When Paul showed up at work with that coffee can full of mice babies, I knew I was perched on the zenith of the best day of my life. Okay, sure, it’s pretty sad that he’d accidentally killed their mother and found the orphaned pups living in his air conditioning unit a day or so later. But we’re talking mouse babies here. White-footed mouse babies at that! In a coffee can nest. I took one look at the heartbreakingly tiny, hungry, pink, blind, creatures with those impossibly teensy handfeet and confiscated the coffee can for my keeping.

I rehydrated them and fed them mouse formula. I groomed them and stimulated their tiny bottoms to use the bathroom. I clicked and cooed at them. I named them Paul and Julie Junior. I marveled at how they grew, from a light dusting of hairs and scrambling claws to coordinated movements and a rich, thick fur. And they were so . . . quiet. Like, too quiet. When they ate they seemed enthusiastic, but never made a noise. I discussed this with my colleague and white-footed mouse expert, Warren Booth, and that’s how I learned there’s much more to mice than meets the ear.

Mice are rarely quiet. In fact, mice are loud. And they have lots to say. We just can’t hear them. Male mice sing bird-like mating songs to their female paramours. Female mice roar in response. Adult mice have excited dialogues when other adults return home with food. Babies complain about their living conditions and happily greet their mothers. But all this discourse takes place in ultrasound, the same ultrasound used by bats to locate prey and navigate darkened skies, the same ultrasound used by dolphins and whales to find each other and a good meal.

Picture sound frequency (the property of sound that most determines “pitch” and measured in Hertz (Hz)) as linear, with low frequencies on one side of the line and high frequencies on the other. We can’t hear the low end of that line, 20Hz and below, and call that sound infrasound. Elephants use those low frequencies to talk over long distances. We can hear above 20Hz all the way up to about 20kHz, and call that range acoustic, or audible sound. But a whole world of noise takes place just outside our hearing abilities, above 20kHz, in the range we call ultrasound.

Sound scale

When I put an ultrasonic transducer (a nifty tool that turns sounds humans can’t hear into sounds we can hear) into the mouse nest, I could hear all-day chatter. My babies greeted me with warbles and shrieks when I came to feed them and fussed over each other with pleasant burbles while they groomed one another. If strangers came near their nest, they grew stone cold quiet.

All these ultrasound shouts and murmurs can save mouse lives.

You know how some animals are more active at night while other animals are more active during the daytime? And how those daytime animals can avoid nighttime animals just by scooting around during daylight hours and hiding at night? Or some animals live underground while others mill about on the surface? And those underground animals can avoid the trials and tribulations of above ground life? Well, the same thing happens with auditory communication.

Just think of the hard knocks my garrulous mice would face if people and predators could hear them chatting up a storm. They’d be found out and dead meat in no time flat. Many mouse predators can’t hear their conversations, though, so mice can gossip and squeal away to their hearts’ content, while keeping it all on the down low.

Paul and Julie cleaning themselves in their coffee can nest. But don’t be fooled, these quiet cuties are actually making a racket.

In case you were wondering, Paul and Julie Junior grew up to be happy, healthy, adorable white footed mice. Julie Junior caused a stir when she broke free one night and spent a few wild days stealing candy from people’s desks and building nests inside their computers. We lured her back home with a tiny Hershey almond bar, which she unwrapped and ate on top of a bookshelf, looking the whole time like a hopeful librarian. Eventually, Warren let them go outside in a grassy white-footed mouse heaven. I imagined they squeaked their tearful goodbyes to us, but I just couldn’t hear them.