Do you have a guess for the probability of planarians being sentient? For context, here are the estimates for the probability of sentience estimated in Bob Fischerās book about comparing welfare across species. I would think about a guess before checking Bobās estimates, and then update the guess.
Planarians may not be a top priority animal to study or to care for
Thanks Vasco! As per my conclusion that planarians show āsubstantial evidenceā, I would estimate the probability of sentience to be on the lower side, especially since I would weight the first four physiological criteria lower than the last four behavioral ones, and we lack research on at least three of them. Maybe 15-20% ? I donāt really know how I would estimate this properly. Instinctively, I would rank planarians lower than insects but higher than nematodes (their behaviors seem less chaotic, but Iām no nematode expert). I would be more interested in finding out if these animals somehow have a ālighterā form of sentience, as Iām not fully convinced sentience (as for current definitions) is either absent or present. Planarians can move around without a head, but they definitely do better with one. So, is there some sort of cognitive emergence only possible in the brain ? I have no idea. I looked at the estimates from the book and it checks out as slightly lower probablity than insects. I would be interested to have the estimates from those that read this post too!
Planarians can definitely be considered numerous as we can find some almost anywhere.
I could not find any estimates for total numbers of individuals or colonies. Here again, instinctively, I would assume that they are still way less numerous than nematodes or even insects. Because they are very small (usually around 1-2 cm depending on the species) they move very slowly and feed on detritus or other very small invertebrates (and thus they donāt proliferate so much), and although they are very small, are known for regenerating and being able to multiply exponentially, they do not seem to form large colonies in the wild. You can find a few under some rocks or dead leaves if you search for them in most river, but I would not consider this āabundantā.
I would rank planarians lower than insects but higher than nematodes (their behaviors seem less chaotic, but Iām no nematode expert).
This is my intuition too considering planariansā number of neurons, and that ānumber of neuronsā^0.188 explains the welfare ranges in Bobās book quite well. Bray et al. (2024) says planariansā ānumber of neurons may fluctuate between ā¼1,000 and ā¼100,000 in a single animal during growth and degrowthā. Based on this, and the estimates below I collected, it looks like planarians have more neurons than nematodes, roughly as many as black soldier fly (BSF) larvae, mealworms, and soil springtails and mites, and up to as many as shrimps, and soil ants and termites.
I would be more interested in finding out if these animals somehow have a ālighterā form of sentience, as Iām not fully convinced sentience (as for current definitions) is either absent or present.
I could not find any estimates for total numbers of individuals or colonies. Here again, instinctively, I would assume that they are still way less numerous than nematodes or even insects. Because they are very small (usually around 1-2 cm depending on the species)
Among farmed/āwild animals only (not considering farmed and wild animals together), smaller animals tend to be more numerous.
Thanks for the interesting post, Guillaume!
Do you have a guess for the probability of planarians being sentient? For context, here are the estimates for the probability of sentience estimated in Bob Fischerās book about comparing welfare across species. I would think about a guess before checking Bobās estimates, and then update the guess.
Animals with a low probability of sentience may still be a priority if they are sufficiently abundant. You may be interested in my post Total number of neurons and welfare of animal populations.
Thanks Vasco! As per my conclusion that planarians show āsubstantial evidenceā, I would estimate the probability of sentience to be on the lower side, especially since I would weight the first four physiological criteria lower than the last four behavioral ones, and we lack research on at least three of them. Maybe 15-20% ? I donāt really know how I would estimate this properly. Instinctively, I would rank planarians lower than insects but higher than nematodes (their behaviors seem less chaotic, but Iām no nematode expert). I would be more interested in finding out if these animals somehow have a ālighterā form of sentience, as Iām not fully convinced sentience (as for current definitions) is either absent or present. Planarians can move around without a head, but they definitely do better with one. So, is there some sort of cognitive emergence only possible in the brain ? I have no idea. I looked at the estimates from the book and it checks out as slightly lower probablity than insects. I would be interested to have the estimates from those that read this post too!
Planarians can definitely be considered numerous as we can find some almost anywhere.
I could not find any estimates for total numbers of individuals or colonies. Here again, instinctively, I would assume that they are still way less numerous than nematodes or even insects. Because
they are very small (usually around 1-2 cm depending on the species)they move very slowly and feed on detritus or other very small invertebrates (and thus they donāt proliferate so much), and although they are very small, are known for regenerating and being able to multiply exponentially, they do not seem to form large colonies in the wild. You can find a few under some rocks or dead leaves if you search for them in most river, but I would not consider this āabundantā.edit: I corrected my mixed up train of thought.
Thanks for sharing your thoughts, Guillaume!
This is my intuition too considering planariansā number of neurons, and that ānumber of neuronsā^0.188 explains the welfare ranges in Bobās book quite well. Bray et al. (2024) says planariansā ānumber of neurons may fluctuate between ā¼1,000 and ā¼100,000 in a single animal during growth and degrowthā. Based on this, and the estimates below I collected, it looks like planarians have more neurons than nematodes, roughly as many as black soldier fly (BSF) larvae, mealworms, and soil springtails and mites, and up to as many as shrimps, and soil ants and termites.
This reminded me of the article āAll animals are consciousā: Shifting the null hypothesis in consciousness science by Kristin Andrews. I agree a greater focus on how instead of whether animals are conscious or sentient would be more helpful to assess their welfare.
Among farmed/āwild animals only (not considering farmed and wild animals together), smaller animals tend to be more numerous.