Gatun Lake Research Articles

Drug resistance

Parasites can invade new ecosystems if they are introduced with their native hosts or if they successfully infect and colonise new hosts upon arrival. Here, we ask to what extent an introduced parasite demonstrates specialisation among novel host species. Infection surveys across three field sites in Gatun Lake, Panama, revealed that the invasive peacock bass, Cichla monoculus, was more commonly infected by the introduced trematode parasite Centrocestus formosanus than were three other common cichlid fishes. Laboratory infection experiments were conducted to determine whether parasitism might be driven by differential encounter/exposure to parasites or by differential infection susceptibility/preference across different host species. These experiments were performed by controlling for parasite exposure in single host (compatibility) experiments and in mixed host (preference) experiments. In all cases, the peacock bass exhibited higher infection rates with viable metacercariae relative to the other potential fish hosts. Our experiments thus support that an introduced generalist parasite shows apparent specialisation on a specific novel host. Further studies are needed to determine whether these patterns of specialisation are the result of local adaptation following invasion by the parasite.

Female–female cooperation in polygynous oystercatchers

Methods Breeding unit size. Donor number is related to the size of the breeding unit (N individuals) as d o Np, where d is the number of different pollen parent genotypes represented in a maternal tree's fruit crop and p is the expected proportion of trees in staminate phase at any given time. Assuming, conserva- tively, that individual trees reproduce asynchronously twice per year 6 and that staminate and pistillate flowering phases each last seven days 20 , p is calculated to be 0.0712. Although these estimates could be effected by mosaicism 21 (the fusion of genetically distinct individuals), its frequency in the species examined here is low. Breeding unit area and radii. These parameters were calculated from paternity-analysis-based estimates of breeding unit size (Nd; Table 2) and the censused densities of adult, reproductively mature trees over 15 km 2 BCI. Based on the long-term censuses of C. Handley and E. Kalko, 6, 108 and 20 adults of F. dugandii, F. obtusifolia and F. popenoei, respectively, are known to occur on BCI. Because these species exhibit little spatial aggregation over the area censused, these densities are assumed to be representative of forested areas surrounding BCI, a conservative assumption given that approximately one-third of the area within 10 km of BCI is occupied by Lake Gatun where figs are absent. Breeding unit radii estimate the distances pollen-bearing, female fig wasps routinely disperse in search of receptive host trees. Although actual breeding populations of figs may deviate substantially from the assumed circular distribution, alternative structures (elliptical, for example) increase estimated wasp dispersal distances.

Asajirella gelatinosa in Panama: a bryozoan range extension in the Western Hemisphere (Ectoprocta: Phylactolaemata)

Asajirella gelatinosa, a freshwater bryozoan known previously only from eastern Asia is now reported in the Republic of Panama. Colonies were collected during April 1992 in the broad reaches of the lower Rio Chagres as it enters Gatun Lake, part of the Panama Canal system. A subsequent collection in 1998 included large colonies and numerous statoblasts from a region upstream of this site in the impounded Lago Alajuela (Madden Lake). Colonies and statoblasts resemble those from Japan in almost every detail. Human transport of statoblasts is a likely cause of this disjunct distribution.

Does Mammal Community Composition Control Recruitment in Neotropical Forests? Evidence from Panama

Patterns of seed predation, germination, and seedling herbivory were investigated in Panamanian forests. We hypothesized that seed and seedling survival would vary with differences in mammal community composition. We tested this hypothesis at five sites in mainland forests adjacent to Gatun Lake, full terrestrial mammalian granivore/herbivore communities with top predators; at five sites on Barro Colorado Island (BCI), also a full mammalian granivore/herbivore community but without the two largest cats; at one site each on five medium-sized islands, with rats, agouti, rabbit, and paca present; and on five small islands that support rats only. Experiments were replicated for Dipteryx panamensis, Gustavia superba, and Virola nobilis, all of which have large seeds. To assess seed removal, seeds were placed in wire exclosure cages and nearby outside the cages. There was no difference in removal rates between forest types, with almost all unprotected seeds removed at all sites. To assess post-removal seed fate, seeds of Gustavia and Virola were attached to threads and placed on the forest floor. All threaded seeds were victims of predation on small islands, whereas 34, 43, and 77% of threaded seeds were dispersed and buried on BCI, medium islands, and the mainland, respectively. To assess seedling herbivory, half of the wire exclosure cages were removed after germination, and seedling survival was assessed after 13–14 mo. Protection from mammals increased seedling survivorship by more than sixfold on the smallest islands, by threefold on the medium islands, by twofold on the mainland, and by less than twofold on BCI. The absence of the two largest cats and the exclusion of poachers from BCI was associated with lower seedling herbivory and higher seed predation than observed on the mainland. In contrast, extreme mammal defaunation on the small and medium islands had large and consistent effects on seedling recruitment, including increased seed predation and increased seedling herbivory relative to sites with more intact mammal communities.

