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Oecologia (1995) 101: 228-233
ORIGINAL PAPER
? Springer-Verlag 1995
Douglas A. Eifler
Patterns of plant visitation by nectar-feeding lizards
Received: 15 July 1994 / Accepted: 30 September 1994
Abstract Geckos in the genus Hoplodactylus visit flow-
ers to feed on nectar. I examined the patterns of flower
visitation exhibited by two gecko species (H. maculatus
and H. duvauceli) having access to two plant species:
pohutukawa (Metrosideros excelsa: Myrtaceae) and flax
(Phormium tenax: Agavaceae). Individual geckos were
not observed to visit both plant species; individuals visit-
ing flax tended to revisit the same plant. Geckos visiting
pohutukawa were larger than those visiting flax and ex-
hibited an early night peak in plant visitation, while liz-
ards on flax displayed a more even pattern of activity
throughout the night. On flax, geckos were more likely
to be found on plants with a greater number of male
flowers. Male flax flowers were of greater diameter than
female flowers and produced nectar at higher rates and
with greater concentrations of sugars. Experimental ma-
nipulation of pohutukawa nectar volumes suggested that
the distribution of geckos is influenced by the pattern of
nectar availability.
Key words Foraging ? Lizard ? Nectarivory ? Gecko
Hoplodactylus
Introduction
The diet and foraging strategies of animals tend to be ef-
ficient, reflecting cost-benefit criteria in such a way that
alternatives are chosen that yield more food per time unit
(Stephens 1990). To forage efficiently, animals need to
adapt their behavior to available food resources, detect-
ing patterns in food dispersion and tracking changes in
resource availability. Response to patterns of food dis-
persion should be evident when dealing with food types
to which an animal is specialized or where a food type
represents an important diet component (Real and Cara-
co 1986).
The ability to assess nectar availability and quality
has been extensively studied for a variety of vertebrates.
Nectarivorous birds can distinguish among nectars dif-
fering in sugar composition or concentration, and among
nectar sources available with differing degrees of vari-
ance (Wunderle and O'Brien 1985; Martinez del Rio
1990). In response to nectar supplementation, humming-
birds alter their activity patterns and focus foraging ef-
fort in areas with abundant nectar (Gass and Sutherland
1985). Members of several families of nectarivorous
birds vary the degree to which they defend territories in
response to manipulation of nectar availability (Gill and
Wolf 1975a; Carpenter and MacMillen 1976; Kodric-
Brown and Brown 1978). Similar behavioral changes oc-
cur among nectarivorous mammals. Some species, such
as the marsupial glider, Petaurus australis, simply
choose to forage at trees with a higher flower abundance
to increase foraging efficiency (Goldingay 1990). Other
species show more complex responses to changes in nec-
tar availability. The long-nosed bat, Glossophaga soric-
ina, switches between territorial defense of nectar sourc-
es and trapline foraging as nectar availability changes
(Lemke 1984). Among tamarin monkeys, the location
and size of territories may be linked to the availability
of seasonally important nectar sources (Terborgh and
Stern 1987).
Most lizard species are exclusively insectivorous or
herbivorous (Greene 1982). Insectivorous and herbivo-
rous lizards may alter their behavior in response to chan-
ges in food availability, modifying home ranges (Simon
1975; Stamps and Tanaka 1981; Guy er 1988), foraging
behavior (Munger 1984), and activity patterns (Wald-
schmidt 1983; Jones et al. 1987) as well as distinguish-
ing among foods of differing quality (Troyer 1984; Ea-
son 1990). Several species of lizard have been recorded
feeding on nectar (Cheke 1984; Whitaker 1987a; Greer
1989). The extent to which nectar is an important food
source for these lizards is unknown. While many animals
specialized for nectar feeding can evaluate and track nee-
D.A. Eitler1
Museum of Comparative Zoology, Harvard University,
Cambridge, MA 02138, USA
1 Present address:
Museum of Natural History, University of Kansas, Lawrence,
KS 66045, USA
229
tar quality and quantity (Kearns and Inouye 1993), the
role of nectar in lizard energetics and the extent to which
nectar availability is a resource that lizards evaluate and
track is unclear (Greer 1989).
