan eating a durian, and to Bryan Brunner from Montoso Gardens in Puerto Rico for allowing us to use his photograph of the flowers of Durio zibethinus. 

- Wolfgang Stuppy -

P.S.: I am not quite sure how but, allegedly, durian acts as an aphrodisiac. As a Malay saying goes: "When the durians fall down, the sarongs fly up!"

Related links

• Further information on Botanical terminology
• Artist Marianne North recounts life in Singapore including the 'Darling Durian'
• Kew Fund: Chocolate, Rainforests and Conservation - find out how Kew's work is helping to protect species like the Durian  

We are looking for two enthusiastic individuals passionate about the environment to join as Seed Processing Assistants. This job is based in the Millennium Seed Bank at Wakehurst Place, West Sussex.

A seed processing assistant at work

In this interesting and varied role you will work with the seeds from arrival, ensure their safe storage, and carry out germination tests. Read more about how we process seeds. 

So if you fancy a unique hands-on opportunity to contribute to the largest ex situ plant conservation project in the world, please apply via the application page:

Seed Processing Assistant application page

Closing date for applications: 13th June 2013

The Millennium Seed Bank Project (MSBP) and four Tanzanian organisations have been working together to conserve the Tanzanian flora through ex-situ seed conservation since 2006. I joined John Elia a botanist at the National Herbarium of Tanzania (NHT) and Lourance Mapunda a seed scientist at the Tanzania National Plant Genetic Resources Centre (NPGRC) on a seed collecting trip to Mount Kilimanjaro in February.

Mount Kilimanjaro, as well as being the highest mountain in Africa at 5,895m high, is also a UNESCO world heritage site, part of Conservation International's “Eastern Afromontane” global biodiversity hotspot, and a popular tourist destination. It has a large flora of around 2,500 plant species with many Tanzania endemic species (i.e. species that are only found in Tanzania)... Lots of potential for a successful seed collecting trip! 

View of the two peaks of Mount Kilimanjaro, Kibo (on the left) and Mawenzi (Image: Emma Williams)

Before the trip began I produced a target list of species for our fieldwork focusing on Tanzanian endemic species and those restricted to high altitudes. Mountain species are particularly vulnerable to climate change. As the climate warms, alpine plant species move higher up the mountain and are at risk of extinction as their area of suitable habitat to live in decreases. I used the recently finished 'Flora of Tropical East Africa' to find descriptions of my target species, and studied herbarium specimens at Kew to find locations on Mount Kilimanjaro where they had been recorded before. 

Dendrosenecio kilimanjari – a Mount Kilimanjaro endemic and one of our target species. Some of these plants could be several hundred years old. (Image by Emma Williams)

I flew to Tanzania at the end of January and spent a few days in Arusha studying specimens at the National Herbarium, organising the trip logistics and obtaining a collecting permit from the Tanzania National Park Authority who manage Mount Kilimanjaro.

The first part of our trip was a trek up the Marangu route on the south east side of the mountain. We began at the park gates at 1,900m in tropical forest and would eventually walk up to 4,100m, at almost the limit of vegetation on the mountainside. We hired porters and a cook to help with the trip; once we started walking up the steep trail we were certainly grateful that we didn't have to carry all our bags ourselves! We stayed in small mountain huts at Mandara camp (2,700m) and Horombo camp (3,700m). Although it was sunny and warm during the day the temperature quickly dropped at night. 

Fieldwork team of Emma Williams (2nd from left), Lourance Mapunda (3rd from left) and John Elia (1st on right) with our porters at the start of the Marangu route. (Image by Emma Williams)

The flora and scenery on the route were spectacular. We found 3 species endemic to the mountain in flower - the stunning giant groundsel (Dendrosenecio kilimanjari), Euryops dacrydioides and Lobelia deckenii which was also fruiting so we made a good seed collection. We made 17 seed collections on our six day trek. For each seed collection we also took herbarium specimens to identify back at Kew’s Herbarium, a GPS recording of our location, and filled in a data sheet with all the information about the collection. 

