Galapagos Iguana Hybrids: What about their Salt Glands?

Last week I gave a talk on salt glands to a local group. I remembered that I had not raised a question here that arises from the existence of some very odd animals on one island of the Galapagos.

Everybody knows there are the Marine Iguanas (Amblyrhynchus cristatus) and Land Iguanas (Conolophus sp.) on the Galapagos Islands. Evidence indicates they had a common ancestor about 4.5 million years ago. Watchers of television wildlife programmes cannot fail to have noticed the entirely different lifestyles. Land iguana eat plants and do not venture into the sea. By contrast, Marine Iguanas eat seaweed often foraged at considerable depths.

Over the past forty years it became evident that on one very small island, Plaza Sur, just off the east coast of Santa Cruz (Indefatigable) there were hybrids present between Marine and Land Iguanas (Conolophus subcristatus). These hybrids are sterile as one might expect. There is good evidence that their fathers are male Marine Iguanas, their mothers Land Iguanas. It seems that the breeding seasons of the two species can overlap there. The habitats are very not very far apart either, the thin vegetation where the Land Iguanas bask and feed being only a matter of yards from the rocks on which the Marines bask.

We took video and still photographs of one of these hybrids on Plaza Sur in January 1012. It was lying with Land Iguanas. Local guides say they are not very active and typically have a ‘spaced out’ look like the one below:

Hybrid Marine x Land Iguana, Plaza Sur, 12 January 2012

For comparison: Galapagos Land Iguana, Plaza Sur, 12 January 2012

Also for comparison: Galapagos Marine Iguana, Plaza Sur, 12 January 2012

Nesting burrows of Land Iguanas. Plaza Sur, 12 January 2012

Vegetation on Plaza Sur, 12 January 2012. The marine iguana-like claws of the hybrid iguanas are said to be advantageous in reaching the fruits of the prickly pear. The land iguanas have to wait for them to drop

The question I have had since then is what do the salt glands of these hybrids do? Marine Iguanas have very active salt glands which operate to remove the sodium chloride ingested during feeding. In addition, the concentration of potassium is about ten times higher than in birds which could reflect the high potassium concentration in the plants of their diet. Marine iguanas on land can be seen snorting secretion from the salt glands out of their nostrils, a phenomenon observed by Charles Darwin who did not appreciate its significance to their survival.

Other, species of iguanid lizard have salt glands that can secrete both sodium and potassium. There is evidence that the ratio can be changed by varying the diet. Lizards on a high sodium diet or given a salt load produced a secretion with a more sodium than potassium. As far as I am aware, the Land Iguanas of the Galapagos have not been studied in this respect.

The hybrids seem to live and eat like their Land mothers although I see there are anecdotal reports of their being seen eating seaweed but not entering the water to do so.

So do the hybrid iguanas on Plaza Sur have functional salt glands? How does their size compare with the Marine and the Land? If the salt gland is functional, what are the stimuli for secretion, and does the ratio of sodium:potassium change to reflect changes in the dietary intake?

Possibilities of studying the hybrids to answer these questions are low but the insights into which genes are involved in salt-gland structure and function would be fascinating outcome of this ‘natural experiment’ but biological dead end.

I  used this drawing from Bill Dunson's paper in our book on salt glands