by Matthew Cobb
This article from the scientific journal Cell Reports, by Eve Schneider and colleagues from Yale, popped into my inbox this evening. I could see that it was about ducks and evolution, so I thought it would interest Jerry. It turns out to be pretty fascinating, and it tells us something important about how the sensory worlds of different species are shaped by evolution, as I’ll explain below. It’s open access, so anyone can read it. Click on the image below if you want to read the PDF.
One of the nice features about this article is that it has a “graphical abstract”. Several of the journals from the Cell stable use these abstracts, and I must admit this is the first one I have actually found useful. You can see why: it RHYMES:The study looked at seven different species of duck (including the mallard) which have different food preferences – some of them dive, some dabble (Pekin ducks [domesticated mallards], it appears, forage solely in darkness). Here they are, in a figure from the paper:
The hypothesis was that all these species, which rely particularly on the sense of touch, would have more cells capable of detecting touch (“mechanoreceptors”, in the jargon) in the trigeminal nerve that innervates the beak. Touch works through a particular gene called Piezo2, which codes for what is called an ion channel—these are the tiny pores in nerve cells that enable them to work.
They found that all these species have more of the Piezo2 neurons than do chickens, which do not seem to rely so much on tactile stimulation. There were also differences between the ducks: the Pekin duck, which forages solely in darkness, had the highest proportion of these neurons, while the wood duck, with its narrow bill, had the smallest number.
This expansion of touch neurons in ducks has not come free of charge, however: the ducks seem to have lost corresponding numbers of neurons that, in chickens, detect pain and temperature. In other words, there is an evolutionary trade-off. You don’t seem to be able to have lots of every kind of receptor. What the downsides are of not being able to feel pain or temperature so intensely if you are a duck is not clear. Whatever they are, they are presumably less significant than the advantage of being able to sense your prey more accurately as you dabble or dive.
This is not only a nice bit of evolutionary biology (and a cute graphical abstract), it is also a great example of how the sensory worlds of different animals provide completely different insights into the world. In the 1920s, the Estonian biologist Jakob von Uexküll described what in German was called the Umwelt or inner sensory world of each species, which was rooted in its ecology. This concept is now a fundamental starting point of modern sensory ecology and helps us understand how natural selection has shaped brains and nervous systems, and, when it comes to other animals, how we think of what it is like to be, say, a duck.
The significance of the idea goes far deeper, as Uexküll acknowledged. He saw a link with an idea developed by the German philosopher Immanuel Kant. In his Critique of Pure Reason, published in 1787, Kant argued that some features of how we perceive are given a priori, that is, without experience.
Although Kant was primarily interested in things such as space, time and moral judgements, and ended up placing a profound divide between perception and the material world, he put his finger on a key feature of what is happening when we —or ducks—interact with the world. Our senses are not open valves that simply allow all stimuli into our brain; instead we perceive only certain parts of our environment.
Many scientists have subsequently referred to what are called in the jargon ‘the Kantian synthetic a prioris’: nervous systems involve an innate cognitive and neurobiological framework that filters and processes raw sensory stimuli to turn them into a picture of the world. And that picture differs from species to species.
I’m not sure Kant would have been impressed that his idea had been validated in ducks, but that is what Eve Schneider and her colleagues have done.
Schneider, E. R. et al. 2019. A Cross-Species Analysis Reveals a General Role for Piezo2 in Mechanosensory Specialization of Trigeminal Ganglia from Tactile Specialist Birds. Cell Reports Volume 26: 1979-1987.,