Lobelia deckenii – a Mount Kilimanjaro endemic, now conserved at NPGRC and the MSBP seed banks. (image by Emma Williams)

For the rest of our field trip we carried out day-trips up part of two other routes on the south of the mountain. On the Machame route we collected seeds in a lush tropical rainforest with tree ferns, impatiens and epiphytic orchids and ferns. Whilst the Shira Plateau route was drier and dominated by Erica, Hypericum (St John's Wort) and many species of Asteraceae (the daisy family).

At the end of the trip we had made 31 seed collections and taken 21 herbarium specimens of plants in flower which we can target on future trips. The seeds will be cleaned at NPGRC and each collection will be divided into two. Half will be kept in cold storage in Tanzania and the other half will be sent to be stored at the MSBP. We are now planning our next trip to Mount Kilimanjaro in December and hope to explore some of the other routes on the mountain.

- Emma -

Related Links

• More about the MSBP Tanzania Project
• Flora of Tropical East Africa
• Conservation International Eastern Afromontane biodiversity hotspot profile
• Tanzania National Parks

Paul Smith, Head of Kew's Millennium Seed Bank, carried out a vegetation survey of the North Luangwa National Park back in the 1990s. Now he's returned to the Park to help with the black rhino reintroduction project. He has been studying the plants and habitats that black rhinos prefer and keeping a daily record of his activities. Read his daily diary below.

Black rhinoceros (Diceros bicornis) (Image courtesy of Save The Rhino International)

Wednesday 27th February

I set off at seven this morning accompanied by two game scouts armed with AK 47s and a rhino tracking device. My assistants are called Chambo and Michael.

Chambo and Michael standing by a shrub, Combretum zeyheri, browsed by black rhino.(Image: Paul Smith)

We find a rhino!

We headed west and then turned south, parallel with the Muchinga escarpment into the territory of a male rhino called Kango.  We located Kango with the tracking device (he has a radio transmitter collar), but he caught wind of us when we were about 40 yards away. He took off with quite a hullabaloo – fortunately, downwind and away from us. Rhinos are big, heavy animals and the last thing you want is two tons of irate rhino coming at you through thick bush. As soon as he had crashed away, we were able to pick up his spoor and backtrack the route he had taken in the night. Black rhino tend to do their browsing (eating shrubs and trees) at night and lie up during the day. His tracks were easy to follow in the soft ground, and Chambo (from the Bisa tribe), in particular, is a good tracker.

We followed the rhino's route for a mile or so, and it was clear that he hadn’t been stopping during that part of his journey because there was no sign of any browse bites on the vegetation along the way. He left regular middens to mark his territory until he got to a small stream. There we picked up lots of evidence of what he had been eating.

What was he eating?

We recorded signs of browsing on the following species: Combretum zeyheri (Mufuko – Bisa); Ormocarpum kirkii (Mupulupulu - Bisa); Dichrostachys cinerea cinerea (Lupangala – Bisa, Senga); Argyrolobium sp.; Duosperma crenatum (a favourite); Lannea humilis; Jasminum stenolobum; Commelina bracteosa (blue flower);Ocimum americanum (Lwena – Bisa); Dalbergia melanoxylon; Pterocarpus rotundifolius (Muchalala – Bisa); Acacia tortilis; Markhamia zanzibarica; Feretia aeruginescens; Cissus cornifolia; Siphonochilus sp.; Premna senensis; and Phyllanthus reticulatus.

Tracking radio-collared black rhino from the air. (Image: Paul Smith)

It is interesting that he prefers to eat in the riverine area where most of these species occur. At this time of year with plenty of forage around, he appears to prefer the soft stuff in the understorey near the ground. We got back into camp at lunchtime with some useful, new data.
 

Thursday 28th February

Up at 5.30 this morning, I flew with Ed across the valley to drop some supplies to game scouts on the other side of the Luangwa river. Beautiful views in the early morning. We saw elephants, zebra, wildebeest, eland and then a pride of 15 lions following a buffalo herd on the Mwaleshi river. Back for breakfast at seven, then out with my companions, the game scouts Chambo and Michael, to do our first rhino browse transect.

