The development of sentience in juvenile animals

The development of sentience in juvenile animals

15 Sep 2021

Determining which animals are sentient is very important for studying the welfare of animals in the wild. Sentience (the ability to experience things consciously, including pain and happiness) is the characteristic that determines which beings can be harmed or benefited. We give moral consideration based on who can be harmed or benefited, so learning which animals are sentient is crucial. Sentience is synonymous or nearly synonymous with consciousness, and we will sometimes use that term here.

If animals of a certain species are sentient, that sentience must develop at a certain point. In order to understand the amount of suffering an animal undergoes in early life, it’s crucial to know at what point sentience occurs. This could be while in development inside of their mother, while inside the egg in egg laying species, or after birth or hatching.

The question of when an animal develops sentience is at least conceptually distinct from the question of when an animal becomes active. An animal, such as Trichoplax (a simple multicellular animal without any organs or internal structure), can act and move in at least some rudimentary ways despite being nonsentient.1 It is therefore possible for an animal to be active but nonsentient, though these traits may tend to co-occur.

The relevance of when sentience develops

The question of when sentience develops in animals is particularly relevant for informing efforts to help them because most animals, like fishes and insects, have extremely high rates of reproduction, and most of them die shortly after coming into existence. It’s crucial to know if these animals are sentient when they die, because if they are then they may suffer from the factors that cause their deaths, such as hunger, dehydration, exposure to the elements, disease, or predation.

Animals who die when they are very young will not have time to have sufficient positive experiences to outweigh not just the harm of death, but also the suffering of the process of dying, so their average welfare will be low. The earlier these animals become sentient, the more of these early deaths will be accompanied by suffering, so the more suffering we can expect to be present in a population. Therefore, in order to target interventions towards animals who most need our help, it is important to understand at what age they become sentient.

If animals who die just after birth are sentient at the age at which they die, then interventions could be effective if they affect the birth of those animals or prevent that early death and suffering. However, if animals at this stage are not sentient, these interventions will not help anyone, so it is important that we come to a better understanding of at what stage in development they become sentient.

In addition, this question has bearing on direct human treatment of animals. For example, adult zebrafish are protected by some minimal animal welfare legislation in many countries while juvenile zebrafish are typically not protected in any way.2 As we will see, this difference in treatment is not justified by adequate evidence. Of course, even adult fishes are not adequately protected and respected by legislation; it is only that some particularly horrific ways of treating them are prohibited. Unfortunately, there has been very little research directly trying to answer the question of the age at which sentience develops. Much of the information we do have comes from secondary use of sources, so it is not as useful as research that directly tries to answer the question. This is further complicated by the fact that we don’t yet have a way to directly measure consciousness, so we can only use a combination of indirect indicators.

Factors affecting the age at which sentience develops

There is an important distinction between precocial and altricial animals. Precocial animals can function and navigate effectively in their environment from a younger age. Because doing this may well require sentience, they are also more likely to be conscious from a younger age.

