Emperor home ranges, habitat use and differences between sexes

Emperors (Anax imperator) have done very well this year in my local area, which has allowed me to become familiar with the different behaviours of males and females. Females are quite stealthy, entering ponds to oviposit on floating vegetation, often flying low over the water. In contrast, males display an obvious territorial behaviour, patrolling high over the water of the large pond, lake or slow running drains where they breed. Males move towards passing birds, and towards any intruders, with spectacular fights and clashes between the males, some times resulting on individuals falling on the water.

Ovipositing female emperor.

A male patrolling alongside the marginal vegetation of a large ditch.

All this means that we are biased in our understanding to these more obvious behaviours, ovipositing and territoriality. But many other questions remain about Emperor's behaviour and ecology, for example, where are females when they are not ovipositing? or males, when they are not patrolling a pond? how far do individuals roam or disperse? where do they roost? how is their behaviour affected by temperature? do they make use of the habitats around ponds? what about sex differences?

Capture-mark-resighting techniques have be used in dragonflies to study individual movements, behaviour or demographic parameters. Individuals of large dragonflies can be marked using alpha-numeric unique wing codes written with permanent markers, which could be read from a distance, with no need from physical recapture (for example using binoculars). This technique, however, requires an enormous field effort to relocate as many individuals as possible.

Another technique that has been used to study migration is to analyse location-specific isotopes in wing samples. These isotopes have signatures specific to geographic areas, which point at the area where the larval stage took place (as the wing tissue is formed during the larval stage). This method has been used to reveal the multi-generational migrations of the Green Darner (Anax junius) a North American relative of the Emperor that is a long-distance migrant. Hydrogen stable isotopes showed that the migration cycle comprises a north migrating generation, a south migrating generation and a resident generation that develops around the Gulf of Mexico. Another study using stable isotopes on the Global Skimmer, Pantala flavescens, revealed its multigenerational migration steps around the Indian Ocean. Although ranging from South Africa to Sweden, and still involved in natural colonisation towards the north, facilitated by global heating, the Emperor is a resident species, so this technique is not of much use.

Radio transmitters in dragonflies?

As technologies have resulted in the miniaturisation of radio-transmitters  in recent years, they have increasingly been used to study more local movement patterns in large dragonflies, like the North American Tiger Spiketail (Cordulegaster erronea) a relative of our Golden-ringed Dragonfly and Green Darners. The individuals need to be captured and fitted with tiny transmitter before release, and they have to be found in the landscape using scanning receivers fitted with an antenna. Size matters as the transmitter must be less than 30% of the weight of the dragonfly not to impede normal behaviour. The Emperor, one of our largest dragonflies, weighs about 1 g, and can be fitted with such small transmitters (check the photo here of an individual fitted with a transmitter). In an early small scale study (5 tagged individuals), researchers looked at home ranges and local movements between roosting sites and pond territories in male Emperors. The furthest moving male travelled 1.5 km from the tagging pond to another pond.

An article published recently sheds some light on home ranges and habitat use of Emperors using radio-transmitters. Marceau Minot and his collaborators chose five ponds in an urban/rural interface in northern France. Over the summers of 2017 and 2018 they marked 87 mature emperors with unique wing codes and visited the ponds at least once weekly to search for marked individuals. They also fitted 54 individuals with radio transmitters, and tracked them daily for up to 15 days.

Capturing individuals to fit radio transmitters or mark their wings has a cost. Both techniques increase mortality in the day after capture, probably due to the stress of the capture. 

Sex differences in behaviour

Females had larger home ranges than males. The furthest distance a female travelled was 1.9 km while males moved less, with the maximum male movement 0.5 km. This could be related to male's territorial behaviour. Presumably females oviposit in several ponds.

Reproductive behaviour of males, but not females, is positively related to temperature.

Flying behaviour in females is positively related to temperature, while resting high in trees is negatively related to temperature.

Both sexes were mostly found on ponds or pond marginal areas, although males spend more time near water.

Resting happened in ponds and trees. Females tend to roost high on trees, more than males. While males tend to rest low in vegetation.

A male emperor resting on marginal vegetation.

The researchers estimated the effect of marking protocol on survival of the dragonflies. The manipulation of the individuals affected their survival, possibly due to the stress during manipulation: 76% of individuals survived 24 after capture and wing marking, while just 56% survived 24h after being fitted with a radio-transmitter. Individuals with proportionally larger wings (not larger body mass) and younger in age survived better throughout the experiment.

Habitat management

The study also highlighted that both rural and urban ponds will benefit from the presence of neighbouring trees as suitable roosting sites, and emperors will benefit from the presence of a network of ponds, rather than isolated ponds.