Game scouts Chambo and Michael helping with the rhino browse transect (Image: P. Smith)

Trying to estimate how much food is available

Black rhinos are exclusively browsers, meaning they eat only woody plants. White rhinos, in contrast, are grazers, i.e. they prefer grass. Unlike yesterday, where we simply back-tracked a rhino to see what he had been eating, the task before us today was to gather information on the amount of available browse in the rhino sanctuaries. To do this we were driven out to our starting point six or seven kilometres from the camp, and we walked back on a 300 degree bearing, recording the available woody plants in each of the three habitats we passed through, noting some exotic local fauna, like this chameleon.

Chameleon on Brachystegia shrub in the miombo woodland (Image: P. Smith)

We started in miombo woodland which is very dense, recording all browse in touching distance of our path (a metre each side), estimating the volume of woody plant material available under two metres, i.e. in reach of a rhino. After recording five pages of miombo browse (around 200 plants) we were interrupted by a herd of elephants, and decided to move on to the next habitat - Combretum-Terminalia wooded grassland. From there we came to riverine woodland and grassland, recording all species within rhino range as we passed. We eventually got back to camp at one o’clock. The same procedure was followed this afternoon in a different part of the park. We got back in at about 5.30, having recorded some 800 individual plants, ready for a cold beer.

Bauhinia petersiana, thicket species favoured by rhino, in flower (Image: P. Smith)

Planning for the future

Our two transects today were marked with orange spray paint on occasional trees along our route, and as waypoints on the GPS. This means that these transects can be walked again in October at the end of the dry season to compare the amounts of available browse. This is when the rhinos are likely to be most hungry, and the data we are gathering will help the park managers to measure the carrying capacity of the current rhino sanctuaries. This, in turn will help ensure that stocking levels are appropriate. If this seems like a lot of trouble to go to, it is worth remembering that black rhinos are critically endangered. In South Africa alone, it is estimated that a rhino is poached every 14 hours for their horns. According to TRAFFIC, the major market is the far east where the horns are used as a hangover cure amongst other things.

Friday 1st March

No flight this morning. We headed out by car south, and then west along the Mwaleshi river. The riverine vegetation here is very productive, being associated with deep, alluvial soils. The result is plenty of vegetative biomass. It is very green at this time of year.

Transect team above the Mwaleshi after a morning's work (Photo: Ed Sayer)

Near the river, on the flood plains is mainly grassland, interspersed with a few shrubs and trees such as Acacia sieberiana and Kigelia africana (the sausage tree). Then, as you move away from the river, you come into the thicket, dominated by Combretum obovatum and then Combretum fragrans and the ‘monkeybread’ Piliostigma thonningii. As you gain higher ground, Terminalia and Combretum zeyheri start to appear. You also see Phyllanthus reticulatus – the ‘baked potato bush’, the tiny flowers of which produce a smell like baking potatoes during the evenings in May through to September. This is one of the evocative smells of the Luangwa valley.

We get scared!

We sampled around 300 plants on our narrow transect, covering perhaps a couple of kilometres up the riverine catena. We finished at about 11 am, and headed back up to the car with Ed Sayer (Chief Technical Adviser in North Luangwa) leading the way, Chambo and Michael behind him and me bringing up the rear. Ed stepped over a Mocambique spitting cobra without noticing it, but Chambo coming second jumped a foot in the air as it raised its head and inflated its hood. We rapidly retreated, and it went on its way. I got a fuzzy photo on full zoom!

A glimpse of a Mocambique spitting cobra. Neither party was pleased to see each other. (Photo: P.Smith)

Back for lunch and then out again this afternoon. I got to try out the newest vehicle in camp – the aptly named Yamaha Rhino, a kind of quad bike but with a steering wheel and a gear stick. Great fun, and seems to be able to go anywhere.

