Marine Iguanas of the Galápagos Islands

Kelly Haisfield
GEOL388
July 12, 2002

Marine Iguanas, Amblyrhynchus cristatus, are endemic the Galapagos archipelago and are the only species of aquatic lizard on earth. These creatures are distributed throughout about the islands, and have a broad range of appearances, with males being more brightly colored than females ranging from grey, to brown, to red, to blue and green, as well as sizes, with males reaching up to 12.3 kg on Isabella, and as little as 1.2 kg on Genovesa (Laurie, 1983). The marine iguana is more closely related to the Galapagos land iguana than to any other mainland iguanid lizard, which leads researchers to believe that the two forms may have diverged from a common ancestor (Dawson et al, 1977).

Feeding

Marine iguanas feed on macrophytic marine red and green algae by either going into the ocean and diving to feed, or by waiting for low tide to expose the algae on rocky surfaces. When feeding they will usually dive into the ocean as deep as thirty-five feet for about five to ten minutes, but can hold their breath for up to an hour if necessary (Cornell). Generally the larger more dominant individuals feed more frequently and at high tide, while the smaller iguanas need to wait till low tide to consume algae on intertidal rocks (Laurie, and Brown, 1990). Because the water is cold from deep-water upwelling, the cold blooded iguanas spend a lot of time sunning themselves on rocks to keep warm before and after feeding to compensate for the drop in body temperature of up to 18 degrees Fahrenheit (Townsend, 1992). To conserve energy the iguanas also drop their heart rate while swimming from about 100 beats per minute, to about 30 beats per minute (De Roy, 1994). While the largest iguanas are able to dive for algae whenever they wish to feed, smaller individuals need to forage for vegetation at low tide.

Biorhythms

The iguanas seem to have an internal biorhythm that allows them to anticipate low tide, even if they are resting in an area where they cannot see the ocean. It seems closely related to the fact that individuals who reach foraging sites earlier, have a higher rate of survival than those individuals that reach the sites later, and so individuals will anticipate low tide by a period of up to four hours in order to graze on the best algae (Wikelski and Hau, 1995). The iguanas prefer to feed during daylight, because they need higher body temperatures to feed most efficiently, maintain high endurance, and be able to escape crashing waves, and also need to warm up after foraging so that they can digest their food (Wikelski and Trillmich, 1994), so when the low tide occurs late in the evening, will often switch their feeding patterns to feed at the next low tide in the early morning instead. This foraging behavior is described as a bitidal rhythm (Wikelski and Hau, 1995). The daily anticipation of low tide, is more greatly influenced by the actual cycles of the tide, than increase or decrease in actual water levels. In a study by Wikelski and Hau (1995), it was discovered that the time when iguanas went to the feeding sites was more closely related to the tabulated times for low tides than by the times when intertidal areas were actually exposed. Even when the intertidal rocks were exposed for most of the day, the animals foraged only around the time of tabulated low tide. The presence of waves and adverse weather conditions can also serve to hinder the feeding of marine iguanas and alter the tides so that the times in which rocky shores are actually exposed changes (Wikelski and Trillmich, 1994). Because iguanas need a long time to digest their food when their stomachs are full, many iguanas do not feed every day, but even after periods of fasting, when individuals could not gather tidal information, iguanas forage at the "correct" time, further implying that there is an internal mechanism, an endogenous circatidal rhythm leading to tide cycle anticipation (Wikelski and Hau, 1995). Iguanas also seem to posses semimonthly tidal information, because they will often rest nearer to feeding sites near neap tides in areas that would be underwater during spring tides, but are dry due to lower tides (Wikelski and Hau, 1995).

Salt Excretion

Due to the lack of freshwater on the islands, marine iguanas have adapted and unusual behavior to rid them selves of the salt that accumulates in their bodies. Iguanas have special glands near their tear ducts that allow them to "sneeze" salt out of their bodies. This salt often accumulates on the snouts of the lizards, causing them to appear more lightly colored. The animals are highly capable of reducing salt and can eliminate them in waters with up to twice the salinity of seawater, or up to 60 parts per thousand (De Roy, 1994).

