When food is scarce, most animals go into survival mode, reducing their activity to conserve energy. But not northern elephant seals (Mirounga angustirostris). Leaving their ocean feeding grounds and hauling themselves onto Californian and Mexican beaches for weeks, the females give birth, nursing their pups, while the males posture as they protect their harems; all of which they do while fasting. Jane Khudyakov from the University of the Pacific, USA, explains that during this time the animals depend on their extensive fat reserves for survival. However, it wasn't clear how the animals coordinate their tissue energy use while going without. ‘The cellular mechanisms that regulate these adaptations are still not fully understood’, she says. Teaming up with collaborators Dan Crocker (Sonoma State University, USA) and Dan Costa [University of California, Santa Cruz (UCSC), USA], Khudyakov and Rachel Holser (UCSC) headed down to the elephant seals’ breeding grounds at Año Nuevo State Reserve, USA, when the animals returned to moult, to find out.
‘They shed old skin and hair within several weeks’, says Khudyakov, adding, ‘it literally peels off in sheets, which makes the beach very smelly’. However, the moulting females are easier to approach than when fasting in the breeding season while protecting their pups. Surrounded by the colony, Holser with colleagues from Costa's lab gently anaesthetised five animals that had just returned from 3–4 month foraging trips, ready for Khudyakov to take small blood, muscle and fat samples at the beginning and end of the moult. In addition, the team weighed the females, recording that the animals lost ∼3 kg per day – totalling ∼28% of their body mass over the ∼33 day period – in the form of skin, hair and fat.
Back in the lab, Khudyakov and University of the Pacific undergraduates Serena Ly and Theron Niel extracted proteins from the muscle and blubber before Craig Vierra and Basma Hasan (University of the Pacific) joined them to analyse the protein content, alongside that of the blood sample. ‘We identified 1053 proteins in blubber, 711 in muscle and 260 in blood, most of which have not been studied before in seals’, says Khudyakov. Yet, when the team compared the composition of the muscle and blubber over the month-long fast, they found they had changed little. ‘This suggests that protein turnover in these tissues may be tightly controlled’, says Khudyakov. However, the quantities of three key proteins involved in fat accumulation reduced in all three tissues as the animals switched from storing to consuming fat.
In contrast, 53 blood proteins involved in controlling and transporting lipids around the body increased dramatically, including hormones that regulate the release and transport of lipids and those that control how the body uses lipids as a fuel. ‘We think that changes in these proteins may help seals quickly switch their metabolism between feeding and fasting without any of the adverse effects that such rapid fat gain and loss would cause in humans’, says Khudyakov. And when the team checked on the quantities of fat circulating in the animals’ blood, they found that triglycerides – a form of fat – and high-density lipoproteins – ‘good’ cholesterol – did not change significantly, ‘suggesting that seals very tightly regulate their fat reserves, despite using them for energy, during fasting’, says Khudyakov.
So, fasting elephant seals are able to sustain an active lifestyle by mobilising fats from their blubber in a controlled way. And Khudyakov is keen to find out how the proteins that regulate fat use change when the females produce full-fat (60%) milk for their pups, although that will require running the gauntlet of the fiercely protective mothers’ teeth.