DOES MAMMAL COMMUNITY COMPOSITION CONTROL RECRUITMENT IN NEOTROPICAL FORESTS? EVIDENCE FROM PANAMA

Patterns of seed predation, germination, and seedling herbivory were investigated in Panamanian forests. We hypothesized that seed and seedling survival would vary with differences in mammal community composition. We tested this hypothesis at five sites in mainland forests adjacent to Gatun Lake, full terrestrial mammalian granivore/herbivore communities with top predators; at five sites on Barro Colorado Island (BCI), also a full mammalian granivore/herbivore community but without the two largest cats; at one site each on five medium-sized islands, with rats, agouti, rabbit, and paca present; and on five small islands that support rats only. Experiments were replicated for Dipteryx panamensis, Gustavia superba, and Virola nobilis, all of which have large seeds. To assess seed removal, seeds were placed in wire exclosure cages and nearby outside the cages. There was no difference in removal rates between forest types, with almost all unprotected seeds removed at all sites. To assess post-removal seed fate, seeds of Gustavia and Virola were attached to threads and placed on the forest floor. All threaded seeds were victims of predation on small islands, whereas 34, 43, and 77% of threaded seeds were dispersed and buried on BCI, medium islands, and the mainland, respectively. To assess seedling herbivory, half of the wire exclosure cages were removed after germination, and seedling survival was assessed after 13–14 mo. Protection from mammals increased seedling survivorship by more than sixfold on the smallest islands, by threefold on the medium islands, by twofold on the mainland, and by less than twofold on BCI. The absence of the two largest cats and the exclusion of poachers from BCI was associated with lower seedling herbivory and higher seed predation than observed on the mainland. In contrast, extreme mammal defaunation on the small and medium islands had large and consistent effects on seedling recruitment, including increased seed predation and increased seedling herbivory relative to sites with more intact mammal communities.

Ecological correlates of trap response of a Neotropical forest rodent, Proechimys semispinosus

ABSTRACTPatterns of trap response in the Central American spiny rat (Proechimys semispinosus), a frugivorous forest rodent, were examined in relation to age, sex, density and resource abundance. Populations on four small islands (designated numbers 4, 8, 14, and 52) in Gatun Lake (central Panama) were sampled by live-trapping for four nights every month for four years. Trappability was calculated as the proportion of all individuals known to be alive that were actually captured. Mean body mass at first capture was within the range of subadult body masses and differed among islands but not between sexes. Trappabilities summed over the entire study were generally low and differed among islands and age classes (juveniles, subadults and adults) and between sexes. In general, adults were more trappable than juveniles and subadults, and females (particularly adults) were more trappable than males. Trappability and the number of captures per individual varied seasonally and were generally greatest during December and January. Monthly estimates of these two variables were examined for autocorrelation, and the order of the autoregressive error model was determined separately for each island population. The arcsine of trappability and the number of captures per individual were regressed on log10 densities of spiny rats and of fruiting trees and lianas after controlling for autocorrelation. Both trappability indices were negatively related to the density of fruiting trees and lianas on islands 8 and 14, indicating that spiny rats may be seasonally food-stressed. The number of captures per individual was related positively to the density of spiny rats on island 52 and negatively on island 8. Results indicate that spiny rat populations must be sampled with greater intensity and duration than temperate forest rodents.

Army ant population dynamics: the effects of habitat quality and reserve size on population size and time to extinction

A simple deterministic model is used to examine the effects of habitat size and quality upon the sizes and chances of extinction of isolated populations of a species of neotropical army ant, Eciton burchelli . The results of the model suggest that equilibrium population size is influenced more by habitat size than quality. Similarly, expected extinction times increase exponentially with habitat size but depend on quality only through its minor effects upon the equilibrium population size. Such effects may be crucial from a conservation point of view, however, because of the strong dependence of extinction probabilities on equilibrium population size. Density-independent effects in the form of queen senescence also have strong effects on expected extinction times but not on equilibrium population size. The results are used to predict which of several islands and peninsulas in and around Gatun Lake, Panama, have a high probability of still being populated by E. burchelli after their isolation from the surrounding tropical rain forest.

