Download Imps of Darkness

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Karina Liker
Darwin, Evolution, and Galapagos
Professor Bill Durham
October 15, 2012
The Amazingly Elastic “Imps of Darkness:”
How and Why Galapagos Marine Iguanas Decrease in Length
The Galapagos marine iguana is one of the archipelago’s most notable endemic fauna for
its many unique adaptations to living in an arid, intertidal environment. They are one of the few
animals that have mastered the thermodynamic balance required to be successful diving foragers
in the cool Galapagos waters, and they survive on the algae that their blunt noses adeptly scrape
from the rocks along the shore and in the water (Romero and Wikelski: 2009).
The marine iguana’s blunt nose is an adaptation for ripping algae off intertidal rocks. This
marine iguana is facing the sun in a unique thermoregulation behavior that helps the iguanas
maintain body temperature, but not overheat, after swimming in the cool Galapagos waters
(Romero and Wikelski: 2009).
However, not even Darwin could have guessed at the ingenious ways in which these creatures
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have evolved to cope with the major climate patterns in Galapagos. During El Niño years of
food scarcity, marine iguanas have actually evolved to decrease in length; they later are able to
regrow during years when more algae is available (Wikelski and Thom: 2000). Most likely, the
evolution of such a trait can be attributed to the equilibrium between two competing forces: the
selection pressure of food scarcity and sexual selection. To test why shrinkage and regrowth
would be evolutionarily favored in marine iguanas, we can examine three hypotheses concerning
the survival of iguanas of different sizes, the success of iguanas in turning to food sources other
than algae, and the advantage of size in reproduction.
The Basics of Shrinkage
Marine iguanas are the only adult vertebrate known to shrink and regrow in response to
environmental stressors, specifically the El Niño weather pattern (Wikelski and Thom: 2000). El
Niño, or El Niño-Southern Oscillation Event (ENSO), occurs every three to seven years and
brings with it increased rainfall and sea surface temperatures (Wikelski and Thom: 2000).
Unfortunately for marine iguanas, the increased ocean temperatures mean that the eastwardflowing Cromwell Current that normally supplies the islands with cold, nutrient-rich water
weakens, and the amount of algae available for the iguanas to consume decreases dramatically
(Wikelski, Carrillo, and Trillmich: 1997). As a result, marine iguanas are stressed to find enough
food to retain their size and survive the difficult conditions. For instance, Romero and Wikelski
observed muscle wastage in the iguanas and later mass population die-offs in the wake of the
1997 El Niño (Romero and Wikelski: 2009).
However, the researchers also recorded a strange phenomenon occurring during this same
period. During the 1997-1998 El Niño event, Wikelski and Thom recorded that the marine
iguanas on Santa Fe and Genovesa shrank by up to twenty percent of their body length over the
course of the year (2000). Furthermore, the scientists found that “body length increased again
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during subsequent La Niña conditions, when algal food was abundant” (Wikelski and Thom:
2000). Researchers were understandably surprised because, not only was this the first recorded
case of shrinkage in body length in an adult vertebrate in response to its environment, but the
entirety of the shrinkage in the iguanas could not be attributed solely to reduction in cartilage
mass. In a marine iguana, cartilage and other tissues make up only about ten percent of its body
length, yet some of the iguanas had shrunk in length by twenty percent (Wikelski and Thom:
2000).
The mechanics of how marine iguanas actually achieve shrinkage have not been precisely
determined, but researchers currently attribute the remaining shrinkage to bone absorption
(Wikelski and Thom: 2000). Using an extrapolation from how bone loss occurs in humans, it is
possible to see how bone absorption might allow marine iguanas to decrease in length. In
humans, bones are constantly being remodeled in three- to four-month cycles (“Cellular
Mechanisms:” 2000). Cells called osteoclasts destroy old bone cells as they travel along the
bone, which leaves small indents where osteoblasts cells later build new bone (“Cellular
Mechanisms:” 2000). When the osteoclasts decompose the old bone cells faster than the
osteoblast cells can replace them with new ones, bone loss or bone absorption (also called
resorption) occurs (“Causes of Osteoporosis:” 2010). While there is an absence of extensive
research on this occurrence in marine iguanas, we can infer that bone absorption could take place
at the ends of vertebrae or bones, which would lead to the observed shrinkage in length.
Researchers also believe that this shrinkage may be linked to levels of stress hormones during
the El Niño periods (Romero and Wikelski: 2009, Romero and Wikelski: 2001).
Body Size and Shrinkage
While the specifics of how marine iguanas shrink remain unclear, why marine iguanas
may have evolved this unusual ability to shrink and regrow in length can be explained as the
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result of two competing natural forces. First, the selection pressure of food scarcity in El Niño
years would have selected for smaller marine iguanas who could subsist for longer periods of
time on less food. Sexual selection then would have selected for larger individuals with a higher
reproductive capacity, leading to the adaptive advantage of shrinkage and regrowth. This
hypothesis is strengthened when we consider the findings of research into three key aspects of
marine iguanas: the advantage of smaller size in the absence of abundant natural resources, the
inability of marine iguanas to turn to other food sources, and the benefits of larger size in leaving
numerous and strong progeny.
