Hormones of humpback whales indicate mating patterns, stressors at sea

SINGAPORE – Scientists who wanted to find out more about humpback whales’ reproductive patterns chanced upon another discovery – the marine mammals’ hormone levels not only indicate breeding periods, but they also point to stressors faced at sea.

“It’s been 40 years of study, and we still don’t fully understand the humpback whale mating system... largely because mating has never been observed,” said Professor Adam Pack from the University of Hawaii at Hilo.

“In the early years, we didn’t know much about how reproductive hormones varied in the whales. We didn’t know how male testosterone concentration varied... which females were in (heat), which females were ovulating or pregnant,” Prof Pack said at a seminar at the NUS Tropical Marine Science Institute in late April.

The renowned whale and dolphin expert had been invited to deliver a lecture on his research into humpback whales.

The unknowns prompted Prof Pack and his colleagues to launch a 20-year study from 2004 to collect more than 1,000 biopsy samples and find out more about how their hormones varied seasonally and what this reveals about their health.

To obtain the samples, Prof Pack used a crossbow to fire an arrow with a stainless steel tip at a whale’s flank. The tips were designed to extract a few centimetres of blubber – the thick, fatty layer under its skin – before the dart bounced off the whale and floated in the water for the researchers to retrieve.

The samples were then shipped frozen to the endocrinology lab of the University of Alaska, Fairbanks, where concentrations of selected sex and stress hormones were determined. 

Data collected between 2004 and 2006 showed that February and March – the middle of the breeding season for the North Pacific Humpback whales – are the peak periods for progesterone levels, which support pregnancy.

The males’ sperm production also peaks in those months.

These support decades of observations that found males becoming more aggressive with other males in winning over a female during the mid-breeding period.

The levels of a chief wooer’s corticosterone, or stress hormone, tend to be higher than in other males, indicating a lot of stress associated with defending his dominance and fighting off his competitors.

Crucially, the samples allowed the scientists to build a baseline of the mammals’ hormone levels, including that of corticosterone.

The migratory giants – as apex predators feeding on krill and small fish – provide clues about ocean health as changes caused by marine heatwaves, pollution and industrial activity can affect their reproductive health and stress levels.

“By describing the baseline levels of hormones, we have a foundation to compare these concentrations during natural and man-made events to determine whether these events lead to disturbances in the whales’ reproductive system and stress,” said Prof Pack.

Australian wildlife scientist Vanessa Pirotta told The Straits Times: “Humpback whales are like the canary in the coal mine. Any changes we see in (their) populations may be indicative of changes in the marine environment.

“With a warming ocean, we may see changes to where these whales choose to calve and nurse. Will these traditional breeding grounds become too hot for these whales and see a shift in where they travel to in the future?”

The humpback whale is a conservation success story, but climate change and environmental degradation could reverse that.

Decades of commercial hunting had wiped out about 90 per cent of the population before 1966, when the International Whaling Commission protected the North Pacific whales.

By 2006, the population of whales grew to over 21,000, up from 1,500 before they were protected.

Their numbers continued to grow to over 30,000 by 2013, before a severe marine heatwave depleted food resources, leading to a loss of nearly 7,000 whales.

Mummy whales can handle more stress

Mothers with calves also experience high amounts of stress as they fast in the breeding grounds.

When the mums are nursing, their fat reserves deplete rapidly as they lose more than 95kg a day. Over a gruelling 45 days, for instance, they can lose around 4,600kg.

The pressure increases when mum and calves are pursued by a group of males.

However, they appear to have adapted to handle this type of stress as their cortisol levels are significantly lower than injured or ill females, Prof Pack pointed out.

Recently, Prof Pack and his team came across two unwell females off Maui. One was covered in whale lice while the other had been entangled in fishing gear earlier and was emaciated, with tiger sharks trailing behind her, ready for the kill. 

Their biopsy samples showed that their cortisol levels were 143 times higher than mums with calves.

Prof Pack’s ongoing research is seeking to better understand humpback males’ song, one of the most complex communication displays in the animal kingdom. Interestingly, male singers tend to attract other eavesdropping humpback whale males.

He said: “Findings can help us further understand nuances of the humpback whale mating system as well as how whales socially network in their breeding grounds.”