ON THE EDGE OF EXTINCTION - A - THE TIGER (Panthera tigris)

[peter]

I never read anything about the structure of tiger skulls, meaning I'm on my own here. Entering the realm of speculation, that is. But I don't mind giving it a try.


INDIAN TIGERS - LOWER SKULL DEVELOPMENT, RESULTING IN A LIFTED AND STRENGHTENED OCCIPUT AND ROBUSTNESS

V. Mazak (1983) wrote the concave profile of the sagittal crest was quite typical for Indian tigers. He added it was more often seen in skulls from Nepal and northern India. As tigers in these regions could be a bit larger than those of central and southern India, it could be related to size. Amur tiger skulls, however, are as long or longer and they don't show the concave profile. This means size, most probably, isn't the only reason.

Tigers in northern India and Nepal live in or close to alluvial floodplaines. This is not true for central and southern India. Alluvial floodplaines attract large herbivores and tigers hunt them. There are different ways to overcome the resistance of very large animals, but it isn't about how they do it. Skullstructurewise, it would be about the way they use the skull to kill these large animals. It definitely would take a lot of time, meaning the skull would need to be reinforced. In what department?

My take is the lower part of the skull (the mandibula) in particular. The upper skull is used to get a grip, meaning the canines have to be long, strong and curved. This is needed to be able to withstand a large force for a long time. The lower skull, however, is used to get to the right position and finish the job. The victim, more often than not, is killed by suffocation. As herbivores are large, this will take time. Tiger Raja, when he killed the female gaur, showed it is an exercise in both technique and strength. Endurance also is important.   

I'm not saying the mandibula is instrumental, but it could be close. If the mandibula is used in this way, the lower part of the sagittal crest will need to be reinforced. This could be the reason why the occiput is slightly elevated and not too wide. A wide occiput is not needed. It is about that part used to enable the lower jaw muscles the exercise maximum pressure for a prolonged period of time. A wide occiput means the muscles have to make a kind of detour. This will result in loss of strength. It could also result in damage to the tendons. The zygomata need to be reinforced as well, as they are instrumental in keeping the skull together. Experienced, older, tigers have the typical concave profile because they, more than younger animals, hunt large herbivores.

All in all, one could say strength is instrumental for Indian and Nepal tigers hunting large herbivores in alluvial floodplaines. Skulls of large and experienced males adapted to this way of hunting. One result is a strengthened and elevated, but somewhat narrowed occiput. Another is an outspoken vault, a relatively short face, wide arches and a strong crest, especially in the posterior part (near the occiput).


AMUR TIGERS - UPPER SKULL DEVELOPMENT, RESULTING IN A LONGER AND MORE ELEVATED SAGITTAL CREST AND LONGER UPPER CANINES

According to V. Mazak, Amur tigers have slightly longer skulls than Indian tigers. Although the arches could be a bit wider, they seem to be relatively less robust. Another difference is Amur tigers have a relatively long, straight and elevated face, very long, robust, but straight upper canines and a straight sagittal crest. The crest, in contrast to Indian and Nepal tigers, starts directly after the os frontalis. The occiput usually is a bit more rounded and wider. Why is that?

My take is it is a result of a more extensive use of the upper skull. Indian tigers use the upper skull to grab and hold their victim, which resulted in long and robust, but slightly curved upper canines. If we include the facial part of the skull, the curve becomes a bit more pronounced. The upper canines of Amur tigers, on the other hand, are longer, straighter and a bit less curved. When you include the front part of the skull, where the upper canines are anchored, the conclusion is the angle, compared to Indian tiger skulls, is less outspoken. In other words: the upper canines in Indian tiger skulls are used to cause damage, to hold and to withstand a strong force, whereas the upper canines in Amur tiger skulls, more than anything else, are used to cause significant damage. One could say Indian tigers are grabbers and holders, whereas Amur tigers are stabbers. Why the difference?

One is Amur tigers hunt other animals. Deer are a major part of their diet, but so are wild boars and bears. In some seasons, in some parts of Russia, bears are more important than deer or wild boars. Deers are quickly killed, but wild boars and bears are robust and agile animals able to return the favour. Large herbivores can injure and kill tigers, but wild boars and bears probably do it more often. As a result, Amur tigers need them in a different way.   

What is known about the way Amur tigers hunt wild boars and bears? 

Sizewise, there isn't much to choose between male wild boars and male Amur tigers. Male Himalayan black bears usually are smaller than male Amur tigers, but male brown bears are larger. Amur tigers prefer to hunt animals a bit smaller than they are. Experienced tigers, however, also hunt animals of similar size (weight) or animals slightly larger than themselves. 

Although many think a tiger would need a clear advantage to stand a chance in  fight with a boar or bear, an ambush wouldn't make a big difference in a fight with a robust and agile opponent. The reason is large wild boars and bears of, say, 150-200 kg. (332-442 lbs.), are too strong to be killed outright.

Although some fights have been witnessed or pieced together, not much is known about confrontations between animals of (near) similar size. Based on what I read, I don't think it's likely a tiger will kill a bear in the same way as a large herbivore. In India, a hunt often culminates in an ambush, a rush, contact, a large herbivore going down and a prolonged struggle. It's advantage tiger most of the time. In Russia, chances are a hunt more often has the character of a fight. I don't know if Amur tigers use a hit and run strategy, but it's more than likely contact will be interrupted and resumed every now and then. Every time contact is resumed, the tiger will try to get a good hold (hence the large fore-arms and paws) and try to inflict as much damage as possible. The upper canines are not used as anchors, but to penetrate and damage.

