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Boating NZ : FREE TO READ March 2014
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They are among the heavy-weights of marine mammals and the males have trunk-like noses for roaring at mating time; it's easy to see how the southern elephant seal got its name. Beach bums CONVERSATIONS ON CONSERVATION BEN GLADWELL 72 O n land, the males of the species are up to 5000kg of lumbering blubber – that’s equivalent to two, 7m fibreglass trailer boats – but at sea, their body's special adaptations for delving into the depths make the southern elephant seal one of the most highly evolved mammals in the ocean. In the chaos of flippers and bodies on the beaches where elephant seals breed, it may seem difficult to discern individuals from the bunch but there are several distinctions in their appearance and behaviour within a group depending on their age, sex and status. Southern elephant seals are the most sexually dimorphic of all mammals, meaning that there are huge differences between the males and females. Males measure 4.2 to 5.8 metres in length; females generally grow to 2.6 to 3 metres. The average mature male weighs in at 3,175kg; an average female elephant seal weighs 771kg – although males often weigh up to six times more than their prospective sexual partners. The automotive equivalent would be an armour-plated Humvee mating with a Smartcar. Males are sexually mature at three to six years – if they make it that far. Most never breed, as 90% of males die before reaching sexual maturity. If they do survive through puberty, only the largest in a colony will have the opportunity to breed and then not until they are about 10 years old as they need to be bigger and more aggressive than other males in the group to win a harem. The rewards are considerable though – dominant males often mate with up to 100 females in a season. Females are sexually mature at two to four years old and may give birth annually for 12 years. The elephant seal is the biggest member of the Pinniped family, which includes seals and sealions, and the largest living member of the order Carnivora, outweighing even the massive polar bear five to one. When at sea, they spend more than 80 percent of their time below the surface and although they are not known for their speed and grace on land, they are able to swim at up to 25km/h (13.5 knots). Not surprisingly, given their size, they are less manoeuvrable than many seals in the water, but they are the best divers, even exceeding the capabilities of many species of dolphins and whales. Their dives generally exceed 20 minutes, with breathing stints of two to three minutes. Their dives are typically to between 400 and 1000 metres deep, and they have been known to dive to 2,133m. They can achieve such depths thanks to adaptations to the heart and arteries to regulate blood pressure and the flow to brain at high pressure, as well as other physiological adaptations that allow them to survive for up to two hours without surfacing. While exploring the depths, they can have a good look around, thanks to excellent vision at low light levels. Southern elephant seals have extraordinarily high tolerance to low levels of oxygen, known as hypoxia. They remain comfortable when the oxygen levels in their blood drop to as little as 5% of that seen at the surface – significantly lower than levels that would cause humans to black out. They have several techniques to reduce the rate at which oxygen is consumed, such as a reduction of their heart rate and the ability to shut off non-essential metabolic processes and tissues from the main circulatory system. These either switch over to locally stored oxygen supplies or, in the case of certain muscles and organs, operate anaerobically – that is, without oxygen. In these instances, the muscles produce large amounts of lactic acid that, because they are shut off from the rest of the body, are isolated in the muscles until they surface. Southern elephant seals can carry more than three times the amount of oxygen for every unit of body mass than humans, but they store just 4% of their oxygen in their lungs with the rest in their blood and – the southern elephant seal Photo:HughLansdown/Shutterstock.comPhoto:JanelleLugge/Shutterstock.com 72 Boating New Zealand March 2014 Feature_bens conservation_March14.indd 72 18/02/2014 2:06:48 p.m.
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