Monitor Lizard Growth Strategies (Frýdlová et al., 2020)

📖 16 min read~2857 words

What makes the Komodo dragon the largest lizard alive? The answer lies not in its unique biology but in a spectrum of growth strategies shared across the entire monitor-lizard family — a spectrum that a series of studies from Charles University Prague has spent two decades mapping, bone by bone. This is an original editorial summary of peer-reviewed research by Petra Frýdlová, Daniel Frynta, and colleagues; readers are encouraged to consult the primary sources listed below for full methodology and data.

Table of Contents

• Quick Facts
• Paper Overview
• The Growth-Plate Discovery
• Sexual Size Dimorphism and Extended Growth
• Varanids in Broader Squamate Context
• Habitat, Ecology, and the Road to Gigantism
• What This Means for the Komodo Dragon
• Myths vs Facts
• Key Takeaways
• Frequently Asked Questions
• Sources & Further Reading

Quick Facts

Feature	Detail
Primary paper (Frýdlová et al. 2020)	Proceedings of the Royal Society B, vol. 287, no. 1941, article 20202737. DOI: 10.1098/rspb.2020.2737
Primary paper (Frýdlová et al. 2019)	Scientific Reports 9, 18913 (2019). DOI: 10.1038/s41598-019-54573-5
Institution	Department of Zoology, Faculty of Science, Charles University, Prague (with Czech Technical University & University of Salzburg)
Core method	Micro-computed tomography (micro-CT) of femoral growth-plate cartilage across lizard species
Scope (2020 paper)	194 specimens across 85 lizard species; ancestral-state reconstruction across 164 species
Key conclusion	Determinate (bounded) skeletal growth is ancestral in squamates; indeterminate growth in large varanids is a derived exception tied to ecological gigantism
Komodo body mass range	Hatchlings ≈ 100 g; adult males up to ≈ 90 kg — nearly three orders of magnitude
Body-size span within Varanus	Approximately four orders of magnitude, from pygmy monitors (≈ 10 g) to the Komodo dragon

Paper Overview

For most of the twentieth century, reptiles were textbook examples of indeterminate growers — animals that never fully stop growing, merely slowing with age. Monitor lizards, with their enormous size variation and strikingly dimorphic adults, seemed to confirm the stereotype. A research programme led by Petra Frýdlová and Daniel Frynta at Charles University Prague has systematically dismantled that assumption using micro-computed tomography (micro-CT) of femoral growth-plate cartilage — the precise anatomical marker of whether longitudinal bone growth is still possible.

The series produced three interlocking papers: Frýdlová et al. (2017) in Zoomorphology (DOI: 10.1007/s00435-016-0338-3), which established the varanid baseline; Frýdlová et al. (2019) in Scientific Reports (DOI: 10.1038/s41598-019-54573-5), which rejected the universality of indeterminate growth across Iguania; and Frýdlová et al. (2020) in Proceedings of the Royal Society B (DOI: 10.1098/rspb.2020.2737), which reconstructed ancestral growth state across 164 squamate species. The combined finding: determinate growth is ancestral in lizards; extended growth in large varanids is a derived exception.

The Growth-Plate Discovery: Varanids Under the Scanner

The key anatomical structure in this research programme is the growth-plate cartilage (GPC) — a thin zone of proliferating chondrocytes at the end of each long bone. While it persists, the bone can elongate; once it fuses, longitudinal growth ceases permanently. In the Prague group's work, GPC presence or absence in the proximal femur became the operational marker separating determinate from indeterminate growers.

Their 2017 Zoomorphology paper surveyed thirteen varanid species using micro-radiography and micro-CT. The headline finding was a body-size gradient: small-bodied monitor species showed growth-plate fusion in adults, whereas large-bodied species — including those in the size range of Varanus komodoensis — retained open growth plates well into adulthood. Komodo dragons are therefore not simply "ordinary lizards that got very big": their growth-plate biology has been secondarily modified to support a prolonged skeletal growth window that most squamate relatives lost early in evolution.

