Nature Notes: Large Litters Feed the Food Chain
During spring, we eagerly look forward to the return of migrating or hibernating animals as well as a new generation of babies emerging. It won’t be long before small spotted fawns curl up in the grass, cubs follow a hungry mama on their search for food, or fox kits emerge blinking from their dens. But why is it that deer typically have one or two fawns while it seems like there is a steady production of adorable baby bunnies in my yard until winter returns? There are certain patterns to mammal fecundity (the ability to produce offspring) and these patterns have a very vital role in ecosystem functions.
In general larger mammals produce few offspring while small mammals produce many. However, reproductive strategies are based on more than an animal’s size. Larger animals tend to have fewer predators. A bear cub is much less likely to get eaten than a vole or a rabbit. Species with higher predation rates need to produce more offspring to ensure at least one survives to pass on genetic materials.
Interestingly, bats are a perfect example to counter the “smaller size means more offspring” rule. Midwest bats typically give birth to one pup per year, but they also have a low predation rate for their size due to their nocturnal flight habits.
Larger animals also tend to have a longer developmental period that requires more parental care. An infant (baby chimp!) remains close to its mother for 4-5 years after weaning to learn social behaviors, hunting and foraging techniques, tool use, and other complexities of chimp life. In comparison, a house mouse can give birth to up to 15 pups and have another litter just 25 days later!
Order: Lagomorpha - Rabbits & Hares
Minnesota and Wisconsin are home to three Lagomorphs - the eastern cottontail, the snowshoe hare, and the white-tailed jackrabbit (actually a hare). What’s the difference between a rabbit and a hare? Hares are precocial, meaning they’re born with fur, with open eyes, and can run within a few hours. Rabbits are altricial, meaning they’re born naked, with closed eyes, and are rather helpless.
Despite this greater need for parental care, eastern cottontails win the lagomorph fecundity category, with each female capable of producing up to 25 leverets (baby rabbits/hares) a year in up to 5 litters, earning them the well known euphemism. The next generation can begin reproduction after 3 months. Unfortunately, very few leverets will live longer than a year. One strategy to help? Rabbits and hares engage in coprophagy. Yup, that’s right. That means they eat their own poop in order to maximize nutritional uptake from their cellulose-based diets.
Order: Rodentia - Rodents
Rodents make up approximately 40 percent of the mammal group. The order Rodentia comes from the Latin word rodere, meaning “to gnaw” and all members must have a pair of upper and lower front incisors that never stop growing. (Think beaver teeth - if they didn’t constantly chew things, their teeth would grow too long to even close their mouths!) In Minnesota & Wisconsin, we have larger rodents (i.e. beavers, porcupines, & woodchucks) and smaller, easier to prey upon rodents (muskrats, squirrels, chipmunks, gophers, rats, lemmings, voles, & mice) collectively occupying (and providing food) in every habitat type in our region.
Let’s focus on the insane reproductive potential of the smallest rodents - lemmings, voles, and mice - but first, what’s the difference? The easiest way to tell these small rodents apart is by their tail - mice have long tails, voles short tails, and lemmings nearly no tail.
These rodents all have short gestation periods, short periods of parental dependency, can reproduce all year if there is adequate food through winter, and have “post-partum breeding” that allows them to mate again within 24 hours of giving birth. Females are just weaning one litter as they get ready to give birth to the next. Put all this together and these small critters are production factories for the food chain!
Our most common type of vole, the meadow vole, is capable of producing litters of 3-10 pups up to 17 times a year. That means being pregnant up to 17 times per year and having up to 170 babies! Can you even imagine?! Females can become pregnant right away after giving birth and offspring are capable of reproducing after just 3-4 short weeks on this planet.
Those numbers should add up, right? But author Bernd Heinrich explains what happens next in his book, Winter World: “At such reproductive potential, it would not take long for them to carpet the earth. Luckily, such horrors of exponential growth are seldom realized. Instead, the voles’ role in the economy of nature is, like that of hares, to convert vegetation into the protein-rich dietary staple of many predators that rely on them in winter, principally foxes, weasels, fishers, coyotes, and bobcats. The summer shift includes hawks and snakes.” And don’t forget about the owls!
Populations of voles are cyclic and in a high-density period, there can be 250 voles per acre! After the snow melts in the spring, go out to a meadow or grassland and search for vole holes! Voles have elaborate systems of trails and tunnels, creating well-worn pathways through thick grass and surrounding habitats. Look for cut grass near holes and tunnel openings, as well as piles of vole scat! Spring cleaning is in full effect. Check it out in the video below!
So the next time you find an annoying mouse nest in a cupboard, a vole tunnel through your yard, or rabbit damage in your garden, just remember that these animals play an important role, too. When I first hung my bird feeders and started a compost pile at our new house, I noticed a definite increase in mice and voles around our yard and garage. Now do you know what I see? I see numerous types of owls, hawks, foxes, coyotes, and bobcats all in the woods right near our house. I see an ecosystem that I’m now a part of, with a stable food source not only for the prey species, but for the predators up the food chain. That’s the real beauty of those pesky exploding small mammal populations.
Left: One of the great horned owlets growing up behind our house. Right: Owl pellet, near the compost.