Life Table Analysis for Black-striped wallabies

Pre-Assignment 1 Practical: Aging of Kangaroos and Wallabies Using Molar Progression

Aging of Macropods Using Molar Progression

Molar progression occcurs only in the marsupial genera Macropus, Petrogale and Peradorcas (Jackson 2003). These marsupials are among only a relatively few worldwide whose teeth errupt ar the posterior end of the jaw, and migrate foward along the jaw during life (the others are the elephants). As the teeth wear down and become less useful for grazing, they have moved sufficiently anterior in the jaw that they can fall out, 'pushed' from behind by newly errupted teeth. In this way, macropods can maintain good functioning teeth with high cusps for grazing on tough fibrous grasses throughout life. This 'molar progression' is a handy way to age kangaroo and wallaby skulls, and will be demonstrated in this practical using a range of different-aged skulls from the eastern grey kangaroo, Macropus giganteus.

The position of teeth are estimated relative to a reference line drawn across the skull in line with the anterior limits of the orbits. Ten stages of molar progression are recognized and given decimal notation in tenths (see figure below, from Jackson 2003). In the example below (fig 1a), molar M1 has progressed beyond the orbit, while molar M2 is given a score of about 0.7, according to figure (b). Therefore, the skull has a molar index (MI) of 1.7. Consulting Table 4.1 below of published estimates from macropods, we see that this animal was somewhere between age 2 and 3 (in years) when it died.

Figure 1. Skull showing reference line for age determination, and one-tenth division in length for molar teeth of kangaroos and wallabies. Figure from Jackson (2003). Click image for a larger view.

Use the following rules when calculating molar index (MI):

1. If there is a premolar tooth, then the next one along in the tooth row is the M1. If there are not 4 molars, some of the teeth (i.e., M4, M3) have not yet erupted.

1. Some anterior teeth can be missing. If there is no sign of a premolar, then treat the last tooth that you can see in the skull as the M4, and count backwards from that to the M1.

As an example, I have anotated the images (Fig 2) below to indicate the two conditions that you might come across.

In the left-hand panel, the premolar remains in place and the skull has three erupted molars (M1-M3), plus (presumably) an additonal one that cannot be seen (M4), becuase it has not yet erupted. We therefore begin calculating MI at the boundary between the premolar (P2) and the first molar (M1) from 0.0, to yield a MI of 2.3. This corresponds to an age of about 3 years for this animal according to Table 1 (below).

In the second, much older animal (right-hand panel) the premolars have been shed along with the first molar (M1), and the remaining molars (M2-M4) have fully erupted. Therefore, we begin calculating the MI at 1.0, and count to the end of M4, to give a MI of 4.0. This corresponds to an age of about 12 years for this animal according to Table 1.

Table 1. Molar indices (MI) of progression for age (in years) in Macropus giganteus. Table adapted from Jackson (2003) - for more species, click here.

Age	MI
1	0.4
2	1.4
3	2.0-2.3
4	2.4
5	2.7
6	3.0
7	3.2
8	3.4
9	3.6
10	3.8
11	3.9
12	4.0
13	4.1
14	4.2
15	4.3
16	4.5
17	4.6
18	4.6
19	4.7
20	4.8

Now its your turn. Examine the following photographs of eastern grey kangaroo (Macropus giganteus) skulls. For each skull, determine the age of the individual using the rules we have established for molar progression, and Table 1 above. For each skull, we have drawn in the reference line that you will need to use for aging acording to the molar progression method outlined above. The label given to each of the skulls corresponds with the museum specimen label in our collection in Ecosystem Management.

Skull 1 [NR0782]

Skull 2 [NR1321]

Skull 3 [NR1607]

Skull 4 [NR2096]

Skull 5 [NR2159]

It is probably best that you make notes of molar Index (MI) and age of each skull (using the NR reference number) on a scrap of paper. Once you have done this, exit and click on 'Take the skull quiz!' to see whether you have got the hang of skull aging. Remember, this quiz is NOT ASSESSED - it is purely for your benefit.