Pipevine Swallowtails Still Gregarious at Sixth Instar?

Our Pipevine Swallowtail caterpillars (Battus philenor hirsuta) have been gregarious their entire lives, even now that they are sixth instars! In this post we go over some of the possible causes.

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It's natural that, as social animals ourselves, we take particular notice of social behavior in other species. Of course, social insects have piqued the interest of entomologists for centuries. When most people say social insect, the first thing that usually comes in mind are the Hymenopterans (ants, wasps, bees, and sawflies) and termites (family Isoptera) which are consistently rated as being the most intelligent and sophisticated creatures in the insect world, comparable to the intelligence gap between humans and other primates. Like us, the Hymenopterans and termites are incredibly advanced and hold the highest status of animal sociality and complexity, called eusociality or truly social. They are capable of caring for their young (and have overlapping generations) and assigning specific jobs within the colony. But what of the other insects, like caterpillars? 

Caterpillars are also capable of some social behavior to some extent, usually gregariousnism. Gregarious caterpillars hatch from eggs laid in clusters and band together for a certain amount of time. There have been countless proposals as to what the exact function of gregariousness in caterpillar serves and why it would evolve in the first place. While the reason can differ quite greatly from species to species (more often then not, social caterpillars evolved independently of each other in convergent evolution), some common theories are that it can help them defend against predators, eat tough leaves, increase growth rate, improve thermoregulation capabilities (caterpillars are cold-blooded), or enhance the effectiveness of aposematism (warning appearance as opposed to cryptic). 

In Pipevine Swallowtails (Battus philenor), the first reason that comes to mind is aposematism since they have aposematic caterpillars. After all, they are well-known for sequestering the incredibly toxic aristolochic acids produced by their pipevine host plants (Aristolochia). In fact, the caterpillars themselves can die from ingesting leaves that contain too high a level of the toxins such as mature leaves that have been fed on (damage stimulates a defense response in most all plants and for pipevine, this means releasing more toxins) or tropical varieties of pipevine such as A. gigantea and A. elegans. The horns aligning the bodies of the caterpillars are a conspicuous and very aposematic bright red, the most frightening warning color in the animal kingdom. By banding together in groups, they increase the amount of red there is and therefore increase its defensive property. 

However, in most social caterpillars, gregariousnism is mostly prevalent in the younger instars; the mature caterpillars are usually solitary by choice. Certainly, this would make sense with aposematism since larger caterpillars are already more aposematic. The benefits of being gregarious would simply be too little at this stage to outweigh the cost of intraspecific competition (basically, overcrowding). But now that our Pipevine Swallowtails have reached sixth instar, they are still mildly gregarious. For every instar up until now (1-5), they have been very gregarious, purposefully lining themselves up into very neat and compact parallel rows to rest, feed, or even molt. Now they still do this, just with less individuals per group (2-3) since they are so large. Perhaps it is because, in the wild, the Pipevines would have suffered massive losses in the colony by the time they reach maturity and don't really even have the choice to be gregarious anymore that they appear to be solitary when in reality they still have the instinct (genetic and/or hormonal). When there are a whole bunch are stuffed in a tub together, they do have the choice.

On the other hand, gregariousnism can be actually a lot more complex than it seems. It is hard to imagine that aposematism alone would create such intense selective pressure for the butterfly to lay her eggs in clusters and have the caterpillars overcrowding themselves when they hatch. Most Lepidopterans still lay their eggs singly because there are usually a myriad of potential disadvantages that come from overcrowding. An interesting Springer scientific article by Allen M. Young called On the evolution of egg placement and gregariousness of caterpillars in the Lepidoptera cites a whole bunch of elements required for the evolution of gregariousness in caterpillars. Let's take a look at an excerpt:

. . . cluster oviposition and larval gregariousness are more likely to evolve when (a) the spatial patchiness [ecological distribution] of the larval food plant is high [very spread out], relative to the dispersal ability of the dispersal abilities of the fecundated butterfly; (b) dispersal ability of the average adult female phenotype is low and other factors are promoting home-range behavior  [the specific butterfly's local range where she lays her eggs is small]; (c) for the range of phenotypes present in each generation of the butterfly population, larval feeding is restricted to a single to a single or few food plant species owing to genetic and physiological factors determining feeding factors [such as phytochemicals required to elicit feeding]; (d) the age specific fecundity curve for the average female is one characterized by moderate or low egg production over a relatively large segment of adult lifespan; and (e) physical and chemical anti-herbivore properties of the food plants are more readily overcome by the communal activities of the larvae. 

Our Pipevine Swallowtail really fits the bill here if Mr. Young's scientific conjecture is correct. The Pipevine Swallowtails are a relatively new arrival in California and already they have evolved into a distinct subspecies hirsuta which is known to lay larger egg clusters than the species philenor. The selective pressures that Young names are at work: the caterpillars have are harder time eating California Pipevine (Aristolochia californica) than other species due to its tough and hairy leaves, and live all pipevines, it contains the toxins that are potentially dangerous to the caterpillars (and by eating in one spot of the plant rather than, say, twenty, they decrease the defensive response of the plant) (e). In addition, the California Pipevine is the only host plant of the subspecies since they will not eat the ornamental tropical pipevines that are too poisonous and no other native pipevine grows here (c). And because the host plant is scarcely distributed and the butterfly can only fly so far, the butterfly has not much of a choice to lay all its eggs at once (a and b). The Pipevine Swallowtail is also relatively long lived for a swallowtail, living up to a month as an adult but does not nearly have the capacity to lay eggs everyday (d). 

So, in coclusion, I guess its okay for our little guys to keep having their sleepover parties even when they are all grown up at sixth instar. Its just that ingrained into them genetically. 

- Brian