Size Variation in B. philenor hirsuta

Now that we have collected to many Pipevine Swallowtails (Battus philenor) we have begun to notice some incredibly drastic size differences between individuals, much more than we have ever seen in our Anise Swallowtails (Papilio zelicaon)!

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Today we collected fourteen new Pipevine caterpillars and two green morph chrysalises when we returned to Tilden Regional Park District to pick fresh pipevine leaves. We were amazed that between our new total of 26 caterpillars and four chrysalises and a prepupa, there were some pretty insane, somewhat unexplainable size differences.

When we had collected our first fourteen Pipevine caterpillars on 6/18, we had gave our best judgement from observing caterpillars for years and assigned each of our caterpillars an instar based on their sizes relative to each other. However, we were completely caught off guard when what had originally appeared to be a third instar unexpectedly had runny stool and became a prepupa today. Clearly, this particular species can have some pretty major variations in size and according to some sites the average wingspan can range from 2.5 - 5 inches, with the smaller end of the range being half that of the larger.

So, using a fairly accurate digital scale (unfortunately it was not a micro scale and could only measure whole grams) we decided to do some testing around. First, we weighed the chrysalises -- the huge one from the largest of the fifteen original caterpillars, the two medium sized wild-caught ones, the small brown one, and the tiny prepupa from today. Because it is very difficult for the scale to detect low measurements, we first weighed the two medium-sized wild caught chrysalises together (they were virtually the exact same size) and got 3 grams. After that, we weighed the largest chrysalis to be 2 grams and got 5 grams when combined with the wild caught chrysalises. However, after adding the small brown chrysalis, it remained 5 grams meaning that no matter how the mass had been rounded it was less than 1 gram. Since we did not want to remove the prepupa from the enclosure before it pupated we did not try to get a mass from it but it was even smaller than the small brown one so we would say that it is at least 2-3 times smaller than the huge one!

On another note, the today's newborns appear to be noticeably larger than the ones that hatched yesterday. . . . 

After this, we were also curious to compare our findings with Anise Swallowtail chrysalises which we have always found to be relatively uniform in size, with any major size differences being correlated with gender (females are generally larger than males). In addition, we were having doubts that the Pipevine Swallowtail butterflies were really much larger than Anise Swallowtails since the Pipevine chrysalises didn't appear to be significantly larger (the small brown one and the prepupa definitely appear smaller than the average Anise chrysalis). Plus, our Pipevines are the Northen California subspecies Hirsuta which are supposed to be smaller in size than the species anyway. 

When we put seven Anise chrysalises on the scale we got the total to be eight grams, meaning on average they a little over 1 gram each in mass (the seven chrysalises appeared to be about the same in size). This came out a lot less than we thought, and we concluded that trying to eyeball chrysalis size can be quite deceiving since we had gotten a total of 5 grams for the two wild-caught and huge pipevine chrysalises! At the same time, it is true that while their chrysalises share many similar physical characteristics as they are both Swallowtail species (though not in the same genus), the dead-leaf mimicking Pipevine chrysalises are much wider and flatter than the skinny Anise chrysalises which are not leaf mimics so it can be hard to compare than with the naked eye. 

However, when you consider that if we were to weigh all five chrysalises together that we had it probably would have came about to be about 6 grams which is also, on average, a little over 1 gram. In the end, we would say that generally P. zelicaon are no smaller than B. philenor hirusta butterflies but the latter is still capable of producing the largest individual to break the record. 

Finally, we took the masses of a few caterpillars. With the 25 Pipevine caterpillars, we selected the three very largest ones (found today) that were probably very near or at their peak size before going to pupate. We also selected the three largest Anise caterpillars. For the Pipevine caterpillars, we got a mass of 3 grams for each caterpillar and total mass of 9 grams for all three. The Anise caterpillars were only 2 grams each and 6 grams together, about two thirds the size of Pipevines that are on the larger end of the spectrum! 

To conclude, it is possible that the Pipevine Swallowtails come in such an enormous range of sizes because they are relatively new to Northern California and it has not been long enough for the new selective pressures to completely weed out individuals one way or the other. Or perhaps it is case of disruptive evolution and eventually the species will diverge into small and large (in any case, it is hard to imagine that a 2.5 inch butterfly would be able mate with a 5 inch one and vice versa due to the differences in genitalia size!) while Anise swallowtails exhibit stabilizing selection. But, of course, we shouldn't forget about the other universal factors besides genetics such as overall health of the caterpillar and obvious factors as disease or parasitism (but we don't think this is the case in our particular situation). Caterpillar size is proportional to how much it eats, so if a certain individual doesn't eat enough during this "critical" growing period in its life cycle it will reach its genetic potential. Likewise, a caterpillar that is fed undesirable leaves or a less preferred host plant will be less likely to eat and turn out to be smaller than it could have been. Honestly, the possibilities are endless.

- Brian