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🕊️ Mumbai’s skies wouldn’t be the same without pigeons. With feeding bans, these gentle birds face starvation. 💔 Human Mumbaikars turned into “pigeons” to remind everyone pigeons belong here too. 👉 Join us in urging Hon’ble CM Devendra Fadnavis to stop starving pigeons with feeding bans:

3,812,703 görüntüleme • 10 ay önce •via X (Twitter)

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There is a new paper in Science proposing a mechanism for how homing pigeons navigate on cloudy days. (Hint: It's magnetic fields.) As with many magnetobiology papers, though, I'm skeptical of their proposed mechanism. These researchers, from Germany, found that pigeons have macrophages in their liver that accumulate lots of iron. They confirmed this with staining and other analyses. These macrophages also tend to cluster near nerve fibers in the liver (this is important for later). Then they did a really interesting experiment: - Train 34 pigeons to fly a particular 19-kilometer route. Ensure they all do this well. - Split the pigeons into two groups: treatment and control. - Inject the treatment group pigeons with liposomes loaded with clodronate. The macrophages eat these liposomes, and then clodronate kills the cells and scatters the iron. - Release the pigeons, from both groups, on an overcast day (it's thought that pigeons use magnetic fields to navigate when there is no sun). - All of the control pigeons reached their destination within 70 minutes, but the treated pigeons scattered in random directions. - (Important control experiment: The treated pigeons, released on a sunny day, flew like normal and reached their destination.) - This is taken as evidence that ??? macrophages --> iron --> navigation ??? via magnetic fields. But the mechanism is fuzzy. This experiment is super interesting, and it's clear that the treated pigeons really are unable to home to their destination using a magnetic field. But I'm not entirely convinced by the mechanism these authors propose. The main claim is that these iron-loaded macrophages "align" in a magnetic field, and that they shift according to the bird's orientation so that it can fix its direction. These macrophages (somehow) send signals to the nerve fibers in the liver, which then pass the messages to the brain, which allows the bird to navigate. The news coverage for this story suggests how this might happen: "One idea is that as the bird shifts its position relative to Earth’s magnetic field lines, the ferritin changes orientation and tugs on the web of fibers within a macrophage, possibly triggering the release of signaling molecules." (All you need to do is read the 2016 Meister paper, from the images below, to understand why such a mechanism is physically dubious.) The problem, though, is that the authors show (in their own study) that the iron in these pigeons' livers only act as a stable magnet at super low temperatures, below about 12 degrees Kelvin (or -260 degrees Celsius). At normal, physiological temperatures, the iron would be scrambled by the thermal motions of the tissue. Every measurement in the paper is taken at cryogenic temperatures, but a bird's body temperature is much higher, which means heat would likely destroy any magnetic alignment. The authors claim that MILLIONS of iron particles in the liver are all acting together to escape this effect, but they don't demonstrate the mechanism convincingly at all. If this claim is true, why not take homing pigeons (control vs. macrophage-depleted) and then rotate a magnetic field around them? You could record their neurons to see if there is some kind of signal coming from the liver.

Niko McCarty.

22,396 görüntüleme • 2 ay önce