Cool New Paper: Ants, Pheromones, and Temperature

I justified writing a post on long-distance visual navigation in ants by saying that “laying pheromones is out of the question because they evaporate immediately in the heat and the road to a new food source is too tortuous for a pheromone trail to be followed”, and I left it at that without a citation. Those two facts are common knowledge to myrmecologists, but a new paper I was reading today deals with exactly this issue. I’ll give you some background info on ant pheromones in a bit, but first I want to say a bit about ants and temperature (copy-pasted from a paper I have in review on this subject, to spare myself some work).

Ants are generally thermophilic, making efforts to keep nest temperatures from dropping below 20°C, as most are unable to reproduce at lower temperatures (Hölldobler & Wilson, 1990) – this is the reason why ant diversity drops with increasing latitude on both a global scale (Kusnezov, 1957) and a regional one (Cushman et al., 1993); of course, foraging ants also die of dessication when ground temperatures are too high.

Brood development is temperature-dependent (Roces & Nunez, 1995), as is the rate of colony growth (Kaspari, 2005); life expectancy of ant workers decreases with increasing temperatures due to increased activity leading to increased metabolic rates (Calabi & Porter, 1989). Whether a larva enters the worker or queen developmental pathway depends mostly on environmental conditions, including temperature (Wheeler, 1986; but see Schwander et al., 2010), and workers will actively measure temperatures inside the nest and move the developing brood to optimal sites (Porter & Tschinkel, 1993). Caste differentiation may proceed only above a specific temperature threshold, as in the soldier caste of Pheidole pallidula, which doesn’t develop unless temperatures are above 24°C (Passera, 1974). Ant phenology is affected by temperature (Dunn et al., 2007), as is foraging activity and behaviour (Azcarate et al., 2007). Ecological interactions may also be affected: foraging time by coexisting ant species is determined primarily by temperature (Cerdá et al., 1998; but see Wittman et al., 2010) and not competition (Gibb & Hochuli, 2004; King & Tschinkel, 2006). Dunn et al. (2007) show that in the months with higher temperatures, more ant species are active.

The specificity of ants’ temperature preferences should not be underestimated. For example, Del-Claro & Oliveira (1999) showed that coexisting ant species forage on the same resource (in this case, hemipteran honeydew) at different times of the day due to differences in temperature tolerance. Some species purposely create sheltered foraging trails in order to lessen the impact of high temperatures during the day, e.g. Solenopsis geminata (Perfecto, 1994). The temperature preferences of ants are not hardwired but somewhat flexible, depending on the conditions experienced during the larval and pupation stages (Weidenmüller et al., 2009).

Temperature-mediated alterations in social behaviour can be traced back to a biochemical level, as cuticular hydrocarbons used for communication and nestmate recognition (Akino et al., 2004) have specific melting points depending on their molecular weight and chemical structure (Gibbs, 1998). Wagner et al. (2001) show that cuticular hydrocarbon structure is different in foragers and nest workers, with the former having more linear alkanes with higher melting points, an adaptation to the higher temperatures encountered while foraging.

So, long story short, ants have a complex relationship with temperature. The plagiarised passages above (does it count as plagiarism if it’s myself I steal from?) summarise the generalities. For this post and paper, we’re interested particularly in the relationship between ant pheromones and temperature.

Pheromones are generally volatile chemicals, so it is obvious that the higher the temperature, the faster they evaporate and the more useless they become. In ants, pheromones are used for very specific purposes and at very specific times, and are used only for communicating. For example, alarm pheromones are used only in or near the nest, and are manufactured to precise proportions to regulate defensive behaviour (Fujiwara-Tsujii et al., 2006). Other types of pheromone include:

• sex pheromones: obvious;
• spacing pheromones: set individuals apart so foraging is more efficient and there is no competition between individuals;
• Aggregation pheromones: bring individuals together, sometimes used in conjunction with sex pheromones;
• Territorial pheromones: mark the territory and range of the colony:
• Surface pheromones: used for direct communication with nestmates – surface refers to surface of the insect, and these are perceived directly by nestmates touching each other with the antennae.

And finally, there are trail pheromones. Most ants use these to define trails that lead to a foraging source or to a new nest site (Attygalle & Morgan, 1985); this has actually been known for over 200 years, with Bonnet (1779) having first observed that ants get confused on their foraging trail if you wipe across it with your finger, and it was Santschi (1923) who showed that the source of the chemicals was the gaster (the “butt” of the ant); the only exception is in the genus Crematogaster, where the pheromones are produced in the tibia. The reason is simple: trail pheromone production is a multi-gland affair, and they’re all located in the gaster.

A pheromone trail is continually maintained by the ants travelling along it. Ants that get to a food source judge whether the food source is worth returning to. If so, then they will lay the pheromones on the way back to the nest, establishing/maintaining a trail. If the food is dwindling, then they will not lay the pheromone, and soon enough the trail will evaporate due to the pheromones’ volatile nature – you have to imagine that only nanograms are deposited by each ant. The first trail pheromone chemical identification, by Tumlinson et al. (1972), required a whopping 3.7 kg of ants to produce enough chemical to be detected by the methods of the time.

All this leads to several conclusions about how ants choose to navigate: pheromone trails or vision. The first factor is, of course, phylogeny. Some ants just have better vision/olfaction, and thus will rely on the respective system more. The other factor is numbers of foragers. If you have many, then a pheromone trail is practical is every case, since it will constantly be maintained. Vision is then only used when there aren’t so many foragers, and it is hot and/or humid (since the pheromone will dilute). This is why in the navigation post, all the examples used either desert ants or rainforest ants.

Anyway, that’s just about all the background info needed for this new paper I was reading today:

Van Oudenhove L, Billoir E, Boulay R, Bernstein B & Cerdá X. 2011. Temperature limits trail following behaviour through pheromone decay in ants. Naturwissenschaften 98, 1009-1017.

The researchers studied the effects of temperature on foraging in the ant Tapinoma nigerrinum. The observation that kicks the whole thing off is that this species’ foraging time is limited only by temperature – they don’t like foraging at temperatures above 30°C; the authors hypothesise that this is due to the pheromones not surviving higher temperatures.

To test the idea, they set up several possible trails. The ants were let go on all of them. After a bit, they were taken out, and each trail was heated to a different temperature (up to 60°C), with a control trail left of course. After cooling, the ants were let loose again. What happened was that the ants didn’t choose to go on those trails that had been heated to temperatures above 40°C, and the best explanation for this is that there is no pheromone left for them to follow, so why should they go on this unknown trail when there is a reliable one next to it.

This is a decent explanation for why they forage less at temperatures above 30°C in the wild, since it’s backed up by basic chemistry as well. It admittedly doesn’t sound like an exciting paper, but I love it when one can take an observation, set up an experiment to test it to the extremes, then reapply the knowledge to the natural system.

References:

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