Data on size differences between marine iguana populations and on shrinkage during El
Niño years help determine whether smaller size is an advantage among marine iguanas. Marine
iguanas, among their many curious traits, vary in size considerably from island to island. For
example on Genovesa Island, the average length of marine iguanas in 1992 was 250mm from
snout to vent, while individuals on Santa Fe Island averaged 400mm in length (Wikelski,
Carrillo, and Trillmich: 1997). In response to why there is so much variation between
populations, Wikelski, et. al., found that “food abundance…explained differences in adult body
length and mass between islands as a result of energetic limitation” (1997). On Santa Fe, there is
a much higher level marine productivity due to the effects of the nutrient-rich waters of the cool
Humboldt Current from the south and the Cromwell Current from the east (Wikelski, et. al.:
1997). Genovesa is not as affected by these currents, which results in lower marine productivity
(Wikelski, et. al.: 1997). This limits the foraging efficiency of larger iguanas because “food
intake per bite per gram of body mass was about twice as high for small iguanas as for large
iguanas” (Wikelski, et. al.: 1997). As a result of lower marine productivity, it is more difficult
for larger iguanas to maintain their body size in conditions of limited food supply because they
are less efficient feeders when compared to their smaller counterparts. Thus, marine iguanas’
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length and size are directly related to the amount of algae available for them to consume and how
great their foraging efficiency is for their size. The graph below shows the difference in foraging
efficiency for the populations of iguanas from Santa Fe (the upper line) and Genovesa (Romero
and Wikelski: 2003). Foraging efficiency decreases as body length increases for both islands.
Reseachers have consequently determined that this foraging performance, which depends on how
much algae is available, is the main factor influencing the average length of marine iguanas
(Romero and Wikelski: 2003).
(Romero and Wikelski: 2003)
In addition to marine iguanas’ length being tethered to food availability, data on
shrinkage during the 1997-1998 El Niño event also shows that food scarcity favors shorter body
length. In data gathered during this particular El Niño, Wikelski and Trillmich found that the
longest male marine iguanas are the first individuals to die in El Niño seasons (1997). The
effects of algae scarcity during this season meant that the largest marine iguanas had even more
severely depressed foraging efficiencies compared to body size than shorter iguanas did, and thus
they were selected against in conditions of food scarcity. Larger marine iguanas also shrank by a
higher percent of their body length, again supporting the argument that larger animals are more
vulnerable in El Niño years because of their large body size and correlating decreased foraging
efficiency (Wikelski and Thom: 2000). Because all the iguanas in a population do not
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necessarily shrink, it is significant to note that the iguanas that did shrink were more likely to
survive El Niño than those that did not shrink (Wikelski and Thom: 2000). In the chart below, it
is also evident that the marine iguanas that were able to shrink more survived longer (Wikelski
and Thom: 2000).
(Wikelski and Thom: 2000)
In other words, higher rates of shrinkage correlated with higher survival rates. Thus,
shrinkage would be a favorable adaptation in marine iguanas that is selected for by
environmental conditions. Iguanas that possess the ability to shrink to account for the dearth of
algae during El Niño would be more likely to reproduce and pass on this trait in the aftermath of
this extreme weather pattern.
Repercussions of an Inflexible Diet
A second clue into why marine iguanas would evolve to shrink in response to El Niño
events is that marine iguanas are not able to successfully turn to new sources of food, such as
terrestrial plants, when algae is scarce (Wikelski and Wrege: 2000). When they do find new
sources of food, the results can be disastrous. For instance, on North Seymour Island, some
individuals in a population of marine iguanas began eating a low-growing beach plant called
Batis maritima (Wikelski and Wrege: 2000). However, these iguanas became so dependent on
Batis that when the Batis on the island was drowned in high tides, every individual that had been
consuming Batis perished instead of returning to eating algae (Wikelski and Wrege: 2000).
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Wikelski and Wrege hypothesize that this reliance on only Batis for sustenance is a result
of “extreme conservatism in local foraging traditions, which is characteristic of all marine iguana
populations” (Wikelski and Wrege: 2000). This oddity is probably better explained by
examining how marine iguanas digest algae. A marine iguana digests bacteria with the aid of
bacteria in their gut that are specialized in breaking down marine plants (Romero and Wikelski:
2009). It may be that when the marine iguanas began repeatedly eating Batis, the few bacteria
able to successfully process this terrestrial plant were able to thrive and out-compete the bacteria
that could not subsist on terrestrial vegetation. After some time, the bacteria colonies would
evolve and only be able to break down terrestrial plants, which could explain why the Batisdependent marine iguanas did not return to consuming algae in the absence of Batis.
Marine iguana eating algae off rocks in Puerto Ayora, Santa Cruz Island.
