"Females of almost all species except man will mate only during their fertile period, which is known as estrus, or heat..." Helena Curtis (1975). Biology. Worth Publishers. pp. 1065. ISBN0879010401.
Shein; Zyzanski, SJ; Levine, S; Medalie, JH; Dickman, RL; Alemagno, SA; et al. (Spring 1988). "The frequency of sexual problems among family practice patients". Fam Pract Res J. 7 (3): 122–134. PMID3274680.
Dinitia Smith, "Central Park Zoo's gay penguins ignite debate", San Francisco Chronicle (February 7, 2004). Article is mainly about gay penguins but also mentions homosexuality in dolphins, and also says 'In bonobos, she noted: "you see expressions of sex outside the period when females are fertile."' Available online at http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2004/02/07/MNG3N4RAV41.DTL.
