The skin of a Grey is very similar to that of dolphins in texture, and in most cases, coloration. On Terra, cetaceans evolved legs and arms to function on land, but when they returned to the oceans, the arms and legs evolved into flippers and tails. X-ray analysis clearly show the residual appendages in modern day dolphins and whales as being comparable in skeletal structure to that of land-dwelling mammals in placement and types of bones. It is possible that the Greys evolved much like Terran dolphins did, but did not return to the oceans, but continued to develop as a land based mammal. Next is the skull shape: The Greys have a large rounded skull region area on the upperanterior and posterior sections of their skulls. An examination of the dolphin skull shows very similar large, rounded anterior and posterior areas. The eyes of Greys are described as large and black, with what appears to be a protective shielding covering the eye. Many Terran aquatic animals have developed protective outer eye shields for underwater use.

Another factor is the dark coloration of this eye shield. It suggests an eye structure suited for a reduced light environment. In the oceans, this would be natural for a creature living underwater were the planet's primary's light does not penetrate very deep.

The dark coloration would in indicative of eyes that absorb nearly all the light that enters it, creating the appearance of blackness. On Terra, dolphins and other cetaceans developed sonar ability as their primary navigational aid, and thus do not rely as much on sight while underwater. Dolphins can emit very powerful and focused ultrasonic bursts from the anterior section of their skulls, from an organ knows as a 'melon'. This is used to stun prey and enemies, such as sharks and barracuda physical creating paralysis and disorientation. When hunting, the ultrasound beam could be directed at small fish and amplified to lethal levels,causing instant death. Some cetacean researchers believe that this is alsoused when dolphins ram sharks. Autopsies of the shark corpses after a dolphin attack reveal the sharks internal organs have been visibly shattered. The beam can also be controlled so as to produce a stun effect, as in the case of the barracuda. Nearly every abductee states that when they are abducted, they are paralyzed by the Greys, and become mentally disorientated.

The description of the Grey's head is usually large, triangular and rounded in front and back. This suggests a large and rounded rear and frontal brain lobe structure. This is exceedingly similar to the physical structure of a cetacean brain. It is the main difference between cetacean and human brains. The 'Stare" as abductees call it, is merely the positioning of the Grey's anterior skull area, the projection area where the Grey equivalent to the cetacean 'melon' is located, to achieve maximum beam concentration on the abductee.  This large rounding is thought to be an adaptation of the cetacean brain to handle the enormous data flow received from echo-location and other sound-based communications faculties. It should also be noted that on the rare occasions that abductees hear verbal sounds from Greys, it is described as high-pitched, sometimes chattering or staccato clicks or beeps. These are all similar to the air vocalizations of dolphins.