Trying out the latest in off-road vehicles - the aptly named Yamaha 'Rhino'.(Photo: Ed Sayer)

We did another riverine transect this afternoon without encountering anything more dangerous than a herd of zebra. Roughly a thousand plants sampled in our transects so far. It will be interesting to see how heavily they have been browsed by the end of the dry season.

Saturday 2nd March - I get covered in insects!

This morning we headed out to the east of the park into Combretum-Terminalia woodland and thicket. We were on the trail of two rhinos: Londekeni, whose name means ‘Lost but found’ in Bemba; and Mwaiseni (‘Welcome’). We travelled out to the thicket in the Yamaha Rhino, which you will recall from yesterday has no windscreen. While I was prepared to take a lot of grass seed on board, I was less prepared for the wide range of spiders, stick insects, beetles and caterpillars that live on the tall grass stems in the middle of the road.

Flyblown!

At one point we had to stop to let a golden orb spider out – 10cm across and with a nasty bite. Apart from this minor drawback the vehicle did very well – no terrain was too difficult to cross. We arrived in the rhino’s territory at about 9am, and climbed an observation tower to scan for rhinos. We found Londekeni first, and tracked him to where he was hiding in some thick vegetation. I caught sight of his broad back as he beat a retreat when he heard us coming. We were able to track his movements back into the thicket, though - from where he had slept to his forays for food.

Tracking rhinos from an observation tower

The species we recorded with rhino bites were: Baphia massiensis, Maerua pritwitzii, Phyllanthus sp., Combretum molle, Catunaregum spinosa, Aneilema nicholsonii, Commelina africana, Holarrhena pubescens, Boscia angustifolia, Strychnos potatorum, Excoecaria bussei (a favourite), Barleria prionitis and one other Acanthaceae as yet unidentified. We then carried out a browse availability transect in the thicket area.

Morgan with rhino-browsed Holarrhena

Lots of interesting species around at this time of year, and I was able to take some good photographs for a book on the trees and shrubs of the Luangwa valley that I am working on. We also found two new species for the park’s checklist – the Strychnos potatorum mentioned above, and Philenoptera bussei, another shrub/small tree with signs of elephant browse damage. Back at 1pm for lunch, and then out again to do a riverine transect on the Lubanga river.
 

Sunday 3rd March

Out with my team again at 8am this morning, this time on the trail of a rhino called Bukwele (‘rhino’ in Bemba). We set out in the Landcruiser today because the river has risen a couple of feet, and we would have drowned the Yamaha had we tried to cross in it. The added bonus was no flies in the face today. Bukwele still receives some supplementary feed, so we started at his trough, which is out in the scrub savanna vegetation about 10 km from the camp. According to his radio collar signal, Bukwele was a long way to the south, so we didn’t see him. Instead, we turned north, and put a long transect in through mopane scrub, Combretum-Terminalia-Diospyros wooded grassland and a riverine gully that a large herd of buffalo had recently vacated. Lots of mud and cowpats to avoid.

Elephant on the Lubanga river

Early March is a great time to be in the valley. Everything is very green with many species in flower. Gladiolus dalenii (orange) and Gladiolus gregorius (purple) dot the landscape along with many pink Ipomoea (morning glory), and Hibiscus species of various colours. Bukwele had been eating Duosperma crenatum, a small but abundant herb, which we had noted as a favourite of Kango’s on day one.

Gladiolus gregarius

My game scout assistants have a detailed knowledge of their local flora and we have been comparing local and Latin names. The one that has flummoxed them is Pseudolachnostylis maprouneifolia, a tree that is simply known as ‘Msolo’ in these parts. You can guess which name we have been using on the transect forms.

Hot and humid today, with storms gathering over the escarpment. We were glad to head for home at lunchtime. This afternoon was spent writing up the data of the past few days.
 

Monday 4th March - the final day

Today marks my last day in the valley, so my time was spent tying up loose ends.

