What is the research behind "these animals return to the same spot"?

August 19, 2023 12:54 PM

A trope of nature documentaries is "these turtles have traveled thousands of miles to come back to the very same beach where they hatched to lay their eggs". Similar statements are made about salmon and other species. I want to know more about the accuracy of animal navigation. How "the very same" are we talking?

What is the average distance between a green sea turtle's nest and the nest they hatched from? What percentage of salmon make it back to the same creek where they hatched, vs another tributary?

I'm interested in learning about any far-traveling species, it doesn't have to be turtles or salmon specifically.

posted by tylermoody to Science & Nature (8 answers total)

Best answer: It may be helpful for you to know that this is called "natal philopatry"
posted by matkline at 2:13 PM on August 19, 2023 [5 favorites]

And we verify it — plus other stuff — by literally tagging individuals, eg

Link
posted by clew at 2:15 PM on August 19, 2023 [5 favorites]

With regard to Monarch Butterflies, there are specific groves of trees they return to like the one at Natural Bridges State Park in Santa Cruz. One group of Monarchs lives six months and an alternate generation lives 18 months and travels to Mexico. They have been monitored for a while now by humans and marked. There should be information on their life cycle through the Natural Bridges Website.

This is sort of adjacent to your question: Arctic Terns make the longest documented migration of seabirds. Also pigeons have been extensively studied recently since it is thought that they can see the Earth's magnetic field, and use it to navigate.
posted by effluvia at 3:16 PM on August 19, 2023

Besides tagging in the case of salmon we know that a localized disaster, like say a stream bed get blocked for a while by a landslide, that prevents fish from spawning in a particular year will be disastrous for returns four years later. Not every salmon will take four years to return so it will slowly get better as stragglers or early birds and fish that just get lost from other years show up but it takes a long time to even out.

Also you can take hatchery fry and dump them in a stream and they come back there and not wherever the eggs were harvested from.
posted by Mitheral at 3:39 PM on August 19, 2023 [2 favorites]

From a quick scan of the literature, it looks like stray rates for Pacific Salmon species vary from 3%-15% in most cases (minus a couple of outliers). Some species have a higher stray rate (pink, usually), some a lower one (sockeye, usually). But there are a lot of other factors that can impact stray rates.

For Pacific salmon, stray rates are calculated by the individual stream, as each stream is considered to have a genetically distinct population. Hatchery fish straying into wild spawning grounds and mixing with the wild population is a huge problem for wild salmon genetics. But even when it's a wild population straying into the stream right next door, that's usually considered a negative impact on the genetics of the recipient population.

So, to sum up, between 85-97% of salmon return to the exact same stream where they were born.
posted by 100kb at 3:50 PM on August 19, 2023 [7 favorites]

Best answer: Two species of anguillid eels found in SW Victoria, Australia: the Long-finned Eel (Anguilla reinhardtii) and Short-finned Eel (A. australis) transform, hit the seas, travel for thousands of kilometers, but only their spawn come back.

Adults spend many, many years (decades) in freshwater habitats (such as rivers and wetlands) feeding and growing before metamorphosing into silver eels. The silver eels migrate downstream, often during high river flows, and into the sea to spawn. During these oceanic spawning migrations, adult eels travel several thousand kilometres to warm tropical waters. After spawning, the adults die and the newly hatched leaf shaped larvae (called leptocephali) commence a journey toward the coast, drifting on ocean currents and developing into glass eels before eventually entering rivers. The young eels, now known as elvers, migrate further upstream into freshwater, developing into yellow eels and eventually becoming adults.

A substantial number of the short-finned eels would return to the creeks and rivers of their parents. Archeological evidence and the oral history of the the Gunditjmara people demonstrates that eels have been corralled and trapped in that part of Vic. for over ten thousand.

More info:
Koster, W. et al (2021) First tracking of the oceanic spawning migrations of Australasian short-finned eels (Anguilla australis), Scientific Reports
ABC news On the tail of the eel
An article in The Age: 'One eel of a story: the slippery truth of a fishy underground migration'
An article in The Citizen (University of Melbourne): 'From Port Phillip Bay to the Coral Sea, tracking the travels of the eels'
posted by Thella at 6:09 PM on August 19, 2023 [4 favorites]

Best answer: Ascension Island’s green turtles have been tracked about 2000km to waters near Brazil on their annual migration, and then returning to Ascension. The island itself is only 34 square miles, and there are only 3 beaches the turtles come and lay eggs on.
posted by penguin pie at 6:19 PM on August 19, 2023 [3 favorites]

Anecdote in support of spitbull’s comment: I have barn swallows every year. They send a sentinel swallow in late April, arrive en masse in May, raise at least two hatches, and depart right around now. They reuse nests from year to year. And every time there are only a few left, I talk to them and remind them to come back next year. Every time I catch the sentinel, I tell him to tell the others. So far, so good, and I do believe the babies remember and come home.

“ Each year, about 44 percent of all barn swallows will return to nest in the same area they nested the previous year. If the birds decide to renovate their old nest, they begin by throwing out and replacing old nesting material and adding more mud around the nest's rim.”
posted by MonkeyToes at 6:22 PM on August 19, 2023 [9 favorites]