Biting is very different from holding. Holding an agile and powerful animal like a bear or a boar head on would be too risky. The tiger most likely wil try to get to the side or back and target the neck or shoulder. If the opponent weakens, the tiger will try to get a hold. If not, it could develop into a hit and run strategy. It's advantage tiger most of the time: if the fight isn't going his way, he can always get out. This is not true for the bear.

I'm not saying the Russian Far East is a battleground with tigers and bears duking it out every few weeks or so, but it is likely Amur tigers who hunt bears and wild boars might get involved in bouts they would have preferred to avoid. Based on the information provided by Russian researchers, I'd say problems with wild boars and, in particular, bears often just can't be avoided. Just read the information about the 3 adolescent tigers released a year and a half ago. Well before they reached adulthood, they hunted large male wild boars and had to learn how to defend their kill from scavenging bears.

A large size is helpful, but it wouldn't be decisive in a fight with a dangerous opponent. It could even be a handicap. Fights in this category are not only decided by sheer strength. Besides, Amur tigers, as a result of the testing conditions, have no need for 500 pounds and over. This is why only some parts of the body have been adapted. 


CONCLUSION

All in all, I'd say Amur tigers, more than Indian or Nepal tigers, use their skull to bite as much as to hold. When biting with as much force as possible, pressure is not exercised at the muscles used to hold and chew. It is exercised at the top of the skull, just behind the os frontalis.

Try it yourself. When you bite with force, you use different muscles and feel pressure at the top of the skull. When you chew or hold, pressure is exercised at the back of the lower jaw. This difference could result in the profile of the sagittal crest seen in both subspecies.

Some confirmation can be seen in skulls of Caspian (male) tigers, who also specialised on wild boars. In some skulls, the crest is even more pronounced (elevated) than in Amur tiger skulls. One reason could be that wild boars in southwestern Asia are at least as large as those in southeastern Russia.

The shape of the upper skull in Amur and Caspian tigers might be a result of the way it is used. A sagittal crest rising immediately behind the os frontalis could be a result of using muscles used to bite with maximum force at the tips of the canines. If the sagittal crest, on the other hand, is elevated near the occiput, it points towards intensive use of the lower jaw muscles. This idea is supported by the width of the zygomata in Indian tigers (pointing towards prolonged struggles with large animals) and bone density, resulting in a more robust, heavier, skull.

The differences between the skulls of Caspian and Amur tigers could be a result of specialisation. Caspian tigers have a very shortened (bulldoglike) snout and more curved upper canines. As they specialised on wild boars, it could mean wild boars are killed in a different way than bears. More a question of combining stabbing, holding and jerking, perhaps.

Amur tigers have relatively long faces, elevated snouts, a more horizintal axis of vision and less curved upper canines, meaning they, more than Caspian tigers, are biters. A biter doesn't need a curved upper canine, but a long, straitish and robust canine and, if possible, a bit of pressure on top of the canine to add more weight. It is about penetrating and causing significant damage. In order to get there, a strenghtened rostrum is needed. The rostrum in Amur tigers, although their skulls are less robust than skulls of Indian tigers, is very wide and strong. If we add the elevated and straight (horizontal) face, the elevated sagittal crest and the large size, the result is acute angles and vertical lines dominating horizontal lines. Exactly what a biter needs. In Indian tiger skulls, more sloped angles and horizontal lines dominate. This is what a powerhouse needs.

One could say that a solitary big cat hunting large herbivores needs brute (skull) strength, resulting in a very powerful mandibula, a narrow and elevated occiput and wide arches; that a solitary big cat hunting wild boars needs a shortened face and snout and a well developed sagittal crest; and that a solitary big cat hunting (or fighting) bears of about similar size needs a combination of size (skull and upper canines) and agility. This results in a long and elevated skull dominated by vertical lines and a powerful anchor (referring to the rostrum) for the immense upper canines. Bone denseness and mass, apparently, are not needed.       
             
Sumatran tiger skulls, in a way (referring to a long and straitish face and very long upper canines), resemble Amur tiger skulls, but their rostrum isn't reinforced and neither is the sagittal crest. This means the long canines are more than enough to get the job done. A tiger faced with large bears needs strength, but in a different way than a tiger hunting large herbivores. The reason is one bite isn't enough. In order to be able to bite more than once, the tiger needs to be able to hold his opponent. This is why Amur tigers have exceptional fore- and upper-arms and large paws (large paws most probably not only are an adaption to snow, as other cats have relatively smaller paws).  


A FEW PICTURES

a - Caspian tiger (Panthera tigris virgata). Turkmenistan. The wild boar hunter. A biter. Watch the sagittal crest rising immediately behind the os frontalis, resulting in a convex profile:

*This image is copyright of its original author

b - Indian tiger (Panthera tigris tigris) from Nepal. Animals living in alluvial floodplains in Nepal and northern India often hunt large herbivores. Upper skull profile very vaulted and occiput strengthtened, moderately wide, triangular and elevated, resulting in a concave profile:

*This image is copyright of its original author

c - Amur tiger (Panthera tigris altaica). Profile more like in the Caspian tiger, but larger. A biter with exceptional upper canines hunting deer, wild boars and bears. Upper skull profile straight. Vault limited. Sagittal crest long, elevated and straight:

*This image is copyright of its original author

d - This was the model used a long time ago. Didn't work as expected, as prone to breakage (Berlin Natural History Museum). Modern big cats, for this reason, have shorter and conical teeth:

*This image is copyright of its original author

e - Wild boar from Turkey. The angle is deceptive, but he was large. As large as they come (781 lbs.). Not saying he would have been hunted by Caspian tigers, but they did hunt large boars. This is why conical teeth developed: 

*This image is copyright of its original author

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