Lekking long-horn moths

 In a walk by a wooded area on my way to work yesterday, I came across a few small swarms of dancing long-horn moths. I watched and waited until one of them sat to rest on the foliage: a Yellow-barred Long-horn moth, Nemophora degeerella. Long-horn moths are truly spectacular micromoths. Males are not only beautifully marked with metallic golden, chestnut and blue metallic stripes and a broad diagnostic yellow band across the wings, but their antennae are extremely long, several times their body length. It is astounding they can actually fly! Not only they can fly, but they perform a dancing flight, males bobbing up and down in a small area in shady wooded glades where they gather forming mating swarms or leks. These swarms, and possibly pheromones the males emit presumably serve to attract females and mate. Do females prefer males with longer antennae? These are day flying moths and I've watched the swarms in the morning or afternoon in warm sunny days in May and June. The antennae are white and so stand out as the moths fly. The antennae are strongly sexually dimorphic as the females, which I've never seen, are similar to males, but with short antennae. Their caterpillars feed on leaf litter.

 This is a short video of one of the moths appearing to sip honeydew from a leaf.

UPDATE 28/06/2017
A large swarm (over 20 moths) in the University Woodland area


via GIPHY

The curious Autumn spider mating tactics

Female spiders tend to be larger than males. Large females are thought to be selected for as they can produce more eggs. The relative size differences between males and females depend on the type of competition between males, that is, on the importance of intra-sexual selection. When males do not compete much with one another they are smaller than females, whereas when there are male-male contests for females, then large males are selected for, and size differences with females become smaller. The spider, Metellina segmentata, is an orb spider common during September and October, when most mature individuals are found. Males are more powerfully built and have longer legs than females, suggesting that male-male contests occur in this spider. When young, males build their own webs to hunt insects, but upon reaching their final moult, they mature and start searching for female webs. Once they encounter a receptive female web - likely through detecting a pheromone - they adopt a curious sit and wait strategy. They have to wait, often days, until a large fly falls in the web. Only when the female starts eating the fly does he starts his courtship, taking advantage of the diminished cannibalistic tendencies of sated females. Of course, during this wait, other suitors might arrive to the female's web and when two males encounter each other fights follow. Males are often injured or killed in these fights, where the larger male has an advantage and this is what provides the selective pressure for powerfully built males.
 In addition, males are able to monitor several female webs when they are nearby and this results on males guarding webs becoming larger as the season progresses, and a pool of smaller wandering males.
 The mature female Metellina on the top photo has hung her web on a tomato plant since at least mid september. I haven't noticed any males but I shall keep an eye for these in the next days.

Reference
Prenter, J., Elwood, R., & Montgomery, I. (2003). Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment Animal Behaviour, 66 (6), 1053-1058 DOI: 10.1006/anbe.2003.2266

Small white or small ultraviolet butterfly?

Graced with a few black spots and a shade of yellow in their underwings, the plain wings of white butterflies contrast with the colourful and rich patterns of other butterflies. Unfortunately, this is a reflection of the limitations of our visual system. Male and female Small White butterflies, Pieris rapae, have different upper wing colours, especially in the ultraviolet (UV) spectrum: males absorb UV strongly but reflect most visible light, so they look a brighter white than females to us, but have a dark tone in the UV spectrum, which we cannot see.

A Small White bilateral gynandromorph, with visible female coloration on the left and male coloration on the right (top). Below, a false-colored representation of how these wing colors might be perceived by small whites, which are able to see UV light (photo courtesy of Nathan Morehouse)

Wing colour patterns in pierid butterflies, the family of the Small White, depend on beads made up of pigments called pterins, which are deposited on wing scales, and in the Small White are UV-absorbing. Males vary widely in their brightness and UV reflectance. Males able to produce more beads have darker UV patterns and appear brighter to our eyes. The amount of pterins a male can produce depend of the diet they had as caterpillars. Pterins need a lot of nitrogen to produce - they are the most nitrogen-rich pigment in animals - and the caterpillars can be nitrogen limited, so a brighter male is an indication of a good caterpillar diet. A brighter male also is also expected to be more desirable to a female, as during mating, male white butterflies transfer nutrients to the females in the form of infertile sperm, a nuptial gift which will increase the females life expectancy and therefore will be able to lay more eggs. A male with a higher quality diet can afford to be brighter and to produce bigger and more nutritious nuptial gifts.
 These butterflies are also able to see UV patterns, they have a rich repertoire of visual receptors in the short wavelength, including three types of photoreceptors that peak at UV, violet and blue. Crucially, being aware of their ability both to emit and detect UV allows us to understand their mating behaviour. Nathan Morehouse  and Ronald Rutowski carried out some elegant experiments to test the hypothesis that females choose males with more contrasting UV patterns. They carried out two experiments, in both of them they used lab-reared virgin females, which they presented with some males in a large cage. In the first experiment they used males that were from wild origin, and therefore had a natural range of colour patterns, in the second experiment they used laboratory reared males, but dipped the males' wings in a chemical that partially removes the beads, this way they created a range of variation that mimicked the natural one, but removing other possible confounding effects such as age, nutritional state, effects of encounters with predators or mating history. They removed the mated males to carry out measurements and they allow each female to mate three times. Then they also measured males that hadn't been mated. Females are able to exercise mate choice as they can adopt the "mate refusal posture" by which they lower their wings and raise their abdomen as shown in the photo below when they don't want to mate. Males showcase their wings during their ritualised courtship flight

Courting Small White butterflies. The female, underneath the male is rejecting him by lifting her abdomen and lowering her wings.

Their experimental results (below) together with further modeling of the female's visual system indicated that females, as expected, prefer to mate with brighter males. Sexual selection therefore favours brigher males.

Spectral reflectance of male and female Pieris rapae in study 1 (A) and study 2 (B). Average female phenotype (dashed lines), average mated male phenotype (solid lines), average unmated male phenotype (dashed and dotted lines), and minimum and maximum male values (dotted lines) are displayed. Histograms to the right are for all males, with the proportion of mated males (solid region) and unmated males (open region) indicated in each bin (from Morehouse & Rutowski 2010).

 However, not everything is rosy (or ultraviolet if you wish) for these males, as the same colour patterns that make males more attractive to females, make them more visible to their main predators, birds, which are also able to see in the UV range. This study shows how sexual selection and natural selection through predators can work in an opposite direction, and reminds us that beauty is in the eye of the beholder.

References

Morehouse NI, & Rutowski RL (2010). In the eyes of the beholders: Female choice and avian predation risk associated with an exaggerated male butterfly color. The American naturalist, 176 (6), 768-84 PMID: 20942644

Water slater embrace

Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
 Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males.  Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
 Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.

(from Verrell, 1985)


References
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127