Mating and Survival: Male Competition

Males are quite possessive of their females, which resemble harems, and often head bob, to threaten, or head-butt other males who come too close (Cornell). During breeding season, the males separate themselves from each other to develop areas that they patrol, to prevent other males from entering and attempt to court and mate with females entering their territories (Townsend, 1992). Males go through elaborate displays to prevent other males from entering their territories and mating with their females. The fighting ritual described by Townsend (1992) begins with the males approaching each other near boundary limits, and one will inflate his body and stand high up on his legs. They begin nodding their heads rabidly and open their mouths flashing their tongues. Often this leads one male to retreat, if not, they keep bobbing their heads at each other and eventually charge at each other like rams, attempting to push each other out of a given territory. This shoving can last for several hours, but may be interrupted if an especially aggressive iguana bites the other on its legs or jaw or violently shakes his opponent. Usually there is not harm to either lizard, and the weaker male will leave the area, or lay down in a submissive position. About a month after copulation females lay from one to six eggs, with number of eggs positively correlating to size of female (Rauch, 1985). The eggs incubate in nests dug between 30 and 80 centimeters deep in volcanic ash or sand for about three months (Laurie and Brown, 1990).

Sexual Dimorphism

Marine iguanas are sexually dimorphic, with the males weighing up to 70% more than their female counterparts. A possible reason for this dimorphism is the fact that females mature more quickly than males, and after they mature, they devote a lot of their energy that could be used to increase body mass, to produce eggs (Wikelski and Trillmich, 1997). The probable evolutionary history for maintenance of sexual dimorphism in marine iguanas is described by Wikelski and Trillmich (1997). Because all currently living iguanid lizards show sexual dimorphism, it is presumed that marine iguana ancestors also exhibited this trait. Upon their arrival on the islands, they most likely found little to eat in the volcanic areas, but a large food source near the coasts, so they colonized near the intertidal areas. Larger animals were more likely to be favorable due to limited foraging areas and the larger animals ability to stay warmer in the cold water, and increased resistance to wave action. Since the iguanas all grouped around suitable foraging areas, it allowed male iguanas to court larger numbers of females, and females had the option of being more picky, when choosing a mate. Just as occurs now, the females selected for larger males, and because females only need to get large enough to produce eggs, their mass stabilized, while male mass increased over generations as larger males genes were passed on more frequently than small males.

Sexual Selection Contrasting Optimal Survival

Marine Iguanas exhibit interesting sexual behavior, that seems to be contrary to optimal survival rates. They participate in lek mating on some islands, a process where males gather in an area and females choose mates based on a male display, most females will mate with one dominant male, and select for large body size, but on the islands, dominant males generally only receive about thirty-five percent of total copulations (Wikelski et al, 1996). However, it is the largest of the males that die off first in adverse environmental conditions, even though they are the first to be selected for mating (Wikelski, and Trillmich, 1997). The selection for larger body size, a highly heritable trait, may help explain the large difference between sizes of iguanas on different islands (Wikelski, and Trillmich, 1997). One would presume that larger males would have higher survival rates than their smaller relatives due to increased fighting abilities, and greater intimidation, however in marine iguanas, larger size does not mean an organism will live longer. The larger iguanas have a harder time maintaining energy levels in periods of starvation because the energy needed to sustain life of a more sizeable animal is greater than that needed to sustain a smaller creature (Wikelski, and Trillmich, 1997).

Pre-Copulatory Ejaculation

Another unusual behavior that Wikelski and Baurle (1996) describe is that marine iguanas demonstrate pre-copulatory ejaculation. Females will only mate once per season, and this causes high competition between males to mate with females. A copulating male needs approximaltely 3 minutes to copulate and inseminate a female, will be interrupted between one and four times while trying to place sperm inside the female. Larger individuals are rarely removed before ejaculation and finish copulation in ninety-five percent of mating attempts, but smaller, less dominant males are removed from a female by a larger male twenty-nine percent of the time before they have finished mating in the study by Wikelski and Baurle in 1996. To compensate for this competition, small males will ejaculate at the sight of a female iguana, and store their sperm in a pouch at the base of the tail, where it remains viable for up to a day. Then, when they do mount a female, they insert the sperm before beginning copulation, so that if they are removed before they have finished, they have still successfully inseminated the female. This masturbatory behavior increases the mating success of small males by forty-one percent.

Female Choice

Marine iguana females do not all choose to mate with the "best" male, even though female choice is strong, and leks are prevailent. In the few days prior to mating, females visited about five more males than they usually encountered on their foraging routes (Wikelski et al, 2001). They generally approached large males, and would usually mate with the male that had the highest display rate of all the males visited. In a dense cluster, females lost more body mass, possibly due to greater interaction with males and changes in activity due to male attention, and mated with more active males than females in more dispersed areas of biomass (Wikelski et al, 2001). Female choice is most likely not unanimous due to the fact that gathering complete information on all males in an area would be too costly for females, and would outweigh positive effects of selecting the "most-fit" male (Reynolds and Gross, 1990). In Wileski et al (2001) females copulated with males whom they actively visited, not ones they merely encountered during foraging trips. On average the male they selected was the one with whom they spent the second most amount of time, and statistically, they spent more time with their selected mate that would be expected due to randomness.

