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16 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 NAVA

Adult male Puerto Rican Dwarf Gecko, Sphaerodactylus macrolepis guarionex: Like other very small lizards, Dwarf Geckos struggle with
heat and water loss, avoidance of which requires sophisticated exploitation of available structural, thermal, and temporal microhabitats.

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Size of an animal is in many ways its most important biological
trait. According to J.B.S. Haldane’s principle, “sheer size very

often defines what bodily equipment an animal must have.”
Lizards are a diverse group of animals with extremely diverse mor-
phologies and biologies. Lizards vary greatly in size from the largest
living lizard, the Komodo Dragon (Varanus komodoensis), which
measures up to 3 m from head to tail, to the smallest living lizard,
the Jaragua Dwarf Gecko (Sphaerodactylus ariasae), which meas-
ures only to 28 mm from head to tail. Many exhibit amazing
adaptations relative to their sizes. However, extremes in body size
can incur considerable consequences. Particularly, as size decreases,
physiological functions such as water balance and heat regulation
can become exigent on small lizards’ biologies. Herein I discuss
the physiological challenges facing Dwarf Geckos (genus
Sphaerodactylus) and describe how these remarkable lizards not
only cope, but also thrive in suitable environments.

The genus Sphaerodactylus is comprised of over 90 species
that are widely distributed throughout the West Indies, occur-
ring on every major island in the Caribbean, as well as in north-
ern South America through Central America into parts of

México as well as southern Florida. Species of Sphaerodactylus
occupy a wide variety of ecosystems that range from montane
rainforests and hilly scrub woods to coastal, desert-like habitats.
Species range in size from 16 mm snout-vent length (SVL) to
40 mm SVL. A few species reach the lower limits of amniote
body size and are considered to be the smallest terrestrial verte-

Size Does Matter
Saúl S. Nava

Department of Biology, Indiana University, Bloomington, Indiana, USA (email: snava@indiana.edu)

“We are prisoners of the perceptions of our size, and rarely recognize how different the world
must appear to small animals.”

Steven Jay Gould
Natural History, January 1974

IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 17SIZE DOES MATTER

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Adult male Sphaerodactylus macrolepis macrolepis from St. Croix, U.S.
Virgin Islands: S. macrolepis is a polytypic species with nine currently
recognized subspecies, all endemic to the Puerto Rico Bank.
Sphaerodactylus macrolepis is the most abundant and widely distributed
Dwarf Gecko in Puerto Rico. Populations in different areas exhibit
great variation in morphology, microhabitat use, and osmoregulation.

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Sphaerodactylus ariasae, discovered in 2001 on Isla Beata, a small
Hispaniolan satellite island, is the smallest known amniote (a group that
includes all reptiles, birds, and mammals). Extremely small body size
imposes severe challenges in regard to thermo- and osmoregulation.

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brates discovered to date. Adults of the smallest species,
Sphaerodactylus ariasae, from Isla Beata, off the southern coast of
Hispaniola, can curl up on a dime! Furthermore, population
densities can be phenomenally high. One species
(Sphaerodactylus macrolepis), which is endemic to the Puerto
Rico Bank, is known to reach densities to 67,000 lizards per
hectare, quite possibly the highest population density for any
lizard in the world!

For such tiny lizards, Dwarf Geckos exhibit considerable
diversity in morphology, ecology, behavior, and general natural
history — including the fact that many violate a general rule of
gecko biology. Unlike most geckos, which are nocturnal, species
of Sphaerodactylus, with only a few exceptions, are diurnal or cre-
puscular. Another departure from general gecko habits is that most
species are ground-dwellers, with very few known to be primarily
arboreal or saxicolous. The paucity of arboreal forms may be
attributable to the fact that Dwarf Geckos lack the typical large
flattened “toe pads” used by most other geckos for climbing. These
toe pads sport tiny rows of plate-like projections called lamellae,
each of which is equipped with thousands or even millions of
microscopic hook-like hairs called setae. These make it possible
for most geckos to climb on nearly any surface, even ceilings or
vertical panes of glass. However, as the name implies,
Sphaerodactylus (sphaero = round and dactylus = digit) feet bear
round lamellae that are restricted to the tips of their toes.

