Researchers wanted to learn whether using two legs instead of four helps jumping rodents escape their predators.
Introduction
Predators have many different ways of capturing prey. Some predators chase their prey, while others wait and strike suddenly. It is important for prey animals to be able to avoid or escape predators. Prey can have different shapes, sizes, and speeds to help them. For example, a fly has wings that help it avoid a lizard’s quick tongue. The number of legs an animal uses might also be important for avoiding predators. Do you think more legs or fewer legs help an animal escape?
Most mammals are quadrupedal, which means they walk on four legs. Cats, bears, and mice are all quadrupedal. Using four legs at once may help them run fast. But some mammals are bipedal. They walk on two legs. Bipedal animals usually have larger back legs and smaller front legs.
The kangaroo rat is bipedal and has strong back legs. It is even famous for its high jumps – like a kangaroo! Being bipedal may help the kangaroo rat jump really high. It may even help them avoid predator attacks from animals like owls and snakes. But are bipedal rodents better than quadrupedal rodents at avoiding predators? To find out, we compared the jumping ability of kangaroo rats with three types of quadrupedal rodent.
Methods
We went to deserts in southwestern North America. We captured 150 rodents. These included bipedal kangaroo rats and quadrupedal pocket mice, woodrats, and ground squirrels. We gave them tags and fur dye so that we could identify them. We then released the rodents and allowed them to return to their home burrow.
At their home burrow, we used a rattlesnake strike simulator (Fig. 1) to scare each rodent. This fake snake is made out of a plastic tube, a spring, and an elastic string. We cut the string so the tube moved toward the rodent. We filmed the rodent’s reaction. We later watched these videos to see:
how many of the rodents jumped (jump frequency),
how quickly each rodent reacted (reaction time),
how fast each rodent took off from the ground (takeoff time), and
how high each rodent jumped (jump height).
Results
1. We found that almost all of the kangaroo rats and pocket mice jumped when the rattlesnake strike simulator scared them. Only a few woodrats or squirrels jumped. The rest of them “scrambled” or ran away.
2. We found that the desert kangaroo rat and the woodrat reacted faster to the fake snake than the other species on average.
3. We saw the rodents had different takeoff times. Pocket mice had the shortest takeoff time, followed by kangaroo rats. The woodrats and ground squirrels had a slower takeoff time (Fig. 2).
4. There were no large differences in jump heights. A few of the kangaroo rats did jump nearly half a meter (1.6 feet) high! But most kangaroo rats did not jump higher than the quadrupedal rodent species.
Discussion
We saw that the bipedal kangaroo rats were good at reacting to the rattlesnake strike simulator. We were surprised to see that pocket mice were also good at jumping away quickly. This tells us it’s possible to be a good jumper without being bipedal.
Kangaroo rats and pocket mice are both part of the heteromyid rodent family. This means they are closely related. Even though pocket mice are not bipedal, they have strong hind legs like kangaroo rats. Strong hind legs can help rodents jump away from predators.
Heteromyid rodents like kangaroo rats and pocket mice also have good ears. This helps them hear predators. This might explain why pocket mice and kangaroo rats jumped more often and took off faster than the other rodents. In contrast, woodrats and ground squirrels end up being slower than a rattlesnake strike as they take time to run away.
We know that time is important when prey responds to a predator attack. Even taking just a few milliseconds longer can be the difference between life and death! If jumping is so helpful, why don’t other quadrupedal rodents like the woodrat and ground squirrel jump as much? One reason is these rodents are also bigger, so they might not be as afraid of the snakes as the pocket mice and kangaroo rats are. They are also less sensitive to rattlesnake venom. We want to do more studies to figure out the details of why kangaroo rats are bipedal and why other rodents don’t jump as well.
But traits like size and venom resistance must have advantages, too, right? Every animal has a unique combination of features. We can understand why these combinations exist when we look at how these features help the animal survive.
Conclusion
Looking at differences among animals can help us understand how and why a certain trait evolved. We saw how jumping ability may have evolved to help small rodents avoid predators. Now think about your own body. Why are humans bipedal? Why do we have an ear on each side of our head and two eyes facing forward? And why have humans developed the ability to read – like you’re doing now?
You can use the same methods as we used for rodents to think about this. What challenges have humans faced? And what sets us apart from other similar animals with different skills or habitats?
