1. Evolution of Size and Sexual Size Dimorphism in Galapagos
Marine Iguanas and Flightless Cormorants
Olivia Chambliss
Galapagos Academic Institute of Arts and Sciences, University of San Francisco Quito.
Introduction
The evolution of body size in organisms is a
source of inquiry for many biologists. There
are a few theories to explain body size in
animals that have evolved on continents.
One such theory is Bergman’s rule which
purports that endotherms increase in body
size as latitude increases. However, studies
for this rule have been biased towards
rodents and carnivores (Wikelski M, 2005;
Dayan T et al., 2004.) Bergman’s rule also
fails to take into account exothermic
organisms. Cope’s rule is a little more
inclusive in reference to animal body size.
This rule states that animals generally
evolve to be larger in size. However, the
trend is not so clear when comparing
median body size and mean body size
(Solow AR & Wang SC, 2008.)
The question of body size is
especially intriguing for organisms that
have evolved on oceanic islands far away
from the mainland. The Galapagos
archipelago is made up of oceanic islands
that vary in age from 3 to 5 million years
old (however animals on the islands may
have split from their continental ancestors
as many as 15 million years ago.) Because
of the Galapagos’ unique environment, it
appears that body size is influenced by a
variety of factors specific to each species
and its location on the various islands.
Wikelski notes that body size is an
adaptation of life history for an individual.
He explores four main hypotheses for body
size in Galapagos Marine Iguanas (GMI): 1.
Size specific predation – this theory
suggests that animal size is due to
interactions between predators and their
prey. Animals grow to a certain size to
escape or obtain prey. 2. Habitat area –
animals are larger when they have a larger
habitat. 3. Average resource productivity –
the availability of food and space directly
affects body size. 4. Variability in
resources/productivity – how consistently
these resources are available also affects
body size. (Wikelski M, 2005.) While these
theories may have a hand in shaping body
size, they do not explain the mechanisms
that cause a difference in body size between
animals.
Galapagos Marine Iguana
Iguanas and many other reptiles
experience continuous growth throughout
their lifetime. Nevertheless there are
constraints to this growth (Wikelski M,
2005). For Galapagos Marine Iguanas
(GMI), maximum body size differs greatly
between islands on the archipelago. This
species occupies thirteen on the largest
islands on the archipelago. They have no
major predators or competition. The iguanas
feed on intertidal algae which is available to
them during low tide in a “scramble
competition” for food. GMIs have

2. specialized hindgut bacteria that digests
algae walls (Romero LM & Wikelski M,
2003.) Larger individuals fed more often
than their smaller counterparts and in some
cases were forced to dive for more algae.
Larger males were sometimes outcompeted
in the scramble for food. There was some
early speculation that the main reason for
body size of the Galapagos Marine Iguana
was due to a payoff between energy intake
and energy expenditure. Individuals must be
able to eat enough algae to maintain their
body size (Carrillo V et al. 1997). This
theory has been further investigated and
modified due largely to the research of
Martin Wikelski.
The four theories of animal body
size were investigated in the Galapagos
Marine Iguanas. The first hypothesis
(predation) can be thrown out because
GMIs suffer no major predation and have
no prey. In one case study on the island of
Santa Fe, a negligible amount of predation
on GMIs was found due to Galapagos
Hawks. However, the hawk only preyed on
small individuals such as hatchlings and
some females (Wikelski M, 2005.) The
second hypothesis suggests that larger
habitat area will result in larger individuals.
But the average size of Galapagos Marine
Iguanas has increased on all islands in the
past century (Wikelski M, 2005.)
Hypotheses three and four do a slightly
better job at explaining differences in body
size. The relationship between resource
productivity and body size was relevant
during El Niño events (Wikelski M, 2005.)
A new hypothesis was suggested
after the first four were found to be
insufficient for Galapagos Marine Iguanas.
Wikelski proposes that body size in GMIs is
due to an interaction between natural
selection and sexual selection (Trillmich F
& Wikelski M, 1997; Wikelski M, 2005.) A
lek mating system is utilized by Galapagos
Marine Iguanas. Leks are gatherings of
males the defend territories that do not
contain resources (Partecke et al., 2002.)
Large males are usually able to obtain
territory. Males with territory bob their
heads to warn away intruders. When male-
male encounters turn into fights, the larger
male usually wins. Large males in turn have
more mates than smaller males. Natural
events, such as El Niño, confound sexual
selection for large males (Wikelski M,
2005.) El Niño oscillations devastated the
GMIs main food supply, algae.
Environmental temperatures increased,
killing most of the iguana’s main food
source. Evidence shows that the larger
Figure 1 Open symbols for females; circles represent
Santa Fe males; squares for Genovesa males.
males were affected the most by the food
shortage. In a 1991 El Niño event,
individuals with a snout to vent length of
less than 250 mm died on Genovesa while
90% of similar sized individuals survived
on Santa Fe. On Santa Fe these individuals
would be considered medium sized.

