On a hoof, a wing, and a fin – National Geographic’s ‘Great Migrations’

From a very early age, we are taught to place disparate aspects of nature in discrete conceptual boxes. Children’s field guides and Facts on File-type cards tell us that particular animals live in specific places, eat particular foods, and otherwise give simple, easy-to-remember summations of the essence of nature. But the natural world – past and present –  is not so easily simplified. Herbivores, such as hippos, sometimes scavenge meat, and many dinosaurs – traditionally characterized as being “good reptiles” – were marked by a slew of characteristics that we once thought were unique to birds. Among extant organisms, however, there is scarcely a better test of our preconceptions about animals than their constant cycles of travel over the planet, and National Geographic now celebrates these phenomena with their new miniseries Great Migrations.

A great white shark. Image courtesy National Geographic.

The show’s section of the migration of the great white shark is a good example of how species don’t always do what we think they should. These massive predatory fish are well-known denizens of North America’s Pacific coast (among other places), but they do not spend the whole of their lives haunting the shorelines. As a globally distributed species, different populations have different migration routes, but the vignette presented by Great Migrations is of sharks which hunt elephant seals off Mexico’s island of Guadalupe but often travel over 3,000 miles to Hawaii (a trip which takes about seven months to complete). Just why the sharks undertake this long journey is unclear, but the fact that they do forces us to reconsider what we thought we knew about their natural history.

Numerous other examples of nature’s complexity could be picked from the ranks of migrating animals featured in the show. The red crabs of Christmas Island, the wildebeest of eastern Africa, North America’s monarch butterflies, and the little red flying foxes of Australia are among the many familiar creatures which are tracked as they move from place to place. Among the most impressive of the show’s examples, however, is contained entirely within Jellyfish Lake. This marine lagoon, located on Palau’s Eil Malk Island, is home to scores upon scores of golden jellyfish – a subspecies of the more common spotted jellyfish which has evolved in isolation within the lagoon’s confines. Inside them live tiny zooxanthellae which internally nourish the jellyfish, but to keep their symbionts healthy the jellyfish must track the movement of the sun at the surface of the lagoon. This would seem like a pretty safe lifestyle, cycling according to the movements of the sun each day, but it is not so simple as that. As they float along some jellyfish end up getting caught by white sea anemones – another tentacled cnidarian anchored onto a surface rather than floating free – and meet their demise.

Golden jellyfish. Image courtesy National Geographic.

Narrated by Alec Baldwin and organized into four main parts – “Born to Move”, “Need to Breed”, “Feast or Famine”, “Race to Survive”  (along with supplemental, behind-the-scenes features), the programs feature a variety of migratory events of differing scales. Given that this is a prime-time, cable flagship event, the show is more about narrative than hard science – viewers wanting to know the details of how birds navigate and what creates the “super generation” of monarch butterflies which fly almost the length of North America will be slightly disappointed – but the show is to be praised for collecting a wide array of transitory tales spanning army ants to elephants. Each program provides an extraordinary look into the lives of various creatures, from the familiar feast of the Nile crocodiles during the annual wildebeest migration to the search of Botswana’s zebras to find salt in the baking Kalahari desert.

A male pronghorn, photographed on Antelope Island, Utah.

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Bee hives, with their regularly arranged honeycombs and permanently busy workers may seem like the picture of order. But look closer, and hives are often abuzz with secret codes, eavesdropping spies and deadly alliances.

Bees release alarm pheromones that draw small hive beetles towards the hive.African honeybees are victimised by the parasitic small hive beetle. The beetles move through beehives eating combs, stealing honey and generally making a mess. But at worst, they are a minor pest, for the bees have a way of dealing with them.

They imprison the intruders in the bowels of the hive and carefully remove any eggs they find. In turn, the beetle sometimes fools the bees by acting like one of their own grubs, and gets a free meal instead of imprisonment. In Africa, both species have found themselves in an evolutionary stalemate.

But in 1998, American beekeepers spotted the beetle in hives of their local European-descended honeybees. These insects are gentler versions of their aggressive African relatives, and in them, the beetles found more vulnerable victims.

