I’ve braced for months against the cold and been sobered by the long, dark nights.

As spring approaches, I feel like a clenched fist relaxing.

I wonder what those who watch the natural world for a living — the naturalists and botanists — notice this time of year. What are the earliest signs of spring they see?

One notices the mad dash of male squirrels after females gone into heat. Another says it’s the small salamanders squashed on the pavement.

A third hears the songs of cardinals and chickadees.

Another speaks of the swelling buds of wind-pollinated elms and silver maples.

If I heard as well as God, I might even hear the sugars and water rising from the roots of trees to the tips of twigs.

My instructors on the first signs of spring in mammals, reptiles and birds are Robert Timm, John Simmons and Mark Robbins, at Kansas University’s Biodiversity Research Center.

My plant experts are Craig Martin, professor of botany, and Craig Freeman, a scientist at the Kansas Biological Survey.

For most of us, longer days herald spring’s arrival. It may also be those longer days that cause the sexual readiness of female squirrels. The males can smell it.

Robert Timm says, “Several may pursue a female up and down branches and from one tree to the next.”

For most humans, rising temperatures also give hope of spring. And smallmouth salamanders also follow this cue.

Drab, gray, a few inches long, they spend the winter underground. But give them a temperature of 45 and a good rain and out they come, Simmons says.

In Lawrence they travel north across 31st Street to mate in the Haskell Wetlands — or die trying.

“As you might imagine,” Simmons writes in an e-mail, “the poor, little salamanders get squashed by the hundreds — make that the thousands.”

The next heralds of spring are the amphibian cries of the Western Chorus Frog. Some people call them spring peepers, and they live in ponds, lakes, swamps, ditches and buckets.

More hours of daylight, once again, figure into the early bird sounds we hear. They drive the songs of cardinals, chickadees and tufted titmice, whose gonads also grow larger around this time.

Many of us notice geese flying north. Robbins says we should also look for the arrival here of the eastern Phoebe, a drab brown bird. Their sign: mud nests under bridges and beneath the eaves of old buildings.

The buds of trees are a sign of the sap that rises every spring. Martin tells me that food and water are routed through a tree by two paths, called xylem and phloem.

The xylem path is built of dead cells that form hollow tubes. It’s ordinarily a path through the tree for water. The phloem path is made of live cells. It usually routes sugars through a tree so it can grow.

But in early spring, for reasons botanists don’t understand, sugars and water both take the xylem road.

This arrangement helps them reach the growing tips of twigs so leaves can sprout.

How does the tree know it’s time to do this?

“It’s all hormonally driven,” Martin says “in response to … well … the spring.”

After months of dark and cold, it will soon be time again for the common stir, time for us all to sing and bloom and frolic, cued by light and warmth and hormones.