3. Moreover, it can be concluded that medium
sized individuals fare better than the
smallest and the largest individuals
(Wikelski M, 2005.)
It should also be noted that El Niño
oscillations are not the only natural
selective factors that affect body size in
Galapagos Marine Iguanas. Foraging
efficiency contributes greatly to body size
(Wikelski M, 2005.) Body size is also
restricted by thermal constraints. Larger
individuals require more food to maintain
body temperatures. The larger an iguana
grows, the more food it requires to sustain
its body size (Wikelski M, 2005.)
Sexual size dimorphism is an
important factor in determining body size.
In reptiles, males are usually larger than
females (Madsen T et al., 2002.) This is true
for the Galapagos Marine Iguana. In fact,
male GMIs are twice as heavy as their
female counterparts. This is due to the
aforementioned fact that female marine
iguanas prefer larger males. This preference
pushes male body size to almost extreme
proportions. Galapagos Marine Iguanas do
not seem to have a fixed optimal body size.
Thus female preference has a big effect on
male body size. Female size is a little more
difficult to explain. The proximate cause of
female size is the need to export resources
to egg production. It has been suggested
that females need only to grow to the
minimal size necessary to produce eggs.
There does not seem to be any sexual
selection for body size in females (Trillmich
F & Wikelski M, 1997.)
Galapagos Flightless Cormorant
The model for size and sexual size
dimorphism in marine iguanas is not
necessarily transferable to other Galapagos
species (such as the Flightless Cormorant.)
The Galapagos Flightless Cormorant (GFC)
is the largest bird in the cormorant family
(Valle C, 2014.) On average, males weigh
15% more than females (Tindle R, 1984.)
Sexual size dimorphism does not appear to
be a result of sexual selection for the GFC.
Males do not compete for females. Females
will often compete for males but this has
not resulted in female biased sexual size
dimorphism (Valle C, 2014.) The difference
in size may be due to intersexual
competition for food. It is unclear whether
differences in size produce different feeding
niches or if the differences in feeding niches
drive sexual size dimorphism. It has been
speculated that the difference in size
between sexes is due to ecological factors
acting more heavily on male GFCs. There is
a genetic correlation in body size for
Flightless Cormorants. This means that this
particular species grows bigger as a result of
genetics and not food availability. It is clear
that GFCs do not follow Rensch’s Rule for
sexually dimorphic species (Valle C, 2014.)
The rule states that across species within a
lineage, size dimorphism will increase with
increasing body size (male bias) and will
decrease when there is a female bias.
Differences in sexual size do not follow the
logarithmic patterns of Rensch’s rule. The
ultimate causes of sexual size dimorphism
in Galapagos Flightless Cormorants are still
unknown but it has been suggested that it
may be simply due to the species large size.

4. Conclusions
The evolution of size and sexual size
can be due to any number of factors
depending on species, location, and
environment. It may be difficult in future
research to find any consistent pattern, even
within a species. Furthermore, more
research is required for a definite answer.
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