A service dog during a training exercise in 2017. The U.S. Department of Transportation has proposed several rule changes regarding animals on airplanes. (The Minnesota Star Tribune)

Since they don't enunciate, we can't say for sure, but probably there isn't a dog, cat or any other nonhuman vertebrate that would freely board an airplane if it understood the implications. Even the number of humans who enjoy air travel has dwindled after years of heightened security and diminished personal space.

Yet a pig has flown, as have a horse, a marmoset monkey and a miniaturized breed of Allosaurus. (That's not actually true, the part about the dinosaur. The other examples, however, have been documented.)

Of course, airplane passengers have long traveled with their pets, either in the cargo hold or in the cabin — in limited numbers determined by airlines — if the animals fit into carriers that slide under seats.

Also sometimes present are service animals — the "fully trained" dogs, as Star Tribune Travel Editor Kerri Westenberg wrote, "that help the likes of veterans with PTSD and the visually impaired."

Increasingly, though, floor space and passenger laps have been occupied with animals that offer "emotional support" for their owners. Unlike service animals, comfort animals are not specially trained to provide help for a disability. They have had access to planes, however, because they are included in the definition of service animals under the Air Carrier Access Act, which prohibits discrimination in air travel on the basis of disability.

The federal Department of Transportation administers the act, and last week proposed changes that would bring its definition of a service animal more closely in line with that of the Justice Department's implementation of the Americans With Disabilities Act, which applies in other settings where members of the public mingle — including, ironically, airports. If approved, the changes also would mean more stringent documentation than is currently required — critics have noted the ease with which a letter purporting the need for an emotional-support animal can be acquired — and more latitude for airlines in setting limits.

The number of emotional-support animals on commercial flights rose to 751,000 in 2017, from 481,000 the year before, the Washington Post reports via the trade group Airlines for America. According to the Transportation Department proposal, passengers complained to U.S. and foreign airlines about service animals (under its broader definition) 3,065 times in 2018, up from 719 in 2013.

Readers won't need us to remind them that air travel is confining. But for perspective, an average person — length times width — takes up 7.19 square feet lying down or standing. The average airline seat offers that same person 3.7 square feet to fit into, partly folded. Such is among the prices to be paid for the convenience of hurtling toward one's destination at nearly 600 miles an hour, but contrast it, again for perspective, with another spot where people might render themselves bodily inert for several hours: a living-room recliner, with an average 21.8 square feet.

(A digression: No Earthling is ever truly inert, what with the planet spinning at 1,000 miles an hour on its axis, circling the sun at 67,000 mph and riding the galaxy at half a million in that measure.)

Meanwhile, flights have been quite full in recent years. There's simply not a lot of room at the levitating inn for furry or feathered friends.

The Transportation Department's newly pragmatic approach is needed. And if the case for broader access for emotional-support animals is to be made, scientific documentation of their benefits is needed.

But the wisdom of the crowd is also needed. The public has two months to comment on the proposed changes and can do so at regulations.gov, docket number DOT-OST-2018-0068.