Over a decade of comparative studies, researchers have found that rudimentary numerical abilities are widespread among vertebrates. While experiments in mammals and birds have employed a variety of stimuli (visual, auditory and tactile), all fish studies involved visual stimuli and it is unknown whether fish can process numbers in other sensory modalities. To fill this gap, we studied numerical abilities in Phreatichthys andruzzii, a blind cave-dwelling species that evolved in the phreatic layer of the Somalia desert. Fish were trained to receive a food reward to discriminate between two groups of objects placed in opposite positions of their home tank. In Experiment 1, subjects learned to discriminate between two and six objects, with stimuli not controlled for non-numerical continuous variables that co-vary with numbers, such as total area occupied by stimuli or density. In Experiment 2, the discrimination was two versus four, with half of the stimuli controlled for continuous quantities and half not controlled for continuous quantities. The subjects discriminated only the latter condition, indicating that they spontaneously used non-numerical information, as other vertebrates tested in similar experiments. In Experiments 3 and 4, cavefish trained from the beginning only with stimuli controlled for continuous quantities proved able to learn the discrimination of quantities based on the sole numerical information. However, their numerical acuity was lower than that reported in other teleost fish tested with visual stimuli.
A.B. and C.A. designed the study; C.T. and C.A. collected the data; A.B., C.T. and C.B. analyzed and interpreted the results; and A.B., C.B., A.F. and C.A. wrote the paper.
The authors declare no competing financial interests.
This study was supported by ‘Futuro in Ricerca (FIRB) 2013’ grant from Ministero dell'Istruzione, Università e Ricerca (MIUR, Italy) [protocol: RBFR13KHFS to C.A.]; ‘Progetto Giovani Studiosi 2010’ research grant from University of Padova [protocol: GRIC101125 to C.A.]; and ‘Progetti di ricerca di rilevanza nazionale (PRIN) 2009’ from MIUR [protocol: 2009WZXK7T to A.B.]. C.B. and A.F. are supported by University of Ferrara research grants.
- © 2014. Published by The Company of Biologists Ltd