In New Zealand, nocturnal Hoplodactylus geckos take
nectar from a variety of plants (Whitaker 1987a). The
purpose of this study was to determine the extent to
which patterns of flower visitation by Hoplodactylus
geckos reflect patterns of nectar availability. I focused on
geckos visiting two flowering plant species: flax (Phor-
mium tenax: Agavaceae) and pohutukawa (Metrosideros
excelsa: Myrtaceae), identifying the patterns of activity
displayed by geckos visiting flowers, the distribution of
geckos within and between plant species, and the rela-
tionship of nectar availability and plant characteristics to
gecko distributions.
Materials and Methods
Study site and study organisms
The study was conducted on Korapuki Island, an 18 ha island ap-
proximately 10 km east of the Coromandel Peninsula, North Is-
land, New Zealand. Pohutukawa is the dominant canopy plant,
while flax is the dominant plant bordering beaches. Pohutukawa is
a large tree (up to 20 m) found mainly along sea coasts and lake
shores (Laing and Blackwell 1950). The shallow cup-like flowers
remain open for 6-8 days and are arranged in cymes; flowers
within the cyme open sequentially so that a single inflorescence
blooms for up to 12 days (Whitaker 1987a). Individual trees may
be in flower up to 6 weeks, and within a population the flowering
season may last 6-10 weeks (Whitaker 1987a). Pohutukawa trees
produce large amounts of nectar and are visited by birds and bees
(Moore and Irwin 1978). Geckos are known to accumulate at
flowering pohutukawa trees in large numbers, with as many as 5-8
geckos/m2 of canopy (Whitaker 1968, 1987a,b).
Flax is a large monocot endemic to New Zealand. The tubular
flowers are arranged in raceme-like stalks averaging approximate-
ly 3 m in height; flowers are protandrous hermaphrodites produc-
ing large quantities of relatively dilute nectar (Craig and Stewart
1988). Male and female flowers may be present on the same inflo-
rescence (Craig and Stewart 1988). Nectar feeding birds common-
ly visit flowers, preferring to visit male flowers (Craig 1989).
Male flowers in populations of flax near Auckland contain higher
volumes of nectar of higher sugar concentration than female flow-
ers (Craig and Stewart 1988). Geckos have been observed to feed
on flax nectar. At some sites every flower stalk may have geckos,
with densities as high as 4-6/stalk (Whitaker 1987a,b).
Hoplodactylus geckos are nocturnal lizards using terrestrial and
arboreal habitats and are considered to be primarily insectivorous
(Bull and Whitaker 1975). Two species, H. duvauceli and H. macu-
latus, occur on Korapuki Island (Towns 1991). H. duvauceli is the
larger of the two study species, with adult snout-vent lengths
(SVL) ranging from 100 to 160 mm and weights ranging from 40
to 118 g (Whitaker 1968; Barwick 1982). Previous reports of adult
H. maculatus SVL range from 45 to 71 mm and weights range
from 4 to 14 g (Whitaker 1982a; Gill 1986; Towns 1991). A tran-
sect running approximately 200x5 m along the beach and shoreline
comprised the study area. Observations focused on geckos visiting
flax that lined the transect and on the lower 2 m of a single poh-
utukawa tree on the transect that was in flower during the study.
Nectar and floral characteristics
Standing nectar crop was measured with 100 ?? pipettes, and sugar
concentrations were measured with a Reichert 10430 temperature
compensated refractometer (Leica, Buffalo, NY). Measurements
on standing nectar crop and nectar concentration were made from
2100 to 2400 hours. Rates of nectar production and sugar concen-
tration of new nectar for male and female flax flowers were com-
pared by first draining the nectar from pairs of male and female
flowers on the same stalk, enclosing the flowers in white mesh
bags, and measuring nectar volume and concentration after 24 h.
The size of male and female flax flowers was compared using the
maximum flower diameter below the point where the sepals
formed a continuous tube.
Gecko activity and distribution patterns
I determined activity periods for geckos by walking a predeter-
mined census route every hour from 1 h before dark until sunrise
and recording the number of lizards present on flax and poh-
utukawa. The size distribution of lizards on the two plant species
was assessed by collecting geckos from plants and weighing them.