Sexual Size Dimorphism and Extended Male Growth

Before the micro-CT era, an earlier contribution from the same Prague group established the proximate developmental mechanism behind sexual size dimorphism in varanids. Frynta, Frýdlová, and colleagues published "Ontogeny of Sexual Size Dimorphism in Monitor Lizards: Males Grow for a Longer Period, but not at a Faster Rate" in Zoological Science (2010; DOI: 10.2108/zsj.27.917), following 22 male and 13 female Varanus indicus (mangrove monitor) from hatching to 33–40 months at Prague Zoo, with repeated measurements of snout-vent length and body mass. Sexing was confirmed by ultrasound.

The central finding was deceptively simple: male and female mangrove monitors grow at statistically indistinguishable rates during early development. Dimorphism does not emerge because males sprint ahead; it emerges because females taper off sooner. Males continue adding body length and mass for several additional months after female growth has effectively plateaued. The study's title distils the result perfectly: it is the duration of growth, not the rate, that separates the sexes.

This has a direct corollary for understanding large-bodied varanids. In Varanus komodoensis, where adult males routinely reach 70–90 kg against female averages of 40–50 kg, the same mechanism appears to operate at a larger scale and over a longer timeframe. Males retain open growth plates and continue growing beyond sexual maturity; females, whose reproductive investment is dominated by egg production rather than further somatic growth, arrest skeletal growth earlier. The Prague data suggest this pattern is phylogenetically deep in varanids, not a Komodo-specific quirk.

Sex	Growth Rate (juvenile phase)	Duration of Active Growth	Primary Driver of Size Difference
Males	Similar to females	Longer — extends post-female-plateau	Extended growth window, open growth plates
Females	Similar to males	Shorter — plateau coincides with reproductive maturation	Earlier growth-plate closure; reproductive allocation

A complementary study on sex-specific growth arrest in the gecko Paroedura picta (Kubička et al., iScience, 2022) showed that female growth-plate closure is triggered by rising estrogens at reproductive maturation — providing the hormonal mechanism consistent with the Prague group's anatomical observations in varanids.

Varanids in Broader Squamate Context

The 2019 Scientific Reports paper moved the Prague programme into Iguania. Its micro-CT survey found that most adult pleurodont specimens ("iguanas") showed complete absence of GPC, indicating early growth arrest, while most acrodont specimens (agamas, chameleons) retained apparent GPC. The paper's title is its thesis: "Universality of indeterminate growth in lizards rejected."

The 2020 Proceedings of the Royal Society B paper synthesised all previous data into a phylogenetic framework. Examining 194 specimens from 85 species and performing ancestral-state reconstruction across 164 lineages — including the tuatara — the authors concluded that complete GPC loss is the predominant ancestral state in squamates. Extended growth with retained plates has evolved independently multiple times, clustering in lineages characterised by large adult body size or intense sexual selection. Large-bodied varanids fall squarely in that derived category: the Komodo dragon's capacity for three-order-of-magnitude growth is an unusual evolutionary privilege, not a default lizard birthright.

Habitat, Ecology, and the Road to Gigantism

The Prague physiological data interlock with macroevolutionary analyses of body-size evolution in Varanus. Collar, Schulte & Losos (2011) demonstrated that habitat use is the dominant predictor of size trajectory: terrestrial varanids evolve toward gigantism; rock-dwellers toward miniaturisation; arboreal species occupy an intermediate range. The Komodo dragon is the apex expression of the terrestrial trajectory, reinforced by its insular setting (see our Island Gigantism page for the ecological dimension).

What the Frýdlová/Frynta research adds is the proximate developmental mechanism that makes the terrestrial trajectory achievable. Natural selection can favour large size, but unless the skeletal programme permits prolonged growth, selection cannot translate into phenotype. The finding that growth-plate persistence scales with eventual adult mass — large-destined species retain open plates longer; small-destined species close them early — is not merely a consequence of size but a precondition for it. The genus spans roughly four orders of magnitude in adult body mass (V. brevicauda at ~10 g to V. komodoensis at ~90 kg). That range is biologically possible because varanid growth biology accommodates it.

What This Means for the Komodo Dragon

Taken together, the Prague group's papers reframe Varanus komodoensis from a biological anomaly into the extreme endpoint of a well-understood biological axis. Several practical implications follow.

