Module 5: Termites & Social Insects
Termites are eusocial cockroaches—an independent origin of complex sociality from the Hymenoptera (ants, bees, wasps). With colonies of millions, they build mounds that rival human architecture in sophistication, cultivate fungus gardens, and digest the most abundant organic polymer on Earth through intimate symbioses with microorganisms. This module explores the physics and biochemistry of their remarkable biology.
1. Eusociality Without Haplodiploidy
Termites (order Blattodea, infraorder Isoptera) represent the only non-hymenopteran lineage to evolve full eusociality with morphologically distinct castes. Unlike ants, bees, and wasps, termites have a diploid sex determination system—both males and females are diploid (2n). This is significant because Hamilton's kin selection theory (1964) originally invoked haplodiploidy to explain eusociality in Hymenoptera: in haplodiploid systems, sisters share 75% of their genes (relatedness \(r = 3/4\)), making it potentially more advantageous to raise sisters than daughters (\(r = 1/2\)).
Hamilton's Rule
Altruistic behavior evolves when:
\[r \cdot B > C\]
where \(r\) is the coefficient of relatedness between altruist and recipient, \(B\) is the fitness benefit to the recipient, and \(C\) is the fitness cost to the altruist. For termites with standard diploid genetics (\(r = 1/2\) for full siblings), eusociality requires\(B > 2C\)—the benefits of helping must be at least twice the cost.
Several factors may satisfy this condition in termites: (1) nest inheritance—workers may eventually inherit the colony and reproduce, (2) fortress defense—the nest is both home and food source (wood), making defense highly valuable, (3) chromosomal translocation—some termite species have extensive translocations that effectively increase \(r\) among siblings, and (4) symbiont transmission—gut symbionts required for cellulose digestion are only transmitted through proctodeal (anal) trophallaxis, requiring social contact.
Caste System
A mature Macrotermes colony contains: a single queen (physogastric, abdomen swollen to 10 cm, laying ~30,000 eggs/day), a king (lifelong mate, unlike Hymenoptera where males die after mating), workers (sterile, both male and female), and soldiers (defensive caste with enlarged mandibles or chemical spray nozzles). Total colony size can exceed 2 million individuals.
2. Cellulose Digestion: Symbiotic Biochemistry
Cellulose (\((\text{C}_6\text{H}_{10}\text{O}_5)_n\)) is the most abundant organic polymer on Earth, yet almost no animal can digest it alone. Termites have solved this problem through two independent symbiotic strategies.
Lower Termites: Flagellate Protists
Basal termite lineages harbor obligate mutualistic flagellate protists (e.g., Trichonympha, Pyrsonympha) in their enlarged hindgut. These anaerobic protists phagocytose wood particles and hydrolyze cellulose using endogenous cellulases. The hydrolysis reaction is:
\[(\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \to n\,\text{C}_6\text{H}_{12}\text{O}_6\]
The glucose is then fermented anaerobically to short-chain fatty acids (primarily acetate):
\[\text{C}_6\text{H}_{12}\text{O}_6 \to 2\,\text{CH}_3\text{COOH} + 2\,\text{CO}_2 + 8[\text{H}]\]
The termite absorbs acetate across the hindgut epithelium and oxidizes it aerobically, capturing \(\sim 200\,\text{kJ/mol}\) from fermentation and the subsequent aerobic oxidation of acetate (\(\Delta G^\circ = -870\,\text{kJ/mol}\) for complete oxidation of glucose).
Higher Termites: Fungus Farming
The subfamily Macrotermitinae (representing ~70% of termite biomass in African savannas) has evolved an external digestion system: they cultivate the basidiomycete fungus Termitomyces in elaborate fungus gardens (fungus combs) within the mound. Workers collect plant material, chew it into a substrate, inoculate it with fungal spores, and feed on the fungal nodules (primordia) and partially digested substrate.
