“…On the other hand, they may obtain benefits from each other's presence such as a more favorable thermal environment (Alberts, 1978;Sokoloff & Blumberg, 2001) or enhanced sensorimotor stimulation (Alberts & Cramer, 1988). In the study of sibling interactions, considerable attention has been given to competition, particularly in birds (Drummond, 2001;Mock, Drummond, & Stinson, 1990;Mock & Parker, 1997), but also increasingly in mammals (Drummond, Vázquez, Sánchez-Colón, Martínez-Gómez & Hudson, 2000;Frank, Glickman, & Licht, 1991;Fraser & Thompson, 1991;Hartsock & Graves, 1976;Mock & Parker, 1997;in humans, Sulloway, 1996; but see Solomon & French, 1997, for evidence of sibling cooperation).…”