Sexual selection often actually works against natural
selection. In situations of mainly female parental care, the reproductive
success of the male is almost completely in the hands of the female. So even if
a male is well camouflaged and can survive for a long time, if he doesn’t
impress the female, he wont have any babies, and his genes will cease to be
passed forward. In this way female choice selects for traits that sometimes
actually increase the survival risks of the males. For example the agama lizard
of east Africa have lost any camouflage capability when compared with their
female counterparts.
Here in Lake Malawi many cichlids have evolved a unique form
of attraction. Over 200 cichlid species here build special structures to
attract their females. These structures are called bowers, named after the structures
built by the famous bowerbirds of Papua New Guinea. Like the bowerbirds these
structures are built by male cichlids solely for the purpose of showing the
girls what they can do. In essence, the
size and structure of the bower is a direct indicator of the fitness of the
male. This is because, after all, if you are able to spend time digging and
pushing around sand rather than finding food, you must be a strong individual
with good genes to pass on.
There are two general bower types; either castles or pits
and they are often built in large groups or congregations of many males called
leks. Here, on Kande island there are species that build both types of bowers,
but the most impressive, in my opinion, are those belonging to the species Dapidiochromis kawinge. The males of
these species dig large pits often with diameters and depths greater than one
meter. They do this by pushing the sand out of the way with their snouts. Every
week here we do a survey of a lek near the island, and we try to track when the
bowers are usually
inhabited.
After the bowers are constructed, the males will remain in
or above the structure and display for any passing female. If the courtship is
successful, the female will lay her eggs and the male will fertilize them
within the bower. The female will then scoop up the eggs with her mouth and
carry them away to care for them herself.
Female cichlids do not have to build any nests or
structures, as many of them have evolved an amazing ability to brood their babies
in their mouths!!! This behavior allows the mother to be mobile all the while
providing round the clock parental care.
Here at the island we have found a couple mouth brooding resident
mothers of the species Tyrannochromis
nigriventer who we love to go watch whenever we have a chance. Cichlids are
some of the most caring mothers of the animal kingdom, and this puts immense
evolutionary pressure on the males to be more brightly coloured, or have better
mating displays, or build better bowers. Cichlids can mouth brood their
offspring for upto a month! Once the babies grow up, this sexually selective
cycle repeats itself. As new tastes are developed or enhanced over generations
whole new species with different preferences arise, continuing the ancient
process that has led to the species and behaviours that we see today.
