Why birds have an unusual appetite for sour foods

Mammals usually avoid sour flavors, but a robin can devour berries so tart they would make most of us pucker in pain. How do birds pull off this dietary feat?

A new study answers the riddle by showing that evolution has rewired birds’ sour-taste machinery.

The discovery not only reveals why avian diners relish fruits with pH values as low as lemon juice, it also hints that a molecular tweak in taste may have opened new ecological doors during bird evolution.

The sour switch in birds

To decode avian acid tolerance, scientists at the Kunming Institute of Zoology, the Chinese Academy of Sciences, and the Max Planck Institute for Biological Intelligence focused on OTOP1. It is the only known vertebrate receptor that detects sour flavors.

In mammals – including people – OTOP1 channels fire vigorously when acidity rises, sending a sharp warning to the brain.

In many birds, however, OTOP1 does the opposite: as the surrounding fluid turns more acidic, the channel shuts down. With the bitter alarm silenced, beaks keep munching.

The team began by cataloging bird feeding habits across Asia, Europe, and South America. The data confirmed that many songbirds, pigeons, and parrots thrive on fruits whose pH dips below 3 – a level similar to vinegar.

Genes that change taste

Genetic sequencing of their taste receptors revealed a cluster of four amino acid changes – H239, L306, H314, and G378 – that alter how the OTOP1 protein behaves under acid stress.

Functional tests delivered the clincher. When researchers used CRISPR to edit these mutations into the OTOP1 gene of laboratory mice, the rodents’ brain cells fired far less in response to sour liquids.

Conversely, when they blocked OTOP1 in pigeons and canaries with a targeted inhibitor, the birds promptly abandoned their usual acidic fare. The receptor is therefore both necessary and sufficient for avian acid tolerance.

Sour and sweet together

Tracing the family tree of OTOP1 revealed another twist. The last of the four mutations – G378 – arose only in songbirds, a lineage that today accounts for nearly half of all living bird species.

The timing is suggestive. Earlier work had shown that songbirds also gained a sweet-taste receptor not found in most other birds.

The new study raises the possibility that upgrades in both sour suppression and sweet perception co-evolved, enabling ancestral songbirds to exploit nectar and acidic fruit simultaneously. That ecological flexibility might help explain the group’s modern success.

Big impacts from taste

Why should anyone beyond ornithology circles care about a bird’s taste tolerance? For one, the findings illuminate how sensory systems evolve under dietary pressures.

Sour taste originally signaled unsafe or unripe food. Turning that alarm off required a molecular workaround – achieved with just a handful of amino acid swaps. This shows how rapidly chemosensory genes can reshape animal behavior and niche use.

There are broader ecosystem implications too. Birds that devour harshly acidic fruits are pivotal seed dispersers in many tropical forests. Birds ability to stomach such fare ensures that plant species with defensive sour pulp still find willing couriers.

Understanding the molecular roots of that partnership refines predictions about how changing climates or fruit chemistry could cascade through food webs.

A blueprint for taste

The study also provides a roadmap for probing taste evolution in other animals. Similar OTOP1 edits might help explain why some primates relish citrus while others avoid it, or why certain fish tolerate acidic streams.

Meanwhile, the CRISPR-mouse model offers a living platform to test how sour aversion shapes diet, metabolism, and health.

For agricultural science, the insights could guide breeding of poultry or songbird populations used in pest control, tailoring feed formulations to their altered taste sensitivities.

Plant biologists might likewise engineer fruit acidity to attract or deter specific avian consumers.

Taste and bird evolution

Birds captivated biologists with their songs, colors, and aerial exploits. Now, thanks to a little proton-gated channel in the tongue, they have added another evolutionary marvel: an internal dial that mutes the sting of acid.

Researchers found that tweaks to sour taste may have co-evolved with sweetness, reshaping how birds sense and seek food. Together, these changes helped propel birds into ecological niches overflowing with nectar and tangy fruit.

The work highlights a recurring theme in evolutionary biology: subtle molecular changes can yield sweeping macro-scale outcomes.

A quartet of amino acid substitutions may sound trivial, yet it probably influenced what early songbirds ate, where they migrated, and how they diversified.

The next time you watch a thrush gulp down a crabapple or a tanager feast on still-green berries, remember their dauntless palate is the legacy of eons of microscopic tinkering. That subtle adaptation continues to shape forests and fields around the globe.

The study is published in the journal Science.

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