a skeletal specimen and a few stranded specimens). Among the results of this study was the conclusion that the male's teeth are actually a secondary sexual characteristic, similar to the antlers of male deer. Each species' teeth have a characteristically unique shape. In some cases, these teeth even hinder feeding; in the strap-toothed whale, for example, the teeth curve over the upper jaw, effectively limiting the gape to a few centimeters. Females are presumed to select mates based on the shape of the teeth, because the different species are otherwise quite similar in appearance. The social structure is not well known, but sightings of single males accompanying multiple females suggest a polygynous mating system, which would considerably limit the number of males a female can choose. Taxonomy Beaked whales comprise at least 22 species. Several species have been described only in the last two decades. Six genera have been identified. The beaked whales are the second-largest family of cetaceans (after the dolphins). They were one of the first groups to diverge from the ancestral lineage. The earliest known beaked whale fossils date to the Miocene, about 15 million years ago. Order Cetacea Suborder Odontoceti: toothed whales Family Ziphiidae Incertae sedis Genus †Anoplonassa Genus †Caviziphius Genus †Cetorhynchus Genus †Eboroziphius Genus †Pelycorhamphus Basal forms Genus †Aporotus Genus †Beneziphius Genus †Chavinziphius Genus †Chimuziphius Genus †Choneziphius Genus †Dagonodum Genus †Globicetus Genus †Imocetus Genus †Messapicetus Genus †Ninoziphius Genus †Notoziphius Genus †Tusciziphius Genus †Ziphirostrum Subfamily Berardiinae Genus †Archaeoziphius Genus Berardius B. arnuxii, Arnoux's beaked whale B. bairdii, Baird's beaked whale Berardius minimus, Sato's beaked whale †B. kobayashii Genus †Microberardius Subfamily Hyperoodontinae Genus †Africanacetus Genus †Belemnoziphius Genus Hyperoodon, bottlenose whales H. ampullatus, northern bottlenose whale H. planifrons, southern bottlenose whale Genus †Ihlengesi Genus Indopacetus I. pacificus, tropical bottlenose whale Genus †Khoikhoicetus Genus Mesoplodon, mesoplodont whales M. bidens, Sowerby's beaked whale M. bowdoini, Andrews' beaked whale M. carlhubbsi, Hubbs' beaked whale M. densirostris, Blainville's beaked whale M. eueu, Ramari's beaked whale M. europaeus, Gervais's beaked whale M. ginkgodens, ginkgo-toothed beaked whale M. grayi, Gray's beaked whale M. hectori, Hector's beaked whale M. layardii, strap-toothed whale M. mirus, True's beaked whale M. peruvianus, pygmy beaked whale M. perrini, Perrin's beaked whale M. stejnegeri, Stejneger's beaked whale M. traversii, spade-toothed whale M. hotaula, Deraniyagala's beaked whale Genus †Nenga Genus †Pterocetus Genus †Xhosacetus Subfamily Ziphiinae Genus †Caviziphius Genus †Izikoziphius Genus †Nazcacetus Genus Tasmacetus T. shepherdi, Shepherd's beaked whale Genus Ziphius Z.