8.9 KiB
terrain, substrates, and pheromone decay
how environmental factors affect pheromone persistence. relevant to feature #7 (substrate-dependent decay) and the worldBlur shader's per-cell decay rate.
substrate effects on persistence
the key study is Jeanson et al. (2003) on Monomorium pharaonis:
substrate chemical half-life behavioral preference half-life plastic ~9 min ~25 min paper ~3 min ~8 min
a 3x difference in chemical half-life between two smooth artificial surfaces. mechanism: paper is porous and wicks the compound away from the surface, reducing the airborne concentration ants detect. plastic is non-porous, so the compound sits on top and remains available.
no comparable controlled study exists for natural substrates (soil, rock, sand, leaf litter, wood). inferred from physical chemistry:
- porous substrates (soil, sand, wood, leaf litter) behave more like paper — absorb compounds, accelerate apparent decay
- non-porous substrates (rock, packed clay) behave more like plastic — keep compounds on the surface, slower decay
- soil moisture complicates things further (see humidity section)
species variation in baseline trail longevity
trail pheromone persistence varies enormously across species:
species trail longevity notes Solenopsis invicta ~100 sec / <2 min extremely volatile compounds Monomorium pharaonis ~9 min (plastic) multiple pheromone types Aphaenogaster albisetosus minutes short-lived Pachycondyla sennaarensis ~30 min to half gone in 1 hr Monomorium spp. (general) ~1 day optimal varies Camponotus (carpenter ants) days hindgut-produced Daceton armigerum 7+ days poison gland secretion Eciton spp. (army ants) weeks long-chain, low-volatility
the range is >100x. species in stable environments with permanent food sources use long-lasting compounds. species exploiting ephemeral food use volatile ones.
Pharaoh's ants also use multiple pheromone types with different decay profiles: a long-lasting attractive pheromone, a short-lived attractive pheromone, and a short-lived repellent pheromone (~78 min vs ~33 min half-life).
temperature
the definitive paper is van Oudenhove et al. (2011/2012), studying Tapinoma nigerrimum and Aphaenogaster senilis.
key findings:
- above ~40C: workers cannot discriminate marked substrate — pheromone is effectively destroyed
- above ~30C: foraging activity drops independently, partly because trails decay too quickly to be useful
- between 25-40C: decay accelerates but trails remain functional
- the 40C threshold is a behavioral cliff, not a smooth curve
species differ in thermal resilience:
- T. nigerrimum (mass-recruiting): secretions highly volatile. most compounds vanished even at 25C. only iridodials persisted up to 55C.
- A. senilis (group-recruiting): secretions less volatile, resisted elevated temperatures better. at 55C, only nonadecene and nonadecane (long-chain hydrocarbons) persisted.
pheromone persistence = f(temperature, time since deposition). both interact — higher temperature accelerates decay at all time points.
diurnal implications: hot midday temperatures degrade trails laid in morning. desert species tend to use less volatile compounds (evolutionary compensation via chemistry rather than behavioral compensation via deposition rate).
humidity and moisture
less quantitative data than temperature.
- higher humidity slows evaporation of polar pheromone components
- a cuticle covered with water may hinder both reception and emission of pheromones — wet conditions impair laying AND sensing, not just persistence
- army ants (Eciton burchellii) increased speed by 30% in response to increased humidity and rain sounds near the trail, but watering the trail directly did not cause load-dropping
- extreme humidity (either direction) suppresses foraging entirely
wet porous substrates (damp soil, wet leaf litter) would absorb pheromone faster than dry porous substrates, but no controlled study confirms this quantitatively.
no direct evidence of ants selecting substrates specifically for pheromone persistence, though trail-clearing behavior (see below) effectively creates favorable substrate.
physical trail infrastructure
leaf-cutter ant highways (Atta spp.)
