12 April 2013

Science Summarized! Are 2 heads are better than one for prey trying to escape salticids? (Sourakov 2013)

This first post of our new Science Summarized feature will be a bit unique from our future posts in the series.  While it is about salticids, they are not the main focus, but rather convenient predators used in this paper. 

"Two heads are better than one: false head allows Calycopis cecrops (Lycaenidae) to escape predation by a Jumping spider, Phidippus pulcherrimus (Salticidae) is a recent paper by Andrei Sourakov (2013).  Published in the Journal of Natural History, this paper focuses on the butterfly subfamily Theclinae, which are commonly known as the "Hairstreak" butterflies.

The prey, Calycopis cecrops. Borrowed from Bugguide.net

These butterflies are known for both the thin white stripes on the underside of the abdomen that give them their common name, and the "false heads" on the abdomen.  Unlike many lepidopterans (moths and butterflies) which freeze when approached by a predator, hairstreaks display an interesting vertical motion ("bouncing up and down") of the hind-wings when approached by a predator.  Sourakov hypothesizes that this behavior, and the coloration of the abdomen, combine to provide an effective anti-predator deterrent. To test this hypothesis, Sourakov collected a local predator, Phidippus pulcherrimus, and presented it with a variety of lepidopteran prey items.

The predator, Phidippus pulcherrimus. Borrowed from bugguide.net

Of the 12 species tested, only Calycopis cecrops successfully evaded capture every time.  Sourakov ascribes this successful evasion  of spidery death to the unique behavior of C. cecrops described earlier.  Sourakov also suggests that had the spider managed to successfully attack and latch onto C. cecrops, the butterfly still would have been likely to escape.  According to Sourakov, the escape would be facilitated by a thinning of the hind-wings near the false head region that would have allowed the section to break off of the butterfly and leave the spider with a mouthful of wing parts.

While the conclusions drawn from this study are both interesting and plausible, there are two major flaws with the work that cannot be overlooked.  First and foremost, the author only tested 15 butterflies total (one of each species with three exceptions that had two animals tested).  In other words, the escapes by  C. cecrops individuals tested could easily have been either flukes or traits unique to those individuals. To be convincing, I would have much preferred to see at least 30-40 individuals tested per species, and ideally many more than that given the abundance of these animals.

The second, and equally egregious, flaw was that only a single specimen of Phidippus pulcherrimus was used to test all of the butterflies. Given my own research experience with animals of this genus (Phidippus), as well as the work produced by other salticid researchers, I can say with absolute certainty that this animal is not representative of its species, let alone its genus or family.  Why?  Let's put it this way, in my own experiments I have found salticids to be as behaviorally variable as humans.  In other words, if allowed to attack a prey item, some animals will fail over and over and over again, while other will succeed first time, every time.  To imply that there is no behavioral variation in these animals, which is what Sourakv has done, is roughly equivalent to saying that North Americans all have the same political views because we are all Americans.

While I had high hopes for this paper, I am saddened to say that the study, as published, was quite a let down.  The hypothesis Sourakov proposes has strong merit, and in spite of the lack of evidence presented, I for one believe it could be successfully demonstrated with some more effort.  I fervently hope that future work in this area will be more scientifically rigorous.