Introduced Species

Until human intervention introduced new species to the islands, the iguanas had few natural predators, and had thus developed relatively few defenses leaving them susceptible to feral cats and dogs preying on young and occasionally adults, while introduced rats consume marine iguana eggs, so populations in more highly populated areas are declining, because most young lizards are eaten before they can develop into adults (Cornell). In a colony without introduced predators 53 percent of all iguana hatchlings survived their first six months, but in a nearby colony plagued by cats, fewer than one percent of hatchlings survived (De Roy, 1994).

El Niño

Every few years a warm current travels along the coast of South America bringing warm, nutrient poor, low salinity waters to the Galapagos Islands. These events cause drastic population crashes in many species on the islands including iguanas. Marine iguanas were studied by Laurie (1990) before and after the El Nino event in the Galapagos Islands in 1982-1983. The nutrient poor waters cause the normal food supply of the marine iguanas, red and green algae, to be replaced by less favorable brown algae. The lack of food supply cause the death of sixty percent of iguanas, and those individuals that did survive were in poor condition. While almost no females were able to breed in 1983-1984, since unhealthy animals usually devote most of their resources to reproduction, ninety percent bred in 1984-1985, and 1985-1986. The emphasis on reproduction was also evident in clutch size as average number of eggs laid per nest increased from two to three.

Recent News

On January 17, 2001 an oil tanker named the Jessica spilled three million liters of oil into the ocean when she struck San Cristobal Island at the Galapagos Natural World Heritage Site. Although the immediate effects from the tanker were not as damaging to the marine iguanas as was feared, as the animals fed and acted normally and the only difference were the abnormally high levels of the stress hormone corticosterone, after a year, two thirds of the iguanas had disappeared. Researcher Martin Wikelski believes the iguanid deaths are due to oil contaminated food, which could have killed the bacteria in iguanas guts, and led to inability of individuals to digest algae, however there is not data available to support his claims (Milius, 2002). In a study by Wikelski et al. (2001) animals were sampled before and after the oil spill, and oil residues were found on the skin of over seventy percent of the iguanas. Marine iguanas are believed to be sensitive to oil contamination because the increased levels of stress hormone in blood of all iguanas exposed to contamination, was as high as individuals that died within two to four weeks of an El Nino event, and the hormone is linearly correlated to survival rates of the species (Wikelski et al., 2001).

Literature Cited:

Dawson, W. et al,.1977, A reappraisal of the aquatic specialization of the Galapagos marine iguana (Amblyrhynchus cristatus) Evolution, 31:891-897.

De Roy, T. 1994, Welcome to the lizard show. International Wildlife, 24:16-23.

Laurie, W. 1983, An ill wind for iguanas. New Scientist, 100:108.

Laurie, W. & Brown, D. 1990, Population biology of marine iguanas (Amblyrhynchus cristatus). II. Changes in annual survival rates and the effects of size, sex, age and fecundity in a population crash. Journal of Animal Ecology, 59:529-544.

Milius, S. 2002, Plight of the iguanas. Science News, 161:357.

Rauch, N. 1985, Female habitat choice as a determinant of the reproductive success of the territorial male marine iguana (Amblyrhynchus cristatus). Behavioral Ecology and Sociobiology, 16:125-134.

Townsend, C. 1992, Galapagos surf and turf. Natural History, 101:40-43.

Wikelski, M. et al. 1996, Lekking in marine iguanas:female grouping and male reproductive strategies. Animal Behavior, 52:581-596.

Wikelski, M. et al. 2001, Why is female choice not unanimous? Insights from costly mate sampling in marine iguanas. Ethology, 107:623-638.

Wikelski, M. et al. 2001, Marine iguanas oiled in the Galapagos. Science, 292:437-438.

Wikelski, M & Baurle, S. 1996, Pre-copulatory ejactulation solves time constraint during copulations in marine iguanas. Proceedings of the Royal Society of London, Series B: Biological Sciences, 263:439-444.

Wikelski, M. & Hau, M. 1995, Is there and endogenous tidal foraging rhythm in marine iguanas? Journal of Biological Rhythms, 10:335-350.

Wikelski, M. & Trillmich, F. 1994, Foraging strategies of the Galapagos marine iguana Amblyrhynchus cristatus: Adapting behavioural rules to ontogenetic size change. Behaviour, 128:255-27.

Wikelski, M. & Trillmich, F. 1997, Body size and sexual size dimorphisim in marine iguanas fluctuate as a result of opposing natural and sexual selection: an island comparison. Evolution, 51:922-936.

Marine Iguanas, Cornell University, http://www.geo.cornell.edu/geology/GalapagosWWW/MarineIguanas.html