Podcast Summary: Unexplainable – "Animals in the Year 20202025"

Podcast: Unexplainable (Vox)
Episode Date: August 25, 2025
Main Guests/Contributors: Noam Hassenfeld (Host), Benji Jones (Vox Science Writer), Manding Nguyen (Producer), various scientists (see credits)

Episode Overview

In this episode, Unexplainable explores a mysterious trend in the animal kingdom: the shrinking of many animal species, particularly birds, over recent decades. The hosts investigate why this is happening, the scientific theories behind it, and even speculate – with the help of scientists – on what animals might look like millions of years in the future, including a playful imagining of future life on Earth and beyond.

Key Discussion Points and Insights

1. The Discovery: Birds Are Shrinking

• [01:24] David Willard, from the Field Museum in Chicago, recounts how he discovered dead birds around McCormick Place, a glass-covered convention center near Lake Michigan. This incidental finding grew into a long-term research project collecting over 100,000 dead birds for study.
• [03:05] Scientists noticed “birds over the last 35 years have shrunk.”
• [03:14] “...on all of the species, whether it was a thrush, a warbler, a sparrow, they were showing this declining size. And the question of just exactly why is an open one.” — David Willard

2. The Pattern: It's Not Just Birds

• [04:38] Benji Jones explains that the shrinking phenomenon isn't isolated to birds; studies also report it in salamanders, fish, wood rats, deer, sheep, and common toads.

3. Scientific Explanations: Why Are Animals Shrinking?

• [05:01] There is no single answer, but climate change is a leading suspect.

• [05:34] Benji introduces Bergmann’s Rule: “Across the range of a warm-blooded species...you will see smaller versions of that animal in hotter climates and larger versions...in colder climates...It's just much easier to cool down if you're small.”

  “In a hot environment because you would potentially overheat.” — Benji Jones [06:39]

• [06:50] Cold-blooded animals (e.g., amphibians) are also shrinking, even though Bergmann's Rule shouldn’t apply to them. Instead, “when it’s hotter, their metabolism speed up...they are smaller by the time that they're adults.” — Benji Jones [07:32]

4. Exceptions and Complications

• [08:04] Not all animals are shrinking; some, like the American marten and Eurasian otter, are actually getting bigger, possibly due to increased food availability or lower energy needs for keeping warm.
  • “It’s just really hard to say why there is this variation.” — Benji Jones [08:46]
• [08:58] The trend is species- and context-specific. Predicting outcomes is challenging.

5. Evolution or Plasticity?

• [09:12] Is the shrinking genetic (evolution) or a response during development (plasticity)? For birds, some evidence suggests evolution, but for cold-blooded animals, plasticity (changes within an animal’s lifetime) is likely.

  “That would be what scientists call a plastic change...for cold-blooded creatures, it seems that these kind of like within lifetime changes might be responsible for the shrinking effect.” — Benji Jones [10:01]

• [10:23] Whether shrinking is evolved or not has major consequences for species' survival, especially with the current rapid pace of climate change.

6. Ecosystem Implications and Uncertainty

• [11:10] The effects of climate change on biodiversity are “way more complicated...and...really hard to predict.” — Benji Jones

Memorable Quotes & Notable Moments

• Why so fast?

  "A 2.6% change, we would normally expect to see that over, like, a much longer period than 40 years, right?" — Noam Hassenfeld [04:18] "When I think about evolution...that often takes place over thousands of years, certainly not over 40 years. So it's pretty wild." — Benji Jones [04:26]

• Uncertainty highlight:

  "Trying to predict what will happen to a given species is really, really difficult." — Benji Jones [11:10]

Timestamps for Major Segments

• Bird Shrinking Discovered: [01:24] - [03:12]
• Extent of Shrinking Across Species: [04:02] - [04:55]
• Potential Causes & Bergmann's Rule: [05:01] - [06:50]
• Cold-blooded Animal Phenomena: [06:50] - [07:56]
• Exceptions to Shrinking Pattern: [08:13] - [08:58]
• Evolution or Plasticity Debate: [09:12] - [10:15]
• Ecosystem and Predictability Issues: [10:15] - [11:53]

"Nature Documentary of the Future" – Speculative Segment (Post-Commercial Break)

7. Imagining the Distant Future (With Some "Sci-Fi")

• [14:27] Manding Nguyen creates a "nature documentary" for the far future, combining scientific principles with imaginative speculation.

