What happens when scientists start changing the brain – the very system through which an animal’s mental states are expressed in behaviour?
In a recent study published in Nature, researchers implanted human brain organoids – small, simplified structures grown from human stem cells – into the brains of living mice. Over time, these organoids developed and integrated with the mouse nervous system.
Transplanting organoids composed of human cells into animals, including their brains, is not new. However, previous experiments faced an obstacle: human brain cells grow much more slowly than mouse brain cells, and so organoids are quickly “out-competed” before they can fully integrate with the mouse brain. In the new study, scientists genetically altered mice so that parts of their brain did not develop, then transplanted the human organoids into the vacant space.
The idea of creating human-animal chimeras – animals containing human cells – raises ethical questions, particularly when involving the brain. Various philosophers have argued that as the proportion of human brain tissue increases, we should consider whether the animal could acquire human-like mental capacities – and whether this should change how we treat it. But I argue that serious questions about our ability to recognise animal suffering will arise long before mice gain increased intelligence.
‘Super-intelligent’ mice?
What is it like to be a mouse?
The philosopher Thomas Nagel asked a version of this question about bats. He argued that conscious beings have a specific experience of the world, but that others cannot access that experience from the outside. Instead, we must infer what others experience. With animals, we do this through their behaviour and physiology. But this approach assumes that we understand how what we observe relates to what the animal experiences.
There is nothing morally important about a cell simply because it came from a human. Imagine two mice with identical mental lives and cognitive abilities, one produced using transplanted human cells and the other through a modification involving only mouse cells. It is difficult to see why the first should matter more simply because some of its cells are human.
If chimeras pose a moral problem, it is more plausibly because of changes to their cognitive capacities, than because they blur a species boundary.
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There are good reasons to doubt that a mouse containing human brain cells could develop anything resembling human intelligence. This is because human-like cognition does not emerge simply from having enough human neural cells. A mouse containing human cortical neurons still has mouse sensory systems, a mouse body and develops according to species specific processes. All of these factors impose constraints on the kinds of cognitive capacities it might acquire.
Animal welfare science depends on interpretation. A mouse cannot report that it is stressed or in pain, so researchers rely on behavioural and physiological indicators. Changes in nesting, social interaction or facial expressions associated with pain, for example, can provide evidence about mouse welfare.
Changing an animal’s brain complicates these interpretations. This is because it alters the very biological system responsible for generating an animal’s experiences, and for producing much of the behaviour through which we infer those experiences.
A mouse might still seem to behave “like a mouse” even as we become less certain that its behaviour accurately tells us how it feels.
In the recent Nature study, researchers reported that mice which received the organoid implant performed on a par with healthy control mice on short-term memory tasks (such as navigating a maze). The researchers did not observe any evidence of altered behaviour or increased intelligence.
This is reassuring. But suppose a chimera grooms less than an ordinary mouse. Researchers can still measure that difference accurately, but interpreting it as evidence of distress means we’re assuming that grooming retains roughly the same relationship to the animal’s experiences as it does in an ordinary mouse.
The same applies to the observation of “normal” mouse behaviour. Since we don’t know exactly how changing the brain will change behaviour, this could make it harder to recognise if the indicators of poor welfare we rely upon are no longer accurate.
What follows for research?
Organoids grown in a dish have been important tools for studying human brain development and disease. But a dish cannot reproduce the environment of a living brain. Transplantation allows researchers to study human brain cells as they develop within a functioning nervous system, potentially providing better models of brain disorders and new ways to test treatments.
This does not, by itself, settle the debate over whether research using chimeras should stop. Uncertainty about an animal’s welfare does not automatically mean that its welfare is worse. It could, in principle, be better. Abandoning potentially valuable research also carries a cost.
Proposals researchers have made for overseeing studies involving chimeras already emphasise careful behavioural monitoring. But this may not be enough. Researchers may also need to establish that the indicators they measure continue to tell us what we think they do. As neural interventions become more extensive, researchers may therefore need to re-validate their methods of welfare assessment.
What happens if we eventually create an animal with human-like intelligence or consciousness? We may never reach that point. But we do not need to reach it before chimeras present a difficult ethical problem. Long before we create an animal that thinks like us, we may create one whose behaviour we no longer understand.