In the last decade, they have spread through hives on the East Coast, causing much more destruction than they normally get away with. In some cases, the damage is so severe that the bees are forced to abandon their hive. As the bees suffer, so do the economically vital crops they pollinate.

Now, scientists from the International Centre of Insect Physiology and Ecology and the University of Florida have uncovered the secrets behind the beetle's destructive ability.

Small hive beetles  hunt down beehives by hijacking their communications. When bees are stressed or confronted by threats, they release alarm pheromones into the air to alert their hive-mates of impending danger. But the beetles can also detect these chemical signals and use the bees' own early warning system to locate their hives.

Baldwyn Tonto and colleagues found that the beetles are sensitive to much lower levels of these pheromones than the bees themselves are, and can detect a much wider ranger of airborne chemicals from the hive. With their superior senses, the beetles can home in on beehives before the bees themselves can sense the alarm.

But that's not the whole story. Tonto found that honeycombs infested by beetles, but free of worker bees, were emitting a strange smell. It mimicked the bees' alarm pheromones and strongly attracted even more beetles. But it wasn't coming from the parasites themselves.

Instead, the source of the smell was a type of yeast that hitches a ride with small hive beetles into the bees' home. Tonto found that the fungus was fermenting the pollen collected by the bees, and releasing chemicals that closely mimic the beetle-attracting alarm pheromones.

Domesticated European honeybees have been bred into helplessness against the small hive beetle.The beetles' keen sensitivity to the bees' chemical messages allows them to initially home in on a hive. As they arrive, they bring the yeast along for the ride and distribute it among the hive's pollen stores. The yeast ferments the pollen and releases chemicals that mimics the bee's alarm pheromone, attracting even more beetles .

Soon, the infection reaches critical mass, and the bees are forced to abandon their homes. They leave behind a sizeable store of pollen and honey, ideal breeding grounds for the unwitting partnership of yeast and beetle.

But the yeast also exists in Africa, where it is similarly spread to hives by hive beetles. Why does the alliance not wreak such havoc there? Tonto believes that domestication is the answer. Because of years of selective breeding, the European honeybee is a slightly dopier version of the African bee - more docile and less prone to swarming.

It faces a larger number of pests and problems that prevent it from concentrating on imprisoning invading beetles. And its poor sensitivity to its own alarm chemicals allows the beetle-yeast alliance to gain a strong foothold before the bees recognise the threat.

With bee populations mysteriously dying off across America, the threat of the small hive beetle and its fungal partner may be even more pressing than before.

Reference: Torto, Boucias, Arbogast, Tumlinson & Teal. 2007. Multitrophic interaction facilitates parasite-host relationship between an invasive beetle and the honey bee. PNAS 104: 8374-8378.

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If I have one criticism of the show, however, it is that I was hoping for the influence of humans on these migrations to be made more starkly apparent. Nature documentaries are often filmed in such a way that humanity does not enter the shot, as if there are still places of pristine wilderness not bordered by cities, cattle ranches, or shipping lanes. David Wilcove highlighted how our species is altering the migration patterns of other species in his excellent book No Way Home, and while the show often mentions human influence it is only rarely that the effects our activities are having on migrations is truly made tangible. Nevertheless, the episode “Race to Survive” contains several stories which meet my expectations, including an excellent look at the plight of American’s pronghorn. The last remnants of a once more prolific group of artiodactyls, these migratory mammals are being squeezed by human development along their migration routes, having to duck through backyards and dodge eighteen wheelers as they hoof it across Wyoming. I would have liked to see the migrations of other animals in the show documented in a similar way, but during a time when conservation messages can be a tough sell for documentary producers I think National Geographic should be applauded for taking such steps to put animal migrations in the context of the world we are quickly altering.

There is still much we do not know about animal migrations. We can track organisms and document where they go, but the reasons why and how they do it are still incompletely understood. And, with every passing day, we are creating barriers to these migrations, forcing animals to find new paths or risk crossing paths with our species at their peril. Great Migrations documents some of the most wonderful events in the natural world, but how much longer these ancient migration patterns are followed is something that our species will increasingly influence as our relationship with global ecology continues to change.

Great Migrations starts this Sunday at 8PM on the National Geographic Channel. Also see this month’s issue of National Geographic magazine for a tie-in article by David Quammen.

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