1. Prolonged male growth underpins dominant male size. The largest wild Komodo males — those holding territories and monopolising mating access — may still be growing in their second decade. Dominance hierarchies partly reflect which males have maintained an open growth window longest. Removing large adult males through illegal collection therefore removes not just individuals but the animals farthest along a multi-year developmental trajectory.

2. Female reproductive investment competes with somatic growth. The shorter female growth window documented in V. indicus likely applies across large varanids. Females that begin reproducing early trade somatic growth for reproductive output — which may explain why female Komodo dragons rarely match male size even in resource-equivalent environments.

3. Growth-plate biology constrains captive management. A female that appears to have "stopped growing" may simply have closed her epiphyses on schedule, not stalled in development. A male still gaining length in his second decade is following his species' programme, not over-growing pathologically. Zoos and breeding centres benefit from sex-stratified, species-specific growth benchmarks.

4. Evolutionary uniqueness demands conservation priority. The growth programme permitting Komodo gigantism is the product of millions of years of selection under conditions (large island, megafaunal prey, reduced predator competition) that cannot be replicated. The loss of the species would extinguish a singular developmental strategy, not merely a large lizard.

Myths vs Facts

Common Misconception	What the Research Shows
Reptiles are classic indeterminate growers — they never truly stop.	Frýdlová et al. (2019, 2020) demonstrate that permanent growth-plate closure is ancestral and predominant in squamates. Most lizards are determinate growers.
The Komodo dragon's giant size is typical for a large lizard.	Komodo dragons retain an extended growth window that most lizards — including many varanid relatives — have lost. Their size is a derived biological exception.
Male Komodo dragons grow faster than females, which is why they are bigger.	Frynta et al. (2010) show growth rate is similar between sexes; dimorphism arises because males grow for a longer period after females plateau.
Body size in monitor lizards is primarily determined by food availability.	Habitat type (terrestrial vs. arboreal vs. rock-dwelling) is the dominant macroevolutionary predictor of Varanus body size (Collar et al. 2011); growth-plate biology provides the developmental capacity to fulfil each trajectory.

Key Takeaways

• Determinate growth is ancestral in squamates. Complete growth-plate closure — a hard ceiling on body size — is the norm in lizards, not a mammalian peculiarity.
• Large varanids are derived outliers. Open femoral growth plates in large-bodied monitor species are a secondarily evolved feature that enables prolonged skeletal growth and gigantism.
• Sexual dimorphism is a duration effect, not a rate effect. Males and females grow at similar rates; males continue after females plateau, a pattern rooted in sex-hormone regulation of growth-plate closure.
• Habitat ecology and developmental biology are inseparable. Terrestrial habitat selection favours large size; retained growth plates are the developmental mechanism that makes that trajectory achievable.
• Conservation must protect age structure, not just numbers. A population lacking large adult males has lost decades of irreplaceable developmental investment that cannot be quickly rebuilt.

Frequently Asked Questions

What exactly is "determinate" versus "indeterminate" growth?

Determinate growth means skeletal elongation is bounded — growth-plate cartilage fuses and longitudinal bone growth permanently stops, usually near sexual maturity. Indeterminate growth means plates remain open and the skeleton can in principle continue elongating throughout life. Mammals and birds are classically determinate; most lizards were assumed indeterminate. The Prague group demonstrated that most lizards are, in fact, determinate growers.

How did micro-CT improve on earlier methods?

Earlier approaches relied on histological sectioning (destroys specimens), basic X-ray (low resolution for cartilage), or external measurements (cannot reveal internal plate status). Micro-CT generates three-dimensional images of bone architecture non-destructively, allowing the Prague team to survey large numbers of museum and zoo specimens and dramatically expand the comparative dataset.

Does the Komodo dragon truly have indeterminate growth?

Large varanids retain functionally open growth plates throughout most of their lives — growth slows but the anatomical capacity persists for decades. Whether it ever reaches absolute zero is unresolved. Practically, the growth window is vastly longer than in most lizards, which explains the species' enormous adult size range.

Why do female Komodo dragons stop growing sooner than males?

Based on the Prague group's V. indicus data and supporting hormonal research, females close growth plates earlier — likely triggered by rising estrogens at reproductive maturity. Once a female is large enough to reproduce successfully, allocating resources to egg production rather than further skeletal growth yields higher fitness. Males, where large size directly improves combat success and mate access, benefit from a longer window.

