Abstract
We investigated numerical discrimination and numerical rule-learning in ring-tailed lemurs (Lemur catta). Two ring-tailed lemurs were trained to respond to two visual arrays, each of which contained between one and four elements, in numerically ascending order. In Experiment 1, lemurs were trained with 36 exemplars of each of the numerosities 1–4 and then showed positive transfer to trial-unique novel exemplars of the values 1–4. In Experiments 2A and 2B, lemurs were tested on their ability to transfer an ascending numerical rule from the values 1–4 to novel values 5–9. Both lemurs successfully ordered the novel values with above chance accuracy. Accuracy was modulated by the ratio between the two numerical values suggesting that lemurs accessed the approximate number system when performing the task.
Highlights
Over the last 100 years, a great many studies have investigated the numerical abilities of non-human animals and have suggested that a broad range of species are capable of attending to the numerical attributes of the world around them
In order to determine whether lemurs were using non-numerical cues to order the stimuli, we conducted separate analyses on trials when (1) cumulative surface area was held constant for both stimuli, (2) element size was held constant for both stimuli, and (3) element size and cumulative surface area varied randomly from trial to trial
We found that accuracy was above chance regardless of whether the cumulative surface area of the stimuli was equated, when the size of the elements in each stimulus was equated, and when there was no systematic relationship between the element size or cumulative surface area and numerosity
Summary
Over the last 100 years, a great many studies have investigated the numerical abilities of non-human animals and have suggested that a broad range of species are capable of attending to the numerical attributes of the world around them (for reviews, see Brannon and Cantlon, 2009; Merritt et al, in press). Monkeys and apes can learn numerical rules (Brannon and Terrace, 1998, 2000), match numerical values across sensory modalities (Jordan et al, 2005, 2008), and even make simple arithmetic calculations (e.g., Rumbaugh et al, 1987; Boysen and Berntson, 1989; Olthof et al, 1997; Beran, 2004; Anderson et al, 2005, 2007; Addessi et al, 2007; Cantlon and Brannon, 2007a; Evans et al, 2009). Brannon and Terrace (1998, 2000) used a touch-screen procedure with rhesus monkeys to carefully control for alternative stimulus dimensions and to test whether monkeys were capable of abstract numerical rule-learning. By varying the size and shape of the elements across stimulus sets and testing the monkeys with trial-unique stimuli, Brannon and Terrace showed that the monkeys could order the values based on numerosity alone. Both monkeys successfully transferred the ordinal rule they learned with the values 1–4 to the novel values 5–9 (see Brannon et al, 2006; Cantlon and Brannon, 2006a)
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