I individually marked lizards with a paint pen, then recorded the
individual plants on which they were resighted to determine indi-
vidual patterns of plant use. When observing lizards on flax
plants, I recorded the sex of flowers visited and whether the liz-
ards fed at the base of flowers or in the corolla. Observations were
made using red filtered light.
The relationship of nectar availability and plant characteristics
to gecko distributions
To identify characteristics of flax plants visited by geckos, I mea-
sured variables from 50 flax stalks on the transect; on the evening
following these measurements I censused the plants to see which
were visited by geckos. During the evening of censusing, I also
observed geckos on nine unmeasured stalks. These stalks were
measured the next morning. The following characteristics were
identified for the 59 flax stalks: stalk height (m), number of flow-
ers of each sex present, nearest stalk (m; at 1.5 m height), nearest
stalk from another flax plant (m; at 1.5 m height), and number of
stalks belonging to the plant. Sometimes the nearest stalk was also
the nearest stalk from another plant.
I evaluated the effect of flower distribution on the distribution
of geckos visiting pohutukawa by comparing gecko sightings at
six discrete patches of inflorescences, varying in inflorescence
number from 8 to 63, for 3 nights of censusing. As patches varied
noticeably in canopy surface area, I tied away nearby branches so
that the patches were not contiguous with the canopy surface to
avoid simply documenting an area effect. Lizards had to enter
each inflorescence patch from the interior of the tree along main
branches (similar to an animal choosing among the arms of a radi-
al maze).
I experimentally manipulated pohutukawa nectar availability to
examine its effect on lizard distribution. From the above six inflo-
rescence patches, the three with the most inflorescences were each
partitioned into a manipulated and unmanipulated half. Prior to
manipulation, the three patches were censused for 2 nights and the
number of lizards visiting each patch half was determined. Next,
artificial nectar (56.5% sucrose solution; approximately the aver-
age pohutukawa nectar concentration observed during this study)
was used to supplement the nectar available in the flowers of the
manipulated half of each patch. Sucrose solution was added to
15% of the flowers and was added until the flower was full of liq-
uid (an addition averaging approximately 25 ?? sucrose solu-
tion/flower). Sucrose solution was added to flowers just before
sunset for 3 days. During the night following the last 2 days, I
censused patches to determine the number of lizards visiting ma-
nipulated and unmanipulated patch halves.
Results are presented as means ?SE. Statistical tests used are
identified in the text and were performed using Minitab (State
College, Pa.).
230
Results
Nectar and floral characteristics
The concentration and volume of standing flax nectar
were determined for male and female flowers at the be-
ginning of the night. There was no significant difference
in nectar sugar concentration between male flowers
(21.6 ? 0.34% sugar) and female flowers (20.3 ? 0.88%
sugar; Mann-Whitney test: U = 164, ? = 9 and 13, re-
spectively, ? = 0.34). Nor was there a significant differ-
ence in the standing nectar volume of male versus female
flowers (Mann-Whitney test: U = 72, ? = 6 and 9, respec-
tively, ? = 1.0). Male flowers contained 95.3 ? 31.8 ?? of
nectar whereas females contained 83.1 ? 30.3 ??. Male
flax flowers had a greater maximum diameter than fe-
male flowers (11.8 ? 0.2 mm versus 10.0 ? 0.2 mm;
Mann-Whitney test: U = 215, ? = 8 and 14, respectively,
? = 0.0003).
Male flax flowers exhibited a significantly greater
rate of nectar accumulation than female flowers of the
same plant (Wilcoxon's signed rank: ? = 55, ? = 0.006);
the 24-h nectar production of males averaged almost 3
times that of females (males flowers 264.3 ? 35.6 ?? ver-
sus females 98.5 ? 35.5 ??). Although sugar concentra-
tion in standing nectar crop does not vary among male
and female flax flowers, sugar concentration in new nec-
tar produced by male flax flowers was significantly high-
er (19.62 ? 1.07% sugar) than that produced by female
flowers (15.36 ? 1.48% sugar; Wilcoxon's signed rank:
T= 27, ? = 0.035).
