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Late Summer: How many
molts? |
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![]() Male Nephila clavipes reared in the laboratory start out indistinguishable from their sisters. They are the same size as babies, build webs, eat flies, and grow by molting. But after somewhere between 4 and 6 molts, they reach sexual maturity and never grow again. In contrast, their sisters continue molting - up In this picture, a mature female N. pilipes rests on my hand - she weighed over 6 grams, more than many hummingbirds. |
![]() But when we look at animals that are not mammals, we find that in the vast majority of species, females are larger than males (among vertebrates, the most famous are the deep-sea angler fish, where males are reduced to minature external parasites of the females). The Nephila spiders exhibit the ![]() In this photo, the male on
the left has stolen a prey item from the small juvenile female.
photo courtesy of M. Kuntner |
For males and females to evolve
to such different developmental pathways indicates that the benefits
and costs of being small or large are very different. We have a
much better idea of why female spiders should be very large than why
male spiders should be very small, so I'll talk about females first. |
Size matters. Darwin in 1871 proposed that large females are favored through a form of selection he termed "fecundity" selection: larger females can lay more eggs. In Nephila species I've studied, larger females lay many more eggs than smaller females - the figures below show my data for N. pilipes (left) and N. clavipes (right). Bigger spiders (measured by the length of one of the leg segments) lay many more eggs than smaller spiders, supporting Darwin's hypothesis. (That big spider on my hand is the upper-most point in the left-hand graph). ![]() ![]() If you look carefully at these figures, you can see that the X and Y axes are very different: the smallest N. pilipes (left) is bigger than most N. clavipes (right), and lays proportionaly more eggs. Within each species, the largest females are one or two molts larger than the smallest females. Yet our best measure of evolutionary success – the number of eggs laid – increases when a female delays maturing by going through additional molts. Why, if getting bigger is so advantageous, should any female mature at a small size? |
The price of delay. In a nutshell the answer to this question is that there is an end to all good things, and in Mexico this is the end of the growing season. Nephila clavipes populations live in habitats with definite changes in rainfall and temperature. Spiders caught by the shift in seasons die. How does this alter their size? We can use a graphical model to predict changes in development. ![]() |
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I tested this model by looking at data I've collected over the years in different seasonal habitats. There is a lot of variation in how well spiders do at capturing insects (mostly determined by their luck at choosing a good spot for their web). If my hypothesis is right, then in seasonal environments, late-maturing female spiders will be smaller than early-maturing spiders. I don't expect this to be the case for males, because they mature so early that they have no problem with growing too slowly. |
Testing the model. When I looked at the size of females maturing at different times over the growing season in Mexico, the data clearly support the model of slowly-growing females making the best of a raw deal. In all sites, late females are smaller than early females. Below are the data for females from all the Mexican sites and Panama (lower right panal). Although the effects are slight, across all Mexican sites the late females are significantly smaller than earlier females. To reinforce the idea that the length of the season is important, the spiders are bigger in places with longer seasons (coastal Veracruz: Los Tuxtlas, Playa Escondida and Nanciyaga) and smaller in places with shorter seasons (Fortin, Tehuacan, Arroyo Frio, and Chamela). This contrasts with the data from Panama, where there is no end to the growing season (although the wet and dry seasons are distinct) and there is no change over time in spider size. However, the wet season spiders capture more food, and they mature at a larger size. ![]() ![]() (from Higgins 2000 Oecologia 122:51-59) |
So
why
are
males tiny? As pointed out by Lande and Slatkin in the
1980s, males and females share a lot of genetic
information. Therefore, selection for enormous female size should
(all else being equal) "pull" males along - the males should also be
evolving to larger size. Working with Jon
Coddington, Matjaz Kuntner, and Charles Goodnight, I tested this by
comparing male and female size across all species of Nephila. ![]() (Lande 1980: Evolution 34:292-307; Slatkin 1984: Evolution 38:622-630). |
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Publications
related
to
the material in this web page L. Higgins. 2000 The interaction of season length and development time alters size at maturity. Oecologia 122:51-59. L. Higgins. 2002 Female gigantism in a New Guinea population of the spider Nephila maculata. Oikos 99:377-385 L. Higgins and M. A. Rankin. 1996 Different pathways in arthropod post-embryonic development. Evolution 50:573-582 L. Higgins. 1992 Developmental plasticity and fecundity in the orb-weaving spider Nephila clavipes. Journal of Arachnology, 20:94-106 |