Tuesday 27 October 2020

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 migrationBiol. Lett. 15, 20190327 (2019).

Levett, S. & Walls, S. Tracking the elusive life of the Emperor Dragonfly Anax imperator Leach27, 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)

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