This is a remarkable case of convergent evolution with leaf-cutter ants (Attini), which independently evolved fungus farming ~50 million years ago. Termitomyces and the leaf-cutter ant cultivar (Leucoagaricus) are not closely related—the agricultural strategy evolved independently.
Enzyme Efficiency Comparison
Fungal cellulases (from Termitomyces) show superior kinetics compared to bacterial cellulase systems, following Michaelis-Menten kinetics:
\[v = \frac{V_{\max}[\text{S}]}{K_m + [\text{S}]}\]
Termitomyces achieves \(V_{\max} \approx 12\,\text{g/L/h}\) with \(K_m \approx 5\,\text{g/L}\), while bacterial cellulosomes typically show \(V_{\max} \approx 6\,\text{g/L/h}\) and \(K_m \approx 15\,\text{g/L}\). At typical substrate concentrations (~10 g/L), fungal cellulases are approximately twice as efficient.
3. Mound Ventilation: Living Architecture
Macrotermes mounds can reach 9 meters tall and house 2 million individuals with a combined metabolic output of ~100 W—comparable to a human. The mound must exchange \(\sim 10^3\) liters of air per hour to maintain \(\text{O}_2\) above 18% and \(\text{CO}_2\) below 3%. The Eastgate Centre in Harare, Zimbabwe (architect Mick Pearce, 1996) was inspired by termite mound ventilation principles, achieving passive cooling without conventional air conditioning.
Stack Ventilation: Bernoulli's Principle
The driving force for mound ventilation is the stack effect. Warm, CO\(_2\)-enriched air from the colony metabolism is less dense than ambient air, creating a buoyancy-driven pressure difference:
\[\Delta P = \rho_{\text{air}}\,g\,\Delta h\,\frac{T_{\text{in}} - T_{\text{out}}}{T_{\text{out}}}\]
where \(\Delta h\) is the effective chimney height (~3 m for a typical mound),\(T_{\text{in}}\) is the nest air temperature, and \(T_{\text{out}}\) is the ambient temperature. For \(\Delta T = 5\,\text{K}\) and \(\Delta h = 3\,\text{m}\):
\[\Delta P = 1.225 \times 9.81 \times 3 \times \frac{5}{300} \approx 0.6\,\text{Pa}\]
This small pressure difference (~0.006% of atmospheric) is sufficient to drive measurable airflow through the mound's network of channels, with velocities of 1–10 cm/s.
Luscher's Convection Model (1961)
Martin Luscher proposed the first mechanistic model of mound ventilation: warm, stale air rises through a central chimney, flows through thin-walled peripheral channels near the mound surface (where CO\(_2\) diffuses out and O\(_2\) diffuses in), then descends and returns to the nest. This model explained the cathedral-like structure of Macrotermes mounds with their prominent central chimney.
Turner's Metabolic Gas Exchange Model
J. Scott Turner (2000) refined this picture by showing that ventilation is not a steady-state convective loop but rather a tidally oscillating system driven by diurnal temperature cycles. During the day, solar heating of the mound surface drives air inward (reversed from Luscher's model); at night, metabolic heat from the colony drives air outward. Gas exchange occurs primarily through the thin-walled mound surface channels, which act as a lung-like exchange surface.
\[\dot{V} = \frac{\Delta P \cdot A_{\text{channel}}}{\mu \cdot L / D_h^2}\]
where \(\dot{V}\) is the volumetric flow rate, \(A_{\text{channel}}\) is the total cross-sectional area of ventilation channels, \(\mu\) is air viscosity,\(L\) is the channel length, and \(D_h\) is the hydraulic diameter.
Termite Mound Cross-Section & Ventilation
4. Fungus Farming: Convergent Agriculture
The Macrotermitinae–Termitomyces mutualism originated ~30 million years ago in African rain forests. The fungus provides three critical services: (1) it breaks down lignin and cellulose that the termites cannot digest, (2) it concentrates nitrogen into protein-rich nodules (3–8% N vs ~0.1% in wood), and (3) it converts plant secondary metabolites (tannins, phenolics) into non-toxic forms.