Tropical Forest Fragmentation and Isolation Promote Asynchrony Among Populations of a Frugivorous Rodent

1. Spiny rat Proechimys semispinosus populations on eight small islands in Gatun Lake (central Panama) were censused monthly for 1 year by live-trapping to examine seasonal patterns of demography in a fragmented landscape. 2. The annual fluctuation in density, breeding activity, age structure, and sex ratio from all populations combined was similar to populations studied previously in central Panama. Densities were lowest at the beginning of the rainy season and peaked in the late rainy season. Breeding occurred all year, but the number of births was greatest in April. The proportion of adults peaked in mid-dry season and was lowest in mid-rainy season. 3. Each separate population also showed distinct fluctuations, but the timing of peaks and lows varied among populations

Tallying the Tropics

C asual hikers probably don't notice the faded orange and green ribbons lying half buried in the leaf litter of the tropical forest on Barro Colorado Island (BCI) in Panama. Likewise, visitors to this island reserve probably don't miss the birds that no longer live here or the herds of whitelipped peccaries or the big cats such as puma and jaguar. Those animals seem to have disappeared after this 1,500-hectare (3,700-acre) hilltop was cut off from surrounding land by engineers creating Lake Gatun, part of the Panama Canal. Only because this reserve has served as a mecca for tropical biologists for the last 70 years do the data exist to reveal the changes in species likely brought about by isolation. And even if one of the ribbons did catch a watchful eye, how could that hiker guess its purpose? Two ecologists began tying ribbons around tree trunks no bigger than their thumbs more than 12 years ago. At the same time, they tacked metal plaques onto larger trees. Each plant so decorated received a code number. Stephen P Hubbell, now at Princeton University, and Robin B. Foster, now at Chicago's Field Museum of Natural History, surveyed more than 238,000 trees in this 1 kilometer by 0.5 km rectangle, placing each within a grid marked every 20 meters and again every 5 m. The name, size, and location of each became part of a computer database. Then considered an ambitious if not outrageous undertaking, the 'Tropical Forest Dynamics Plot now serves as a model for similar study sites being established across the globe (see sidebar). At first, the ribbon-tying scientists just wanted to monitor species diversity: They planned to count the kinds of trees present at regular intervals. But before long, many more people started trekking up to the plot, an hour's walk from BCI's buildings. For more than a decade, senior scientists, students, even amateur researchers have been watching these trees grow and produce fruit and have monitored the seedlings in order to learn what controls the organization of the forest. Hubbell and Foster have led two more censuses on the 50-hectare plot, one in 1985 and another in 1990. Meanwhile, other investigators have ventured elsewhere on the island and to nearby peninsulas, tracking fauna as well as flora. Bit by bit, the countless hours of note taking are building a new body of knowledge. ecology has gotten to the point where a lot of the easy things have been done, says Gregory S. Gilbert, a scientist at the Smithsonian Tropical Research Institute (STRI), which runs the island reserve. Now we must get to the underlying mechanisms.

A one-year study on carbon, water and nutrient relationships in a tropical C3 -CAM hemi-epiphyte, Clusia uvitana Pittier.

