BRENTWOOD -- When you're a teacher trying to convey a point to a roomful of teenagers with short attention spans, the ick factor can be a useful tool -- especially when it involves dead meat and flesh-eating bugs.

Welcome to Spencer Holmes' biology class, where Heritage High School students can observe the process of decomposition up close and personal.

"Initially they're grossed out. It's like 'Eeew! They stink! (You're) a freak!' But then that wears off into curiosity, and they're just pressed up against the glass," said Holmes, whose cheerful personality and freshly scrubbed appearance seem at odds with someone whose hobby is collecting roadkill.

Sitting under a glass-enclosed fume hood just inside the door to his classroom are two portable picnic coolers where Holmes stores the wildlife he scrapes off area roads.

Inside one of them is the skull of a small mule deer, complete with antlers; the thriving colony of beetles Holmes keeps in the tubs has all but finished its job, and only a tuft of fur and skin on the crown remains.

Holmes runs his fingers through the bed of wood shavings, sending some of the hundreds of small black beetles living among them scurrying to the surface.

These dermestid beetles take over where turkey vultures and other first-on-the-scene scavengers leave off, Holmes explained.

Whereas those creatures rip apart a carcass to get at the soft tissue, the beetles' mandibles enable them to chew the dry, tough skin, he said.

Skulls 'n' skeletons

Holmes has amassed about 60 animal skulls from a broad range of species.

His display cabinet contains the remains of a badger, bobcat, raccoon, black-tailed jack rabbit, ground squirrels, foxes and skunks as well as deer, coyotes and opossums.

Fourteen are from birds -- egrets, herons, hawks and a crow.

Not all the exhibits were accident victims. The pig skull was once someone's 4-H project; the otter is a family heirloom. His emu head was culled from a breeder's ranch, and Holmes obtained the wallaby along with a brush-tailed possum in a "skull swap" with some biologists in New Zealand.

The benefit of using roadkill is that it does not cost him a cent.

Without this ready source of skeletons, Holmes said he would either be depleting the science department's budget to buy plastic models or have to settle for a limited number -- and that would stunt class discussions.

"The skulls just bring (up) more topics," says 16-year-old Austin Aeschbach, who took Holmes' class last year and still remembers one talk about why cows have more molars than humans.

"It made the lesson more clear because we could see (an) example of what he was talking about," he said.

Holmes uses his collection as a show-and-tell when explaining how the bone structure of different species reflects their lifestyle in the wild.

Look at a coyote's incisors, designed to rip and tear its prey apart, whereas its rounded molars pulverize muscle and cartilage into a more easily digestible form, Holmes said.

By contrast, a wallaby's back teeth are all but flat, which makes them better at grinding grass, he said.

A hawk, on the other hand, does not have any teeth because it relies on its sight to hunt, Holmes said.

"About half their head is just pure eyeball," he said, producing the skull of a red-tailed hawk to show its large orbital cavity.

Animals that depend on their noses for survival have long snouts, Holmes added.

He invited a visitor to peer into a coyote's nasal cavity, where a honeycomb structure of fragile-looking bone increases the surface area of its membrane covering -- thereby maximizing the number of chemical receptors that give the animal an acute sense of smell.

Cycle of life

Holmes first began collecting roadkill as a wildlife biology major in college, where he set out to find a quick way to determine the age of Virginia opossums in the field other than counting their teeth.

In the course of a year he salvaged about 150 opossums that had been killed by vehicles, initially using scalpels and forceps to get at the bones.

Holmes could not process them as fast as he was finding them, however, so he borrowed the university's collection of dermestid beetles and discovered that they did the dirty work in a fraction of the time.

Once he became a teacher, Holmes obtained the state and federal permits required to salvage roadkill -- the government considers dead wildlife public property and as such regulates its removal in the same way it does by issuing hunting and fishing licenses, he explained.

About once a month Holmes will stop to collect an animal that met its fate trying to cross a road and, using the stash of latex gloves, plastic bags and the large pocket knife he keeps in the trunk of his car, cuts off the head.

Holmes' most unusual find was a badger on Vasco Road nearly four years ago. He wrapped the entire body in multiple plastic bags and, sealing them tightly, popped it in the freezer at home "right next to the green peas," Holmes chuckled.

To show students the badger's muscular structure, however, required eventually thawing the partially decomposed mass.

"It was my worst dissection. Whoo! It was actually revolting," Holmes said, recalling the stench.

Although his wife benefits from his quirky pastime by borrowing skulls for the biology classes she teaches at Liberty High School, Holmes now stores all his finds in a classroom freezer dedicated to specimens that still have flesh on them.

"I could tell the hobby was getting old for her, so I just took her cue," he said.

As for the carcasses he occasionally takes with him, Holmes buries the parts he does not use in his backyard, where he happens to grow tomatoes, beans, peas and other vegetables.

Come fall, he shares his harvest with students as he explains how decomposing organic matter enters the soil and nourishes new life in a different form.

"The whole grand idea is nutrient cycling. It's food going from one organism to another," Holmes said. "They really get the idea."

Reach Rowena Coetsee at 925-779-7141 or rcoetsee @bayareanewsgroup.com.