These tiny lizards inhabit a variety of microhabitats in just
about every type of terrestrial environment in the West Indies.
They are commonly found in dense leaf litter, inside termitaria,

or under dead palm fronds, agaves, fallen trees, large rock spills,
and human debris, such as wooden boards and galvanized steel
sheets. A few species (e.g., Jamaican S. semasiops and S. oxyrhi-
nus) are known to occur only in the complex microhabitats of
arboreal bromeliads.

The Surface Area-to-Volume Ratio
A major physiological challenge that affects all small animals is
a high surface area-to-volume ratio (SA/V), which, in turn,
affects basic physiological processes like osmoregulation and
thermoregulation. As a lizard’s body size decreases, the relative
surface area increases. This increase occurs because surface area
decreases as length squared, whereas volume decreases as length
cubed. Consequently, volume decreases more rapidly than sur-
face area, resulting in a high ratio. For tiny sphaerodactyls, the
high SA/V has profound effects on how they live their lives and
function in the environment. 

Problem 1: Evaporative Water Loss
One vital challenge associated with a high SA/V is controlling
water loss to the environment through evaporation. Evaporative
water loss (EWL) plays an important role in the patterns of
behavior and activity for lizards of all sizes, but most especially
the smaller species. The rate of EWL depends primarily on three
factors: Size of an animal, rate of oxygen consumed, and physi-
cal properties of the skin. Due to their incredibly high SA/V,
small lizards inevitably experience very high rates of cutaneous
evaporative water loss. This is particularly true of Dwarf Geckos.

18 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 NAVA

Adult male Sphaerodactylus macrolepis mimetes from Puerto Rico: Note the toe tips with round lamellae for which the genus is named.

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IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 19SIZE DOES MATTER

In the few EWL studies on Sphaerodactylus, researchers found
that these lizards exhibit incredibly high rates of EWL. In fact,
some of the rates reported for Sphaerodactylus are among the
highest recorded for any lizard. These studies suggest that, in
comparison to other lizards inhabiting similar or identical envi-
ronments, Dwarf Geckos have modest physiological adaptations
capable of compensating for high EWL. Additionally, EWL
rates are generally higher for reptiles inhabiting humid habitats
than species inhabiting arid habitats. In terms of humidity,
species of Sphaerodactylus occupy all types of environments.
Some, like S. klauberi, which is endemic to Puerto Rico, are
found only in very mesic (moist) montane ecosystems, whereas
others, such as S. parkeri, from Jamaica, occupy xeric (dry)
coastal environments that have all of the characteristics of
deserts. Although all species demonstrate high EWL rates, one
study found that eggs of S. cinereus, a Hispaniolan form, might
have the lowest EWL rate of any reptilian egg. More research on
EWL of eggs is needed to further explore this adaptation.

Problem 2: Thermoregulation
Thermoregulation is also difficult when faced with a high SA/V.
Dwarf Geckos are poikilothermic, meaning that they have no
internal metabolic mechanism for regulating body temperatures,
which consequently vary with the environment. As in any small-
bodied animal, heat is gained and dissipated relatively rapidly,
negating any ability to effectively regulate body temperatures
using behavioral mechanisms, such as those used with amazing
efficiency by many larger reptiles. Large surface areas result in
greater exposure, and the relatively small body volume retains
heat ineffectively. Thus, sphaerodactyls are extremely vulnerable
to heat stress. On the other hand, unlike larger lizards, Dwarf
Geckos absorb heat rapidly, and, unlike most larger forms, do
not need to bask in order to raise their temperatures. The net
effect, however, is that these diminutive species cannot effectively
deal with variations in environmental temperatures.

The Solution: Microhabitat Selection
So, what defenses do Dwarf Geckos have against dehydration or
heat stress imposed by their diminutive sizes? Although evidence
shows that their bodies are not adapted physiologically to min-

Adult Sphaerodactylus klauberi from a montane rainforest in south-
eastern Puerto Rico: S. klauberi is endemic to Puerto Rico and is one
of the larger species of Dwarf Gecko, reaching sizes to 39 mm SVL.
This species is found only in mesic, montane habitats.

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Adult Sphaerodactylus nicholsi from the Bosque Seco, an extremely arid
region in southwestern Puerto Rico: This species is the smallest Dwarf
Gecko endemic to Puerto Rico and lives in sympatry with the much
larger S. roosevelti.

