Sexual dimorphism is extreme in sperm whales:
- Adult males may be over 18 metres in length, whilst females are up to 12 metres (Jefferson et al, 2008).
- Physically mature males may weigh between 35,000 and 50,000 kg, with females weighing around one-third as much (Nowak, 2003).
Longevity of sperm whales has been studied using tooth-sections. Dentinal growth layers are counted to determine age, with some animals estimated to be up to 70 years old (Rice, 1989). Females are thought to live longer than males (Ralls et al, 1980).
Females reach sexual maturity at around age 9 years, whilst males may be as old as 20 years before they become sexually active (Best, 2007). Gestation has been calculated variously at 14 to 19 months (Nowak, 2003). During the breeding season, mixed age and sex schools of sperm whales are joined by large, older males. These schools then become known as harem schools, with one adult male for every ten adult females (Nowak, 2003).
Sperm whales are a top predator. However, they may sometimes be attacked by killer whales and other odontocetes (Jefferson et al, 2008). Tablespoon-sized scoop marks and oval scarring on the back and flanks of sperm whales are evidence of attack by small sharks such as the cookie-cutter shark Isistius (Best, 2007). Other extensive scarring often found on the head and body may represent interactions with giant squid or other sperm whales.
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