In contrast, altricial animals are those born in a state where they are less able to navigate their environment and survive independently. They are sometimes initially dependent on their parents for survival as they continue to develop. As the inverse of precocial animals, altricial animals are less likely to show signs of sentience from a younger age. Of course, this is only a tendency and there are notable exceptions, such as humans who are sentient from birth despite being altricial. These categories are a matter of degree, with animals being more or less precocial relative to others.3 Larger animals show a clear tendency to be altricial, while smaller animals (who also tend to have higher rates of reproduction) are usually more precocial in comparison.4 There are exceptions such as eusocial insects who are born as helpless larvae and are cared for by members of the hive. There are also many larger animals who are precocial, such as brush turkeys who can fly within a day of birth, or blue wildebeests who can stand within six minutes of birth.5 Also, most species of fishes are altricial despite often being small. Even among closely related taxa of animals, some may be precocial while others may be altricial. The distinction between egg laying (oviparous) and live birthing (viviparous) is also relevant here. There is evolutionary pressure for animals developing in their mother’s womb to be in a state of reduced activity to avoid harming their mother or their siblings. In sheeps, this state of reduced activity comes about through generally lower levels of oxygenation and the presence of neuroinhibitory substances like adenosine in the fetus or embryo.6 If this level of oxygenation is raised artificially, the developing animal will become more active.7 In mammalian species such as sheeps, sensory pathways from the peripheral nervous system and many other nervous system functions are well-developed prior to birth.8 It seems that the potential for sentience is there, but it is possible it is suppressed prior to birth. Independent breathing immediately after birth may lead to an increase in oxygenation, ending the state of reduced activity. It is possible that this reduced activity indicates unconsciousness, but it is not necessarily the case.9 These animals still sometimes move in the womb, and reduced activity is still compatible with consciousness. EEG readings of sheep in the womb suggest they’re generally in a state of sleep, but potentially with some periods of consciousness.10 The same is not true for animals who develop in eggs. Since there is less risk of damaging the egg, they are generally more active and therefore more likely to be sentient from an earlier stage in development.11 In fact, since animals who were born in eggs need to break out of the egg to hatch, they must become active prior to birth. On the other hand, there may be evolutionary advantages for animals to be conscious prior to birth. One of these advantages is the ability to learn from stimuli received before birth or hatching.12 There is evidence to suggest that some larger animals can learn in this way. Another advantage for egg laying animals is that they may be able to break open the shell and hatch early when the situation calls for it. This early hatching behavior was observed in skinks, who can hatch in a matter of seconds in response to cues from predators. It has also been observed in some fishes, amphibians, and invertebrates.13 This indicates that for some period before hatching, these animals are able to initiate hatching, potentially navigate their environment, and escape from predators. These tendencies point to smaller animals with high rates of reproduction, all else equal, being more likely to be sentient from a younger age than larger animals with lower rates of reproduction. However, these are are only tendencies, so knowing which of these traits a species has does not settle the question about what stage in development they develop sentience. To begin to answer the question of which animals are sentient, we need to examine their particular cases, which we will now turn to.

Case study: Juvenile zebrafish

Zebrafish are a group of small fishes that produce hundreds of eggs per spawning.14 Adult zebrafish have more neurons, about 10 million,15 and juveniles have around 100,000 neurons.16 The 100,000 neurons of a juvenile is a lower number than in adults, but juvenile zebrafishes display similar behavior to adults. Juvenile zebrafishes show shoaling (similar to schooling) behavior, 17 and from a couple of days after hatching they begin to show hunting behavior.18 They are able to avoid predators, which involves processing cues associated with the predator as noxious and responding appropriately.19 They also have individual differences in how risk seeking versus risk avoidant they are.20Also from a couple days after hatching, larval zebrafishes show more activity in a dark area than in an area with more light.21 Brighter areas may represent areas where there is a greater danger of predation and so might cause anxiety in the fishes, which could cause them to be less active to avoid drawing attention to themselves. Anxiety showing these general characteristics is experienced consciously in humans, and it may be experienced consciously in these fishes as well. Juveniles of this age also show what is described as a “startle response” to some sounds and vibrations.22 Juvenile zebrafish have also been found to reduce activity after exposure to noxious stimuli.23 This is relatively good evidence of a non-reflexive response to noxious stimuli, since it is a longer-term reaction, not merely an automatic escape response. There is further behavioral evidence suggesting a nuanced, non-reflexive reaction, such as a reduction in activity for days following exposure to noxious stimuli. 24 These responses are similar to pain responses in adult zebrafish, suggesting that juveniles feel pain in similar circumstances.25 One of the roles that conscious pain plays in humans is facilitating nuanced long-term responses to noxious stimuli in this way. Conscious pain can be traded off against other interests the animal has, so can incentivize the animal to reduce certain activities that might cause reinjury and facilitate the process of healing without stopping the animal from engaging in all activity. These are non-reflexive responses. On the other hand, reflexive responses to noxious stimuli are simpler and occur unconsciously in humans (indicating they do not require consciousness), so they do not provide evidence of consciousness. It is not always possible to tell whether a response is reflexive or non-reflexive, but aversive responses that lead to long-term behavioral changes suggest non-reflexive, and therefore conscious, responses.

