Articles...      

An Ecological Life History of the
Mexican Fritillary on Jamaica

© 2000, Phil Schappert

Part 2 (continued)

In captivity, males locate and repeatedly visit pupae. They do not assemble in masses on the pupa as their near relatives the Heliconiines do, but visit more frequently as the chrysalis approaches eclosure. Captive females are always mated within about 5 minutes of emergence, well before their wings have had a chance to expand and dry. Unmated females are probably very rare in nature. Evidence for this comes from watching attempted courtship advances by males that are rejected by females. What actually ensues can only be described as harassment!

Two scenarios are possible: the male finds a female nectaring or in flight or he finds one resting or thermoregulating. If the male locates a perched female, he will "flutter" above and slightly behind her, wings beating rapidly, for a few seconds. The female responds by lifting her abdomen and partially closing or opening her wings (depending on her previous stance) in a classic rejection posture. The male will continue to hover above her but will only attempt to land beside her if she ceases, or does not initially respond with, her rejection posture.

Similarly, males will approach a female in flight and flutter above and slightly in front of her in an attempt to entice or force her to land. She has the option of escaping, if possible (males are very tenacious, hence "harassing"), or— more usually— landing and responding with a rejection posture. I have only observed a single successful courtship in the field, of a sedentary female, in the many dozens that I have witnessed. My impression is that "sexual harassment" is common and that it may be partially responsible for male-biased sex ratios by potentially causing females to emigrate from populations. However, wing damage of captured females provides evidence that they may experience higher predation rates than males and this may actually be responsible for the differences in "lifespans" or "residence time" between sexes.

One of the major predators, perhaps the major predator, of both the caterpillars and the butterflies of Mexican Fritillaries on Jamaica are Anolis lizards. Caterpillars are able to sequester the defensive chemicals of the host plant and use them to effectively protect themselves from the lizards. These compounds are always present in the alternate host plants but the preferred host plant varies for their presence and for the quantity of compounds available. Therefore, caterpillars may be at a disadvantage if they feed on unprotected plants of the preferred host. The adult butterflies, however, do not appear to retain these compounds from the host plants so all butterflies are equal regardless of which host they fed on as caterpillars. The lizards attack butterflies, when they are resting on vegetation or on the ground, whenever they can but they show a marked preference for larger butterflies.

Originally I thought that their preference for larger butterflies might be due to sexual size differences because females are about 25% larger than males. However, when I examined the data for the two sexes separately I found identical trends: the lizards choose larger prey regardless of the sex of the butterfly (an alternate explanation might be that larger butterflies must rest more frequently so are more often encountered by searching lizards). Regardless, this means that larger butterflies are at a disadvantage and, in fact, I found that smaller butterflies actually live longer in the wild. So, it turns out that there are advantages to being a small butterfly but – and this shouldn't surprise you – there are also benefits to being a large butterfly. Females in many species prefer large males, presumably because they can provide a larger nuptial gift, while large females may carry larger egg loads and be physically capable of laying greater numbers of eggs. How do these potential reproductive advantages balance with an apparent survival disadvantage?

Let's consider a mental exercise. My research on the Mexican Fritillary shows that a "small female" from the preferred host is able to carry and lay about 200 eggs in its normal 7 to 8 day lifespan. Let's assume that a butterfly from an alternate or secondary host that is 25% larger can carry 25% more eggs, a total of 250 eggs. We would expect that the larger butterfly will need more nectar resources to support its larger body, muscle mass and flight machinery and could also be subject to overheating faster in the tropical sun. Similarly, we would expect that the smaller butterfly will require less nectar but may have to spend more time basking to maintain its flight temperature. On average, these factors, and many others, will probably balance each other so the real difference likely comes down to who lays the most eggs per day over how many days.

Females from the primary host plant are capable of laying about 27 eggs per day so they need a total of 8 good days, their entire lifespan (on average), to lay their entire clutch. If larger females from the secondary host plant are capable of laying 25% more eggs per day (about 33 eggs per day, assuming the resource base to support this) then they would also need 8 days to lay their entire clutch. If the large female has, say, a 50% chance of surviving for the 8 days then the most she will lay, on average, will be 125 eggs. If the smaller butterfly has exactly the same probability of survival then she will only lay 100 eggs. The larger butterfly has the advantage if their chances of survival are the same. But if the smaller butterfly has only a 25% better chance of surviving for the 8 days (75% chance) then she will lay 150 eggs, on average. The "small female" off of the preferred host plant has the advantage if predators choose larger prey.

This poses an interesting question: do butterflies trade-off the use of preferred vs. alternate host plants based on their perception of how many or what kind of predators are occupying the same habitat? In other words, if females do not detect lizards or other potential predators near a host plant or in a population of host plants, will they choose an alternate host to maximize the number of grandchildren they have or a preferred host to maximize the lifespans of their children? I can't say I know the answer to this one (yet) but I do think it would be a lot of fun to find out!