  • Key Sci-Fi Creatures:

    • Giant insects: Due to higher atmospheric oxygen, e.g., “...praying mantis the size of a cocker spaniel” that hunts “giant cockroaches the size of Pomeranians.” [16:37]
    • Whale rat: A fully aquatic rodent, “its tail has maybe become more like an eel’s tail...its arms and legs...evolved into flippers.” — Scientist voice [18:45]
    • Sailing bat: “A bat that lives in the ocean and has evolved very extreme anatomy to sail and soar...wings are long, narrow, they're stiff...” — Scientist voice [19:37]

  • Science Underpinning the Speculation:

    “You can have the same starting conditions and just one little change to the DNA, one little happenstance about what predators are around, what pollinators are around, that can change everything in the course of what’s gonna happen over the next many millions of years.” — Scientist voice [21:29]

• [22:09] Is this realistic or science fiction?

  “It’s definitely more speculation...but a lot of the scientists' answers were rooted in science.” — Manding Nguyen

  • Some predictions are based on known evolutionary patterns (e.g., size and oxygen, aquatic transitions), while others (giant Martian slime molds) are more tongue-in-cheek.

8. The Value of Speculating About the Future

• [24:59] Why do scientists indulge in these thought experiments?

  “Evolutionary biologists are very used to thinking about long timescales in the past, but it's somehow a lot harder to envision the future...in that short time that we've been on the planet, we've completely altered these basic aspects of geochemical cycling. And so, as a consequence, how evolution proceeds for the Earth's inhabitants has changed as well.” — Liz Alter, scientist voice [25:07]

• [25:41] Impact of human choices:

  “Life is probably going to persist, and any future animal or ecosystem ... is going to be impacted by what happens today.” — Manding Nguyen

  “It makes it a very sobering thing to think about the long future ... especially because we're at a point as a species where we're going to have to make some hard choices to avoid a future Earth that's uninhabitable, at least for us.” — Scientist voice [25:49]

Notable Quotes (with Timestamps & Attribution)

• “Birds over the last three, 35 years have shrunk.”
  — David Willard [03:05]

• “When I think about evolution...that often takes place over thousands of years, certainly not over 40 years.”
  — Benji Jones [04:26]

• “It's pretty widespread...there are lots of things that seem to be getting smaller over time.”
  — Benji Jones [04:38]

• "In a hot environment ... you would potentially overheat."
  — Benji Jones [06:39]

• "Trying to predict what will happen to a given species is really, really difficult."
  — Benji Jones [11:10]

• “You can have the same starting conditions and just one little change to the DNA...that can change everything in the course of what’s gonna happen over the next many millions of years.”
  — Scientist voice [21:29]

• “Life is probably going to persist, and any future animal or ecosystem... is going to be impacted by what happens today.”
  — Manding Nguyen [25:41]

• “It makes it a very sobering thing to think about the long future...especially because we're at a point as a species where we're going to have to make some hard choices to avoid a future Earth that's uninhabitable, at least for us.”
  — Scientist voice [25:49]

Overall Tone & Takeaways

• The episode strikes a balance between genuine scientific curiosity, creative speculation, and a lighthearted approach to imagining distant futures.
• The phenomenon of shrinking animals is complex and deeply connected to climate change, with unpredictable and ecosystem-wide consequences.
• Thought experiments about evolutionary futures highlight both the wondrous possibilities of life and the profound impact of today’s environmental choices.
• The guest scientists' willingness to indulge in speculative forecasting helps communicate the awe and open questions at the edge of biological understanding, while also subtly warning of the consequences of unchecked human influence on Earth.

For full credits and a list of participating scientists, see [26:45] and end of transcript.