Shearwaters: pelvis and legs

The shape and proportions of the shearwater's pelvis and legs are strongly depending on the way it forages. The more aquatic species, such as the Puffinus species and Ardenna tenuirostris and grisea have strong legs adapted for underwater propelling by use of their feet. The more aerial species like all Calonectris species and Ardenna bulleri and pacifica are less well adapted for diving and show simliar features as the petrels that are also known to be not well adapted to this life style, such as the fulmarine petrels.

Pelvis

The pelvis of the true divers are relatively long and slender with much room for strong and long muscles. The aerial shearwaters have a pelvis that is wider and shorter, indicating less powerfull strokes while diving. Te pelvises of the Cory's Shearwater Calonectris borealis (left) and of Sooty Shearwater Ardenna grisea (right) are good examples of both ends of the scale.

Legs

The adaptations in shearwater legs are shown in all large leg bones. The more a species is a diver the femur is shorter and more curved. In the aerial species the femur is more straight and realtively long to the tibia. The tibia is longer in the diving species. The most conspicious feature is the extending processus rotularis, the 'flag' that forms a lever to which the strong thy muscles are attached. This process is extremely long in the Divers Gaviidae which are the foot-propellers par excellence, but in the Shearwaters this adaptation is also found to a certain extend. In Calonectris the this process is short (about 11% of the length of the tibia between the articulating surfaces i.e. the effective length of the tibia) and resembles that of the fulmarine petrels. In the diving shearwaters this percentage goes up to 25% as in Puffinus yelkouan (see table). Below the tibia and femur of Calonectris borealis (lower) compared with with Ardenna grisea (upper).

The table below shows the relative size of the proccesus rotularis in percentages in the various shearwater species. The lower the value, the less adapted to diving.

Species Range in %; average SD; N
Calonectris borealis 10.2-12.3; 10.9
0.01; 19
Calonectris diomedea 10.3-12.3; 11.2 0,00; 14
Calonectris edwardsii 10.1 -;1
Calonectris leucomelas 10.5 -;1
Ardenna carneipes 16.9 -;1
Ardenna creatopus 15,6-17.0; 16.1 0.23; 4
Ardenna bulleri 15.0-16.2; 15.4 -, 4
Ardenna gravis 15.4-17.2; 16.4 0.56; 11
Ardenna grisea 20.3-20.6; 20.5 0.17; 3
Ardenna tenuirostris 21.7 -; 1
Puffinus puffinus 20.8-23.7; 22.5 0.13; 6
Puffinus yelkouan 20.6-25.1; 22.6 0.02;3
Puffinus mauretanicus 22.0-23.1; 22.1 0.01;3
Puffinus gavia 22.0 -;1
Puffinus huttoni 21.5 0.0; 2
Puffinus baroli 19.9-20.1 -;2
Puffinus elegans 21.2 -;1

The tarsus in diving species is laterally flattened. The flatness of the tarsus in the flesh is very much defined by the soft parts, but the bone shows the flattening as well. The feet in shearwaters are not very specialized. Three webbed toes forward and a minuscule fourth hind toe.

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