OF MICE ANDMASSONIAS

The pollination of flowers by rodents was thought to beconfined to the Proteaceae but recent discoveries show that

the,Namaqualand lily Massonia depressa, and mostcertainly others like it, are also pollinated by rodents.

by Steve Johnson, School of Botany and Zoology,University of Natal, Pietermaritzburg,

Anton Pauw and Jeremy Midgley, Department of Botany,University of Cape Town

Above A flower head of Massonia depressa nestles between two large leaves lying flat onthe sun baked surface of the Knersvlakte in Namaqualand. Photo: A. Pauw.

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T he Knersvlakte is a vastexpanse of flat gravelly landthat stretches north from

Vanrhynsdorp. During spring it iscarpeted with flowers and this isthe season when tour buses can beseen traversing the national road insearch of the ultimate photoopportunity. We had come to theKnersvlakte during the cold monthsof winter, long before the veld turnsinto a kaleidoscope of colour. Ourmission was to uncover the polli­nation mysteries of an unusual lilywith the most unprepossessingflowers imaginable. Massoniadepressa, the object of our attentionon this trip, has greenish brownflowers at ground level, nestedbetween a giant pair of flat greenleaves. We suspected that theseflowers are pollinated at night, notby insects, but by mice, which are,by and large, nocturnal animals.And so, a group of slumberingKaroo sheep were startled by thesight of three botanists picking theirway among the vygies in the deadof a winter's night. It is an eerieexperience to wander about theKnersvlakte after dark; colder thanone might imagine and, in theabsence of city lights, luminescentwith stars.

More than twenty years havepassed since John Rourke ofKirstenbosch and his Americancolleague Delbert Wiens made thesensational discovery that a groupof Protea species is pollinated byrodents. These proteas have dullflower heads situated near groundlevel, quite unlike their bird-polli­nated relatives which have brightlycoloured conspicuous flower heads.They also have a strong yeastyodour that makes it possible formice to find the flower headsduring the night. We have long beenintrigued by a small group of liliesin Namaqualand that have featuresreminiscent of rodent-pollinatedproteas. The flowers of these lilies,like Massonia depressa andAndrocymbium dubium, are dulland grow low on the ground. Couldthey represent an as yet undocu­mented case of rodent-pollination?

We had located a large floweringpopulation of Massonia depressa inlate July 1998 and laid out anumber of rodent traps, baited withpeanut butter and rolled oats. Wehad also been taking measurementsof the nectar, which is unusuallyviscous and jelly-like and collectsin a pool at the base of the bowl­shaped flower. We noticed that thenectar in each flower increased

Veld &- Flora December 2001

towards dusk - an indication thatthe plants were preparing for anocturnal visit. As eveningprogressed the flowers developed astrong scent that permeated the airaround us. The scent is not pleasantand was likened by one of us to thesmell of a damp sheep! Now wewaited, shivering as the temperatureplunged to near freezing point.

By midnight the first of the trapshad been sprung. These traps werenot lethal spring-loaded 'mouse­traps', but small tunnels into whichthe rodents venture but cannot exituntil set free by their humancaptors. Inside we were surprised todiscover a hairy-footed gerbilGerbillurus paeba, and not a mouseas we had expected. We dabbed itsfur with sticky tape to collectpollen, and most importantly,collected the droppings that hadaccumulated in the trap. Three moregerbils were trapped and later themicroscope revealed that theirdroppings were packed withMassonia pollen. This was the proofwe needed to show that they hadbeen visiting the flowers. Aftervisiting flowers, rodents becomedusted with pollen which they lickoff their fur during the groomingprocess. In this way pollen isingested, but as the pollen wall isvirtually impervious to digestion,it eventually ends up in thedroppings.

We took one of the gerbils to ournearby campsite and placed itcarefully in a darkened terrariumcontaining several flowering plantsof Massonia depressa. The firstreaction of the gerbil was to sniffthe air and then,without hesitation,dash across to aMassonia plant andbegin lapping nectarfrom the flowers. Itpushed its snout inamongst the anthersand stigma of eachindividual flowerwhen draining the

The ground levelposition of Massoniaflowers makes it easy forrodents like this hairy­footed gerbil GerbjJluruspaeba to get to thenectar. Photo: Steve Johnson &

Anton Pauw.

nectar, and did not nibble ordamage the flowers while doing so.The rodent's snout was soon heavilyencrusted with pollen, which wasliberally daubed onto the stigma ofeach flower that it subsequentlyvisited. Having witnessed thisexciting interaction betweenMassonia and its unlikely polli­nator, we released the gerbil back atthe field site and retired exhausted,but happy, at 3 am.

Although the results of this firstseason were promising, we wantedto obtain more data from otherpopulations, and so in the followingwinter we travelled to theNiewoudtville escarpment whichoverlooks the Knersvlakte. We setrodent traps among giant boulderson a rocky ridge where plants ofMassonia depressa are plentiful.Several individuals of the Namaquarock mouse Aethomys namaquensisand spiny mouse Acomyssubspinosus were found in the trapsand, on examination, they tooproved to have Massonia pollen intheir fur and faeces. After beingreleased in the early hours of onemorning, a Namaqua rock mouseeven brazenly lapped nectar fromMassonia flowers just a few metresaway from where we stood.

Insects hardly ever visit theflowers of Massonia depressa.One reason might be the jelly-likeconstituency of the nectar, whichwould make feeding difficult forinsects with their narrow syringe­like mouthparts. (Rodents easily lapup the nectar with their broadtongues.) To establish with certaintywhether rodents are the primary

pollinators of Massonia, we decidedto cover plants with wire meshcages that would exclude rodents,but allow insect access to theflowers. At the end of the floweringseason we returned to the site andfound that almost no seeds wereproduced by the covered plants,while uncovered plants hadproduced plenty of seeds. We nowfelt that we had enough evidence toconclude that Massonia depressa isindeed adapted for pollination byrodents.

Pollination by small 'non-flyingmammals' (to distinguish them frombats) we now know, occurs onseveral continents. Marsupials visitthe flowers of various Australianshrubs including Banksia, which arecousins of our South Africanproteas. In Madagascar, lemurs haverecently been implicated in thepollination of several plantsincluding Ravenalia, a close relativeof Strelitzia. Proteas were thought tobe the only plants in southernAfrica that rely on rodents for polli­nation, but now it seems that aguild of South African lilies can beadded to that list. In all, we predictthat about a dozen Namaqualandlily species with ground-huggingflowers are pollinated by rodents.Wi

Further readingWiens, D., and J. P. Rourke. 1978. Rodent

pollination in southern African Prateaspp. Nature 276, 71-73.

Johnson, S.D., Pauw, A. & Midgley, J. 2001.Rodent pollination in the African lilyMassonia depressa (Hyacinthaceae).American Journal of Botany (in press).