Case study: Fruit fly larvae

Larval fruit flies exhibit a defensive reflex roll in response to noxious stimuli. This behavior may have evolved to help them escape from attacking parasitic wasps.26 However, this roll behavior appears to be reflexive, rigid, and stereotyped.27 Because reflexes are unconscious even in humans, this does not provide real evidence of consciousness. This behavior does show that they have nociceptive cells to detect noxious stimuli.28 Nociception is the detection of noxious stimuli by specialized nerve cells called nociceptors.29 Nociception is not itself a type of pain experience, though it can potentially trigger pain experience in the brains of animals who are capable of experiencing pain. These nociceptive cells could allow for conscious pain, and may even be a necessary condition for it, but this example does not show it by itself. This evidence does not strongly suggest sentience in fruit fly larva, but it shows that sentience cannot be definitively ruled out, particularly considering that there is evidence of sentience in adult fruit flies.30

Looking at the indicators reviewed in these case studies, we can begin to determine the strength of the evidence of sentience. This information, together with knowledge of whether the animals are precocial or altricial and other indirect evidence, can help us estimate the likelihood of sentience at young ages. However, this is only taking into consideration the evidence we currently have. There is much more to learn about sentience in animals with high rates of reproduction.


1 Smith, C. L.; Pivovarova, N. & Reese, T. S. (2015) “Coordinated feeding behavior in Trichoplax, an animal without synapses”, PLOS ONE, 10 (9) [accessed on 15 September 2021].

2 Sneddon, L. U. (2018) “Where to draw the line? Should the age of protection for zebrafish be lowered?”, Alternatives to Laboratory Animals, 46, pp. 309-311 [accessed on 2 September 2021].

3 Augustine, S.; Lika, K. & Kooijman, S. A. (2019) “Altricial-precocial spectra in animal kingdom”, Journal of Sea Research, 143, pp. 27-34.

4 Muir, G. D. (2000) “Early ontogeny of locomotor behaviour: A comparison between altricial and precocial animals”, Brain Research Bulletin, 53, pp. 719-726.

5 Starck, J. M. & Ricklefs, R. E. (eds.) (1998) Avian growth and development: Evolution within the altricial precocial spectrum, New York: Oxford University Press. Estes, R. D. & Estes, R. K. (1979) “The birth and survival of wildebeest calves”, Zeitschrift für Tierpsychologie, 50, p. 45-95.

6 Mellor, D. J. & Diesch, T. J. (2006) “Onset of sentience: The potential for suffering in fetal and newborn farm animals”, Applied Animal Behaviour Science, 100, pp. 48-57.

7 Broom, D. M. (2014) Sentience and animal welfare, Wallingford: CABI, pp. 108-111.

8 European Food Safety Authority (2005) “Opinion of the Scientific Panel on Animal Health and Welfare (AHAW) on a request from the Commission related to the aspects of the biology and welfare of animals used for experimental and other scientific purposes”, EFSA Journal, 292, pp. 1-136 [accessed on 16 August 2021]. Broom, D. M. (2014) Sentience and animal welfare, op. cit.

9 Ibid.

10 Broom, D. M. (2014) Sentience and animal welfare, op. cit., pp. 108-113.

11 Ibid.

12 European Food Safety Authority (2005) “Opinion of the Scientific Panel on Animal Health and Welfare (AHAW) on a request from the Commission related to the aspects of the biology and welfare of animals used for experimental and other scientific purposes”, op. cit.