Diel (24 h) courses of CO2 and water-vapour exchange of leaves of hemi-epiphytic plants of Clusia uvitana Pittier (Clusiaceae) were measured under natural tropical conditions in the semi-evergreen moist forest of Barro Colorado Island, Panama, from January 1991 until January 1992. Plants were studied at two sites, in the crown of a 47-m tall tree (Ceiba pentandra) and on the shore of Lake Gatun, at a height of about 2-4 m. The following results were obtained: (1) Diel carbon gain was mainly a function of photosynthetic photon fluence rate (PPFR) on individual days. PPFR also strongly affected CAM activity. A leafless period of the host tree Ceiba pentandra resulted in higher incident PPFRs and slightly lower nighttime temperatures in the canopy of C uvitana; this led to increases in both daytime and nighttime CO2 fixation. (2) In fully mature sun leaves from the two sites, nocturnal net uptake of atmospheric CO2 occurred on almost all of the 71 days measured and nocturnal carbon gain was enhanced during the dry season. (3) In C. uvitana at the Lake site, 24-h carbon gain during the wet and dry season was similar to C. uvitana at the Ceiba site during the leafless period of the host tree. Overall CAM activity was lower at the Lake site. (4) Recycling of respiratory CO2 was a major route for nocturnal acid synthesis. Nocturnal net uptake of atmospheric CO2 was closely correlated with changes in titratable acidity, but accounted for only about 30% of the nocturnal increase in organic acids. (5) Mature shade leaves performed CAM only during the dry season, whereas in the wet season they showed atmospheric CO2 uptake exclusively in the light. (6) Independent of exposure or season, leaves less than about 12 wk old showed a C3 pattern of diel gas exchange and the level of titratable acidity was high day and night. (7) The annual carbon budget of outer canopy leaves of C uvitana in the Ceiba site was 1780 g CO2 m-2 a-1 and the average long-term water-use efficiency was 23 × 10-3 g CO2 g-1 H2 O. (8) Vegetative growth was strongly seasonal. Branch length increment and leaf area development was much higher in the wet season. Mineral element contents in these evergreen plants showed no age-related changes, but a significant proportion of elements was retrieved before abscission.

Methane emission by bubbling from Gatun Lake, Panama

We studied methane emission by bubbling from Gatun Lake, Panama, at water depths of less than 1 m to about 10 m. Gas bubbles were collected in floating traps deployed during 12− to 60−hour observation periods. Comparison of floating traps and floating chambers showed that about 98% of methane emission occurred by bubbling and only 2% occurred by diffusion. Average methane concentration of bubbles at our sites varied from 67% to 77%. Methane emission by bubbling occurred episodically, with greatest rates primarily between the hours of 0800 and 1400 LT. Events appear to be triggered by wind. The flux of methane associated with bubbling was strongly anticorrelated with water depth. Seasonal changes in water depth caused seasonal variation of methane emission. Bubble methane fluxes through the lake surface into the atmosphere measured during 24−hour intervals were least (10–200 mg m−2 d−1) at deeper sites (>7 m) and greatest (300–2000 mg m−2 d−1) at shallow sites (<2 m).

The decline of tree diversity on newly isolated tropical islands: A test of a null hypothesis and some implications

Six islands, each less than a hectare in area, were isolated in about 1913 from the mainland of central Panama by the rising waters of Gatun Lake. By 1980, the diversity of trees on all but one of these islands was far lower than on mainland plots of comparable size. A restricted subset of tree species has spread on these islands, notablyProtium panamense, Scheelea zonensis, Oenocarpus panamanus andSwartzia simplex. We constructed a null model to predict how chance would change tree diversity and the similarity of tree species compositions of different islands, assuming that each mature tree has equal chances of dying and/or reproducing, regardless of its species. This model cannot account for the diminished diversity of the changes in vegetation on these islands: some factors must be favoring a particular set of tree species.

Nest Predation and Differential Insular Extinction among Selected Forest Birds of Central Panama

Barro Colorado Island (BCI) is a 75—yr—old tropical forest isolate in Gatun Lake in the Republic of Panama, where depredation of understory bird nests is hypothesized to be more intense than in nearby mainland forest. Eight of 12 bird species in the terrestrial insectivore guild, with historical breeding records from BCI, are now extinct on the island. I tested the hypothesis that interspecific variation in nest design and placement underlies differential avian extinction from BCI. Selecting five species from the terrestrial insectivore guild, two that are extinct and three that persist on BCI, I constructed 200 hand—made mimics of their nests (40 each of five types) using species—specific nest materials collected in local forest. Artificial nests, containing Coturnix quail eggs as bait, were placed on two island and two mainland plots for representative incubation periods (15 d) and checked at 3—d intervals for predation. Experiments were repeated in February, late April, and June of 1988 to assess seasonal shifts in predation pressure in relation to timing of breeding in understory birds, and temporal constancy of interspecific differences in nest lost rates. BCI nest losses were higher than on the mainland in two of the three experiments and predation intensity varied seasonally. Peak losses occurred in April—May (late in the dry season), immediately before the onset of breeding in understory insectivorous birds, suggesting nest predation may influence the timing of nesting activities. Two nest types designed to mimic the nests of species now extinct from BCI suffered lower predation on BCI and on the mainland than three nest types of species persisting on BCI. However, ratios of island to mainland predation rates were disproportionately higher for BCI—extinct nest types than for BCI—persistent nests in two of the three samples. Results suggest that over evolutionary time species—specific rates of nest loss may determine clutch replacement capability, thereby causing variation in tolerance of inflated insular nest predation. In this way, interspecific variation in nest design and placement among terrestrial insectivorous birds may underlie the pattern of differential extinction from Barro Colorado Island.

