Bueno-Guerra, N., Völter, C. J., de las Heras, Á., Colell, M., & Call, J. (2019). Bargaining in chimpanzees (Pan troglodytes): The effect of cost, amount of gift, reciprocity, and communication. Journal of Comparative Psychology, 133(4), 542-550. doi:10.1037/com0000189.