A recent research by Dr Dmitri Logunov, the Curator of Arthropods of the Manchester Museum, has been devoted to a group of large burrowing wolf spiders (Lycosidae) from Central Asia. A new, unknown to science genus of the wolf spiders, with five new species, has been discovered. Some of the new species exhibit the pronounced differences in sizes between males and females. Males are two or more times smaller than the corresponding females.
Physical differences existing between males and females of the same species are called Sexual Dimorphism. For instance, differences in body ornamentation could be so elaborate that males and females may even look like different species (Figure 1). Sexual dimorphism also includes body size differences, from moderate to extreme, referred to as Sexual Size Dimorphism. This phenomenon is widespread among spiders. At least seven hypotheses have been proposed to explain the factors that may give rise to size dimorphism in spiders; two of them are mentioned here.
Figure 1. Male and female of the ladybird spider (Eresus cinnaberinus) exhibit a peculiar sexual dimorphism both in colouration and size. ©Vladimir Timokhanov.
Giant females – fertile and attractive
The most spectacular cases of sexual size dimorphism occur in the orb-weaving spiders (Figures 2-3), where dwarf males of some species can be 10 times smaller and 100 times lighter than the females. It is believed that size dimorphism in orb spiders is the result of females becoming giants rather than males becoming dwarfs.
Figure 2. A mating couple of the black-widow spider (Latrodectus dahli) from the Middle East; tiny male is on top. © Barbara Knoflach.
Figure 3. The giant orb-web spider (Nephila fenestrata) from the Gambia, displaying typical sexual size dimorphism: females are huge (some 35 mm long), while males are tiny. © David Penney.
The most common explanation is that large size in females could be driven by the selection on female fecundity, acting to increase the number of offspring produced. Such selection could favour large female size, since larger females can produce more eggs and hence more young. With the high level of juvenile mortality, the production of larger numbers of offspring is crucial for survival of the species. Besides, large females can provide better parental care for their brood. Being bigger also means that females may outgrow their enemies or be themselves more effective predators.
Dwarf males – rushing off to females
Some explanations are based on ecological reasons driving small size of spiders, for instance the differential mortality model. This hypothesis illustrates the evolution of sexual size dimorphism in spiders living at low densities, with large sedentary females and dwarf roaming males (Figures 4-5). It is based on the assumption that due to contrasting life-styles of adult males and females, males suffer higher levels of mortality. This leads to a non-proportionally large number of adult females in the population and, as a result, a reduced intensity of male-male competition. Instead, selection by scramble competition favours males with special traits and/or strategies that enable them to reach females faster. An early maturation of males at a smaller size is advantageous because the quicker males mature, the better their chance of mating. The advantage lies not so much in the small size itself, as in the shorter individual development of males. Males mature in fewer moults than females. This model generally predicts not an absolute selection for the reduction of male size, but for the relative sizes of the two sexes.
Figure 4. A couple of South African tarantulas (Augacephalus junodi); tiny male is on the left. © Richard Gallon.
For instance, true dwarf males occur in certain tarantulas (Figures 4) and burrowing wolf spiders, living in hazardous habitats characterized by high seasonal aridity and extreme summer temperatures or periodic flooding. In such environments, the burrowing females are safe in their burrows and less at risk than the roving males, which are subject to higher adult mortality. Small males can easier avoid hostile conditions. The reduction of male size could be one of the major adjustments in adapting to such high-mortality habitats.
Figure 5. A tiny male of the crab-spider (Thomisus sp.) from the Gambia sits on the abdomen of the female waiting to mate. © David Penney.
The extreme Sexual Size Dimorphism in spiders is the end result of a complex interplay of various selective pressures. No single hypothesis can fully explain this phenomenon. Each pattern requires its own explanation. Scientists need more life-history and developmental data on dimorphic spider species in order to solve the puzzle of extreme Sexual Size Dimorphism.
Finally, I wish to thank all my colleagues who have kindly provided me with the images used in this post.
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