Distribution and abundance of a tropical rodent, the spiny rat, on islands in Panama

ABSTRACTThe insular distribution and abundance of the spiny rat (Proechimys semispinosus) were studied by live-trapping rats on 50 small forested islands in Gatun Lake, Republic of Panama. Rats were found on 29 of these islands. Rat abundance was not correlated with island size or isolation, in contrast to temperate rodents, and may be due to the highly patchy nature of tropical forests and the differing floristic compositions of the study islands. Rat occurrence on islands was related positively to island size and negatively to island isolation. All islands greater than 2.1 ha contained spiny rats, whereas most islands less than 0.9 ha did not contain rats. The relationship between occurrence, size, and isolation are illustrated by a three-dimensional response surface. Results suggest that colonization and extinction regularly occur on these islands and that (1) larger islands with year-round fruit production, regardless of isolation, will have persistent populations of rats, (2) small isolated islands will never or only very rarely have rats owing to a lack of a year-round food source and the difficulty of immigration, and (3) small near islands frequently will have rats when fruits arc present. No other species of mouse-or rat-like rodents were captured on the islands. We suggest that P. semispinosus is a better colonist than other sympatric forest species, is more of a habitat generalist, and has a more flexible demographic structure allowing persistence in heterogeneous and changing environments.

Annual mortality patterns of a mammal community in central Panama

Few data exist on mortality of tropical forest mammals. Though dramatic reductions in population size have been reported for particular mammals in some years, such information is largely anecdotal and generally attributed to an unusual extrinsic factor such as a forest-wide fruit shortage or outbreak of a virulent disease (e.g. Collias & Southwick 1952, Foster 1982, Kaufmann 1962). It would be useful to have annual mortality data from specific mammal populations and communities, both for comparative purposes and to focus investigation on possible causal factors. However, the difficulty of collecting mortality data in the tropical forest is well appreciated. Vultures show a remarkable ability to locate fresh carcasses (Houston 1986) and the warm, humid environment and abundant insect life results in rapid destruction of any remainder. Here I present data collected over a two-year period on dead mammals recovered from a single lowland tropical forest site in central Panama. These data, though not extensive, give an indication of annual mortality in this mammalian community. In addition, for a few species, mortality peaks occurred at approximately the same time each year. Interspecific mortality peaks were not always synchronous, suggesting that factors adverse for some species may have had little effect on others in this same habitat. The survey was carried out from 1 September 1977 through 31 August 1979, a total of 24 months, on Barro Colorado Island, a 1500 ha nature preserve located in Gatun Lake, Republic of Panama. Details of the climate, topography, flora and fauna of Barro Colorado can be found in Leigh et al. 1982. Barro Colorado Island lends itself to the collection of infrequent and unpredictable data such as the occurrence of dead mammals because typically many researchers are active in the forest on any given day throughout the year. This greatly improves the chances that dead mammals will be encountered

Flight physiology of migrating Urania fulgens (Uraniidae) moths: kinematics and aerodynamics of natural free flight

Air speeds and wing kinematics were determined for the Neotropical moth Urania fulgens in natural migratory flight over Lake Gatun, Republic of Panama. Morphological parameters are presented for the same insects filmed in free flight. A quasi-steady aerodynamic analysis was used to show that unsteady mechanisms of lift generation are probably not necessary to produce the forces necessary for fast forward flight. Mechanical power requirements of forward flight were estimated from the biomechanical and morphological data. Over an airspeed range of 1.5 to 4.5 m/s, the mechanical power required to fly is predicted to increase dramatically with forward speed. A comparison of estimated metabolic rates with endogenous lipid reserves suggests that U. fulgens feeds extensively on flower nectar during migration from Central into South America.

