Sunday 26 June 2022

I Would Swim Five Thousand Miles...

Breeding can be an energetically costly business, whether that's the effort put into finding and attracting a mate, or that required to raise young. The latter is a particularly important factor in mammals, which can't simply lay eggs somewhere where there's plenty of food and hope that the hatchlings do well for themselves as, say, an insect might be able to. Therefore, we might well expect that mammals will feed more during the breeding season, to compensate for all that extra energy they will be using. 

For males, there can be a downside, in that all the time you are spending finding and eating food is time not spend wooing and mating with females. Thus, in animals such as deer, we may find that males actually eat less during the mating season than they do at other times because their mind is far too much on other things. But females, given the needs of both pregnancy and lactation, ought to be different.

In ideal world, they probably would be and, indeed, this is the standard pattern. But, where food supply is uncertain or the potential risk of obtaining it is unknown, an alternative approach may have its advantages. This alternative is called capital breeding and it basically involves the female eating as much as possible in the run-up to the breeding season - that is, "banking capital" ahead of time - and then only breeding if that's worked. That way, if something unexpected happens while you're still pregnant, or nursing, you're okay, because you've already banked what you need.

This is not common, because it's by no means the easiest or safest way of doing things, but there are some species that do it. It requires careful balancing of energy budgets so that the mother doesn't starve herself to death after mating, but for some, the benefits apparently outweigh the risks. One particularly strong example is the humpback whale (Megaptera novaeangliae). 

For this species, as for many capital breeders, the process is achieved through long-distance migration. The mother feeds in cold waters where krill are plentiful, then heads to warmer ones to give birth, sacrificing her own feeding to give her calf a better start in life. Only when the calf is old enough to withstand colder weather does she head back to the feeding grounds. In the interim, she's essentially not feeding at all, although, as I discussed a few years back, there may be exceptions.

The result is quite probably the longest annual migration undertaken by any mammal (although there are some birds that travel even further). Humpback whales live across all the world's oceans, being absent only from partially enclosed shallow-water seas such as the Meditteranean and the Baltic and from areas where sheet ice prevents them from surfacing to breathe. This means that some have feeding grounds in the Southern Hemisphere, and head north to give birth, while others feed in the far north, and then head south. The exact feeding and breeding grounds used also vary, although individuals seem to stick to the same pairing from year to year.

The longest of these many migration routes is the one between the Barents Sea and the Caribbean, a distance of about 9,000 km (5,600 miles) narrowly beating the Antarctica to Costa Rica route, which holds the Southern Hemisphere record. Until recently, however, it has at least been possible to argue that this isn't really a single migration route. Certainly, individual whales have been spotted at either end of the route, but not necessarily in the same year, suggesting that perhaps they stop over at other suitable feeding grounds closer to the Caribbean on the way - Newfoundland would be an obvious possibility here. 

I say 'until recently' because on the 8th January 2019, a group of researchers fitted a satellite telemetry tag to a female humpback whale feeding off Kv├Žnangen Fjord in Norway. The tag in question is about 30 cm in length (which isn't much compared with the size of the whale) and is launched towards the target by what, so far as I can tell, is a sort of modified air rifle; it connects to the Argos satellite system, widely used for this kind of monitoring. It continued transmitting every few minutes for the next eleven months, allowing them to follow the exact position of the whale throughout its entire annual migration. And, last month, they published the findings.

The first thing to say is that the whale followed the predicted path, performing the full migration that we would expect. She left Norway in February, heading southwest to Iceland, where she stayed off the south coast for a couple of weeks before beginning the much more arduous trip across the Atlantic on the 1st of March. She reached the West Indies in mid-April, spending a month in warm breeding grounds, devoid of the food that humpback whales normally eat, and swimming off the coasts of the Dominican Republic and Puerto Rico.  On the 17th of May, she began the journey back, taking a longer and more easterly route, but still reaching Iceland in late July where, this time, she did not stop, but headed on directly to what were likely her habitual feeding grounds in the Barents Sea between Norway and Svalbard.

On the 2nd November, she turned south towards the fjord where she had first been tagged, approaching close enough for the researchers to take a photograph of her. This revealed that she was now travelling with a calf. Based on its apparent age, she must have given birth somewhere in the mid-Atlantic on the southward leg of her journey. Examination of her detailed progress from the satellite feed showed that she had paused in her journey for a couple of days starting on the 25th of March, which would fit perfectly. 

What can we tell from this? It's interesting to note that she didn't take the most direct route possible between her feeding and breeding grounds, which would have involved heading down the coast of Greenland and taking a much more westerly route from then on. That may have been partly to give her those two extra weeks of feeding off the coast of Iceland - a little out of her way, but possibly well worth it. 

Even then, the fact that she didn't start the migration until February means that she left the Barents Sea a few weeks after most of her fellows, giving the impression that she "knew" she was pregnant and wanted to get as much feeding time in as possible before making the trip. Taking into account the Barents Sea, the coast of Norway, and the coast of Iceland, this is also the first time that a humpback whale has been definitively observed utilising three different feeding grounds in the same year. This may be less rare than we think.

The researchers estimate that the round trip of 170 days (counting the 14 feeding off Iceland) would have used up 142,000 MJ of energy (34 million nutritional calories). There must be a balancing act here; the fact that she left her feeding grounds late gave her more time to eat, but meant that she had to swim faster to reach the breeding grounds in time. She started off moving at an average speed of 6.5 km/h (4 mph) although she did slow down after that mid-Atlantic stop where she's thought to have given birth. Even then, her speed was faster than expected, but the researchers note that if she had travelled at the normal slow pace nursing mothers are thought to use, she would have lost at least three weeks of time in the West Indies, in the warm environment that her calf would need to prosper and become healthy enough for the journey back.

Another interesting finding from this study is that, from we previously knew, humpback whales tend to give birth in shallow, coastal waters. But in this case, the birth must have taken place far from land, and very probably in the middle of the ocean, almost as far from shallow waters as the mother could have managed. This isn't totally unknown, but it is unusual. At the very least, it indicates that shallow waters aren't essential for young calves (although ones that aren't especially cold might be) and that, on this exceptionally long migration, the extra time taken to feed off Iceland might have been worth whatever risk there is in giving birth mid-Ocean.

Sometimes a mother's gotta do what a mother's gotta do.

[Photo by D Gordon E Robinson, from Wikimedia Commons.]

3 comments:

  1. How come the species name for such a cosmopolitan species is "of New England"?

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    1. Well, quite. My guess would be that it's one of the feeding grounds closest to human habitation, but who knows?. Neither of the people responsible for naming it were American, so you'd think they'd have noticed it closer to home, but if they did, they must have thought it was one of the species that had already been described (a blue whale, perhaps, or a fin whale).

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