My home for the past week

This morning Michael, Chambo and I went back to a couple of our transect sites to identify some of the ground cover species. During my time here in the early 1990s I made photocopies of my herbarium collections. These are still here, and a very useful reference library. We started in the mopane scrub where the main ground cover species are Indigofera schimperi (pink flowers), Cyphostemma gigantophyllum, Ampelocissus africana and Jasminum stenolobum (a rhino favourite). We also found two Vigna climbers in abundance – Vigna vexillata and Vigna frutescens.

From the mopane we went on to the thicket nearer the river and were able to find a couple of early flowering individuals of Hygrophila auriculata (rhino thistle) and Spermacoce princeae, which are both eaten by the rhinos. There we found a beautiful lagoon with a family of Egyptian Geese on it. We counted seven goslings.

Lagoon with Egyptian goose just disappearing out of sight

Then back to camp for an afternoon of data crunching. We have collected data on about 3000 individual trees and shrubs, and that is going to take some time to put on the computer. The whole thing will have to be done again in September/October but at least we have worked out the methodology, so this should be relatively straightforward. The contrast between available browse in the rainy season (now) and the dry season (then) will be very interesting – and will vary between the habitats we have sampled.

Finally, a big thank you to my hosts Ed Sayer and Claire Lewis. It has been great to be back in the Luangwa valley in such good company.
 

- Paul -

Related links

North Luangwa Conservation Project Facebook page

Millennium Seed Bank Partnership

North Luangwa National Park

More about the critically endangered  Black Rhinos

Known as 'castor beans', the seeds of Ricinus communis, a member of the spurge family (Euphorbiaceae), are among the most infamous seeds in the world. This blog is about their beauty, morphology and notoriety.
 

Seeds of Ricinus communis, commonly also called ‘castor beans’ (Photo: W. Stuppy)

Being the only species of the genus, Ricinus communis is a pretty unique plant, and this holds true for many reasons, as you will discover. Botanically, it belongs to the spurge family (Euphorbiaceae) which means it’s closest relatives include economically important plants such as the Para rubber tree (Hevea brasiliensis) and cassava (Manihot esculenta) as well as popular ornamentals such as poinsettia (Euphorbia pulcherrima) and crown-of-thorns (Euphorbia milii).

The castor oil plant (top; Photo: A. McRobb) and four of its relatives in the Euphorbiaceae family (below; Photos: W. Stuppy). The smaller photos show flowers of poinsettia (top left), cassava (top right), crown-of-thorns (bottom right) and a rubber tree tapped for its latex (bottom left)

Originally native to East and Northeast Africa and the Middle East, the castor oil plant (Ricinus communis) has spread throughout the tropics and has even become an invasive pest in places like Hawaii, South Africa, Australia and the Galapagos Islands. A fast growing tropical shrub that can reach a height of up to 5 m or more, it cannot survive our cold winters. Nevertheless, because of its striking large leaves and bold architectural ‘Gestalt’, Ricinus is a popular garden ornamental in temperate climates. It even comes in a range of different garden cultivars, for example ‘New Zealand Purple’ with dark red leaves.

Young fruits of a purple and a green variety of Ricinus communis (Photos: A. McRobb, left, and W. Stuppy, right)

Unique beauties

Easily beating competition from the rather unspectacular wind-pollinated flowers, the most beautiful, interesting, useful and scariest part of the castor oil plant are its seeds, also called ‘castor beans’ (although they have nothing to do with real beans). They come in sets of three enclosed in soft-spiny capsules. The fruits add to the decorative looks of the plant but they eventually dry up as they ripen and explode to scatter their highly characteristic seeds. A distinctive feature of Ricinus communis, its seeds are smooth, shiny and mottled in various shades ranging from white over beige, brown and maroon to grey and black. Just as every zebra has a unique pattern of stripes, no two castor beans ever share the exact same seed coat pattern. The mottling provides the seeds with camouflage against mice and other seed eating animals as they lie on the ground after their explosive expulsion from the fruit. If lucky, they only need their camouflage for a short while as a rescue team of little helpers should soon be on the way...