Morphometrics, Airspeed, Thermoregulation, and Lipid Reserves of Migrating Urania fulgens (Uraniidae) Moths in Natural Free Flight

The flight physiology of migrating Urania fulgens (Uraniidae) was studied under natural conditions. Airspeed, as opposed to ground speed, was measured directly for moths in free flight over Lake Gatun, Panama. The same insects were then captured, and their thoracic temperatures were measured. Detailed morphological measurements were made on all individual insects. Although significantly different in many aspects of body size, male and female moths flew at the same airspeeds and thoracic temperatures. These results suggest that, during a migratory flight, airspeed and thoracic temperature in U. fulgens remain independent across a range of ambient temperatures and morphological sizes. Surprisingly, although males and females differed in all other morphological parameters measured, they did not differ in body length. The invariance of mean body length between males and females suggests that this trait is constrained in some unknown way. It is argued that stored lipid reserves alone are unlikely to provide sufficient energy for the continuous flapping flight characteristic of U. fulgens during migration, and carbohydrates imbibed at flowers are the most probable source of additional energy.

Genetic Structure and Variability in the Green Iguana (Iguana iguana)

A population of green iguanas (Iguana iguana) that inhabits the forests around Gatun Lake in central Panama faces an apparent scarcity of suitable nesting sites. To investigate whether the disjunct distribution of nesting sites in this area and strong nesting site fidelity by female iguanas has contributed to genetic structuring of the population, samples obtained from females captured at three main nesting sites were analyzed electrophoretically. Significant allele frequency differences existed among nesting sites at one of the two polymorphic loci inspected. The pattern of allele frequency differences was consistent with what is known of iguana movement patterns and suggests that the genetic structure of the population has been influenced by nesting site availability and distribution. Additional electrophoretic analyses were conducted on samples obtained from three iguana populations from central Panama, western Panama, and Venezuela. Low levels of heterozygosity and polymorphism were observed in each of these populations, yet significant differences at three polymorphic loci were demonstrated. The management implications of the results are discussed.

Patterns of Abundance and the Form of the Species-Area Relation

There are several reasons for questioning whether derivations of species-area relations from species-abundance distributions apply to insular communities. First, the derivations assume that the summed density of individuals of all species is a constant. In fact, in most archipelagos this quantity varies systematically with island area (Wright 1980). Second, the appropriate species-abundance distributions may not occur on islands. The logarithmic series is characterized by large numbers of very rare species, an unlikely event when there are barriers to immigration. Derivations from the negative binomial assume that a parameter is negative, but in fact it is positive for 102 of the 111 communities that have been analyzed. The lognormal systematically overestimates the abundances of the most common species, and if species-abundance distributions observed for small islands are lognormal, they are severely truncated. Finally, the parameters of species-abundance distributions and species-area relations do not agree for birds in the West Indies or on islands in Gatun Lake, Panama. These analyses do not suggest that species-abundance distributions determine species-area relations for insular communities.

Tropical zooplankton: phenotypic and spatial analyses of polymorphic Ceriodaphnia cornuta (Crustacea: Cladocera)

SUMMARY. 1. The predation‐balanced polymorphism of Ceriodaphnia cornuta (a small‐eyed horned form and a large‐eyed unhorned form) in Lake Gatun, Panama, is re‐examined approximately 12 years after the original study (Zaret, 1972a). We consider morphological variability, distribution and fecundity of the population inhabiting Laboratory Bay.2. The two morphological characters emphasized by Zaret, eye size and horn length, are studied in detail. The C. cornuta population exhibits wide, yet continuous variation; coefficients of variation are 28% or eye diameter and range from 114% to 136% for horn and spine lengths. Phenotypic variation in the population is characterized by: (1) negative correlations between spine lengths and eye size; (2) positive correlations among spines from different body regions (anterior horn, lateral spines, and tail spine); and (3) weak correlations between body length and spine lengths.3. While the horned and unhorned forms continue to coexist in Laboratory Bay. the frequencies of horned forms have increased in the open water areas since the time of the original study. The increase in horned form frequencies may be due to changes in fish predation intensity. Did vertical distribution patterns of the forms remain very similar to each other.4. In contrast to previous laboratory findings, egg ratio calculations from field samples revealed similar clutch sizes for the horned and unhorned forms. The egg development time of the horned form, however, was found to be approximately 30% longer than that of the unhorned form, perhaps because of increased moulting difficulties produced by the presence of spines.