The Senate Standing Committee on Legal and Constitutional Affairs
Invertebrates Feel Pain?
Invertebrates are classically defined as animals, which lack a’
backbone’ or dorsal nerve cord1, such as insects, crustacea (e.g.
shrimp, lobster and crab), and molluscs (e.g. clams, snails, and squid).
Traditionally, these animals have not been included in legislation
concerning cruelty to animals2.
Pain is defined by the International Association for the Study of Pain
(IASP) as “An unpleasant sensory and emotional experience associated with
actual or potential tissue damage or described in terms of such damage”3.
The subjective, emotional component of pain is considered its important
aspect, not the activation of pain sensors (nociceptors) in the body.
The IASP makes this clear “Activity induced in the nociceptive pathways
by a noxious stimulus is not pain, which is always a psychological state, even
though we may appreciate that pain most often has a proximate physical cause”3.
In other words, the only animals capable of feeling pain are those that
can feel fear, anxiety, distress and terror, similar to what humans feel when we
receive noxious stimuli.
Almost all organisms, including bacteria, will attempt to escape from an
aversive stimulus4. Because
bacteria are not thought to be capable of feeling pain (e.g. they lack a nervous
system), possessing an escape response to an aversive stimulus is not enough
evidence to demonstrate that a species is capable of feeling pain.
To infer that a non-human vertebrate (mammals, birds and reptiles) is in
pain, researchers rely on the vocalizations and physiological responses (e.g.
the release of stress hormones) that an animal produces when faced with an
aversive stimulus2. Because
these responses are similar to our own when we are in pain, researchers argue
that, by analogy, animals showing these responses are also in pain2.
This technique cannot be used with invertebrates.
Invertebrate physiology is different from our own1.
The invertebrates diverged from that of vertebrates hundreds of millions
of years ago1.
Scientists have used three lines of reasoning to assess the likelihood
that invertebrates are capable of feeling pain5.
- The evolutionary function of pain
- The neural capacity of invertebrates
- The behaviour of invertebrates
1. The evolutionary function of pain.
In vertebrates pain is thought to be an important educational tool6.
Vertebrates are relatively long-lived creatures and learning shapes much
of their behaviour. Learning from
pain (and pleasure) plays a vital role in the development of their behaviour6.
Almost all invertebrates are short-lived and their behaviour is thought
to be largely genetically determined7.
Therefore, there is less evolutionary pressure selecting for the
evolution of pain in this group of animals6.
2. The neural capacity of invertebrates.
Except for the cephalopods, invertebrates have small nervous systems,
consisting of many small brains (ganglia).
Because of the small number of neurons and the distributed organization
of their nervous systems, invertebrates are thought to have limited cognitive
cognitive capacity is thought to be a prerequisite for the development of an
behaviour of invertebrates
Invertebrates show few, if any, of the behaviours that we would recognize
as evidence of emotion6. Many
invertebrates are cannibalistic, and many eat their young when given the chance.
Most have no social behaviour. Although
they can respond vigorously to noxious stimuli, even this response is
inconsistent. Insects, for example,
will continue with normal activity even after severe injury.
An insect walking with a crushed tarsus (lower leg) will continue
applying it to the ground with undiminished force. Locusts will writhe when
sprayed with DDT. However, they
will also continue feeding while being eaten by a praying mantid6.
Cephalopods are sometimes given special status by animal care committees
(e.g. CCAC) because they have a large, vertebrate-like central nervous system,
which is about the same size as that of a fish8. In the United Kingdom these animals have some legal
protection, however in the United States they do not.
Although they have large brains, all the coleoid cephalopods (squid,
octopus and cuttlefish) have short lifespans8.
Most live less than one year. There
is no parental care8. The
absence of parental care suggests that most of their behaviour is genetically
determined (i.e. they must be able to hunt, hide from predators, communicate
etc. without instruction by others of their species).
They are capable of learning, but their abilities are sometimes greater,
sometimes less than that of fish8,9.
Most are highly cannibalistic, even the schooling squid. We know nothing
about their hormonal response to stress, and therefore we cannot determine
whether they have a physiological response that resembles ours when confronted
by aversive stimuli. We understand very little about their visual communication
system and, therefore, we do not know whether they make any ‘pain-specific’
signals. Given our three criteria
above, we have very little evidence that these animals feel pain.
Nevertheless, it is possible that as we learn more about them, we may
find evidence suggesting that they are capable of feeling pain.
Although it is impossible to know the subjective experience of another
animal with certainty, the balance of the evidence suggests that most
invertebrates do not feel pain. The
evidence is most robust for insects, and, for these animals, the consensus is
that they do not feel pain6.
1. Brusca R
and Brusca G. 2002. The Invertebrates. 2nd edition. Sinauer.
2. Animal Behaviour Society, 2003. Anim. Behav. 65: 649-655
Association for the Study of Pain. www.iasp-pain.org/terms-p.html
4. Berg, H 1975. Nature. 254: 389-392
5. Sherwin, C
2001. Anim. Welfare. 10: S103-S118
6. Eisemann C
et al. 1984. Experientia 40: 164-167
Drickamer L et al. 2001. Animal
Behavior: Mechanisms, Ecology and Evolution. 5th edition.
8. Hanlon R
and Messenger J 1996. Cephalopod Behaviour, Cambridge Univ. Press.
9. Boal J et
al. 2000. Behav. Processes. 52: 141-153