Links schedel-onderdeel (neurocranium) tonijn van het strand van Den Helder, rechts een exemplaar uit de collectie van Naturalis

Mammoth milk tooth and a slimy blob, the best nature questions of 2023

Naturalis Biodiversity Center
30-DEC-2023 - Naturalis' free question service helps young and old to name finds from nature. Each year, we receive about 2,500 questions and this year even more than 3,000. What were the most surprising nature questions of 2023?

Curious about how Naturalis' nature question bank works, or what the best nature questions of previous years were? Then read about the best nature questions of 2020, 2021 and 2022.

Milk tooth of a woolly mammoth

On a spring-like day in February, Fred Sieval found a dark-colored molar on the beach between Kijkduin and Scheveningen. One with a rather worn down and squiggly chewing surface and two solid roots. He emailed the photos to Naturalis, giving the dimensions: height including root is 21 millimetre, the crown is 14 millimetre. We were able to quickly provide clarity: Fred found something super rare. A beautiful and almost complete first milk tooth from the lower jaw of a woolly mammoth. The crown consists of 5 segments, called laminae. The wear is due to the grinding of plant material. So the mammoth calf was at an age where it didn't only drank mother's milk, it had also been eating solid food (grasses) for some time. It is estimated that the animal was around 10 to 12 months old. It may well be that the molar was washed free from the 'Zandmotor', the artificial peninsula just south of Kijkduin. An enormous amount of sand from old seabed layers was used to build it. That sand is rich in fossil bones and shells. On March 1st, we were able to admire the molar up close, during the Ice Age edition of our monthly Nature Conversation. Other visitors watched curiously: "Is that really a mammoth molar? So tiny!"

Milk teeth of a mammoth baby, showing a top view of the chewing surface on the left and a side view of the crown and roots on the right

Bright yellow frog in Leiden garden

Bright yellow, yet this is a European brown frogIn early April, the question service received a call from Elvira Leuering from Leiden. She said that in her front yard, near the pond, she spotted a bright yellow frog dozing in the spring sunshine. Pretty special, right? A few weeks later, her neighbor Giseke Hopstaken managed to take some nice pictures of the little animal. It turned out not to be an exotic species, but a native brown frog with a color deviation. An albino? Hein van Grouw, senior curator of birds at Britain's Natural History Museum in Tring, knows a lot about colour anomalies in vertebrates and could tell more. "This is a brown frog that lacks the dark pigment (melanin) in its skin, but the amphibian's yellow pigment is unaffected," he said. It's a form of leucism, a colour defect caused by a mutation. The frog's eyes appear normal in color, so it is not an albino. In that case the eyes would have been a different colour, but not necessarily red." The question is whether this is more common in isolated populations. Stories of colour deviations in enclosed courtyards and highly isolated heathlands are familiar to RAVON's amphibian experts. These may indicate inbreeding, but there is no certainty.

Old bird ring in dunes of Vogelenzang

Around Easter, Jan Smeltink found an old metal bird ring in the dunes near Vogelenzang. It reads: 'Museum Nat. Hist. Leiden, ring number 134555.' Very occasionally Naturalis gets such an old bird ring back. They were made available by our museum for migratory bird research between 1911 and 1958. It was director Eduard Daniël van Oort (1876-1933) who set up bird ring research in the Netherlands. Since 1958, the Bird Migration Station – now located in Wageningen – has taken care of the organization, distribution and administration of bird rings. Project worker Natasja van Nijen looked up the ring's data: "The ring was put on an Eurasian oystercatcher in Vogelenzang on June 13th, 1933. Unfortunately, there are no more reports of this bird than just the ring catch. At the time of ringing it was a nestling. The ring was put on by J. Duijve. So this is a great find of an almost 90-year-old ring!" Of course, the bird itself did not live to that age. It may even have died soon after ringing, or maybe one or a few years later. Oystercatchers rarely live beyond 20 years of age. The oldest known oystercatcher of at least 46 years was seen on August 1st, 2016 at the Maasvlakte.

