THEY call it boxing, but lobster brawls have more in common with a
well-choreographed wrestling match. The opponents approach head-on and begin whipping
each other with their antennae to size up the competition. Then things really
get physical. Push leads to shove, and soon the lobsters are locked claw to
claw. Sometimes this ritualistic wrestling progresses to all-out fighting. More
often one individual submits before any damage is done鈥攂ut not before both
have squirted large quantities of urine into their opponent鈥檚 face.
鈥淭he more aggressive the lobster becomes, the more urine it produces,鈥� says
Jelle Atema of Boston University. In three decades of studying lobster
communication, he and his collaborators have come to realise that urine carries
crucial messages that shape the social interactions of these animals. And their
recent findings go one step further, suggesting that the urine of each lobster
contains a unique marker鈥攁n individual calling card by which they can
identify and remember each other.
The American lobster, Homarus americanus, lives a fairly solitary
life, sheltering in its own small cave or rock crevice. But when they do meet,
it becomes evident that they have a complex social system. There are hierarchies
among both males and females and, for males at least, social status can make the
difference between whether they get to reproduce or not.
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Boxing is the key to establishing the pecking order. Put a group of
unfamiliar animals together in a tank, and their first reaction is to fight. Two
by two, they go through the ritualistic boxing bouts. Thereafter the winners
assume a dominant position in the social group, while the losers become
submissive, backing away from any aggressive approaches from their superiors.
Somehow lobsters know their place, and so live in relative harmony.
But how is the status quo maintained? There are two possible explanations.
One could be that lobsters have some way of distinguishing dominant individuals
from subordinates. For instance, top lobsters might adopt a certain body
language, or subordinates might produce large amounts of stress pheromones
during social interactions to demonstrate their relative standing. The
alternative is that lobsters have a true pecking order, with each individual
able to recognise the other members of the group and to remember how they fared
in the previous encounter with that individual. But this is unusual among such
animals. 鈥淭he list of invertebrates proven capable of individual recognition is
a very short one,鈥� notes Christa Karavanich of Richland College in Dallas,
Texas.
To distinguish between these two possibilities, Atema and Karavanich
engineered a series of lobster boxing matches. They took pairs of healthy,
well-matched males who had never met before, placed them in a tank together and
videoed their interactions. All the lobsters were aggressive towards strangers.
Even so, most of the fights ended without physical injury after less than 10
minutes when one animal withdrew. 鈥淭he loser seems to determine the end of the
fight,鈥� says Atema.
Then, a day later, Atema and Karavanich arranged a second round of
matches鈥攕ome between previous opponents and some between winners and
losers that had not previously met. They found that the losers backed away from
opponents that had out-wrestled them in the first bout and avoided aggressive
displays, whereas former winners went in with claws and pincers flying. But,
crucially, when animals met an unfamiliar individual in the second round, even
those that had lost the first bout were just as aggressive in the second round
as they had been originally. They seemed unable to get the message that their
opponent was a dominant individual. 鈥淭he loser will not engage the previous
winner, but will fight unfamiliar individuals,鈥� says Atema. 鈥淲e see that as
evidence of individual recognition.鈥�
Atema suspected that urine was the key to a lobster鈥檚 expression of
individuality. Each animal has two bladders on the sides of its body and during
a fight it releases a large amount of urine into a powerful current that it
generates over its gills. This allows it to shoot the liquid at a target that is
up to seven body lengths away. Lobsters are known to use the controlled release
of urine to send messages to one another during courtship. So could the liquid
also carry information about their identity, so that just a little squirt would
allow animals to recognise their previous opponents?
Over the years Atema and his colleagues have come to understand how lobsters
decode urinary signals. In effect, the crustaceans have a very acute nose, with
receptor cells arranged inside the bristles of a toothbrush-shaped structure on
the antennules鈥攖he small antennae. They sniff their environment by
flicking these back and forth. The signals they pick up have an important
influence on their behaviour, as Diane Cowen showed a decade ago when she was a
student with Atema. She found that without antennules lobsters were unable to
form stable hierarchies and there was constant aggression.
And, sure enough, when Karavanich and Atema removed the antennules from some
of their lobsters between boxing matches, former losers pulled no punches in a
second fight against an opponent that had defeated them just 24 hours earlier.
Blocking urine release had the same effect, showing that the critical
information is contained in the urine.
To find out how long lobsters can remember one another, the researchers
waited a week before letting some of the initial opponents fight a rematch. This
time, three out of ten losers from the original round avoided the individual
that had beaten them. The figure was the same whether they had been kept in
isolation or gone back to their communal tanks. Some animals remember who
defeated them for at least a week, says Karavanich, 鈥渆ven if they had numerous
interactions with other lobsters in between the one-on-one bouts鈥�.
But after two weeks all memory of the original encounter seems to have gone,
and pairs that had previously met fought with the same ferocity they had shown
in their original match. What鈥檚 more, all the former losers ended up losing
again. Boxing matches between pairs of females and mixed-sex pairs follow the
same pattern as in males. All lobsters, it seems, are capable of individual
recognition.
The next step is to work out exactly what it is in urine that communicates a
lobster鈥檚 identity. Atema鈥檚 money is on a group of large proteins produced by a
cluster of glands that release their products into the urine as it is squirted
into the water. Proteins are used by other animals as pheromones. 鈥淲e know that
[in lobsters] large amounts of protein are released and we know that the urine
is involved in individual recognition,鈥� says Atema. 鈥淏ut the connection is still
颈苍蹿别谤别苍肠别.鈥�
He is now looking for a way to isolate these large proteins and test their
effects directly on lobster behaviour. Atema is confident that this will prove
to be the first example of an animal using proteins for individual recognition,
but he admits that it will require clever experimental designs to distinguish
between this and the many other messages in the urine. 鈥淚n addition to
individuality, lobsters communicate dominance status, sex and receptivity by
chemical signals in their urine,鈥� he says.