More information

Hallworth, M. T., Marra, P. P., McFarland, K. P., Zahendra, S. & Studds, C. E. Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect. Biol. Lett. (2018).

Hobson, K. A., Anderson, R. C., Soto, D. X. & Wassenaar, L. I. Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern Indian subcontinent. PLoS One 7, e52594 (2012).

Knight, S. M., Pitman, G. M., Flockhart, D. T. T. & Norris, D. R. Radio-tracking reveals how wind and temperature influence the pace of daytime insect migration. Biol. Lett. 15, 20190327 (2019).

Levett, S. & Walls, S. Tracking the elusive life of the Emperor Dragonfly Anax imperator Leach. 27, 59–68 (2011).

Minot, M., Besnard, A. & Husté, A. Habitat use and movements of a large dragonfly (Odonata: Anax imperator) in a pond network. Freshw. Biol. 46, 207 (2020)

Migrant hawkers scramble competition

I'm used to watching migrant hawkers foraging over gardens, leafy streets and sheltered woodland rides, some times in groups. They are immature individuals, gathering energy away from water. Migrant Hawkers, unlike other hawkers, mature slowly, and will move to suitable breeding sites after their long immature period. During this past week I've watched them in their breeding sites in lakes and drains, where mating and egg-laying takes place.

A mature male rests briefly between bouts of searching (Pickering Park, 27th August) 

Males at the breeding site 
At one of my local parks (Pickering Park) last week, dozens of Migrant Hawkers sat or patrolled alongside marginal vegetation around the lake. The males, now fully mature and showing their bright blue spots and eyes and side yellow-green stripes often hovered in a spot, or explored the vegetation, flying well into it, searching for females.

A typical hovering male in a clearing at the marginal vegetation (Pickering Park, 27th August), offering them good views.

Mating
I saw no females, until one was captured by a male: no preliminary or courtship, the male just tackled her and positioned himself to grab her by the head. The female is then able to curve her abdomen and mate, retrieving sperm from the male's secondary genitalia at the base of the abdomen, forming the 'wheel position'. They may fly in wheel position very fast, zigzagging alongside the marginal vegetation edge or briefly rising into the air, before settling on vegetation (above).

Ovipositing female (Foredyke Stream, 1st September)

The moment when the male passes by, and sees the female.

The pair, mating.

Oviposition
A couple of days ago I watched a patrolling male on a ditch doing its usual patrolling routine, rising to inspect any passing individual, even paying attention - briefly rising - to birds flying over. A mated female arrived, unnoticed, and started laying eggs on live leaves well above the water line, I'd say over one metre over the water. She checked leaves and unsheathing her ovipositor, started laying into them (the eggs will overwinter inside the plant leaves, where they are protected from predation). After a couple of minutes, the male noticed her, tackled her and mating ensued. This time the pair settled briefly on plants, which allowed me to take a shot (above). Mating in Migrant Hawkers is longer than in other territorial relatives.

Two males resting near each other (Pickering Park, 27th August)

Nonterritorial males
This species is notoriously non-aggressive, even at the breeding sites. Males will even rest within view from each other (above). A patrolling male will swiftly rise to check a passing one, but the interaction is suggestive of them 'checking' that they are not a female, and letting the other individual go their way if it's a male. There is no defended territory, just males congregating on suitable ovipositing sites and searching for females, a type of mating tactic called 'scramble competition'. This appears to be the reason behind the long copulation. A territorial male mating for a long time may lose the territory to an invader, or miss extra mating opportunities. A nonterritorial male has less to lose, and therefore makes sure that he fertilises as many as possible of the females' eggs, possibly by taking time to remove any previous sperm before transferring his transfer. These patterns have been shown to stand when the copulation duration of territorial and nonterritorial drogonfly species were compared, but I haven't found specific data on the Migrant Hawker. To illustrate the pattern, the Emperor, a territorial species, copulates for an average of 10 min, while the Common Hawker, a non-territorial one, copulates for an average of 67 min. Of course, this lengthy copulation is not necessarily to the benefit of the female, who may be already mated, as the female already laying eggs, and therefore this sets the stage for the evolution of female counter-tactics, such as visiting the water as little as possible and avoiding males if they can, something I have covered before at Bugblog.

More information
Córdoba-Aguilar, A., Serrano-Meneses, M. A. and Cordero-Rivera, A. Copulation Duration in Nonterritorial Odonate Species Lasts Longer than in Territorial Species. Ann. Entomol. Soc. Am. 102, 694–701 (2009).

