Gray Whale Sex, Reproductive Behavior, and Social Strategies

Abstract

AbstractGray whale sexual behavior and copulation are observed throughout their range. The most prominent period for reproductive behavior is during the southward migration from summer feeding areas to wintering areas where some breeding occurs and calves of the year are reared. The seasonal migrations of gray whales are believed to function, in part, to bring together individuals that are otherwise widely distributed during the period of estrus to facilitate mating and reproduction. Sexual behaviors and sexual strategies for this species appear to align closely with those of balaenid (not rorqual) whales, although such comparisons need further investigation. Gray whales are polygynandrous (multi-mate) breeders. There does not appear to be female choice of mates, as groups of numerous females and males aggregate, and multiple copulations occur. Female estrus begins in mid-November and continues to early December; females may undergo a second estrus, extending into February, if they fail to conceive during their first cycle. Male gray whales have large testes and concomitantly produce large volumes of sperm, so they are believed to be sperm competitors; that is, they rely on multiple copulations (and sperm volume) to produce offspring. Multiple copulations with different males during the female estrus period may increase the likelihood that the timing of conception results in the birth of a calf approximately 13 months later near or in the wintering area(s). Mating bouts can last for minutes to hours, interspersed with surface-active-social-sexual behavior. Some all-male groups have been observed with erect penises engaged in social-sexual behavior in the absence of any females. Instances of male aggression toward postpartum females with calves of the year, sometimes resulting in injury or death, have been reported. As a result of dedicated long-term research in the past several decades, the state of knowledge on gray whale reproduction has greatly expanded and updated information on this topic is summarized in this chapter.