On the Move: Nine-banded armadillos and their range expansion in Illinois

Nine-banded armadillos (Dasypus novemcinctus) are among the most unique wildlife to currently inhabit Illinois. With their leathery scaled armor (called scutes), long snout for sniffing out insects, and remarkable ability to traverse rivers by inflating their intestines and floating on the surface (yes, really), they are quite unlike any other mammal within the state. In fact, they have not always called Illinois home, and it is only in recent decades that they have expanded their range into the American Midwest.

Originally from the semitropical climes of Central and South America, the armadillo species began its northward expansion into southern Texas in the mid-19th century and continued their expansion into 14 more U.S. states within the intervening years. This territorial expansion is thought to be aided by human activity, with European settlement and landscape modification removing ecological barriers, while human-induced climate change has increased temperatures on the continent. As a species used to warm and wet environments, they have a low metabolic rate and poor fat reserves for insulation, meaning thermoregulation of their bodies during colder temperatures is challenging. For these reasons, northward expansion into areas with cold winter temperatures and little rainfall was thought unlikely for the species, but their presence in some of these areas has demonstrated the remarkable adaptability of the species. Whether armadillos can exist permanently in these harsh environments and establish breeding populations, however, is another question.

Where in Illinois and Why?

Nine-banded armadillos were first recorded in Illinois in 1994, but observations of the species within the state have risen dramatically since the early 2000s. They are now expected to occur up to the 40th parallel, though they are at their greatest densities in the heavily forested southwest.

When a new species enters the scene, biologists are quite reasonably concerned about how this new entry into the ecosystem may impact native flora and fauna. Assessing how these species interact with their environment and the local wildlife is therefore important information for biologists to obtain. Furthermore, finding out what resource preferences these species have can inform us of what exactly is facilitating their range expansions. Max Larreur, Justin Remmers and other biologists from Southern Illinois University set out to do exactly that!

Focusing on nine-banded armadillos in their hotspot in the southern reaches of the state (which represents the very northern periphery of their continental range), the researchers tapped into an extensive camera-trapping network distributed throughout the 16 southernmost counties of Illinois, deployed January through April of the years 2022 and 2023. In a recently published research project funded by the Illinois Department of Natural Resources, Larreur and team modelled nine-banded armadillo occurrences based on photographs captured by the camera traps to discern which land cover characteristics best explained their distributions here. They then created a heatmap of the expected probability of nine-banded occurrence throughout the southern one-third of the state, based on this data. Furthermore, because camera trap photographs contain the time of day in which the observation took place, the researchers were able to model the daily activity patterns of the species, finding out at what times of day the species is most active.

Expectedly, the armadillos were positively associated with forest cover and the connectedness between forest patches. Forest habitats can provide armadillos with greater food resources, such as bugs, worms, eggs, amphibians and small reptiles, as well as habitats used for denning and vegetation cover for concealment from predators. When forest patches are interconnected, it allows for greater movement between habitats, allowing the species to carry out its life history functions, such as seeking mates and establishing new territories. Crucially, leafy forests also trap heat and moisture, which may be an important factor in the persistence of the species this far north, with forests potentially mitigating the effects of a colder climate and reduced rainfall. Armadillos were also recorded in residential areas, with their presence closely tied with forests within these areas, suggesting the species can tolerate moderate human activity.

A Complex Relationship with Human Spaces

The researchers also found nine-banded armadillo occurrences were closely linked to roadways, with armadillo presence increasing closer to major roads. Roadways may facilitate the movement of armadillos while also aiding thermoregulation. Asphalt absorbs heat from sunlight and traps thermal energy, meaning armadillos may benefit from the warmth of these areas during wintery conditions.

Agriculture, meanwhile, had negative effects on armadillo occurrences. Not only agricultural cover, but the connectivity of agricultural land, appeared to hinder armadillo distribution. Because armadillos are burrowing species, agricultural spaces may not provide adequate or preferred locations for their dens. Furthermore, agricultural practices, such as the use of heavy machinery and early planting of crops, may damage burrows in and around farmland, while insecticides may reduce invertebrate prey that armadillos rely upon.

Activity Patterns

The team also found that armadillos were largely nocturnal in their activity. This is usual for the species, but, as Remmers commented via email “I was surprised to find out that nine-banded armadillos were still nocturnal this far north. I expected them to change their activity or behavior because of the colder temperatures, especially in winter. However, this was not the case.”

Considering Illinois represents the northern periphery of their range, where nighttime temperatures can reach chilling lows, it is surprising to not see an expansion of activity into the day, where the sun’s warmth can help maintain body temperature.

Where to Expect Armadillos

When plotting predicted distributions of armadillos on a map of southern Illinois, the model showed that the highest probabilities of occurrences were in the heavily forested regions, including the Shawnee National Forest. Meanwhile, occurrences were predicted lower in the less forested northeast and northwest regions of the study area, which are dominated by agriculture and where forests occur only in small, isolated patches. Due to the seemingly strong, negative effects of agriculture on nine-banded occurrence, expansion further north into the agriculturally dominated eastern Illinois may be limited, but expansion northward along the forested river corridors of western Illinois may be a more reasonable prediction.

As Larreur commented via email “I think armadillos are only going to continue expanding, with roadways along forested areas providing them with the cover and resources they need, aiding their thermoregulation, and providing dispersal corridors, bolstering armadillo colonization of novel areas that were previously thought to be inhospitable.”

Remmers agreed, adding “I would expect people to see more nine-banded armadillos in the near future.”

Why Should We Care?

As burrowing mammals, armadillos can benefit some species that may be able to use armadillo burrows for their own purposes, but they may also have disrupting effects on the reproductive success of ground-nesting birds, such as bobwhite quail (Colinus virginianus). Unfortunately, much about the behavior of armadillos, and the relationships with other species, is poorly understood within recently colonized areas at the northern edge of their range. This study contributes to our understanding of the factors driving armadillo occurrences in a region previously thought uninhabitable for the species and provides unique insights into some of their behaviors, but more research is needed.

As Larreur commented, “I think armadillos are a species with a lot of unknowns right now in Illinois and taking a first step towards understanding their new role in the environment is beneficial. I think land managers, private landowners, researchers, and state and federal agencies would all benefit from reading the results of this study.”

Remmers also added, “Any time a species begins moving into a new area, it is important to document and investigate what happens. This way we can track how this species influences the environment around it and the other species that already live there.”

With unknown effects on local ecosystems and potential to act as vectors for disease, keeping tabs on this new arrival in Illinois is important for managing our natural resources and public health, as well as determining which factors may threaten the species’ persistence.