Azure Damselflies ovipositing

On Monday, I went to my local wildlife garden to check on dragonflies and damselflies. There were only Azure damselflies, Coenagrion puella. The Azure Damselfly is a common species, frequent in small ponds. Males are blue and black and females greenish and black. It can be distinguished from the Common Blue Damselfly by a forward pointing sput on the sides of the thorax. I walked around the pond and counted three mating pairs in tandem, and many individuals, including two teneral ones - just emerged and still not showing their full colour. The females were egg laying, with the males clinging to their thorax, guarding them as they lay from the attentions of other males. The females insert their eggs into stems of submerged aquatic plants. Females can lay over 4,000 eggs in their lifetime. I noticed that the pairs were not randomly assorted around the pond, but tended to gather in the same spot (top shot). Indeed, ingenious field experiments by A Martens showed that this species form aggregations to oviposit, the pairs particularly attracted to the vertical posture of the male in a tandem. Pinned models of a male in vertical position attracted tandems, but pinned models of females did not. Two model tandems were more attractive to pairs than single tandems, which will start an aggregation.

An adult male resting.

A teneral male, recently emerged.

Two pairs egg laying. 

The exuviae of three damselflies still stuck to the stem of a plant. 

More information

Martens, A. Field experiments on aggregation behaviour and oviposition in Coenagrion puella (L.)(Zygoptera: Coenagrionidae). Advances in odonatology 6, 49–58 (1994).

More skippers!

The sun shone finally after a wet and cool week. We popped in the wildlife garden in the afternoon. The knapweed, a favourite feeding plant for the skippers is now flowering in the meadow. Both species were very active, a male Large Skipper fed on the flowers, showing its long tongue.

A male Small Skippers followed a female fluttering behind her, but then appeared to lose interest, quickly losing her and perching, maybe she was already mated? The female settle to feed almost next to him and I just managed to take a photo with both in the frame: the male is in the foreground.

 

 We watched possibly the same female later as she was searching for suitable oviposition sites, which in the case of Small Skippers is Yorkshire Fog grass.  She settled and started a curious dance on the long grass stems, her swollen abdomen curved and everted, its tip touching the stem, and she moved circling around and up like moonwalking. We didn't actually see any eggs (they must be tiny!), but this confirms that the garden has a breeding population.

This gif showis a short sequence of egg laying behaviour:

Legs for laying eggs

Most Nymphalid butterflies, a group of large species that include the Peacock, Painted Lady, Comma and the Monarch, have modified forelegs, smaller than the rest of the legs and normally tucked in under the head. In the Peacock (above) the forelegs are relatively large, but not used for walking and they even have the same dark colour as the body, giving the impression that the butterfly has only four legs. Why is this? Despite their vestigial appearance, experiments have shown that the forelegs have a very important function, especially in females. In the investigated Nymphalid species (mainly the Monarch and the Queen butterfly), the tips of the reduced forelegs in females - but not males - have sensory organs associated to spines, which they use to recognise specific chemicals from the larval foodplant when ovipositing. They drum the leaves with their forelegs, puncturing the leaves and releasing plant chemicals allowing their receptors to detect them (what is known as contact chemorreception). The antennae and the tips of the other legs also contribute to selecting the foodplant, with tapping with the antennae and drumming with the mid legs also observed when selecting foodplant. The forelegs are part of a very complex sensory system, possibly providing a 'backup' with the sensory spines protected from damage, by the forelegs being reduced and not being used for walking. Butterflies can be very selective in choosing foodplants, as they not only have to determine the species of plant, but also how healthy or how old the leaves are, so it is not surprising that they use a complex sensory system to assess this, which we are only beginning to understand.

This video from Arkive, shows a female Peacock tapping a nettle leaf with its forelegs while laying eggs.



More information

Baur, R., Haribal, M., Renwich, J. A. A. and Staedler, E. (1998). Contact chemoreception related to host selection and oviposition behaviour in the monarch butterfly, Danaus plexippus Physiological Entomology, 23, 7-19 DOI: 10.1046/j.1365-3032.1998.2310007.x

Myers, Judith. 1969 Distribution of foodplant chemoreceptors on the female Florida queen butterfly, Danaus gilippus berenice (Nymphalidae)." J. Lepid. Soc 23: 196-198.

Marmalade fly ovipositing

Aphids are everywhere now, under tree leaves, on herb stems and on tender bush shoots. But they have many enemies. Amongst them, many hoverfly species are busy egg laying, carefully choosing their oviposition sites to be next to rows of fat aphids. This is my first photo of the year of a Marmalade Fly, Episyrphus balteatus: a female laying on a Wood Avens stem full of aphids. It was the only shot I took before she flew away. Soon their voracious larvae will be munching aphids away.