ORIGINAL PAPER Means–end comprehension in four parrot species: explained by social complexity Anastasia Krasheninnikova • Stefan Bra ¨ger • Ralf Wanker Received: 30 July 2012 / Revised: 22 January 2013 / Accepted: 28 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract A comparative approach is required to investi- gate the evolutionary origins of cognitive abilities. In this paper, we compare the performance of four parrot spe- cies, spectacled parrotlets (Forpus conspicillatus), rainbow lorikeets (Trichoglossus haematodus), green-winged macaws (Ara chloroptera) and sulphur-crested cockatoos (Cacatua galerita triton) in standardized string-pulling and string-choice paradigms. We varied the spatial relationship between the strings, the presence of a reward and the physical contact between the string and the reward to test different cognitive skills requiring means–end comprehension. The species tested showed a high individual and inter-specific variation in their ability to solve the tasks. Spectacled par- rotlets performed best among the four species and solved the most complex choice tasks, namely crossed-string task and broken-string task, spontaneously. In contrast, macaws and cockatoos failed to identify the correct string in these two tasks. The rainbow lorikeets were outperformed by the par- rotlets, but outperformed in turn the macaws and the cock- atoos. The findings can be best explained by the variation in social complexity among species, rather than in their ecology. Keywords Ara chloroptera Á Cacatua galerita triton Á Comparative cognition Á Forpus conspicillatus Á Means– end relationship Á Trichoglossus haematodus Introduction Biologists have long assumed that socio-ecological demands explain cognitive variation across species in both mammals and birds (Emery and Clayton 2004; Marler 1996; Seyfarth and Cheney 2002). Foraging (Harvey et al. 1980; Ratcliffe et al. 2006; Reader and MacDonald 2003) and social relationships (Dunbar 1998, 2008) have been identified as the main selective pressures shaping brain size evolution. Provided that ‘‘big brains’’ in birds correlate strongly with lifestyle variables such as social complexity (Burish et al. 2004) and diet (Lefebvre and Sol 2008) and assuming that domain-general cognition correlates with brain size (Sol et al. 2005), there should be demonstrable differences in performance in terms of general intelligence across species differing in their social structure and/or foraging techniques. These technological and social brain hypotheses have increased the recent interest in problem-solving skills. Such skills, however, may not directly reflect ecological chal- lenges (currently) faced by a species, but rather their ability to understand and to solve a novel situation. Problem-solving skills have often been tested with tasks that require some understanding of causal relationships by the subjects without needing to be solved intuitively. One such test is provided by string-pulling tasks. String-pulling requires the integration of a large number of different motions in a precise order to solve a problem. Since the problem is unlikely to have been encountered before in the animal’s life, we can test whether it can be solved without trial-and-error learning. An additional Ralf Wanker: Deceased 6 September 2011. Dr. Ralf Wanker passed away before the final acceptance of the paper. A. Krasheninnikova (&) Á R. Wanker Biozentrum Grindel, Department of Biology, University of Hamburg, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany e-mail: anastacia.k@web.de S. Bra ¨ger Dorfstraße 10, 24211 Schellhorn, Germany 123 Anim Cogn DOI 10.1007/s10071-013-0609-z