Strepsirhines

Grades versus clades; primitive versus advanced; diversity; anatomy; behaviour; Madagascan biogeography; classification.

Classification

Grades versus clades

There are two important classification schemes for what are colloquially called the prosimians. These reflect two rather different classification ideologies. The traditional classification that produces the sub-order Prosimii, reflects the gradistic scheme where animals are grouped depending on morphological similarity with forms that appear at a particular geological age. Thus, animals with prosimian-like features appeared perhaps 50 million years ago in the Eocene. Monkey-like features appeared rather later in the Oligocene - 35 mya. The Prosimii includes the tarsiers. Cladistic schemes require grouping dependent on having a single common ancestor (a so-called mono-phyletic group). It seems likely that tarsiers and the rest of the prosimians do not share single common ancestor that is not also the ancestor of other primate groups, so a new classification scheme has been proposed (and is widely accepted) with lemurs, lorises and bushbabies classified as Strepsirhini and tarsiers, monkeys and apes classified as Haplorhini.. For that reason, today's lecture will be concerned with the anatomical peculiarities of the strepsirhines, and tarsiers will be covered in the forthcoming haplorhine lecture.

Primitive versus advanced

Describing prosimians as primitive (or advanced) is to some extent an anthropocentric hang-over from gradistic classification schemes. It is almost certainly a mistake to describe an animal as primitive. It is probably reasonable to describe a particular feature as primitive, but the term ancestral is almost certainly preferable. Often the term generalised as opposed to specialised is more appropriate.

The problem with this is we tend to think of ourselves as advanced, and anything not like ourselves as primitive. But what we really mean is specialised. Humans are specialised for bipedalism and extremely complex social organism. Try gnawing through a tree trunk to realise how unspecialised our dentition is, and although we are very specialised for walking, we are not specialised for climbing or leaping.

Prosimians are not primitive. They show gross morphological features that appeared early in the fossil record, but they also have some amazing, advanced features too. It is just that their advanced features are different from ours, and we tend to value them less highly. Being highly intelligent (which we are, and prosimians are not) is not necessarily better. It all depends on ones environment and niche.

General Anatomy

Here are some general anatomical features that distinguish strepsirhines:

* Long snout

* Laterally split nostrils

* Rhinarium + philtrum + frenulum

* Epithelio-chorial placentation

* Post orbital bar

* Nocturnal

* Tapetum

* Ectotympanic ring

* Unfused metopic suture (L & R frontal bones)

* Unfused mandibular symphysis (L & R mandibles)

* Coronolateral sulcus (no central sulcus)

* Stapedial artery

Major Groups

Lorisidae

2.1.3.3/2.1.3.3 dental formula. Lower incisors project forward (procumbent) to form tooth comb.

Lorisinae

Lorises: tailess, slow moving, cryptic, nocturnal primates. Found in forested areas in Africa and Southeast Asia. Insectivorous, with adult body sizes from about 200g to 2kg. 2 or 3 paired mammae in female. Males do not have a bacculum. Equal length fore and hind limbs for slow quadrupedal locomotion. Very strong grip, with reduced 2nd digit in hand and reta mirabilia (blood storage channels) to allow the grip to be maintained for long periods. 2nd digit on hind foot has grooming claw rather than a nail.

Galaginae

Bushbabies: named because their cries are supposed to sound like those of babies. Nothing to do with large eyes and general cuteness. Nocturnal insect and gum eaters, with some fruit. 100g to 2kg. Vertical clingers and leapers. Elongated hind limbs, males with bacculum and females have 2 pairs of mammae. There is much current argument about the taxonomic status of many bushbaby species, with many established morphological groupings being expanded to include species differentiated by calling patterns and detailed penis morphology.

Cheirogaleidae

These are the dwarf and mouse lemurs. They are all found only on the large island of Madagascar off the coast of East Africa, are relatively small (50 to 500g) and eat insects, gum and fruits. They look somewhat squirrel like and move by scurrying and leaping. The females have 3 pairs of mammae. Dental formula is 2.1.3.3/2.1.3.3 with well developed incisors.

Lemuridae

Large bodied, diurnal Madagascan strepsirhines. Long heavily furred tails, 2-3kg, eating mostly fruit and some leaves. 1 pair of mammae except Varecia which has 3 sets. 2.1.3.3/2.1.3.3 with small, peg-like upper incisors. The lower incisors and canine project forward as a tooth comb, the lower 2nd premolar is caniniform. Diastema (gap) between upper C1 and P2. Locomotion by leaping and quadrupedalism. Mixed arboreal and terrestrial habit.

