This fierce little handful, only 100 g or so, occupies a niche in the tropical forest just like that of an owl:-
Sit and wait, watching and listening intently, ........then pounce.
The pounce can be awesome, fully as spectacular as the more widely-known owl stoops (see Carstenz Niemitz' "Football-goalie" sketch of a tarsier catching a bird in full flight).
Like an owl, the tarsier has enomous eyes, each larger than the brain and protruding well beyond the lateral margin of the skull. The eyes are virtually immobile, pre-aligned, like an owl, to avoid the problems of trying to align the eyes in very dim light when cooperation between the eyes is essential to achieve good signal-to-noise ratio. The catch-22 of not having enough light to get optimal eye alignment can be escaped by aligning the eyes and fixing them in advance of the dim and difficult conditions, a strategy with which photographers of nocturnal creatures are very familiar.
My own interest in the Tarsier was stimulated by the (still-unresolved..... see below) controversy surrounding its phylogenetic place within the primates....... Not to mention its owl-like features (upon which I could bring to bear some of my expertize on the visual system of nocturnal birds), its inaccessibility, its mystique, and the need for more information about its visual system.
Dispute over the Phylogenetic Position of Tarsius:
There are three possible phylogenetic relationships of tarsiers to the other main primate groups (anthropoids and lemur/lorises). These are shown below. The middle arrangement, with tarsiers as sister taxa to the anthropoids (i.e. monkeys and apes) has certainly been the most popular (although paleontologists have never liked it because the tarsier-like omomyid fossils have an ancient record that cannot be reconciled to anthropoids. Accepting tarsiers as anthopoids has the implication that omomyids are independent of tarsiers, a slightly uncomfortable and convoluted stance).
The evidence is growing that the middle topology is wrong.
Tarsiers are probably only distantly related to the other living groups of primates and most likely arose from the primate lineage well before the lemurs and lorises, and then anthropoids, branched off (left arrangment). The third possibility, with tarsiers as sister taxa to lemurs and lorises, is shown for completeness only (right diagram) as it does not have much support, even though lorises and tarsiers share some "frog-like" anatomical modifications of their hindlimbs to support their tremendous leaps from tree to tree.
In the context of the "flying primate" debate, the reasons for drawing attention to these esoteric points are three-fold:-
1. Phylogenetic disputes still persist, even for such well-studied and
well-understood groups as primates.
One should not therefore be smug about even deeper
branches in the mammalian phylogenetic tree, such as those leading to putative
primate sister taxa such as bats, colugos
and cetungulates.
2. I predicted from brain
anatomy that tarsiers would not be sister taxa to anthropoids.
3. Molecular
evidence now supports this prediction. Tarsiers appear to be basal,
not anthropoid, primates.
I would therefore like to claim some credit for the power of the neural
data set to illuminate deep branches of the mammalian phylogenetic tree,
especially since I have been derided for proposing "flying primates" on
the basis of the brain characters. If tarsiers can now be shown, by both
brain and molecular data, in a radical new relationship to other primates,
one must seriously consider what the same kinds of data have to say about
other possible primate sister taxa............ such as the megabats!
Why Tarsius is not an Anthropoid:
The case for linking Tarsius to anthropoids is largely based on the following 3 features, (together with Matt Cartmill's eloquent persuasion!):-
One might just as well link tarsiers to owls via the orbit as to link tarsiers to anthropoids on this basis.
Dissection of Tarsius bancanus: Note the huge eyes, comparable
in size to the brain and extending laterally beyond the margin of the skull
where a boney shelf supports and protects the eye (arrow).
b. The second feature, a single uterine chamber and discoid placenta, also has a scattered distribution, For example, simplex uteri are found in some phyllostomid microbats (but by no means all of the taxa in this one family out of 17 families of microbats). I am not aware of anyone, flying primates notwithstanding, who would try to link anthropoid primates to a branchlet on one family of microbats.
c. The third feature, a fovea, is also scattered across many phylogenetic
branches. My work on the foveas of birds in dozens of families leads me
to conclude that foveas provide virtually no signal in the phylogenetic
noise.
The thesis that the tarsier's fovea has predictive significance for
its phylogeny and its former lifestyle cannot be supported. The presence
of functional foveas in some nocturnal birds (e.g. owlet-nightjars, frogmouths,
owls) as well as deep sea fish (which inhabit what amount to nocturnal
realms in their very low light levels) is refutation of the argument that
the tarsier's fovea is a remnant of a former diurnal lifestyle. Far from
carrying the stigmata of some hypothetical former diurnal lifestyle, the
tarsier has a superb adaptation to the nocturnal niche, equal in sophistication
to that of the owl. Owl and tarsier both have huge eyes whose immobility
is advantageous in the difficult task of superimposing the two views for
binocular summation and stereopsis. The fovea might be an adaptation to
that niche also, for example to aid in focussing which is prodigously difficult
with a low f-ratio optical system and dim light.There is no need for the
stretched argument that the fovea somehow betrays a diurnal past.
In summary, all 3 so called "anthropoid" features in the tarsier are likely to be convergences. That point of view is strongly supported by the neural traits (reversed magno layers in the LGN, primitive accessory optic system, callosum and cerebellum etc), the growing molecular evidence (alpha-Crystallin, 12SrDNA) and the paleontological data (that links tarsiers to the ancient omomyids), all of which put tarsiers near the base of the primate tree with the colugos.
Sperm Ultrastructure of Tarsius:
Robson SK Rouse GW Pettigrew JD 1997 Acta Zoologica 78:269-278 Sperm
ultrastructure of Tarsius bancanus (Tarsiidae, Primates): Imlications
for primate phylogeny and the use of sperm in systematics.
Tarsiers have unusual sperm. Long and flattened compared with other primates; distal origin of the central singlets (a rare trait that tarsiers share with megabats); a tilted subacrosomal space.
Tarsier sperm do not definitively place them in one of three phylogenetic arrangements shown above, although the prosimian-tarsier link is least supported by the sperm evidence.
