Charles Darwin (1972 [orig. 1871]), p. 105.
Ernst Mayr (1982), p. 97.
Richard Byrne (1995), Jacques Vauclair (1996), Michael Tomasello and Josep Call (1997), James Gould and Carol Grant Gould (1999), Marc Bekoff et al. (2002), Susan Hurley and Matthew Nudds (2006), John Pearce (2008), Sara Shettleworth (2012), and Clive Wynne and Monique Udell (2013).
Werner Heisenberg (1958), p. 26.
Jakob von Uexküll (1957 [orig. 1934]), p. 76. See also Jakob von Uexküll (1909).
Ludwig Wittgenstein (1958 [orig. 1953]), p. 225.
Martin Lindauer (1987), p 6, quoting Karl von Frisch.
Niko Tinbergen, (1953), Eugиne Marais (1969), Dorothy Cheney and Robert Seyfarth (1992), Alexandra Horowitz (2010), and E. O. Wilson (2010).
Preston Foerder et al. (2011).
12 Joshua Plotnik et al. (2006).
Lisa Parr and Frans de Waal (1999).
Doris Tsao et al. (2008).
Konrad Lorenz (1981), p. 38.
Edward Thorndike (1898) inspired Edwin Guthrie and George Horton (1946).
Bruce Moore and Susan Stuttard (1979).
Jan van Hooff (1972), Marina Davila Ross et al. (2009).
Frans de Waal (2000), Nicola Koyama (2001), Mathias Osvath and Helena Osvath (2008).
William Hodos and C. B. G. Campbell (1969).
"Pigeon, rat, monkey, which is which? It doesn't matter." B. F. Skinner (1956), p. 230.
Bonnie Perdue et al. (2011), Steven Gaulin and Randall Fitzgerald (1989).
Bruce Moore (1973), Michael Domjan and Bennett Galef (1983).
Sara Shettleworth (1993), Bruce Moore (2004).