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View Full Version : Week 21 - Tapir



Catiana
October 11th, 2007, 11:28 PM
Introduction

The four living species of tapirs ( Tapirus terrestris- the lowland tapir, T. pinchaque- the mountain tapir, T. bairdii- Baird's tapir, and T. indicus- the Asian tapir) are remnants of an ancient lineage whose evolutionary origin traces back at least fifty million years. Although their stout body superficially makes them look like pigs, and their short fleshy proboscis (trunk) often causes casual zoo-goers to mistake them for anteaters, they are not closely related to either of these groups. In fact, their closest evolutionary cousins among living mammals are rhinoceroses - a relationship well supported by the fossil record. Indeed, some fossil forms occurring near the time of origin of the rhinoceros and tapir lineages are so similar to one another that it is difficult to determine to which group they belong!

However, when we look at extant tapirs, there are many features that distinguish them from rhinoceroses, as well as from all other mammals. Among the suite of characters that differentiate tapirs from other mammals, the most obvious are: the continuous transverse shearing blades (or lophs) on their cheek teeth; their three-toed hindfoot, and four-toed forefoot; and most noticably, their short prehensile proboscis (see discussion below). It is the evolutionary changes in these and other features through time, as documented by fossils, that allow us to unravel the evolutionary history and relationships of tapirs.

Habits and distribution

Tapirs are browsers, and the series of transverse shearing blades (lophs) on their cheek teeth are adapted for chewing fruit, leaves, twigs and vines. Tapirs generally live in forested areas, and their stocky build, with relatively short limbs and stout torso, is well suited for barreling through the underbrush. Although they are large mammals, (adult tapirs weighing from about 150-360 kg [330 -800 lbs] - depending on species), they are characteristically shy and avoid human contact. Because of this, and because they are typically crepuscular (active at dawn and dusk) or nocturnal, they are rarely seen in the wild. Tapirs are generally solitary, although several individuals may share an overlapping home range. They have a long gestation period (about 13 months), and they give birth to one calf at a time. This low recruitment rate makes tapirs highly vulnerable to overhunting. Unfortunately, habitat loss and overhunting are major threats to the long-term survival of all four tapir species. Because of its widespread range, Tapirus terrestris is in the best shape of the four, being listed as 'vulnerable' to extinction by the 2002 IUCN red list. The other three species are listed as 'endangered' on the same list.

The four living species of tapirs have an unusual distribution, with three species living in the New World, and the Asian tapir in southeast Asia. Fossil tapirs are known from Europe, Asia, and North America. Interestingly, tapirs did not appear in South America, their current stronghold, until the Isthmus of Panama emerged from the sea to connect North and South America. Recent fossil recoveries from Brazil indicate that tapirs were present there around 8 million years ago.

Phylogenetic history

Tapirs are perissodactyls (or odd-toed ungulates), a group that also includes the rhinoceroses and the horses. This evolutionary grouping is well supported by both molecular and morphological data. Relationships of fossil taxa within Tapiroidea (defined as all taxa more closely related to Tapirus terrestris than to Rhinoceros unicornis), however, have been more problematic. Despite these difficulties, and the differences of opinions that different researchers have regarding evolutionary relationships among the tapirs, fossil tapirs clearly illustrate a 'progressive' evolution of many of the skeletal features characteristic of the Recent tapirs, particularly those features correlated with the evolution of their muscular trunk.

Phylogenetic analyses of the living tapirs suggest that the New World tapirs are more closely related to one another than they are to the Asian tapir, and that T. terrestris is most closely related to T. pinchaque (Norman and Ashley, 2000). This hypothesis of relationship implies a minimum of one dispersal between the hemispheres to explain their current distribution.

The tapir's trunk

The tapir's prehensile trunk is associated with modifications in their skull. These modifications include an extremely retracted narial incision (or nasoincisive incisure), loss of the bony wall of the nasal chamber, and development of scars and tubercles for muscle attachment. These features have been used to infer the presence of a trunk in fossil tapirs. The trunk is comprised of modified muscles of the upper lip and nose, and is capable of complex movements. The reader is referred to a recent paper by L. Witmer, S. Samson, and N. Solounias for a thorough description of the anatomy of the tapir's proboscis.