Is this research specifically about the Komodo dragon?

The work is primarily comparative, using the full Varanidae and squamate tree as its study system. Varanus komodoensis is the extreme large-body endpoint that illustrates predictions, but conclusions are drawn from dozens of species. The Komodo dragon's growth strategy can only be understood as the apex of a family-wide axis, not a stand-alone anomaly.

How does the Komodo dragon compare to extinct giant varanids?

The Pleistocene Varanus (Megalania) priscus of Australia potentially reached 500–600 kg — far exceeding the Komodo dragon. If growth-plate persistence scales with ultimate size as the Prague data predict, Megalania presumably had an even more extended growth window. The Komodo dragon may represent a surviving intermediate rather than the biological ceiling of varanid gigantism.

What are the conservation implications?

A Komodo population skewed toward juveniles and subadults by poaching loses not just biomass but the ecological function of large dominant males. Recovery to full ecological functioning may take decades because the multi-decade developmental programme cannot be compressed. Age-structure preservation is therefore as important as total head counts in conservation management.

Are the findings relevant to captive breeding?

Captive protocols relying on mammalian growth benchmarks may misjudge health and readiness for reintroduction. The Prague framework predicts that growth assessments should be sex-stratified, that a female reaching plateau is not compromised, and that a male still gaining length in his second decade is developing normally — insights that support more accurate monitoring and better-timed releases.

Sources & Further Reading

1. Frýdlová, P., Mrzílková, J., Šeremeta, M., Křemen, J., Dudák, J., Žemlička, J., Minnich, B., Kverková, K., Němec, P., Zach, P., & Frynta, D. (2020). "Determinate growth is predominant and likely ancestral in squamate reptiles." Proceedings of the Royal Society B: Biological Sciences, 287(1941), 20202737. https://doi.org/10.1098/rspb.2020.2737
2. Frýdlová, P., Mrzílková, J., Šeremeta, M., Křemen, J., Dudák, J., Žemlička, J., Němec, P., Velenský, P., Moravec, J., Koleška, D., Zahradníčková, V., Jirásek, T., Kodym, P., Frynta, D., & Zach, P. (2019). "Universality of indeterminate growth in lizards rejected: the micro-CT reveals contrasting timing of growth cartilage persistence in iguanas, agamas, and chameleons." Scientific Reports, 9, 18913. https://doi.org/10.1038/s41598-019-54573-5
3. Frýdlová, P., Nutilová, V., Dudák, J., Žemlička, J., Němec, P., Velenský, P., Jirásek, T., & Frynta, D. (2017). "Patterns of growth in monitor lizards (Varanidae) as revealed by computed tomography of femoral growth plates." Zoomorphology, 136, 95–106. https://doi.org/10.1007/s00435-016-0338-3
4. Frynta, D., Frýdlová, P., Hnízdo, J., Šimková, O., Cikánová, V., & Velenský, P. (2010). "Ontogeny of sexual size dimorphism in monitor lizards: males grow for a longer period, but not at a faster rate." Zoological Science, 27(12), 917–923. https://doi.org/10.2108/zsj.27.917
5. Collar, D.C., Schulte, J.A., & Losos, J.B. (2011). "Evolution of extreme body size disparity in monitor lizards (Varanus)." Evolution, 65(9), 2664–2680. https://doi.org/10.1111/j.1558-5646.2011.01335.x
6. Kubička, L., Tureček, A., Kučera, T., & Kratochvíl, L. (2022). "Sex-specific growth arrest in a lizard." iScience, 25(4), 104041. https://doi.org/10.1016/j.isci.2022.104041
7. Pianka, E.R. (1995). "Evolution of body size: varanid lizards as a model system." The American Naturalist, 146(3), 398–414. https://doi.org/10.1086/285806
8. Jessop, T.S., Madsen, T., Sumner, J., Rudiharto, H., Phillips, J.A., & Ciofi, C. (2006). "Maximum body size among insular Komodo dragon populations covaries with large prey density." Oikos, 112(3), 422–429. https://doi.org/10.1111/j.0030-1299.2006.13793.x