This inflexibility in diet may directly contribute to bone loss in these marine iguanas. In
humans, bone loss or bone shrinkage has been observed in astronauts and in people who are bedridden (Wikelski and Thom: 2000). Likewise, marine iguanas will “get very little exercise from
feeding during El Niños” because of the need to conserve energy when their food source is so
scarce (Wikelski and Thom: 2000). The similarities between these two cases suggests that
inactivity could be part of what contributes to the bone absorption causing the shrinkage in
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length in the iguanas. In addition, further research on marine iguanas could have implications
for treating osteoporosis and other bone loss disorders in humans, since bone regrowth in
humans is almost impossible (Wikelski and Thom: 2000). Bone absorption in marine iguanas
may then be a by-product of a decrease in exercise as a result of not having the ability to
effectively expand their dietary niche.
Sexual Selection for Larger Body Length
With these advantages that accompany a smaller body size, it seems incredible that
marine iguanas maintain large body sizes at all. However, the force of sexual selection is also at
work in influencing these creatures’ length. Although marine iguanas exhibit strong sexual
dimorphism with males that can be twice the size of females, both females and males are able to
shrink and regrow (Wikelski and Thom: 2000, Wikelski: 2005). Researchers have not been able
Sexual dimorphism in marine iguanas on Santa Cruz Island.
to directly test how body length specifically influences reproductive success; rather, they infer
from data on body size to support the theory that body length as well influences reproductive
success. In females, larger size can confer a significant reproductive advantage. As shown in
the graph below, a larger female will produce “larger clutches with larger eggs, which…often
produce larger hatchlings” (Romero and Wikelski: 2003). These larger hatchlings are then more
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likely to survive (Romero and Wikelski: 2003). Thus, the reproductive advantage for large body
size and length among females lies in the increased fitness of her progeny.
(Romero and Wikelski: 2003)
(SVL=snout-to-vent length)
In males, the advantage for larger body size lies in success in gaining access to mates.
Territory is crucial for marine iguanas in choosing mates. Females choose which males to mate
with based on the size of their reproductive territories, and the outcomes of territorial skirmishes
between males are usually determined by body size (Romero and Wikelski: 2003). Thus, males
with larger body sizes tend to mate more often and thus have greater reproductive fitness.
Another example of sexual dimorphism in marine iguanas on North Seymour, where some of the
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iguanas began eating terrestrial plants.
Concluding Remarks:
Although larger body size and length correlate with increased reproductive success in
both sexes of marine iguanas, body size and length are held in check by the amount of food
available. The ability to shrink and regrow in marine iguanas would have given the individuals
that first possessed the trait an incredible advantage over non-shrinking iguanas. They would be
better able to survive El Niño conditions and other constraints on algae production while still
being large enough to reproduce successfully. One likely hypothesis as to why marine iguanas
have evolved in this way is that this trait is the result of the two opposing forces of sexual
selection for a larger, more reproductively-fit body type and natural selection for smaller body
lengths with a consequentially lower demand for food during scarce El Niño years. Because the
actual mechanisms of how marine iguanas shrink in length are not completely clear, further
research on this topic could have significant effects on research concerning human bone loss and
the connection between activity levels and bone loss. More importantly, additional studies of
this unbelievably well-adapted species can help us to understand and better preserve Galapagos
marine iguanas, especially as the weather patterns most affecting these creatures may be
changing as a result of climate change.
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Juvenile marine iguanas at Tortuga Bay, Santa Cruz Island.
Works Cited
“Causes of Osteoporosis: The Case of the Shrinking Bones.” Pain Reduction Now. 2010. Web.
6 September 2012.
“Cellular Mechanisms of Bone Resorption.” Oulu University Library. Oulu University. 2000,
Web. 31 August 2012.
Romero, L. Michael and Martin Wikelski. “Body Size, Performance and Fitness in Galapagos
Marine Iguanas.” Integrative and Comparative Biology. 43.3 (2003): 376-86. 31 August
2012. Web.
Romero, L. Michael and Martin Wikelski. “Corticosterone levels predict survival probabilities
of Galápagos marine iguanas during El Niño events.” PNAS. 98.13 (2001). 31 August
2012. Web.
Romero, L. Michael and Martin Wikelski. “Marine Iguanas: Life on the Edge.” Galapagos:
Preserving Darwin’s Legacy. Ed. Tui De Roy. Buffalo: Firefly Books Ltd., 2009. 106113. Print.
Wikelski, Martin. “Evolution of body size in Galapagos marine iguanas.” Proceedings of the
Royal Society of Biological Sciences. 272 (2005). 29 August 2012.
Wikelski, Martin and Corinna Thom. “Marine iguanas shrink to survive El Niño.” Nature. 403
(2000): 37-8. Web. 29 August 2012.
Wikelski, Martin and Frtiz Trillmich. “Body Size and Sexual Size Dimorphism in Marine
Iguanas Fluctuate as a Result of Opposing Natural and Sexual Selection: An Island
Comparison.” Evolution. 51 (1997): 922-936. 28 August 2012.
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Wikelski, Martin and Peter H. Wrege. “Niche expansion, body size, and survival in Galápagos
marine iguanas.” Oecologia. 124.1 (2000): 107-115. 28 August 2012
Wikelski, Martin, Victor Carrillo, and Fritz Trillmich. “Energy Limits to Body Size in a Grazing
Reptile, the Galapagos Marine Iguana.” Ecology. 78 (1997): 2204–2217. 31 August 2012.
Web.