Megaladapidae

These are the sportive lemurs (lepilemurs) and the sub-fossil megaladapids. The lepilemurs are somewhat similar to the lemurids, but are smaller (1kg) and lose the upper incisors producing a dental formula of 0.1.3.3/2.1.3.3. The megaladapids are huge (> 50kg), but with the same dental formula. They went extinct about 500 years ago - almost certainly due to the action of European settlers, though this process was almost certainly started by the first human settlers who arrived in Madagascar about 1500 years ago.

Indriidae

Large animals, with a frugivorous/folivorous diet. 1 to 10kg, highly adapted for leaping with elongated hind-limbs and reduction of tail among the Indri. The 1st toes is long, and the other 4 toes are united at the base by a web of skin. Females have a single pair of mammae and males have a penis bone. The dental formula is 2.1.2.3/2.0.2.3 (some workers interpret this as 2.1.2.3/1.1.2.3). The first 2 lower teeth project forward as a tooth comb. There is a great deal of suspensory activity, and sifakas in particular hop bipedally on the rare occasions where they have to move on the ground.

Daubentoniidae

The Aye-aye is the only animal in this family, though there is another, larger, sub-fossil form. Weighs about 2kg, and eats insects and fruit. It has highly specialised hands with extremely long and thin fingers, especially the 3rd finger. All digits have claw-like nails except the first toe. Females have 2 mammae and the males have a penis bone. The teeth are also highly specialised with almost rodent-like incisors. The permanent dental formula is 1.0.1.3/1.0.0.3. They also have extremely large, membranous, naked ears.

Behaviour

The prosimians have much smaller brains for their size than the anthropoids. They are unsurprisingly less intelligent using standard mammal intelligence tests (e.g. Wisconsin general testing apparatus). They are predominately nocturnal, and it is only the nocturnal prosimians that are found sympatrically with anthropoids. The nocturnal varieties are solitary, and the diurnal animals form social groups: generally family groups, but these are extended to form multi-male multi-female groups in the lemurs. Interestingly, there is little or no sexual dimorphism, except for colour dimorphism in black lemurs (only the males are actually black). This is surprising given the level of inter-male competition for female access in the larger groups. These groups thus appear to be female dominated.

Prosimians tend to have rather specialised locomotor patterns compared to anthropoids. Lorises are specialised, slow quadrupeds that never leap, and indri and some galagoes leap almost exclusively.

The smaller animals tend to eat insects, with gum, fruit and flowers becoming part of their diet as they become larger. Leaf eating is only really an option for the larger prosimians. This is in line with the general primate tendency.

Madagascan biogeography

Madagascar is an extremely interesting place for biologists. Because it has been isolated from mainland Africa about 150 mya, any mammals colonising it have had to be rafted across from mainland Africa. Obviously a rare event. The fauna in Madagascar has thus been isolated and (like the Galapagos Islands) had the opportunity to progress down an independent adaptive radiation. It seems likely that this rafting has happened at least twice since the cheirogalids are probably more closely related to the lorisids than they are to the lemurids. Though, which direction this second rafting occurred (Africa to Madagascar or Madagascar to Africa) is not known due to the paucity of the fossil record.

This isolation has made the ecology of Madagascar particularly susceptible to the late arrival of mainland species (including humans), and this is one of the reasons why Madagascar has been described as "without doubt the world's highest major primate conservation priority". 15 species of lemur have gone extinct in the last 2000 years due to the arrival of humans.

Bibliography

Library

 AUTHOR(S)       :Clark, Sir Wilfrid Edward Le Gros 1895-1971
 TITLE           :The antecedents of man : an introduction to the evolution of
                  the primates
 EDITION         :3rd ed.
 IMPRINT         :Edinburgh : University Press, 1971
 SERIES          :Edinburgh University publications. Science and mathematics
                  texts ; 2

 AUTHOR(S)       :Nowak, Ronald M.
 TITLE           :Walker's Mammals of the world
 EDITION         :5th ed. / Ronald M. Nowak
 IMPRINT         :Baltimore London : Johns Hopkins University Press, 1991
      

Other

Conroy, G. C., 1990. Primate Evolution, Norton, London.

Harcourt, C., Thornback, J., 1990. Lemurs of Madagascar and the Comoros. The IUCN Red Data Book. IUCN, Gland, Switzerland.

Mittermeier, R. A., Tattersall, I., Konstant ,W. R., Meyers, D. M., Mast, R. B., 1994. Lemurs of Madagascar. Conservation International Tropical Field Guide Series, Washington.