Part 2: Canine Tooth Biomechanics

        In part 1 of this exercise we reconstructed body masses for three species of extinct Carnivora based on skull measurements. In this part of the exercise we will explore the paleobiology of the same three species further, by performing a functional analysis of their canine teeth.

        Teeth must perform several interrelated tasks. First, they must transmit and concentrate the forces generated by the predator's jaw muscles onto the prey's tissues in such a way as to facilitate capture and degradation. Second, they must resist the counteracting forces generated by the prey's tissues that would tend to damage the teeth. Further, teeth should, if possible, have sufficient strength to survive accidental overloads and be constructed with minimum amounts of material. Under selection, teeth should evolve shapes that maximize the chances of capturing prey, while minimizing both the possibility of tooth damage and the amount of material used in their construction.

        The bending strength of the canines of large mammalian carnivores can be mechanically modelled as cantilever systems of elliptical cross-section (biomechanical model and data from B. van Valkenburgh & C. B. Ruff. 1987. Canine tooth strength and killing behavior in large carnivores. Journal of Zoology (A) 212: 379-397). The method we will use here is based on analyzing the bending strengths of canine teeth in the medial-lateral and anterior-posterior planes. The relative bending strength for medial-lateral flexure (SML) is calculated as:

where CW is the anterior-posterior canine width, CT is the medial-lateral canine thickness, and CH is the canine height. In a similar manner, the relative bending strength for anterior-posterior flexure (SAP) is calculated:
        Log-transformed canine tooth data for an analysis of relative bending strengths is available for a number of living canid and felid species (Tables 3 and 4). Regressions using this data set will be used to evaluate the canine morphology of extinct canids and felids.

        Since some extinct carnivores (most notably Canis dirus) have been reconstructed as primarily scavengers, data for three extant hyaena species have also been included (Table 5). Hyaenas are known to attack live prey in a manner similar to extant canids, but are best known for their ability to efficiently scavenge carcasses. Their strong jaws and teeth can quickly dismember a carcass and crush bones to extract the nutritious marrow. Unlike other Carnivora, which use the postcarnassial molars for crushing bones, hyaenas use their robust canines.

        Canines, of course do not act alone. They only perform part of the capture and processing of food; other teeth in the dentition also have individually specialized functions. Further, the teeth are embedded within sockets in the jaws. Consequently, the jaws also are under selective pressure to maximize prey capture with a minimal chances of jaw damage and minimal amounts of energy used in their construction.

        The functional aspects of jaws can be most easily modified by altering their mechanical advantage. Mechanical advantage is proportional to the ratio between the inforce lever arm and the outforce lever arm:

Listed in Tables 6, 7, and 8 are log-transformed measurements of the temporalis (= inforce) lever arm and the mandible (= outforce) lever arm for canids, felids and hyaenids.
 

Extinct Carnivora

        We will be examining the same four casts of carnivore skulls from the Pleistocene of North America examined in Part 1 of this exercise. The first is the dire wolf (Canis dirus), an extinct relative of the extant gray wolf (Canis lupus). The remaining three species are all felids; the American lion (Panthera atrox) and both juvenile and adult sabertooth tigers (Smilodon fatalis). For each specimen you should measure six features (Figure 2):

Figure 2.  Carnivora biomechanical function measurements: CH = canine height, CW = canine width, CT = canine thickness, TLA = temporalis lever arm, and MLA = mandible lever arm.

1. skull length (SL);

2. canine height (CH);

3. canine width (CW);

4. canine thickness (CT);

5. temporalis lever arm (TLA);

6. mandible lever arm (MLA).

Since skulls may occasionally become deformed during fossilization, independent measurements of TLA and MLA should be made on each side of each skull.

        As in the first part of this laboratory exercise, log-transformed data from extant canids and felids should be used to construct a dedicated spreadsheet for analysis. Data for hyaenids should be analyzed with the canids (since C. dirus is hypothesized to have had a hyaena-like life style).

        This data will be used to examine four different biomechanical relationships in extant Carnivora and then reconstruct feeding behavior in extinct Carnivora. The four relationships to be evaluated are:

1. skull length (SL) vs. canine height (CH);

2. skull length (SL) vs. medio-lateral bending strength (SML);

3. skull length (SL) vs. anterio-posterior bending strength (SAP);

4. mandible lever arm (MLA) vs. temporalis lever arm (TLA).

You must evaluate all four relationships being investigated for both canids and felids. For each relationship you should (1) generate a regression equation and obtain the values of a (constant) and b (x coefficent), (2) plot a graph of the relationship showing data for both extant and extinct species, as well as, the trendline. Consequently, you will generate a total of eight graphs; four canid relationships and four felid relationships.
 

Questions

Is there a relationship between:
        canid skull length (SL) and canine height (CH)?
        canid skull length (SL) and medio-lateral bending strength (SML)?
        canid skull length (SL) and anterio-posterior bending strength (SAP)?
        canid mandible lever arm (MLA) and temporalis lever arm (TLA)?
        felid skull length (SL) and canine height (CH)?
        felid skull length (SL) and medio-lateral bending strength (SML)?
        felid skull length (SL) and anterio-posterior bending strength (SAP)?
        felid mandible lever arm (MLA) and temporalis lever arm (TLA)?

What do the data suggest about the feeding behavior of C. dirus?

What similarites and differences are there between C. dirus and extant hyaenas?

What do the data suggest about the feeding behavior of adult S. fatalis?

How would the feeding of juvenile S. fatalis have differed from that of adults?

How would the feeding behavior of P. atrox have differed from that of S. fatalis?

How would the feeding behavior of P. atrox have differed from that of the extant African lion (P. leo)?