Zebras and castor beans: No two individuals ever show the same pattern in their coat (Photos: W. Stuppy)

Ticks, ants and a strange thing called an elaiosome

The seeds’ shape is rather similar to a tick (e.g. Ixodes spp.) and with 'ricinus' being the Latin word for 'tick', it is pretty obvious where the castor oil plant got its scientific name from. Resembling the small capitulum (comprising the head and mouth parts) at the front end of a tick, castor beans carry a yellowish-white nodule that acts as an elaiosome (‘oil body’) and attracts ants for dispersal. The ants, irresistibly drawn to the fatty nutritious elaiosome, carry the seeds into their underground nests. Here, rather than devouring the precious morsel themselves, they feed it to their larvae. After the elaiosome has been removed, the ants usually abandon the still viable seeds inside the nest or discard them outside the nest on the rich soil of the colony’s refuse piles. In doing so they provide a vital service to the castor oil plant: they help disperse its seeds in order for them to find a suitable place for germination.

Ants collecting the seed of a Cnidoscolus species, a close relative of Ricinus communis, in northern Mexico; note the white nodule (elaiosome) at the right end of the seed (Photo: W. Stuppy)

Others do it too!

The castor oil plant is not the only plant to call on ants to help with the dispersal of its seeds. Lots of other plants have seeds equipped with elaiosomes that pursue the same strategy. This includes many of the castor oil plant’s close relatives in the Euphorbiaceae family as well as other species from across the plant kingdom such as members of the legume family (most notably Australian wattles, i.e. Acacia spp., Leguminosae), milkworts (Polygala spp., Polygalaceae), snowdrops (Galanthus nivalis, Amaryllidaceae), violets (Viola spp., Violaceae), greater celandine (Chelidonium majus, Papaveraceae), and some cacti (Cactaceae).

Collection of SciArt images of ant-dispersed seeds; clockwise from bottom left: greater celandine (Chelidonium majus, Papaveraceae), Lake Logue wattle (Acacia vittata, Leguminosae), sand milkwort (Polygala arenaria, Polygalaceae), sun spurge (Euphorbia helioscopia, Euphorbiaceae), Blossfeldia liliputana (Cactaceae); centre: aztec cactus (Aztekium ritteri, Cactaceae)
[Images from ’SEEDS – Time Capsules of Life’ by Rob Kesseler & Wolfgang Stuppy and ‘FRUIT – Edible, Inedible, Incredible’ by Wolfgang Stuppy & Rob Kesseler; Copyright Papadakis Publisher, Newbury, UK]

The miracle of co-evolution

This close association of seeds and ants is called ‘myrmecochory’, which literally means ‘ant dispersal' (Greek: myrmêx, myrmêkos= ant; khôreô/chorein = to move, to disperse, to wander). Myrmecochory is a beautiful example of how plants and animals have co-evolved to form mutually beneficial relationships: the ant is given a nutritious and reliable source of food while the plant's seeds are dispersed far enough to reduce competition between the seedlings and their parent. By burying the seeds a short distance below the soil surface, the ants not only hide the seeds from predators like mice and other rodents, they also prevent them from being destroyed by fire. The latter may explain the important role ants play as seed dispersers in the dry habitats of Australia and Africa which are swept by seasonal wild fires.

Seeds of snowdrop (Galanthus nivalis, Amaryllidaceae) with a hook-shaped elaiosome at the bottom end of the seed (Photo: W. Stuppy)

A bit of seed morphology

Being a seed morphologist, I have to perform an ‘autopsy’ on the seeds of Ricinus to see what’s inside. Although castor beans may vary in size depending on their provenance, they are typically around 12 mm long. Underneath the brittle seed coat lies a soft, oily nutritious tissue called ‘endosperm’ into which the embryo (i.e. the baby plant) with its short axis and large but very thin and flat cotyledons (i.e. the first pair of leaves) is embedded.

