Recent Use of Signs by Chimpanzees (Pan troglodytes) in Interactions With Humans

Published in the 'Journal of Comparative Psychology', 2005

In light of the controversy about the linguistic properties of chimpanzee signing behavior, the recent sign use of 5 chimpanzees (Pan troglodytes) with long histories of sign use was analyzed while they interacted with longtime human companions. Four corpora from 1992 to 1999 consisting of 3,448 sign utterances were examined. The chimpanzees predominantly used object and action signs. There was no evidence for semantic or syntactic structure in combinations of signs. Longer combinations showed repetition and stringing of object and action signs. The chimpanzees mostly signed with an acquisitive motivation. Requests for objects and actions were the predominant communicative intentions of the sign utterances, though naming and answering also occurred. This recent sign use shows multiple differences with (early) human language.

Kunnen chimpansees een menselijke taal leren? [Can chimpanzees learn a human language?]

Interview by Pouwel Slurink, published in 'Filosofie', 17, 4, 17-18, 2007

Psychological complexity and animal ethics: Choosing between hierarchy and equality

Published in 'Animal Consciousness and Animal Ethics. Perspectives from the Netherlands', edited by M. Dol, S. Kasanmoentalib, S. Lijmbach, E. Rivas and R. van den Bos, 1997, 169-184, Assen: Van Gorcum

The relevance of psychological complexity as a criterion in animal ethics is assessed with respect to hierarchy and equality. This is done by discussing the theories of Singer, VanDeVeer and Regan. These ethicists consider differences in psychological complexity between animals as a valid justification for a hierarchical difference in the treatment of animals. The utilitarian axiology used in Singer's and VanDeVeer's theories, however, is questionable in that it values individuals' experiences rather than the individuals themselves and can lead to negative consequences for individuals. VanDeVeer also risks being speciesist in limiting utilitarianism at the human level. Regan's rights theory shows inconsistencies and unnecessarily limits equality to subjects-of-a-life. It is concluded that when constructing a rational and unprejudiced system of animal ethics, the choice lies between a utilitarian hierarchy for all animals, including humans, and a deontological equality which includes all animals. An egalitarianism based on phenomenal consciousness is suggested and the pracital consequences are specified.

The Question of Animal Awareness and the Culture of Science

Co-authored with T. Rivas, published in 'Science and the Human-Animal Relationship', edited by E.K. Hicks, 1993, 91-93, Amsterdam: SISWO.

The Use of Analogous Reasoning for Assessing Discomfort in Laboratory Animals

Co-authored with F.R. Stafleu, T. Rivas, J. Vorstenbosch, F.R. Heeger, and A.C. Beynen, published in 'Animal Welfare', 1992, 1, 77-84

When assessing discomfort in animals analogous reasoning is often used, namely, that the causes or symptoms of discomfort in people will also apply to animals. This practical rule of thumb can be based on an 'analogy-postulate'. This postulate takes into account the anatomical and physiological similarities of vertebrate nervous systems and the comparability and homology in the behavioural and physiological responses to discomfort of humans and other vertebrates in similar situations. There are theoretical and practical problems with this analogous reasoning. Theoretical objections include claims that feelings do not exist, are irrelevant or that scientific knowledge is not necessary to recognize feelings. Practical problems will occur when assessing the discomfort of animals without proper knowledge of the relevant species-specific information. Nevertheless, we think that there are two equivalent sound reasons for accepting the analogy-postulate. First, there is more evidence in favour of acceptance of the postulate than of its rejection. Secondly, the negative moral consequences of erroneously rejecting the postulate are far greater than those of mistakenly accepting it.

Bewustzijn bij dieren [Consciousness in animals]

Co-authored with Titus Rivas, published in 'Antropologische Verkenningen, 10 (2), 32-40, 1991

In this article the question of subjective awareness in animals is approached from the plausibility of the so-called analogy-postulate; anatomical, physiological and behavioral similarities between men and at least other vertebrates strongly point to the existence of consciousness in such animals. Subjective awareness in animals cannot be empirically proven, but only assumed by use of a postulate. The authors also present a short history and discussion of views on and approaches to animal consciousness. They themselves propose a specific programme based on the analogy principle. Through the use of introspection, knowledge should be gathered on aspects of human consciousness. Empirical research should then decide if the extrapolation of such data to specific animals is justified. Finally, the authors strongly emphasize the ethical implications of animal consciousness for animal use.

Zijn mensen de enige dieren met bewustzijn? [Are humans the only animals with consciousness?]

Co-authored with T. Rivas, published in 'Prana', 1992, augustus/september, 83-88.