Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

Notes from Underground Ant-mimicking jumping spiders never cease to disguise by Ximena J. Nelson Robert R. Jackson What with remarkable eyes, great eyesight, and ability to identify rivals, mates and prey from as far as 30 body lengths away, you’d think being a jumping spider (family Salticidae) was kudos enough, but the 200 or more species in the largest salticid genus (Myrmarachne) add dramatically more to the salticid portfolio by being remarkably similar to ants in appearance. One of the interesting things we have been discovering is that, for Myrmarachne, looking like ants isn’t all fun and games. Although resembling ants seems to function in general for Myrmarachne as Batesian mimicry, this gets complicated because, in order to be a convincing mimic, some Myrmarachne species go to considerable lengths to live in close vicinity of their models. That’s a complication because the company of ants can be dangerous. We have been looking at how Myrmarachne handles this difficulty and the story goes roughly like this. Batesian mimics are palatable individuals that avoid predation by using their resemblance to unpalatable or dangerous models for deceiving potential predators. Would-be predators of salticids often seem to have an innate aversion to ants and we have shown experimentally that this aversion is sometimes generalized to an aversion for the ant-like salticid. Salticids are a great system for experimentally investigating mimicry, with salticids providing both the mimic (Myrmarachne) and the potential predator of the mimic. This is because the Salticidae also includes a sizeable number of species that specialise at eating ants and there are yet other salticids that specialise at eating salticids. Another advantage of working with salticids is how their extraordinary eyesight facilitates experimentation. We know that, for many salticids, ants can be very dangerous in nature but salticid eyesight seems to be up to the challenge. Our experiments have demonstrated that many species of ordinary salticids (species that are neither ant-like nor ant-eating) readily identify by ants sight and then avoid their proximity. Ant-like and ant-eating salticids need to get close to ants, and they apparently have abilities to survive these close encounters better than other salticids. How closely different species of Myrmarachne resemble particular ant species varies considerably, and we have found that accurate mimics of a particular model species survive better than the

1 of 6

9/10/07 5:30 PM

Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

more generalized ant mimics (spiders that resemble ants but have no specific model, see photo). This suggests that accurate mimics have evolved special adaptations that let them survive in the vicinity of their models and this has been one of the topics we decided to investigate further. Myrmarachne assimilis is an accurate mimic of an especially aggressive model, the weaver ant Oecophylla smaragdina. Nonetheless, M. assimilis lives in proximity with this ant, and is very unusual among salticids because it aggregates. Furthermore, the advantage driving M. assimilis’ predisposition to aggregate appears not to be what we would expect from other social spiders. That is, aggregating does not seem to have anything to do enhancing foraging, nor with kin groupings. Instead, with M. assimilis, aggregations seem to come about primarily through adult females seeking each other out and forming crèches. Our work suggests that these crèches function to protect the females’ offspring from predators, with the most notable predator being the salticid’s own model, O. smaragdina. Our experiments have established that Myrmarachne’s mimicry deceives highly visual predators, including mantises, ordinary salticids and, perhaps most interesting of all, salticids that specialise at eating ants. Although, for Myrmarachne, deceiving predators that eat ants sounds like a bad idea, Myrmarachne has evidently found ways to ameliorate mimicry that goes wrong. When an individual of Myrmarachne receives the unwanted attentions of ant-eating salticids, it actively defends itself by revealing its true identity. The mimic switches off its ant-like demeanour and acts like a conventional salticid by raising its legs and displaying at the ant-eating salticid in much the same way as salticids generally threaten each other. Our experiments demonstrated that it is specifically the display posture adopted by the ant mimic that deters the ant-eating predator. This work was interesting as an illustration of how mimicry may be advantageous when it deceives ant-averse potential predators, but disadvantageous in encounters with ant-eating specialists, this being something that may apply more widely than just in the Salticidae. Another interesting implication is how it demonstrates that Myrmarachne’s resemblance to ants holds not only for our human eyes but also in the eyes of salticids. The story gets especially interesting when we consider encounters between Myrmarachne and Portia fimbriata, a salticid that has specific behaviour for capturing other salticids and prefers salticids as prey. Here, it was not obvious which way things would go because P. fimbriata is renowned for having perhaps the best eyesight of any salticid. Experiments using computer animation and virtual prey have shown that an important salticid-identification cue for P. fimbriata is what human taxonomists also use, the large front eyes of the salticid. Like all salticids, Myrmarachne has large forward-facing eyes (see photo). What we found is that P. fimbriata rejects as prey not only ants but also Myrmarachne. An intriguing question remains. Is it that P. fimbriata is simply deceived or do Myrmarachne’s large forward-facing eyes tell P. fimbriata that this is a salticid but with the otherwise repugnant (for P. fimbriata) ant-like appearance of Myrmarachne making P. fimbriata ‘lose’ its appetite? There would seem to be another problem for Myrmarachne. Salticids have long been known for their elaborate vision-guided courtship displays. They are known to be remarkably good at identifying members of their own species and are able to distinguish between males and females