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A Cosmopolitan House Gecko (Hemidactylus mabouia) from Tortola,
British Virgin Islands. This gecko is widely distributed in the West
Indies. Although much larger, House Geckos may share communal
nests with Dwarf Geckos and often are found cohabitating under large
rocks, fallen trees, and logs.

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Sphaerodactylus parvus, from Anguilla: This species can attain huge
population densities in suitable habitats.

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imize water loss and maintain suitable body temperatures,
sphaerodactyls can overcome these limitations by behaviorally
exploiting microhabitats often unavailable to larger species.
Microhabitat selection is undoubtedly the best defense against
dehydration and overheating. Most of the terrestrial habitats
occupied by these lizards are relatively heterogeneous regarding
humidity, and individuals of most species occur under a variety
of complex substrates that appear to preserve high humidity lev-
els. Several studies on Dwarf Gecko ecology have shown that the
two most important factors in microhabitat selection are relative
humidity and structural complexity. For example, S. parvus,
endemic to the Anguilla Bank, demonstrates a clear preferential
use of microhabitats, such as large rocks shaded by dense forest
canopies that provide effective insulation from overheating.
Additionally, leaf litter on the forest floor, for example, can be
relatively deep and dense, sometimes greater than a meter in
depth. This type of substrate provides an ideal microclimate in
which relative humidity levels are substantially higher than
ambient humidity. Microhabitat selection extends also to aspects
of reproduction in Dwarf Gecko ecology. Nest sites are invari-
ably established in humid, sun-sheltered microhabitats (e.g.,
under large, fallen trees, logs, or large rocks). Moreover, Dwarf
Gecko eggs are commonly found in communal nests (one nest
site where multiple females lay eggs), often with eggs of other
species and genera (e.g., Hemidactylus spp.). Even in arid envi-
ronments, Dwarf Geckos typically are found in the most humid
and appropriate microhabitats, usually under trees or cover —
access to which is possible only because of their small sizes. For

example, in Puerto Rico’s “Bosque Seco” (dry forest), which is
in essence a desert, S. nicholsi and S. roosevelti, are typically found
in the leaf litter underneath Sea Grape trees, which is substan-
tially more humid than the ambient climate. By actively select-
ing microhabitats that preserve humidity, these lizards can
endure and even thrive in very arid environments. Additionally,
many species restrict activity to twilight hours, times of day
when ambient temperatures are less severe. Although not com-
mon among species of Sphaerodactylus, presumably because most
forms can effectively exploit the “right” microhabitats, some
species, like S. roosevelti from Puerto Rico, S. parkeri from
Jamaica, and S. sputator from the Anguilla and St. Christopher
banks, have opted for nocturnal lifestyles, reducing even more
their exposure to daytime heat and the consequent danger of
dehydration.

Conclusion
“For every type of animal there is a most
convenient size, and a large change in size
inevitably carries with it a change of form.”

J.B.S. Haldane, 1928

Despite physiological challenges imposed by diminutive size,
geckos in the genus Sphaerodactylus, like many other small ani-
mals, must rely on behavioral and ecological strategies to survive.
Dwarf Geckos overcome the challenges of desiccation and heat
stress by actively selecting the least dehydrating microclimates
available in their macroenvironments. So, the small size that is

20 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 NAVA

Adult female Sphaerodactylus roosevelti from the Bosque Seco in Puerto Rico: This species reaches sizes to 39 mm and is active predominantly at
night. Like many, but not all Dwarf Geckos, S. roosevelti is sexually dimorphic. Note the prominent dorsal stripes, a characteristic seen only in
females of this species.

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IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 21SIZE DOES MATTER

disadvantageous in the context of physiological responses to the
environment is the very thing that facilitates the exploitation of
microhabitats, enabling these tiny lizards to survive and suc-
cessfully radiate across the West Indies. Consequently, I can add
to Haldane’s principle that size “defines what bodily equipment
an animal must have” the caveat that size also defines what
behavioral responses an animals must use.

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Adult male Sphaerodactylus roosevelti from the Bosque Seco in Puerto Rico: Note the reduced prominence of dorsal stripes, which is characteristic
of males.

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