Outside of Westpunt, Curaçao

On the natural history of a peculiar gecko

Unusual adaptations for calcium storage

Geckos are an incredible group of lizards that have managed to colonize habitats that include deserts, rainforests, and even alpine mountains. While the diversity of this group is amazing, the variations between species in ecology and life histories creates a major challenge for predicting their conservation needs.

Since we work on nocturnal reef fishes, late nights of diving mean that we get to spend a lot of time surrounded by geckos when we do fieldwork in Curaçao. On this island native gecko populations are slowly declining, and we are doing our best to opportunistically study these animals to fill knowledge gaps necessary for effective conservation.

Recently our intern, April Lamb, took the lead on a study assessing the role of endolymphatic sacs in the reproductive biology of Gonatodes antillensis.

The need for calcium

Female geckos are faced with challenge when it comes time to lay eggs. They need calcium, and lots of it. Calcium is necessary not just for the development the egg shell, but also to provide the developing baby gecko with enough nutrients to grow. To meet this challenge a variety of gecko species have evolved highly modified endolymphatic sacs that enable them to store calcium for egg development. These are the large sacs on the side of the heads of many geckos you see in pet stores. However, not all species do this. Radiographs of Gonatodes antillensis specimens. (A) Gravid female with a visible egg (circle) and large endolymphatic sacs indicated by arrow. (B) Female with small endolymphatic sacs. (C) Male with no visible endolymphatic sacs. Females that use endolymphatic sacs for reproduction ramp up calcium storage prior to forming an egg.

Documenting which species make use of modified endolymphatic sacs is pretty important. It turns out that stress can inhibit calcium storage in these organs. If animals are stressed, they could potentially lay less thick eggs or not provide enough calcium for development. While the animal may appear otherwise healthy, these sorts of subtle changes can greatly impact the persistence of populations over multiple generations.
Image of a male Gonatodes antillensis. G. antillensis is the only nocturnal species of Gonatodes, making it a great species from which to study how vertebrates evolve dim light vision.

Image of a female Gonatodes antillensis. There are over 1,600 species of geckos in the world. With the gloabl pace of change, documenting basic aspects of their biology essential for conservation management is a race against time.

We investigated whether Gonatodes antillensis makes use of these structures for egg development through the use of X-rays images taken from museum specimens. We compared endolymphatic sac sizes between egg bearing and non visible egg bearing females, as well as females to males.

We found that while both males and females have minerals in their endolymphatic sacs, females have far, far more. Our results support the expectations of endolymphatic sac use for reproduction.
Violin plots with embedded box plots depicting the first, second (median) and third quartiles of Endolymphatic sac size area (mm2) for gravid females (GF), females with no eggs (NVE) and males (M).

We also used egg presence absence data to see when this species is reproductively active. To date, only summer reproductive activity was known. However, we expand the reproductive season into winter, suggesting year-round reproducition.

Gonatodes antillensis is a peculiar gecko, one of the only geckos that is secondarily nocturnal (its ancestor was presumably diurnal as are all other species of Gonatodes). This makes it a great species for understanding dim light vision in vertebrates. However, this species is declining.
Gonatodes albogularis, one of the many diurnal species of Gonatodes.

We recently documented this species being preyed upon by the highly invasive gecko, Hemidactylus mabouia, also discussed in this post. Our study raises the possibility that predation, resource competition, and now reproductive interference through stress may all be impacting G. antillensis. More study is needed on the reproductive biology of this species under duress to see the extent of the impact, however our study adds the another axis of concern for the continual spread of H. mabouia across the neotropics.

Dialogue & Discussion