Christmas
Island makes many a traveller’s bucket list due to the annual spectacular
migration of our showy, shiny red crabs. As kids growing up and learning about
the world from David Attenborough’s documentaries ( before google) many people
attest that the segment they most often can recall is a small island completely
overrun by red crabs - blocking roads, entering houses and filling gardens
whilst on their annual migration to the sea. Little do people realise that
Christmas actually has nearly 2 dozen land crabs, some very easily seen, others
a little more elusive, but the single species that actually makes the greatest
impression on our guests – the giant Robber crabs.
Known
as coconut crabs at other locales around the Pacific and Indian oceans, their Latin
name, Birgus latro, with latro meaning robber, thief, highwayman or brigand, is
the perfect descriptive for these jungle thugs. Tasty or shiny seems to be
their only prerequisites when it comes to identifying suitable bounty. Christmas
Island has little to no crime so when things go missing from your yard, a tool
carelessly left out, shoes missing from your back porch, most locals don’t
instinctively accuse wayward youth and call the police to report their loss.
They start a search, in the nearby vicinity, in hopes that if the thief has worked
out their cache isn’t edible, they will leave it not far from the scene of the
crime. Anyone who has been
out to Dolly beach, with their carefully prepared picnic, quickly finds
themselves surrounded and held at 'claw point' until they surrender their
bounty. And if you don’t acquiesce to their request, they will simply help
themselves. Many a visitor or camper has had to retrieve their gear from the
forest, where the crabs may have scampered away to assess their haul. Our last
trip involved retrieving a plate, a hat, and a thong from the jungle after our
tent had been raided during the night (that was after they had harassed my
partner all night in the tent, and resulted in us standing in the ocean to eat
our breakfast as a number of thugs were still in the vicinity come morning).
A
legendary story exists about an army rifle going missing during a training
session in the forest. Whether true or not, it highlights their indiscriminate thieving
behaviour. One of my favourite stories is from our first Bird & Nature week
event in 2006. Janos, our intrepid Abbott’s booby bird researcher and guide,
had given his small group explicit instructions about etiquette around this
rare and magnificent seabird. He subsequently laid out the gear he needed to
tape a satellite tracker to the bird and then abseiled up to the canopy to
retrieve his subject. Bird secured, he heads back to the forest floor, only to
discover that one of his important pieces of equipment is missing, a large roll
of silver tape. Bird now flapping and unhappy, he has to rifle through his pack
to find another roll of tape, all whilst quietly berating the guests for not
following his instructions about staying still so as not to distress the bird,
and not touching anything. The guests all look at each other, slightly
perplexed, as they know they hadn’t moved and no one had any reason to touch
the equipment. They quietly watched the procedure of attaching the satellite
tracker and Janos returns the bird to its nest high up in the canopy. As the
guests are departing the nesting site, one spots a robber crab, hiding in a large
log, proudly clutching its haul - a large roll of silver tape!
The
island boasts two dozen species of different land crabs, many quite happily
going about their business of scavenging, cleaning the forest floor and occasionally
hunting each other for an easy feed. The robber crab, put simply, is the top of
the food chain on Christmas Island. Like all of our land crabs, the robber
crabs still use the ocean as part of their breeding cycle, the babies emerging
quietly from the sea and using a shell for their first few months on land. Professor
Brian Cox visited the island in 2012, to film a segment to include in his
television series, Wonders of Life. When questioned why he was including robber
crabs, he explained that robber crabs are one of the few species on the planet
that experiences many of the planets physical forces throughout its life span –
‘How many animals born in the sea can also climb a tree?’.
Robber
crabs do have the scientists a bit baffled, as they are known to have evolved
at the same time as many of the crustaceans on the planet, but no one is sure why
they developed their sense of smell to such a high degree. Perhaps a leftover super strength olfactory
sense essential to life in the sea.
For
want of a better description, the robber crab looks like an overgrown hermit crab
but they have evolved to no longer require a shell. They have the remnants of a
tail that tucks back underneath them, and a solid protective exterior. They
have soft, leathery looking skin between their joints for flexibility and their
robotical movement, eyes on stalks and wandering scent antennae make them look
like an alien out of Dr Who. The last thing you would expect to see in a
tropical jungle landscape.
Whilst
the staff at the National Park do their utmost to protect the robber crabs, signage
and educational campaigns to try and cut down the road mortality, they do pose
their own challenges when it comes to park management. Christmas Island has a
number of introduced species – cats, rats and insects, that in their own ways have
impacted on our unique environment. Pest control programs are regularly carried
out, each one designed to target a specific invasive species, employing a
number of techniques to deliver poison or trapping and each one in turn needs
to be modified to cater to the amazing abilities of our robber crabs. Plastic
dispensers to supply ant bait in yellow crazy ant super colonies were subjected
to treatment akin to a gumball machine being violated by some rogue youth. Hanging
the dispensers in trees just added another level of difficulty - but the
outcome was inevitably the same. Toxic sausages to control the feral cats had
to be suspended on fishing line from a specially designed gantry that thwarted
the robber crabs ability to climb, and strung at a height that a cat could
still eat the sausage. Cat traps, set up with meat to attract and trap the cat
were inevitably found the next day with an angry trapped robber crab instead of
the intended species. Old rubbish bins, tipped on their sides, with the cat
trap on top stop the robber crab from being able to climb up and access the food,
whilst still being at a comfortable height for a cat to jump up and hopefully be
trapped. Swedish goshawk traps, deployed to capture and study our endemic
raptor, laced with fresh meat to attract a goshawk, photographed in the field -
naturally covered in robber crabs.
So,
whilst your yearning for a ‘National Geographic’ experience on Christmas Island
with our famous red crabs drives your desire to visit this rocky outcrop – it will
no doubt be the Robber crabs that steal the show – and your heart (and maybe
your flip flops).
Written by Lisa Preston
©
To book your Robber crab experience on Christmas Island visit my website at www.indianoceanexperiences.com.au
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