2 of 6

9/10/07 5:30 PM

Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

using sight alone. But what about Myrmarachne? What is courtship like for an ant-mimicking salticid? The answer is of interest because, after all, Batesian mimicry for M. assimilis depends on making vision-based identification difficult. That’s good if the eyes are on a predator, but not so good when the eyes are on a potential mate. Again we chose M. assimilis as experimental subjects. First we found that this ant mimic has displays dedicated explicitly for use during intraspecific communication. This was good because it meant we could use performance of these displays as a bioassay for determining whether M. assimilis distinguishes, by sight alone, between its ant-like conspecifics and its model, real ants. Our experiments suggest that M. assimilis, as a Batesian mimic of a particularly dangerous model, has the especially good perceptual ability needed for rapidly discriminating between conspecific and model. Now the plot thickens. Besides their ant mimicry, there is something else for which the genus Myrmarachne is renowned. It is as if the males of these salticids have been badly mangled by sexual selection. Myrmarachne males have enormously elongated chelicerae and, at first glance, these would seem to render them relatively unlike ants in appearance. Maybe this is, for Myrmarachne, a cost of sexual selection, but this cost seems to be ameliorated by the Myrmarachne males retaining Batesian mimicry by resembling something more than an ant. The large chelicerae resemble an ant carrying an object in its mandibles. We have called this ‘compound mimicry’, as the model for Myrmarachne males seems to be a compound model, an ant and something being carried by the ant. Resembling an ant that is carrying an object in its mandibles, however, appears to have an unwelcome effect for the Myrmarachne male, as it makes the male more attractive to ant-eating jumping spiders. It turns out that ant-eating salticids prefer ants carrying objects in their mandibles as prey, probably because it is less dangerous than an ant with free mandibles. Ant-eating jumping spiders also ‘prefer’ male over female Myrmarachne.

3 of 6

9/10/07 5:30 PM

Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

Myrmarachne bakeri (a generalised ant mimic).

Photo: Robert Jackson.

Myrmarachne assimilis female (an accurate ant mimic of Oecophylla smaragdina) on leaf with three nests (crèche formation).

4 of 6

9/10/07 5:30 PM

Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

Photo: Robert Jackson.

Myrmarachne assimilis male (a ‘compound mimic’ of Oecophylla smaragdina) approaching female nest in typical courtship posture. Photo: Robert Jackson.

Some recent papers pertaining to ant-spider relationships: Edmunds, M. (2006). Do Malaysian Myrmarachne associate with particular species of ant? Biol. J. Linn. Soc. 88, 645-653. Jackson, R. R. & Li, D. (2001). Prey-capture techniques and prey preferences of Zenodorus durvillei, Z. metallescens and Z. orbiculata tropical ant-eating jumping spiders (Araneae: Salticidae) from Australia. N. Z. J. Zool. 28, 299-341. Jackson, R. R., Nelson, X. J., Pollard, S. D., Edwards, G. B. & Barrion, A. T. (2004). Predation by ants on jumping spiders (Araneae: Salticidae) in the Philippines. N. Z. J. Zool. 31, 45-46. Nelson, X. J. & Jackson, R. R. (In Press). Anti-predator crèches and aggregations of ant-mimicking jumping spiders (Araneae: Salticidae). Biol. J. Linn. Soc. Nelson, X. J. & Jackson, R. R. (2007). Complex display behaviour during the intraspecific interactions of myrmecomorphic jumping spiders (Araneae, Salticidae). J. Nat. Hist. 41, 1659-1678. Nelson, X. J. & Jackson, R. R. (2007). Vision-based ability of an ant-mimicking jumping spider to discriminate between models, conspecific individuals and prey. Insect. Soc. 54, 1-4. Nelson, X. J. & Jackson, R. R. (2006). Compound mimicry and trading predators by the males of sexually dimorphic Batesian mimics. Proc. R. Soc. B. 273, 367-372. Nelson, X. J. & Jackson, R. R. (2006). Vision-based innate aversion to ants and ant-mimics.

5 of 6

9/10/07 5:30 PM

Notes from Underground-fieldobs

http://www.notesfromunderground.org/october2007/jackson/antmi...

Behav. Ecol. 17, 676-681. Nelson, X. J., Jackson, R. R. & Li, D. (2006). Conditional use of honest signalling by a Batesian mimic. Behav. Ecol. 17, 575-580. Nelson, X. J., Li, D. & Jackson, R. R. (2006). Out of the frying pan and into the fire: a novel trade-off for Batesian mimics. Ethology 112, 270-277. Nelson, X. J., Jackson, R. R., Edwards, G. B. & Barrion, A. T. (2005). Living with the enemy: jumping spiders that mimic weaver ants. J. Arachnol. 33, 813-819. Nelson, X. J., Jackson, R. R., Li, D., Edwards, G. B. & Barrion, A. T. (2006). Innate aversion to ants (Hymenoptera: Formicidae) and ant mimics: experimental findings from mantises (Mantodea). Biol. J. Linn. Soc. 88, 23-32.

•••

Date of this version 1 October 2007 ••• All text and images contained on this web site are copyright © 2000 - 2007 Notes from Underground

6 of 6

9/10/07 5:30 PM