Nectar from pohutukawa was more viscous and diffi-
cult to remove from flowers than flax nectar. To sample
flowers more easily, nectar sampling was biased towards
flowers with the most abundant supplies. For the 75
flowers sampled, nectar volume ranged from 25.8 to
61.53 ??. Average sugar concentration for pohutukawa
nectar was 53.22 ? 1.5%. Pohutukawa nectar contained a
higher sugar concentration than flax nectar (flax
21.09 ? 0.42%; Mann-Whitney test: U = 253, ? = 22 and
72, respectively, ? = 0.0001). Inflorescences contained
32.48 ?2.13 flowers.
Gecko activity and distribution patterns
During 3 nights of all-night censusing, 106 sightings of
H. maculatus were made on flax and 131 on the poh-
utukawa. Hourly censuses were conducted from 2000
to 0500 hours; lizards were found during the
2100-0400 hour sampling periods. The distribution of
sightings on flax versus pohutukawa was significantly
different (?2 = 30.45, d.fi =1,P< 0.001; Fig. 1). Lizard
activity on flax did not differ from an even distribution
throughout the night (?2 = 7.66, d.fi =7, ? = 0.36). Ac-
tivity on the pohutukawa was not evenly distributed
(?2 = 54.09, d.fi = 7, ? < 0.001), exhibiting a pro-
nounced peak at 2200 hours followed by a steady de-
cline in activity (Fig. 1). A marked difference in the
Time of
day (h)
POHUTUKAWA
Time of
day (h)
Fig. 1 Distribution of Hoplodactylus maculatus sightings on flax
and pohutukawa plants at different times of day
Flax
Pohutukawa
0-2 4-6 8-10 12-14 16-18
2-4 6-8 10-12 14-16
Body Mass (g)
Fig. 2 Frequency distribution of body mass of Hoplodactylus mac-
ulatus found visiting flax or pohutukawa plants
size of H. maculatus captured on flax versus poh-
utukawa was observed (Fig. 2); lizards foraging on flax
were smaller (5.0 ? 0.7 g) than those found on poh-
utukawa (12.2 ? 2.1 g; population range 1.4-17.9 g;
Mann-Whitney test: U = 996, ? = 37 and 58, respective-
ly, ? < 0.0001). No H. duvauceli were found foraging
on flax; those found on pohutukawa averaged
65.6 ? 4.4 g (range: 57-72 g). Two or three geckos
were often observed on the same flax stalk or poh-
utukawa inflorescence. In these instances, I did not ob-
serve any obvious aggression, avoidance, or other
forms of interaction.
Nineteen H. maculatus were resighted on different
days for a total of 64 observations. No individual lizard
was ever recorded from both flax and pohutukawa. Of
the nine geckos resighted on flax 34 times, five were al-
ways resighted on the stalk from which they were origi-
nally captured and the other four were recorded from
two stalks belonging to the same plant. The process of
capturing and marking appeared to disturb the geckos.
231
Table 1 Characteristics of flax plants at which geckos were and
were not observed. Measurements are presented as mean ? SE;
? = 42 with geckos absent, 17 with geckos present. Statistical
comparisons were made using the Mann-Whitney test
Variable Absent Present U
2.04 ?0.05 1.93 ?0.11 469 0.490
2.36 ?0.37 7.18 ?1.70 684 0.004
18.40 ?1.90 19.60 ?4.10 505 0.940
0.60 ?0.05 0.67 ?0.16 488 0.710
2.62 ?0.48 1.97 ?0.30 527 0.780
Number of plant stalks 2.45 ? 0.19 3.18 ? 0.47 587 0.190
Stalk height (m)
Number
of male flowers
Number
of female flowers
Nearest stalk (m)
Nearest plant (m)
1000
f
E
o
<D
D)
CJ)
O
100
10
10 20 30 40 50
Inflorescence number 60 70
? (Exponential Fit)
Fig. 3 Relationship between the number of inflorescences in a
pohutukawa patch and the log number of geckos observed within
the patch. Regression equation: y = 0.205* + 0.554, r2 = 0.909,
? = 0.003
As a result I stopped the process after 2 nights, only
marking a portion of the population sighted.