Longitudinal (top) and cross section (below) of a castor bean; the embryo with its short axis and large, flat cotyledons (seed leaves) is embedded in copious oily endosperm (Photo: G. Toothill)

Castor oil – natural remedy or tool of punishment?

The oil that is stored in the endosperm of castor beans is meant to provide the embryo with energy during its germination. However, at least 6,000 years ago in ancient Egypt, humans had already discovered the usefulness of castor seed oil and took Ricinus communis into cultivation. Since these ancient times until the present day, ‘castor oil’ is used as a lubricating laxative to relief constipation. Taken in moderation, castor oil does just that but an overdose will inflict explosive diarrhoea. Accompanied by painful cramps, the latter can last for hours, causing unpredictable involuntary bowel movements of considerable magnitude even during sleep. Most people know about the effects of castor oil which is why it makes a very funny joke when Tom the cat (as in the cartoon ‘Tom and Jerry’) is threatened with a bottle of castor oil by his owner Nancy, should he not stay in his bed. As a child, I found this hilarious. [I still find it hilariously funny!]

Bottles of castor oil in Kew’s Economic Botany Collection (Photo: M. Nesbitt) 

Death by diarrhoea

In real life overdosing on castor oil is no laughing matter. For example, force-feeding castor oil to prisoners was used as a means of torture by the Nationalists under the leadership of General Francisco Franco during the Spanish Civil War (1936-1939). Likewise, in Benito Mussolini’s fascist Italy (1930-1943) the paramilitary Blackshirts used castor oil for the very same purpose to deal with opponents of the regime. The severe diarrhoea brought on by the ingestion of large amounts of castor oil led to dehydration which could ultimately cause death.

Beyond lubrication and death

Castor oil is a very valuable oil that is produced on an industrial scale (about 300,000-500,000 tons per year), most of it in India, China and Brazil. Apart from effecting a powerful acceleration of bowel movement, coincidence has it that castor oil also makes a great lubricant in jet and racing car engines. The brand name ‘Castrol’ proves that point and the high-performance engine oil ‘Castrol R40’ is even famed for the beautiful smell it bestows on car races. But the industrial applications of castor oil and its derivatives go far beyond lubrication. In fact, its versatility is almost boundless. Castor oil is used in the manufacturing of paints, dyes, adhesives, inks, soaps, cosmetics, chocolate, hydraulic brake fluids, plastics, waxes, varnishes, sealants and synthetic resins. A product of the latter kind has recently been used to seal the leaking rain gutters of the Millennium Seed Bank. In accordance with Kew’s environmental commitment, the original stainless steel gutters were first lined with a flexible reinforcement fleece made from recycled plastic bottles. The fleece was then saturated with a solvent-free water-proofing resin made from sustainably produced castor oil.

The rain gutter on the roof of the Kew’s Millennium Seed Bank lined with a resin based on castor oil (Photo: W. Stuppy)

The truly dark side of the castor bean

I don’t have a morbid obsession with death but the story of the castor bean would be incomplete without revealing the pretty seed’s most lethal property. As if its potential to inflict death by violent diarrhoea was not enough, the castor bean contains yet another, much more potent substance, the tiniest amount of which can kill man, beast and bug. Notoriously famous for all the wrong reasons, ‘ricin’, as the toxin is called, ranks among the most poisonous substances found in nature, alongside abrin which is found in the seeds of the crab’s eye (Abrus precatorius, Leguminosae). Both ricin and abrin are glycoproteins (= proteins coupled with sugars) that kill individual animal cells by blocking their ribosomes. The latter are the cells' protein 'factories'. By knocking them out, ricin and abrin act like a spanner thrown into the works of the cell. Cell death leads to tissue necrosis which in turn can lead to organ failure and ultimately death.
 

 The crab’s eye (Abrus precatorius, Leguminosae) is even deadlier than the castor bean (Ricinus communis, Euphorbiaceae) (Photos: W. Stuppy)

Don’t try this at home!

