Suburbs pose threat to koala

An engaging combination of the smart, the poignant and the cute, the “Nature” presentation “Cracking the Koala Code” (7 p.m., PBS, check local listings) should not be missed.

Until now, my sole source of information about koalas was the old commercials for Australia’s Qantas airline. A furry fellow would lumber up a tree and complain, in a sad little adenoidal voice, that “Australia was crawling with tourists,” making his life, er, unbearable. Each spot would end with the same memorable tagline from the melancholy marsupial: “I hate Qantas.”

The Qantas spokeskoala may have been on to something, because tonight’s “Code” studies the effects of rapid suburbanization on the creature’s habitat. We’re shown quirky scenes of koalas cutting through backyards, avoiding mean dogs and climbing up on porches and roofs when a eucalyptus tree can’t be found.

The great koala displacement allows local biologists to study them in the semiwild by outfitting them with electronic collars and following them around with camera crews.

Koala alpha males defend a harem of females with their superior size and ferocity. But roaming males continually challenge their status in hopes of dethroning the king. Until that time, these “traveling salesmen” opt for random eucalyptus-tree assignations that guarantee genetic diversity in future generations.

Male koalas have a large mark on their chests that looks like an open sore but is really a scent pad they use to mark territory. So the koala that makes the biggest stink is at the top of the heap. Unlike some animals that have a rutting season, male koalas are perpetually randy. Two koalas featured here have to take time off for treatment of their chlamydia.

That Qantas koala may have sounded like a stuffed-up milquetoast, but alpha koalas are capable of roaring like a gorilla 10 times their size. It’s a startling sight and sound.

What isn’t good for the koala is sharing its forest with streets and highways. Biologists can protect the creatures from STDs, but there is no cure for an encounter with a radial tire. That Qantas koala had every right to be bummed.

• “Bones of Turkana” (9 p.m., PBS, check local listings) explores new skeletal evidence of human evolution found in northern Kenya.

Tonight’s season finales

• Tessa feels left out on Mother’s Day on “Suburgatory” (7:30 p.m., ABC).

• Wedding bells on a two-hour “Criminal Minds” (8 p.m., CBS).

Tonight’s other highlights

• NBA playoffs (6 p.m., TNT).

• The top three perform on “American Idol” (7 p.m., Fox).

• Living art on “Modern Family” (8 p.m., ABC).

• The squad investigates a sordid demimonde of self-mutilators on “Law & Order: Special Victims Unit” (9 p.m., NBC).

• Emily and Jack bond on “Revenge” (9 p.m., ABC).

• The gang bids on an abandoned storage locker on “Hot in Cleveland” (9 p.m., TV Land).