At the end of the study period, I recorded details con-
cerning the behavior of foraging lizards. Prolonged ob-
servations were not feasible; although working with a
red light seemed to reduce disruption of gecko behavior
many animals still sought refuge after only several sec-
onds of observation. Geckos were never observed eating
anything other than nectar during this study. The sex of
flax flowers being visited as well as the method of nectar
extraction was recorded for 15 geckos. Foraging lizards
used different methods while attempting to remove nec-
tar from male versus female flowers. Four lizards were
observed while feeding on male flowers; they all fed by
pressing their head into the corolla. Eleven lizards were
observed while feeding on female flowers; they all fed
by licking the outer base of the corolla. The small ex-
pected values generated by this distribution precludes
statistical analysis of these observations.
Of the 15 geckos for which the sex of visited flower
was determined, 4 were feeding on stalks where flowers
of both sexes were present. All 4 were feeding at male
flowers. There were a total of 6 male and 26 female
flowers on these four stalks. If flower choice were ran-
dom with regard to flower sex, the probability that all 4
lizards would have been found at male flowers is
? = 0.0125 (binomial distribution), indicating that geck-
os were preferentially feeding at male flowers.
The relationship of nectar availability
and plant characteristics to gecko distributions
Six measurements of flax stalks were compared for
plants where lizards were found foraging and those
where lizards were not observed (Table 1). The only
characteristic that differed significantly was the number
of male flowers; the stalks with lizards had significantly
more male flowers than stalks not visited by lizards.
A linear regression of the logarithm of gecko number
against the number of pohutukawa inflorescences in a
patch was calculated using 4 nights of censusing (Fig. 3).
Variation in the number of geckos present in a region of
the pohutukawa tree was sensitive to the number of in-
florescences present (r2 = 0.909, ? = 0.003).
Before nectar manipulation, geckos were evenly dis-
tributed between control and experimental pohutukawa
patches (31 versus 30 geckos per patch). During the ex-
perimental nectar addition, a marginally higher propor-
tion of geckos was found in areas where nectar had been
added (23 in control versus 43 in supplemented;
?2 = 330, d.fi = 1, ? = 0.069).
Discussion
Gecko distributions within plant species
Gecko visits to flowers appear to be based on an evalua-
tion of nectar availability. For flax plants, male flowers
secrete nectar at higher rates and in higher concentra-
tions than female flowers. Geckos tend to return to the
same flax stalks and were observed to feed preferentially
on male flax flowers. By choosing stalks with large num-
bers of male flowers, geckos can take advantage of great-
er rates of nectar replenishment to obtain nectar at high
rates with minimal travel. In some instances, male flower
standing nectar volume and sugar concentration are
greater than female flowers (Craig and Stewart 1988)
and would offer an even greater advantage to the male
flower preference detected in this study. Male flower
preference is displayed by other vertebrates; nectar feed-
ing birds also prefer male flax flowers (Craig 1988).
The distribution of lizards at inflorescence patches
within a single pohutukawa tree also reflects nectar
availability. As inflorescence numbers per patch increas-
es, gecko numbers increase exponentially, indicating an
attraction to greater numbers of flowers. The observed
trend for geckos to switch their attention to areas of nec-
tar-supplemented flowers offers additional support that
geckos monitor and respond to changes in nectar avail-
ability.
232
The distribution of geckos between plant species
Pohutukawa trees appear to offer greater opportunities
for nectar feeding than flax, offering nectar with higher
sugar concentrations and in greater abundance than indi-
vidual flax plants. Several factors need to be considered
concerning the observed distribution of small lizards pre-
dominately on flax and larger individuals on pohutukawa
(Fig. 2). First, the observed distributions could reflect
size-related differences in mobility; perhaps larger geck-
os may be more able or inclined to travel to the poh-
utukawa trees than smaller individuals. Since there was
only one pohutukawa tree flowering in the study area,
most geckos on the study site would have to travel in or-
der to feed on pohutukawa nectar. While the distance
needed to travel may have acted as a hinderance, it is not
likely to be the only factor involved. Small geckos were
found foraging on flax adjacent to the pohutukawa tree.
Hoplodactylus geckos may travel long distances from
pohutukawa trees, sometimes travelling over 50 m (Whi-
taker 1987a). Also, small H. maculatus appear more
likely to move longer distances than larger individuals
(Whitaker 1982a).
Second, the size of foraging geckos may influence
their ability to extract nectar from the two plant species.
