What if there were a way to look back in time and observe living animals interacting directly with their environment? Natural history collections preserve the remains of organisms and the wealth of data that describe them, but there is also a world of information hidden in the ephemeral tracks and traces left behind by animals during their lifetime.
I am a collections technician at the Chicago Academy of Sciences / Peggy Notebaert Nature Museum. Early in the process of digitizing the botany collection, I came across this specimen of Asclepias exaltata, or poke milkweed. This specimen has obvious insect damage: notice the holes in the leaves. Here at the museum, this would normally be a very bad sign! Collections staff regularly monitor our facilities for the presence of insects that might want to feast upon our irreplaceable preserved specimens. Damaged leaves might indicate an infestation of pests in the collection. However, as an amateur gardener myself, I recognized these particular holes as the work of monarch caterpillars, meaning that the damage was done before this plant specimen was collected for the museum. Oh, and the coolest part is that this specimen was collected back in 1871!
What makes us think the damage was done by caterpillars 151 years ago, and not by unwanted museum pests more recently? Milkweed is the only host plant of Monarch butterflies, meaning that they rely on the plant during the larval stage of their lifecycle. Caterpillars grow rapidly, making different traces on the milkweed leaves as they eat, first making tiny holes, then slightly larger ones, and eventually taking big scallop-shaped bites out of the leaf’s edge (Eisman et. al, 2010). Looking at the holes of different sizes, we can imagine watching one little caterpillar develop, taking bigger and bigger bites out of the leaves as its body and appetite grow. Then, along came one of our favorite botanists, Elizabeth Emerson Atwater, who collected this masticated milkweed for her impressive herbarium collection. Atwater’s collection was donated to the Academy shortly after the Great Chicago Fire.
Okay, that’s a cute story, but how do we know holes didn’t get there after the plant was collected? A century and a half is a long time for pests to potentially get in and do their worst. Plants that are damaged in life can actually heal from their injuries. Looking very closely at the holes in this specimen, we can see a thin border line around the edges. This is scar tissue that tells us the damage was done while the plant was still alive (Meineke, 2018). Before this plant was collected, it had the time to heal some of its wounds. For comparison, we can look at plant specimens that were damaged by pests after being collected and dried. Notice where the edges are broken and jagged, without any apparent healing.
This specimen is exciting to me because it gives us a look at a living ecosystem that existed in the 1870s. We’re not just looking at the remains of dead organisms; we are observing the activities of living animals that interacted with their environment.
Tracks and traces can tell scientists a lot about the behavior and the functional anatomy of organisms, especially those that are extinct and thus cannot be observed directly. Ichnology is the study of traces created by living organisms, such as trackways, burrows, and signs of feeding (Buatois & Mángano, 2011). This includes modern traces as well as ancient fossilized traces. Trace fossils can be used to study anything from insects to dinosaurs to humans! Want to see a fun example of animal tracks that are preserved permanently like a fossil? Pay attention to the sidewalk when you walk down the street and you might find the tracks of humans, dogs, and wildlife that walked over wet cement, leaving a trail of footprints permanently in the solid concrete. This is similar to the way an animal’s footprint in soft sediment might be preserved in solid rock for millions of years.
Tracks and traces can also provide clues about the climate and habitat in which organisms existed, which brings us back to the botany collection. An unusual study conducted at the Harvard Herbaria used botany specimens to study changes in insect herbivory over time. Hoping to identify connections between insect activity and environmental change, the researchers analyzed herbarium specimens spanning more than a hundred years. They were careful to separate the specimens that were damaged in life from those that suffered pest damage after collection. Using historical data on temperature and population density for each county in which the specimens were collected, the researchers identified two trends. First, higher temperatures were associated with greater herbivory; second, higher urban population density was associated with reduced herbivory (Meineke, 2018). As cities grow and global temperatures increase, we might expect to see herbivore activity increase in rural habitats but decrease locally near urban centers. This study is the first of its kind and it has received some criticism due to potential sampling bias (Kilzov et. al, 2020), but it highlights something very important about natural history collections in general.
Only a small percentage of museum collections are represented in public-facing exhibits. The large majority of objects exist behind the scenes where they can be accessed by researchers. Digitizing specimens and artifacts in high resolution increases accessibility for scientists and the general public. While digitization opens up a lot of doors for research and public outreach, it is still important to preserve the physical objects in addition to their digital surrogates. We mustn’t assume that we’ve exhausted the potential value of physical specimens. You never know what new technologies and methodologies will enable us to learn from historical specimens in the future. They might still hold the answers to questions that no one has thought to ask yet.
Alex Lorenz
Collections Technician
References
Buatois, L. A., & Mángano M. Gabriela. (2011). Ichnology: Organism-substrate interactions in space and time. Cambridge University Press.
Eiseman, C., Charney, N., & Carlson, J. (2010). Tracks & sign of insects & other invertebrates: A guide to North American species. Stackpole Books.
Kozlov, M. V., Sokolova, I. V., Zverev, V., Egorov, A. A., Goncharov, M. Y., & Zvereva, E. L. (2020). Biases in estimation of insect herbivory from herbarium specimens. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-69195-5
Meineke, E. K., Classen, A. T., Sanders, N. J., & Jonathan Davies, T. (2018). Herbarium specimens reveal increasing herbivory over the past century. Journal of Ecology, 107(1), 105–117. https://doi.org/10.1111/1365-2745.13057
Meineke, E. K., Davis, C. C., & Davies, T. J. (2018). The unrealized potential of herbaria in global change biology. Ecological Monographs. Retrieved from https://static1.squarespace.com/static/5a9ff80c5417fc256c80033e/t/5b157d34f950b782f2709844/1528134965344/Meineke_et_al-2018-Ecological_Monographs.pdf
