Lincoln Park Zoo is home to a wide array of species spanning several taxa, from armies of Madagascar hissing cockroaches to our baby eastern black rhinoceros, King. However, species diversity at zoos goes beyond the wildlife on exhibit; many native and non-native species live within the landscaped vegetation or water features around a zoo, in animal exhibits or in the buildings on grounds.
One such species is the eastern cottontail rabbit (or Sylvilagus floridanus, if binomial nomenclature is your thing). For the last four winters we have been estimating how many rabbits are located on zoo grounds and whether their survival rate differs from rabbits in more natural areas. You see, urban areas can provide additional food and water sources, den sites and possibly a lower abundance of natural predators. For city rabbits this could mean an increase in their over-winter survival rates and allow for densities to be much greater than their rural counterparts.
Rabbits, as well as many other mammals in seasonal environments, are partially limited by over-winter mortality. During the winter much of the foliage they snack on during the warmer months is either gone or covered in snow.
So what do they eat? Much to the chagrin of our hard-working grounds staff, cottontail rabbits often resort to eating bark from shrubs, young trees and other woody perennial plants in order to survive. Furthermore, the eastern cottontail doesn’t dig its own burrow like other rabbit species and either relies on burrows created by other species (including humans) or huddles up underneath woody vegetation to wait out severe weather.
Your chances at seeing a rabbit over the winter are greater at dawn or dusk when the weather is milder. Rabbits will take advantage of times like this to forage as much as possible. However, if you are around Lincoln Park Zoo and take a second to scan the vegetation, you should be able to see a rabbit or two this winter.
So what have we found so far during our study of this little herbivore? First and foremost, there are LOTS of rabbits on zoo grounds and at much greater densities than in natural areas. On average, there are 139 rabbits here during the winter months, which results in a density of about 12 rabbits per hectare (10,000 square meters). In a 1942 study conducted in Michigan, rabbit densities were 0.08–0.35 rabbits per hectare. If we compare the two, rabbit densities at the zoo are 34 times greater!
However, overwinter survival rates for our rabbits were no different than reported values of rabbits from other scientific studies (right around 30%). It may be that rabbits born every spring have a much higher survival rate because of abundant resources and fewer predators, which could result in the greater densities that we see. When the winter rolls around the now abundant rabbits are limited by food availability, and rabbit numbers decline until spring comes around again.
We’re not done with rabbits yet, though. While I may be hanging up my Timothy Hay and catch-and-release box traps for this season, we are starting a new project this winter to determine exactly how rabbits perceive the zoo landscape. A scientific theory called “optimal foraging theory” predicts that a species will no longer forage in an area when its costs (things like risk of predation, energy costs and missed opportunities from looking for food elsewhere) exceed the reward gained from foraging. Thus, rabbits will forage longer in places where they feel safe and forage for shorter periods in places where they do not.
This perceived safety can easily be estimated by measuring the density of food remaining after a rabbit moves on from an area: the Giving-Up Density. By using trays with a small amount of food, placed throughout the zoo, we will be able to determine where rabbits feel most comfortable and where they are the most fearful. In regards to the latter, my money is on the red wolf exhibit.
Once we’re done, we’ll be able to see the zoo through the eyes of a rabbit, which will help us figure out how to keep them from destroying all our beautiful landscaping. If you swing by the zoo from January to March, you may be able to see our food patches in action!