The costs of using different flower types varies with
body size in nectarivorous sunbirds (Gill and Wolf
1975b). Pohutukawa nectar can be easily removed from
its shallow open-faced flowers, but the long tubular flax
flowers may represent a challenge to foraging geckos.
Whitaker (1987a) reports on adult H. duvauceli feeding
on flax and suggested that flax flowers are so robust that
only large geckos could force their petals apart to feed
on their nectar. However, in this study only the smaller
individuals of the smaller species, H. maculatus, were
observed on flax. If larger geckos are able to feed on
flax, then their absence from flax flowers appears to be
from choosing pohutukawa over flax. Because poh-
utukawa trees seem to offer more accessible nectar than
flax, smaller geckos could obtain nectar more efficiently
from pohutukawa. Thus, ability to extract nectar from
flowers does not account for the presence of small geck-
os on flax and larger ones on pohutukawa.
Finally, pohutukawa may be the nectar source best
suited to all size classes but smaller individuals may be
avoiding large H. maculatus and H. duvauceli. The pres-
ence of large gecko species can result in a habitat shift
among smaller species (Petren et al. 1993) and may lead
to pr?dation on smaller geckos (Bauer and DeVaney
1987; Bolger and Case 1992). Competitive displacement
has been suggested for other nectar feeding lizards.
While feeding on flower nectar, large Phelsuma sund-
berg geckos will displace smaller individuals (in Cheke
1984) and interspecific encounters among Phelsuma
geckos may result in the exclusion of smaller species
from access to nectar (in Cheke 1984). In at least one in-
stance, H. maculatus has been found in the stomach con-
tents of H. duvauceli (Barwick 1982) and H. maculatus
(Whitaker 1982b).
Of the three factors discussed above, size-related dif-
ferences in mobility and nectar extraction ability do not
account for the pattern of lizards distributions on the two
plant species. The only explanation consistent with my
results is that larger geckos use the more valuable nectar
source (pohutukawa), while smaller individuals avoid
larger geckos and settle for feeding on flax.
Gecko activity patterns
Differences in the activity patterns of flax- and poh-
utukawa-feeding geckos may reflect differences in quali-
ty of the two plant species as nectar sources. Because of
the larger number of flowers and the more concentrated
nectar, geckos on pohutukawa may be able to obtain sati-
ating levels of nectar quickly, resulting in a more peaked
activity pattern (Fig. 1
). The more dilute flax nectar, rel-
atively fewer number of flowers, and less accessible
nectar may make satiation at flax plants a more time
consuming process resulting in the more protracted, even
activity pattern. Most pohutukawa flowers still contain
nectar at the end of the night, so geckos probably do not
show a shorter span of activity on the pohutukawa be-
cause they have exhausted the available nectar. In nectar-
ivorous bird communities, larger species that control ac-
cess to the most rewarding nectar sources spend less time
foraging (Collins et al. 1990).
The results of my study indicate that geckos tend to
use the most profitable nectar sources and that nectar
sources are subject to intra- and interspecific partition-
ing. While nectar seems to be an important resource for
geckos, further studies are required before this is sub-
stantiated and the nature of the interaction between the
geckos and plants is fully understood. More complete
compositional analysis of the types of nectars taken by
geckos will reveal the extent to which their nutritional
needs are met by consuming nectar. Such studies need
to be coupled with those on the energetics of geckos
and their foraging activities away from nectar sources
to delineate the role of nectar in the life history of these
geckos. Finally, the potential impact of the geckos on
the life history of flax and pohutukawa needs to be ad-
dressed. Pollen collects on geckos visiting these plants
(Whitaker 1987a), suggesting that Hoplodactylus may
play a role in pollination. Furthermore, nectar depletion
resulting from large numbers of geckos visiting flowers
may affect the energetics of these plants or limit
their ability to attract pollinators. The extent of the
interaction between these organisms remains to be in-
vestigated.
Acknowledgements I thank Phil and Sue Thomson for assistance
on Korapuki. The New Zealand Department of Conservation pro-
vided logistical support and granted permission to work on Ko-
rapuki. Tony Whitaker provided advice that made this study possi-
ble. Maria Eifler and two anonymous reviewers provided helpful
discussion and comments on the manuscript. This work was sup-
ported by a Putnam Expedition Grant from the Museum of Com-
parative Zoology, Harvard University.
233
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