Final answer:
The chest structures among ostriches, penguins, and terns differ based on their locomotive behaviors; ostriches have a flat sternum, whereas penguins and terns both possess a keeled sternum to support their necessary swimming and flying muscles, respectively.
Step-by-step explanation:
Differences in Chest Structures Among Birds
In comparing the chest structures of ostriches, penguins, and terns, it's essential to understand how these structures relate to their different lifestyles and adaptations for movement. The correct answer to the question is b) Ostriches lack a keeled sternum while penguins and terns have it. This key difference in anatomy is related to the presence and development of flight muscles in birds.
Ostriches are flightless birds and have evolved to run at high speeds on land. As such, they have a flat sternum with no keel, which means they lack the large pectoral muscles that are necessary for flying. In contrast, penguins, which use their wings for swimming, and terns, which are adept fliers, both possess keeled sterna. This keel provides an attachment point for strong flight muscles.
Penguins have a different adaptation with their wings, known as flippers, to propel themselves in water rather than in the air. Their keel still supports the necessary muscles for strong wing-driven swimming. Terns, being agile fliers, use their keeled sternum to anchor powerful flight muscles that allow them to undertake long migratory flights. The keel is pronounced and serves as the foundation for their pectoralis major and supracoracoideus muscles, responsible for the downstroke and upstroke of the wings, respectively.
The sternum's structure in birds is closely related to their ecology and behavior. The evolutionary pressure exerted by the need to swim efficiently, as with penguins, or to fly, as with terns, has shaped their sternum accordingly. However, the ostrich's sternum structure reflects its terrestrial locomotion where strong pectoral muscles for flight are unnecessary.