Last One!

It's the last blog! My last entry will be a discussion about a masters thesis regarding the dexterity of forelimbs and the reason for this dexterity (Iwaniuk, 1997). The thesis is sectioned into several different research projects, all of which relate to the common theme of forelimb dexterity. The paper starts by defining the methods which will be used to determined dexterity, a scale method created by the author. He goes through and rates a variety of species. He then uses this data to determine which factors are influencing dexterity. This is what he found:

With regard to brain size, there is little relationship between larger brains and increased dexterity, the theory behind this being that evolution has more subtle ways of manipulating dexterity other than increasing the size of the brain. This makes sense. Many small animals (rats, lizards, squirrels) have quite a bit of dexterity for climbing, opening nuts, digging for food, and yet their brain size is constrained by the size of their skulls.

The second factor analyzed was body size. The author predicted that an animal with a large body would have more dexterity. This turned out not to be true, for many of the same reasons as stated above. But also, body size is dependent on many different factors (diet, habitat, phylogeny) and a small animal is just as likely to develop dexterity on the basis of these as a large animal.

Phylogeny was next on the list. It turned out to be correlated strongly with dexterity. Phylogeny refers to the evolutionary history of an animal and which other animals it is related to. The study found that more closely related animals tended to show more similar forelimb use patterns.

It was also predicted that those animals that lived in trees would be more dextrous. This was only related in terms of proximal dexterity. The author claimed that this is because grasping forepaws are not related to climbing trees.

Vertebrate predation is a factor that many claim gives rise to forelimb dexterity. But this study found that it was actually negatively correlated. This is possibly due to the fact that animals that prey on vertebrates also have to chase these animals down (the lion and the antelope, lynx and snow hares, polar bears and seals). Thus these animals forepaws must strike a balance between finally tuned dexterity and raw running power. The result is not always the most dextrous.

AA

Iwaniuk, Andrew N., 1997. "The Evolution of Skilled Forelimb Movements in Carnivoran." Masters Thesis, University of Lethbridge, Lethbridge. 151 p.  

Lions, Tigers, and Crocodiles

So the article for the week is about Crocodylian forelimb musculature. You may be asking yourself how that is related to iguanas, and well ... it's not really, other than it is yet another comparison. My interest of this article relates to its focus on why anybody is even studying Crocodylian forelimbs. The article, "Crocodylian Forelimb Musculature and its Relevance to Archosauria," discusses the forelimb of archosaurs as a functionally diverse anatomical unit, but one that has remained nearly the same through its evolutionary path (Meers, 2003).

Archosaurs are a group of creatures that evolved in the late Permian or early Triassic period. They are diapsid amniotes (meaning they have two holes in their skull, one on each side) and the most modern representatives are crocodiles and birds. This gives you a clue as to why their forelimbs are so interesting. The front limbs of an alligator (arms) vary quite significantly from the front limbs of birds (wings). But though they may look different, their musculature isn't all that distinct.

The author dissected 4 Alligator mississippiensis, 1 Crocodylus siamensis, 2 Crocodylus acutus, 2 Osteolaemus tetraspis, and 1 Gavialis gangeticus. They then compiled in depth muscle descriptions for all of the muscles in the forelimbs of the specimens. The muscles don't vary all that much from the iguanas. A part of this is because crocodylians are squamates, like iguanas, and hence their posture is very similar as are the muscles required to maintain this posture. The species dissected did have more extrinsic muscles on the ventral side. This means that they had more chest muscles (extrinsic meaning the muscles originate on the trunk and insert on the front limbs, and ventral being the side with the animal’s heart.) Also, the study named the deep muscles of the back differently than my sources did, but they were relatively the same.

The conclusion of the study was basically that the American alligator is representative of most Crocodylian species (re-enforcing that the forelimb musculature of Archosaurs has not changed much).

AA

Meer, Mason. "Crocodylian Forelimb Musculature and its Relevance to Archosauria." The Anatomical Records Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology (2003) 274A.2: 891-916. Web. 28 Apr 2012.