Animal Mysteries: 4 Unsolved Questions Scientists Are Still Studying

Somewhere in the Sargasso Sea, far out in the Atlantic, eels from European rivers are thought to meet and spawn. Nobody has ever seen it happen.

Some of the biggest animal mysteries are not monsters or strange lights. They are everyday animals doing things scientists still cannot fully explain: where eels breed, how butterflies find one small patch of forest thousands of kilometres away, how birds sense the Earth’s magnetic field, and how an axolotl regrows a leg. Here is what is known, and what is still open.

Where do European eels breed?

Scientists think European eels breed in the Sargasso Sea, but no one has ever caught eggs or spawning adults there. Johannes Schmidt proposed the site almost 100 years ago, based on finding tiny eel larvae. Adult eels swim 5,000 to 10,000 km (3,100 to 6,200 miles) to get there, says a 2022 study in Scientific Reports.

In that study, researchers fitted satellite tags to 26 eels from rivers in the Azores and tracked them for 40 to 366 days. The eels swam at 3 to 12 km (2 to 7.5 miles) a day. The team called it the first direct evidence of adult eels reaching the Sargasso Sea. How the eels navigate, which routes they take and exactly where they spawn are still unknown.

The answer matters. The same study notes that the European eel is Critically Endangered on the IUCN Red List and that the number of young eels arriving back has fallen by 95% since the 1980s.

How do monarch butterflies find their way to Mexico?

Partly with a sun compass and partly with a magnetic compass, but the full picture is not settled. Eastern monarchs travel up to about 4,800 km (3,000 miles) and most spend the winter in fir forests in the mountains of central Mexico, says the US Fish and Wildlife Service. None of the butterflies that fly south have made the trip before.

A 2014 study in Nature Communications showed in flight simulators that migrating monarchs have a light-dependent magnetic compass, with sensors that appear to sit in their antennae. It may help when the sun is hidden. The butterflies that make the journey also live far longer than normal monarchs: six to nine months, compared with two to five weeks for most adults, says USFWS. Read more in how long a monarch butterfly lives.

How do birds sense the Earth’s magnetic field?

Scientists know that night-migrating songbirds use a light-dependent magnetic compass, but not exactly how it works. The leading idea is that it relies on light-triggered chemical reactions in proteins called cryptochromes in the bird’s eyes. In 2021, researchers showed in Nature that cryptochrome 4 from the European robin is sensitive to magnetic fields in the lab.

The robin’s version was more sensitive than the same protein from chickens and pigeons, which do not migrate at night. That fits the idea, but it was measured in a test tube, not inside a flying bird. Proving that this is how the compass works in a living bird is still an open question.

How does an axolotl regrow lost body parts?

Researchers are still finding out. Axolotls can regrow limbs, the spinal cord and parts of the brain and heart, and they do it without forming scar tissue, says the Marine Biological Laboratory. Labs there are hunting for the molecules and cell processes that make this possible, in the hope of learning lessons for human healing.

The axolotl itself is in trouble. MBL notes it is native to only one lake, Lake Xochimilco in Mexico, and is Critically Endangered in the wild. See why axolotls are endangered.

Why do these mysteries matter for nature?

Each of these animals links habitats that are far apart. Eels carry energy between European rivers and the open Atlantic. Monarchs connect breeding grounds across North America with a few winter forests in Mexico, so damage to one small area affects the whole population. Understanding how these animals navigate helps scientists decide which places to protect, before a species is lost.

Not every strange animal is a mystery

The leafy seadragon looks unreal, but scientists understand it well. Its colours and leaf-like flaps give it excellent camouflage on kelp-covered rocky reefs, and the male carries the eggs under his tail, says the Australian Museum. It lives only in southern Australian waters, usually 3 to 50 m (10 to 165 ft) deep.

Myths about animal mysteries

Myth: Scientists have watched European eels spawn in the Sargasso Sea.

Fact: No eggs or spawning adults have ever been sampled there. Tagged adults have been tracked to the area, which is strong but indirect evidence (Wright et al. 2022).

Myth: Monarch butterflies learn the route from their parents.

Fact: The butterflies that fly south are a new generation flying the route for the first time, says USFWS. They rely on inborn compasses, not teaching.

Myth: The bird magnetic sense is fully explained.

Fact: Cryptochrome 4 is a strong lead, but it has been shown to be magnetically sensitive in the lab, not proven to work that way in a living bird.

Test yourself

1. Where are European eels thought to breed?

The Sargasso Sea in the Atlantic Ocean.

2. Where do migrating monarchs appear to carry their magnetic sensors?

In their antennae.

3. Name two body parts an axolotl can regrow.

Any two of: limbs, spinal cord, parts of the brain, parts of the heart.

Frequently asked questions

Sources

1. Wright, R.M. et al. (2022). First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea. Scientific Reports 12: 15362.
2. US Fish and Wildlife Service. Monarch (Danaus plexippus).
3. Guerra, P.A., Gegear, R.J. and Reppert, S.M. (2014). A magnetic compass aids monarch butterfly migration. Nature Communications 5: 4164.
4. Xu, J. et al. (2021). Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature.
5. Marine Biological Laboratory. Axolotl research organism.
6. Australian Museum. Leafy Seadragon, Phycodurus eques.

Sources checked 12 October 2026.