Global warming and kittens. While it may seem hard to see the connection between the two — a climate phenomenon that melts glaciers and acidifies oceans, and cuddly, 4-ounce balls of fur — experts say there could be one.

Each spring, the onset of warm weather and longer days drives female cats into heat, resulting in a few months of booming kitten populations known as "kitten season."

"The brain receives instructions to produce a hormone that basically initiates the heat cycle in a cat," said Nancy Peterson, feral cat program manager of the Humane Society of the United States, "and those instructions are affected by the length of day and usually the rising temperatures of spring."

Peterson said kitten season generally starts in March or April, as the days get warmer and longer, and the flood of kittens continues throughout the spring and early summer.

What shelter officials and veterinarians have begun noticing, however, is that kitten season is starting to begin earlier and last longer.

"They're mating earlier and we're starting to see them coming into the shelters much earlier in the season," Washington Humane Society Shelter Director Michelle Otis said.

In February 2007, for instance, the Philadelphia Animal Welfare Society recorded a total cat intake of 672. The intake in February climbed to 1,008.

"Right now we're getting absolutely flooded with kittens," said Rory Uhler, president of the Safe Haven Animal Sanctuary in Cincinnati, noting that kitten season usually tapers off in the third or fourth week of June.

Newspaper ads and signs on street corners advertising "free kittens" are popping up all around Cincinnati, where kitten season started a month early this year, Uhler said.

"I don't know any shelter that has any room in Cincinnati," he added.

Where does global warming fit in? Some experts say rising temperatures could be lengthening kitten season by altering cat reproductive cycles.

"It might make sense that if temperatures are rising, cats will go into heat more often," Peterson said.<

On its Web site, the Environmental Protection Agency states that rising temperatures may cause some small mammals to start breeding earlier in the year.

"Domestic cats evolved from African ancestors," said Julie Levy, a veterinarian and shelter medicine professor at the University of Florida. "Although they have adapted to climates throughout the world, it is possible that global warming is mimicking their ancient origins and helping them reach their full reproductive potential."

Other experts disagree, holding that the heat cycle in cats is based on day length and light exposure, not temperature.