the standout example of ants modifying their environment for trail quality:
- colonies clear an average of 2,730 meters of trail per year
- individual trails can exceed 200 meters
- networks extend for kilometers cumulatively
- construction/maintenance costs: ~11,000 ant-hours per year
what they do: remove leaf litter, cut passes through overhanging vegetation, shift soil to level surfaces. selective clearing — flat objects are ignored, upright/folded obstructions are removed.
coordination: trail clearing happens WITHOUT information exchange between workers. independent effort that adds up to emergent infrastructure. clearing is triggered by freshly laid pheromone on an obstructed path.
minim workers as trail maintainers
the smallest workers (minims) are always present on trails but never carry leaves. they deposit pheromone at 83.3% frequency vs 20% for non-minims. they're dedicated trail maintainers — keeping the chemical signal strong while larger workers (2.2-2.9mm head width) handle physical clearing.
physical + chemical reinforcement loop
physical clearing creates smooth packed soil (relatively non-porous) which retains pheromone better than leaf litter. minim workers then maintain high pheromone concentration. cleared trails retain pheromone better -> strong pheromone attracts more traffic -> more traffic means more clearing and reinforcement. positive feedback on two axes simultaneously.
energetics: not always profitable. depends on workforce composition and patrol vs carry ratio. can amortize within days or take weeks/months.
emergent highway formation
the trail network that emerges from substrate-dependent persistence:
- substrate quality: non-porous > porous
- temperature: shade > sun
- physical modification: cleared > uncleared
- traffic: popular > unpopular (reinforcement)
- food quality: rich source > depleted source
a trail across cool, shaded, packed earth near a rich food source dominates over a trail across hot, sun-exposed leaf litter near a marginal source. no ant "decides" this — the pheromone math works out.
trail bifurcation: at branch points, trail asymmetry (angle, width) influences decisions alongside pheromone presence. neither geometry nor pheromone alone dominates — non-hierarchical interaction.
rapid decay as feature: in fire ants, trail pheromone drops below detection in ~2 minutes. this forces continuous reinforcement, meaning only actively profitable routes persist. fast decay = responsive colony.
simulation relevance
for the world texture's terrain type bits (3-5 in world.R):
terrain type decay multiplier real-world analog 0 (default) 1.0x generic surface 1 0.5x (slower) packed earth / rock 2 1.5x (faster) leaf litter / porous 3 2.0x (faster) sand / loose soil 4-7 reserved future use
the blur shader would read terrain type per cell and multiply the base decay rate. ants wouldn't "know" about terrain — they'd just find that their trails last longer on some surfaces, and positive feedback would do the rest.
temperature could be a global uniform rather than per-cell (simpler), or per-cell if the simulation adds sun/shade regions.
sources
Jeanson et al. 2003 Pheromone trail decay rates on different substrates in Pharaoh's ant Physiological Entomology 10.1046/j.1365-3032.2003.00332.x
van Oudenhove et al. 2011 Temperature limits trail following through pheromone decay Naturwissenschaften 10.1007/s00114-011-0852-6
van Oudenhove et al. 2012 Substrate temperature constrains recruitment and trail following J Chem Ecol 10.1007/s10886-012-0130-x
Bruce et al. 2019 Infrastructure construction without information exchange in Atta Proc R Soc B 10.1098/rspb.2018.2539
Bruce et al. 2017 Energetics of trail clearing in Atta Behav Ecol Sociobiol 10.1007/s00265-016-2237-5
Robinson et al. 2008 Decay rates of attractive and repellent pheromones in foraging trail network Insectes Sociaux 10.1007/s00040-008-0994-5
Morgan 2009 Trail pheromones of ants (review) Physiological Entomology 10.1111/j.1365-3032.2008.00658.x
Effect of trail pheromones and weather on Eciton burchellii ResearchGate 225346583
Minor workers maintain leafcutter ant pheromone trails ResearchGate 248591651
Trail pheromone of Pachycondyla sennaarensis PMC3281317
Monomorium trail pheromone longevity ScienceDirect S1226861509001034
Uncovering the complexity of ant foraging trails PMC3291321
Effect of trail bifurcation asymmetry and pheromone on trail choice PMC4204274