So this post
follows on from the article which was posted on Wednesday, which concerned the
idea that islands cause the evolution of weird and wonderful animals, due to
their isolation and other contributing factors.
The animal which
we will be looking at in this post is the Marine Iguana or Amblyrhynchus cristatus, which can only be found on the Galapagos Islands.
This species is unique among modern lizards and one of only a handful of
reptiles which take to the sea to feed. Clearly being a reptile is problematic for
a marine existence as the cold sea water will slowly leaches an iguanas heat
from its body and being ectothermic an Iguana is unable to replace this lost
heat. The iguanas are able to overcome this however by spending a lot of time
bathing in the sun and thanks to their colouration they are capable of quickly
absorbing heat. Along with this iguanas are usually found along rocky areas of
the coastline, this allows them to gain maximum exposure to the sun whilst out
of the water, along with being able to absorb heat radiating from the rocks
they sit on. 
However when
an iguanas body temperature does drop it will like all reptiles become slower
and react sluggishly to stimuli such as the presence of a predator. Therefore iguanas
have evolved a behaviour to deal with this issue, when threatened a cold
individual will display high levels of aggression in an attempt to postpone potential
attacks and create an escape route. However even with these behavioural
responses Marine Iguanas are still in decline as introduced species such as cats,
rats and dogs have been seen to prey upon them and so as of 2004 Marine Iguanas
are rated as vulnerable on the IUCN red list. Also due to the fact a survey on
population size has not been carried out for 12 years and the known negative effect
of climatic phenomenom such as El Nino have on marine Iguanas means that
potentially there are currently fewer Marine Iguanas than is believed.
Events such
as El Nino have a negative effect on population numbers (up to 85% mortality
rates) due to a number of reasons. For instance the event in 1982-83 caused high
rainfall which flooded nests, increased sea levels which meant that a higher proportion
of their food source was found at a depth iguanas could not achieve and finally
increased surface temperatures which reduced the marine productivity of the
area and so reduced food sources yet further.
Marine
Iguanas feed primarily on 4-5 different red algae species along with a less
favourable but more common green algae species with the regional feeding zones
being dictated by the size of an individual. Smaller individuals tend to feed
in the intertidal ranges where food is easily available and diving is not
needed, while larger individuals feed further out. Dives are typically to depth
of 1.5-5m but large adults have been seen to go to depths of up to 3 times
this.
Throughout
their time spent feeding Iguanas are constantly absorb excess salt from the
water. To deal with this issue these iguanas have evolved special nasal glands
which allow them to excrete the excess salt from there system with a method
which looks like they are sneezing. This is extremely beneficial for iguanas as
if the excess salt had to be passed out through their urine they would require
vast quantise of water to allow them to perform such a process.
So overall
Marine Iguanas are a perfect example of how island ecology gives rise to
creates you would never see in mainland populations.
There is a
tonne of other resources out there on the web for anyone who wants to find out
some more on these amazing animals, but I’ve linked two good sites just to get
you started.
Thanks for
reading!
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