Biodiversity

Thursday, May 04, 2006

ADAPTIVE RADIATION: THE LEMUR WAY

Lemurs have been isolated on the island of Madagascar for millions of years and thus adaptive radiation of these creatures has been confined to this island. (1) The fossil record is poor and when exactly the lemurs inhabited Madagascar and when they split from the other prosimian primates would be guesswork, but the fact that Madagascar has been isolated for millions of years, is widely accepted. (1) The lemurs thus have had a long time to adapt to the different environments of the island and in the absence of other primates and relatively few mammals could fill many ecological niches, the island offered. According to Patricia Wright (1999, p. 31), “adaptive radiation of lemurs on Madagascar may have been uniquely characterized by selection toward efficiency to cope with the harsh and unpredictable island environment”. (2)

They differ from anthropoids (monkeys, apes, humans) in several behavioural features, such as female dominance, targeted female-female aggression, lack of sexual dimorphism, strict seasonal breeding and cathemerality (being active during the day as well as night). (2) There are several suggested hypothesis for these adaptations, the latest being the “energy frugality hypothesis” postulating that the majority of traits are either adaptations to conserve energy or to maximize use of scarce resources. (2)

In Africa and Asia other prosimians (bushbabies, pottos, lorises and tarsiers) remained small, solitary and nocturnal, avoiding the diurnal monkeys, to prevent competition. In Madagascar though, lemurs have a wide variety of lifestyles, varying degrees of social structures and they come in all sorts of sizes. One feature they still share with all other prosimians though, is their heavy reliance on their sense of smell. (3)

Some lemurs have become very small and fill a gap in the absence of squirrels and other prosimians. The smallest living primates today are the grey mouse-lemur (Microcebus murinus) and the rufous mouse-lemur (M. rufus) weighing only between 45-90 grams. (3, 4) They are nocturnal and generally solitary when foraging for food. During the day females sometimes group together sleeping in tree holes or nests built of leaves. (3, 4) They are also arboreal and their diet consists of fruit, flowers, nectar, insects and spiders. They have acute noses, which they use to detect food as well as keeping in touch with neighbours. They scent mark twigs by urinating in their cupped hands, wiping it on their feet and then wiping them on a branch, as well as leaving smelly footprints as they move on. (3) Some species of dwarf and mouse-lemurs even hibernate during the winter months, when food is scarce, which is very unusual behaviour for primates. (3)

In the genus Phaner there is a small lemur that has adapted to a specialized diet of resinous sap of certain trees. For this diet its adaptations involve teeth, tongue, nails and digestive system. (3)

The aye-aye (Daubentonia madagascariensis) must be one of the most unusual lemurs. They are nocturnal, about the size of a domestic cat and covered in a thick black fur coat with longer white-tipped hairs giving it a shaggy appearance. Their huge ears are capable of picking up faint sound of larvae inside wood. But it is their long bony third finger that is most remarkable. They move along in trees, tapping along with their fingers on wood and listening until they have found what they have been looking for; grubs! Their chisel-like teeth then quickly gnaw a hole in the wood and the long middle finger is then used to extract the grub. They also eat nuts and even eggs, again using their chisel-like teeth to gain access and then using their long finger to extract the content. (3, 5)

Probably the best known is the ring-tailed lemur (Lemur catta). They are also about the size of a domestic cat and inhabit the dry southern part of Madagascar. Their preferred habitat is the gallery forest that fringes major rivers, but they are found in other habitat as well. They are the best adapted to a partly terrestrial lifestyle of all the lemurs, although a lot of time is spent in the trees, where they mostly feed on fruit, but also leaves, seeds and the odd insect. These lemurs are diurnal and live in sociable societies that are led by females. Sent marking from glands on their wrists, armpits (males) and genital area (both sexes) is part of their daily routine and serves as territorial boundaries. (3)

Some lemurs make the most of both day and night and space out their activities over a 24-hour period. The brown lemur (Lemur fulvus) is one such specie. They mostly feed on fruit but also eat leaves. (3)

The family Indriidae contains the largest living lemurs, namely the indri (Indri indri). They are possibly the most strictly diurnal and arboreal of all lemurs. They live in small family groups and feed on leaves, flowers and fruit. They jump from tree to tree, usually hugging tree trunks, rather than flexible branches due to their weight. The indri is however famous for its singing. They use their eerie chorus to mark their territory. (3, 6)

The sifakas, like the indri are agile jumpers and can jump up to 10 meters. They are smaller than the indri and usually land upright. They do cover ground on the floor sometimes, but cannot walk on all fours, but will rather jump, with arms held high. (3, 6)

Some lemurs have become very specialized in their diet, like the golden bamboo lemur (Hapalemur aureus), the grey bamboo lemur (Hapalemur griseus) and the greater bamboo lemur (Hapalemur simus). What baffled scientists is that they all can all live together in close proximity eating from the same plants. It was then established that they prefer different parts of the same plant and so prevent competition. What is further remarkable is that the golden bamboo lemur prefers young bamboo shoots, containing cyanide and highly toxic, but this lemur has managed to overcome that obstacle. (3)

Today Madagascar has between 23-52 living species of lemurs (depending on the source one uses). (3, 8) But many more, especially the larger ones, have already gone extinct and due to continued habitat destruction and fragmentation, all remaining species are at risk. (3, 8)


References:

1. Martin RD. 2000. Origins, diversity and relationships of lemurs. International Journal of Primatology 21 (6): 1021-1049
2. Wright PC. 1999. Lemur traits and Madagascar ecology: coping with an island environment. American Journal of Physical Anthropology 110 (S29): 31-72
3. Preston-Mafham K. 1991. Madagascar. A Natural History. Pages 141-188, Chapter 7. The Lemurs. Cape Town. Struik Publishers.
4. Wikipedia contributors. Gray Mouse Lemur [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 9, 18:09 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Gray_Mouse_Lemur&oldid=47725424
5. Wikipedia contributors. Aye-aye [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 28, 05:04 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Aye-aye&oldid=50537743.
6. Wikipedia contributors. Indriidae [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 24, 16:02 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Indriidae&oldid=45277136.
7. Wikipedia contributors. Golden Bamboo Lemur [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 27, 18:21 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Golden_Bamboo_Lemur&oldid=50456767.
8. Wikipedia contributors. Lemur [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 2, 01:41 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Lemur&oldid=51141271.


Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za

Web http://brit-journal.com/karen2006bcbnisl/

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