Enzyme Efficiency: Fungal vs Bacterial Cellulases
The catalytic superiority of Termitomyces cellulases can be quantified through the specificity constant \(k_{\text{cat}}/K_m\), which measures catalytic efficiency at low substrate concentrations:
\[\eta = \frac{k_{\text{cat}}}{K_m} = \frac{V_{\max}/[E]_0}{K_m}\]
Fungal cellulases achieve \(\eta \approx 2.4\,\text{L/(g}\cdot\text{h)}\) compared to \(\eta \approx 0.4\,\text{L/(g}\cdot\text{h)}\) for bacterial cellulosomes—a 6-fold advantage. This efficiency gain arises from the fungal strategy of secreting free cellulases that can access cellulose fibrils from multiple directions, versus the bacterial cellulosome's tethered architecture that restricts access angles.
Convergent Evolution with Leaf-Cutter Ants
The independent origins of fungus farming in termites and ants provide a powerful test of convergent evolution:
| Feature | Macrotermitinae | Attini (leaf-cutters) |
|---|---|---|
| Origin | ~30 Mya, Africa | ~50 Mya, South America |
| Cultivar | Termitomyces (Agaricales) | Leucoagaricus (Agaricales) |
| Substrate | Dead wood, leaf litter | Fresh leaves |
| Food from fungus | Nodules + processed substrate | Gongylidia (swollen hyphal tips) |
| Weed control | Less studied | Pseudonocardia antibiotics |
| Transmission | Sexual spores (horizontal) | Clonal, queen carries (vertical) |
Fungus Garden Architecture
Simulation: Mound Ventilation & Stack Effect
Click Run to execute the Python code
Code will be executed with Python 3 on the server
Simulation: Cellulose Digestion & Colony Energetics
Click Run to execute the Python code
Code will be executed with Python 3 on the server
References
1. Hamilton, W. D. (1964). The genetical evolution of social behaviour. I & II. Journal of Theoretical Biology, 7(1), 1–52.
2. Thorne, B. L. (1997). Evolution of eusociality in termites. Annual Review of Ecology and Systematics, 28(1), 27–54.
3. Luscher, M. (1961). Air-conditioned termite nests. Scientific American, 205(1), 138–145.
4. Turner, J. S. (2000). Architecture and morphogenesis in the mound of Macrotermes michaelseni in northern Namibia. Cimbebasia, 16, 143–175.
5. Turner, J. S. (2001). On the mound of Macrotermes michaelseni as an organ of respiratory gas exchange. Physiological and Biochemical Zoology, 74(6), 798–822.
6. Brune, A. (2014). Symbiotic digestion of lignocellulose in termite guts. Nature Reviews Microbiology, 12(3), 168–180.
7. Aanen, D. K., Eggleton, P., Rouland-Lefevre, C., Guldberg-Froslev, T., Rosendahl, S., & Boomsma, J. J. (2002). The evolution of fungus-growing termites and their mutualistic fungal symbionts. Proceedings of the National Academy of Sciences, 99(23), 14887–14892.
8. Mueller, U. G., Gerardo, N. M., Aanen, D. K., Six, D. L., & Schultz, T. R. (2005). The evolution of agriculture in insects. Annual Review of Ecology, Evolution, and Systematics, 36, 563–595.
9. Pearce, M. J. (1997). Termites: Biology and Pest Management. CAB International.
10. Korb, J. (2003). Thermoregulation and ventilation of termite mounds. Naturwissenschaften, 90(5), 212–219.
11. Nobre, T., Rouland-Lefevre, C., & Aanen, D. K. (2011). Comparative biology of fungus cultivation in termites and ants. In Biology of Termites: A Modern Synthesis (eds. D. E. Bignell et al.), pp. 193–210. Springer.
12. King, H., Ocko, S., & Mahadevan, L. (2015). Termite mounds harness diurnal temperature oscillations for ventilation. Proceedings of the National Academy of Sciences, 112(37), 11589–11593.