Lab experiments with human cell cultures have shown that penetration of just a single molecule of ricin into the cytoplasm of a cell is lethal. This explains the extreme toxicity of ricin. It is understandably difficult to carry out scientific experiments in order to establish the exact dose that is required to kill a human being. However, according to a report by the Federation of American Scientists entitled ‘Ricin: Technical Background and Potential Role in Terrorism’, the lethal dosage of ricin is as low as 3 to 5 micrograms per kilogram body weight if inhaled or injected. Uptake of ricin through the gastrointestinal tract is less effective so the lethal dose for ingestion is higher, something around 20-30 mg per kilogram body weight. In absolute terms this means that between 3-8 (some suggest up to 20) seeds chewed and swallowed are needed to kill a human.

Die hard – or how many beans kill a horse?

Reports about the toxicity of castor beans vary widely as to the number of seeds required to induce death. According to a 2004 report of the Centre for Food Security and Public Health at Iowa State University, among domestic livestock, horses are the most sensitive to ricin and can die from the ingestion of 4-7 seeds. Chickens and ducks are much more resistant and can allegedly take up to 80 seeds before they are knocked out. However, following the death of several thousand wild ducks due to castor bean poisoning in the Texas panhandle between 1969-1971, a scientific study from 1981 found that the LD50 (i.e. the dose that kills 50% of the ‘contestants’ participating in the experiment) for mallards (Anas platyrhynchos) appears to be just 3-4 seeds. Likewise, other sources claim that it takes at least 60 seeds to kill a horse. These wildly differing results could at least partly be due to variations in the quantity of ricin present in various strains of castor beans.

4-7 castor beans are allegedly enough to kill a pig (Photo: W. Stuppy)

Murder à la James Bond

Probably the most famous murder victim to be killed with ricin was Georgi Markov, a dissident Bulgarian writer who, at the time of his death in 1978, lived in London working for the BBC World Service. A prominent critic of the then communist regime in Bulgaria, Markov was assassinated in true James Bond-style. Whilst waiting at a bus stop on Waterloo Bridge, he suddenly felt a sharp sting in his right thigh and noticed a man speedily walking away holding an umbrella. Three days later Markov died at a hospital, aged just 49. A thorough autopsy revealed that Markov was shot with a tiny (1.7 mm) platinum pellet that had an x-shaped cavity filled with ricin. The sophisticated gun needed to launch the tiny pellet was built into the umbrella carried by the assassin, very much like one of ‘Q’s gadgets. Sadly, Markov must have died a very painful death. A lethal amount of ricin injected directly into the bloodstream causes severe internal bleeding followed by organ failure and death after 3-5 days of suffering.

One last thing...

Ardent consumers of castor oil need not worry about ricin. It is only soluble in water and not in oil. This means that the toxin will remain in the press cake and not cross over into the oil during the extraction process. By the way, ‘castor’ is Latin for ‘beaver’. Not that Ricinus communis has got anything to do with beavers. The name is a reminder of ‘castoreum’, an oily secretion from the beaver’s abdominal ‘castor sacs’. Combined with the beaver’s urine the animal uses castoreum to mark its territory. Since ancient Greece the aromatic smelling castoreum was used both as an aphrodisiac (perfume) and medicinally, for example, as a laxative. In the mid eighteenth century the oil from the seeds of Ricinus was (re-)discovered to be a much better laxative than castoreum. Thenceforth, the name was transferred, purely for reasons of pharmaceutical marketing. Today castoreum (‘beaver juice’) is still used in perfumes, cigarettes, candies and ice cream. Yum!

Acknowledgements

I want to thank my colleague John Adams who provided me the inspiration for this blog. Thanks to the industrial qualities of castor oil, as John pointed out to me, we will no longer have to put buckets under the few leaks in the corridors of the Millennium Seed Bank when it’s raining. Three other colleagues also very kindly contributed: Gemma Toothill took on the perilous task of cutting open and photographing the seeds of Ricinus communis, Mark Nesbitt specifically photographed some of the castor oil bottles in our Economic Botany Collections, and Andrew McRobb lent me some of his Ricinus photos.

-Wolfgang-