Size Matters: Pesticides in Large Mounted Vertebrate Specimens

Written by Becky Desjardins (Senior Museum Preparator & Conservator), Georgia Kay & Kim König (MSc students Museums & Collections – Leiden University; Naturalis Interns), Naturalis Biodiversity Center.

Back in 2013, Naturalis conducted a research project about arsenic in the museums’ specimens. The goal was to determine if arsenic was spreading from the collection areas into staff and or public areas of the museum. We tested many specimens with an XRF but also tested the elevators, door handles, floors, shelves, keyboards, etc. From this testing we developed protocols about handling specimens and how we use the spaces in the collection. You can read all about that project over here.

What didn’t get tested were the large mounted vertebrates. Back in 2013 the Naturalis collections were spread over a number of warehouses around Leiden. Because these external buildings were considered depots only (meaning no offices/canteens in these spaces) there was less concern about arsenic contamination in non-collection areas. The large vertebrates were considered to be high risk specimens (so very toxic), and were handled as such, they never had their moment with the XRF.

Fast forward to 2021 and after a massive renovation to our main building, nearly all the collections are under one roof and we have time to do some conservation and restoration work. We identified 25 specimens in need of stabilization and restoration. These include: ocean sunfish, hippopotamus, a gavial/gharial, a few different types of buffalo, an elephant, and mounted Cetacea skins, among others. The majority of these specimens are between 100 and 200 years old. In anticipation of this restoration work, we wanted to find out just how much arsenic, lead and mercury these specimens contained. 

As was done in 2013, we used the XRF to look for arsenic, lead and mercury. Each specimen was tested in multiple places: eyes, ears, around the mouth, along the belly seam, any cracks in the skin, and the feet/hooves. Each of the specimens was tested in as few as 3 and as many as 9 places; as it is essential to test several areas because negative test results can be obtained, even if a specimen is contaminated. This time we recorded consistently high arsenic levels, which was not a surprise, what was a surprise was how much mercury and lead was found compared with 2013. Back then we’d found high levels of lead (higher than 600ppm) in only 5 of the 36 specimens that we tested, and the 2013 testing turned up no detectable levels of mercury in any of the tested specimens. What was going on?

In short, we think that size makes the difference, our testing indicates larger specimens received larger amounts of pesticides. Arsenic and mercury (mercury chloride) were used as pesticides in taxidermy throughout history until the 1980’s and there are records of many taxidermists liberally using toxic cocktails for preservation. Perhaps thicker skins, such as that of an elephant or hippopotamus received more pesticides than a sparrow hawk or blackbird (species tested in 2013) would have gotten.

The variation in high amounts of lead is not so easy to explain. Lead was used as lead paint on the soft parts of mounted animals and sometimes was used to model cartilage (in ears, for example) of mounted animals. In general we found if a specimen had a lot of lead, then it had a lot of lead everywhere. Perhaps hides of animals with sparse or no hair were painted with lead paint: for example, we did find more lead in the cape buffalo and ocean sunfish, and less in the zebu. This was different for the arsenic and mercury; which were mostly present in the soft tissue parts of the bodies: around the eyes, mouths, ears and genitals. However, it was unexpected that the arsenic was also very heavily present around the horns.

The next step is to create a protocol of how to handle these specimens when they are undergoing restoration. One of the challenges we are currently facing is that Naturalis does not have a dedicated work space for large restoration projects and we are trying to figure out where this work could be safely done. The specimens are in a depot but we are nervous about cross contaminating nearby specimens, however, due to their large size, just moving these objects out of the depot door is a challenge. One option would be purchasing a sort of party tent so that we can keep the dust contained. We welcome suggestions from readers who have experience in the area of large specimens/pesticides/restoration or want to join us in our biohazard party tent.


Contaminated Collections: Preservation, Access, and Use. Shepherdstown: Society for the Preservation of Natural History Collections, 2001.

Desjardins, Rebecca. (2016). Arsenic and pre-1970s museum specimens: using a hand-held XRF analyzer to determine the prevalence of arsenic at Naturalis Biodiversity Center. Collection Forum, 10.14351/2015.02.15.

Marte, Fernando, Amandine Péquignot, and David W. Von Endt. “Arsenic in taxidermy collections: history, detection, and management.” Collection Forum 21, no. 1-2 (2006), 143-150.