13 Doody, J. S. & Paull, P. (2013) “Hitting the ground running: Environmentally cued hatching in a lizard”, Copeia, 1, pp. 160-165.

14 Spence, R.; Gerlach, G.; Lawrence, C. & Smith, C. (2008) “The behaviour and ecology of the zebrafish, Danio rerio”, Biological Reviews of the Cambridge Philosophical Society, 83, pp. 13-34.

15 Hinsch, K. & Zupanc, G. K. H. (2007) “Generation and long-term persistence of new neurons in the adult zebrafish brain: A quantitative analysis”, Neuroscience, 146, pp. 679-696.

16 Ferro, S. (2013) “Scientists capture all the neurons firing across a fish’s brain on video”, Popular Science, Mar 20 [accessed on 1 June 2021].

17 Engeszer, R. E.; Da Barbiano, L. A.; Ryan, M. J. & Parichy, D. M. (2007) “Timing and plasticity of shoaling behaviour in the zebrafish, Danio rerio”, Animal Behaviour, 74, pp. 1269-1275 [accessed on 25 June 2021].

18 Bianco, I. H.; Kampff, A. R. & Engert, F. (2011) “Prey capture behavior evoked by simple visual stimuli in larval zebrafish”, Frontiers in Systems Neuroscience, 5 [accessed on 3 August 2021].

19 McHenry, M. J.; Feitl, K. E.; Strother, J. A. & Van Trump, W. J. (2009) “Larval zebrafish rapidly sense the water flow of a predator’s strike”, Biology Letters, 5, pp. 477-479.

20 Andrew, R. & Budaev, S. (2009) “Shyness and behavioural asymmetries in larval zebrafish (Brachydanio rerio) developed in light and dark”, Behaviour, 146, pp. 1037-1052.

21 Bos, R. van den; Mes, W.; Galligani, P.; Heil, A.; Zethof, J.; Flik, G. & Gorissen, M. (2017) “Further characterisation of differences between TL and AB zebrafish (Danio rerio): Gene expression, physiology and behaviour at day 5 of the larval stage”, PLOS ONE, 12 (4) [accessed on 26 June 2021].

22 Ibid.

23 Sneddon, L. U. (2018) “Where to draw the line? Should the age of protection for zebrafish be lowered?”, op. cit.

24 Lopez-Luna, J.; Canty, M. N.; Al-Jubouri, Q.; Al-Nuaimy, W. & Sneddon, L.U. (2017) “Behavioural responses of fish larvae modulated by analgesic drugs after a stress exposure”, Applied Animal Behaviour Science, 195, pp. 115-120.

25 Sneddon, L. U. (2003) “The evidence for pain in fish: the use of morphine as an analgesic”, Applied Animal Behaviour Science, 83, pp. 153-162; (2018) “Where to draw the line? Should the age of protection for zebrafish be lowered?”, op. cit.

26 Fiala, A. (2008) “Neuroethology: A neuronal self-defense mechanism in fly larvae”, Current Biology, 18, pp. R116-R117 [accessed on 5 August 2021].

27 Hwang, R. Y.; Zhong, L.; Xu, Y.; Johnson, T.; Zhang, F.; Deisseroth, K. & Tracey, W. D. (2007) “Nociceptive neurons protect Drosophila larvae from parasitoid wasps”, Current Biology, 17, pp. 2105-2116 [accessed on 19 August 2021].

28 Fiala, A. (2008) “Neuroethology: A neuronal self-defense mechanism in fly larvae”, op. cit.

29 Smith, E. S. J., & Lewin, G. R. (2009) “Nociceptors: A phylogenetic view”, Journal of Comparative Physiology A, 195, pp. 1089-1106 [accessed on 11 July 2021].

30 Waldhorn, D. R. (2019b) “Invertebrate sentience table”, Rethink Priorities, Jun. 14 [accessed on 19 June 2023].