Metal bird ring nearly 90 years old from the dunes of Vogelenzang

Leaf of cherry laurel with beautiful patterns by Cercospora leaf spot disease, a fungus

Leaf with an unusual pattern

In May, Peter van der Lende emailed a photo of a leaf with a beautiful pattern of concentric lines. His elderly mother found it in the garden of the nursing home in Vorden, near Zutphen. At first we thought it were all little passageways (leaf mines) of insect larvae. About 800 species of insects in the Netherlands are known whose larvae live off the intermediate tissue of leaves. However, the expert on the subject, Willem N. Ellis, informed us that they are definitely not leaf mines. "Those are hollow, there are larvae in them and they also often leave their faeces in them." The line pattern has another cause. "It comes from leaf spot, a fungus," William said. "A fungal infection of a leaf usually happens because the fungus enters through a stomata. From there, the fungus spreads in all directions, killing the leaf tissue, creating a circular spot. On the border between fungus and still healthy tissue is often a darker-colored combat zone. Sometimes the fungus re-expands from there, with a new dark zone surrounding it again. If that happens a few times you get the pattern of a large leaf spot with a series of concentric lines." Mystery solved, and in closing, Rogier van Vugt of the Leiden Hortus Botanicus was able to tell which plant the leaf spot came from: cherry laurel, an evergreen often used as a hedge plant.

A slimy 'blob'

In August, the Dutch Jeugdjournaal contacted us because children had found a brown jelly blob the size of a soccer ball in a watercourse near Peizermade, in the Kop van Drenthe. Daan, Nina, Eva and Leah were swimming when they suddenly felt something weird. It looked like a giant slippery potato, or a slimy 'blob.' But according to their grandfather, it had to be a freshwater moss creatures (Pectinatella magnifica). Werner de Gier, researcher at Naturalis, was able to confirm this and explained in Jeugdjournaal that 'the blob' is actually a large fused colony of lots of small moss animals (bryozoans). These sit together in groups and form sort of whorls in the thin, slimy crust. The gelatinous mass provides strength. Underwater, the colonies often attach to something hard, such as a stone or wooden pole, but sometimes they become loose and then float through the water. When water temperatures drop after summer, the colonies die off. The species is an exotic species that once arrived in Europe from North America. 2004 marked the first discovery in the Netherlands. Since then, the water bryozoans are seen more and more often, especially in lakes and rivers where it the waterflow is not too fast. Unlike its appearance suggests, the organism is far from soft. It can adapt well to different environments and temperatures, the animals are hermaphroditic and they produce small capsules (statoblasts) in which a type of spore can survive the winter. Biologists would like to continue monitoring its distribution, so please report your observations through

Large colony of water freshwater bryozoan from a watercourse near Peize

Strange skull on the beach of Den Helder

In mid-November, Danny Marees spotted a peculiar bone on the beach while running. Rather flat, with a few large holes, and judging by the pink color still very fresh. He could not take the bone with him, but he took beautiful pictures of it and sent them to the question service of Naturalis the same day. Danny wrote: "For imaging purposes, shoe size 43 is next to it." That's roughly equivalent to 28 centimetres. After research in Naturalis' fish collection, it became clear that the bone is a piece of tuna skull. It is the upper part: the neurocranium. We suspect from a bluefin tuna. In the 1960s this impressive fish had disappeared from the North Sea, probably due to overfishing, but since 2017 it has been seen again occasionally. On September 24th of this year, one of 240 centimetres had washed ashore near Ritthem (Vlissingen municipality). That one is included in our scientific collection. On November 18th, a 190 centimetre specimen was found on the beach at Zoutelande, but unfortunately it has not been preserved. This loose skull part from Den Helder beach may be more likely to be a remnant from fish processing. Interestingly, another tuna neurocranium was found a week later on Texel, by Ank Hobo. About the same size, but it appears to be different in details from Danny's specimen.

Left skull part (neurocranium) tuna from Den Helder beach, right a specimen from Naturalis' collection

More information

Text: Alice van Duijn, Naturalis Biodiversity Center
Photos: Danny Marees and Naturalis (lead photo: tuna skull); Fred Sieval; Giseke Hopstaken; Jan Smeltink; Peter van der Lende; Jolande Verbree