Fiddlers on the mudflats

As I’ve mentioned before, I spent a little time on the mudflats of Darwin a while back with Pat Backwell and her team of postdocs and PhD students. Pat invited me to complete a project that had already been started in a previous field season, looking at coercive mating in a very charismatic fiddler crab, Uca mjoebergi, sometimes called a banana crab.

If you’ve spent much time hanging out on beaches or estuaries in the tropics, you may have noticed hundreds of tiny little crabs with one oversized claw waving like mad – a rather peculiar sight. These are the aforementioned fiddler crab, a group of around 100 species that are united by their possession of one tiny claw and one giant Popeye-esque claw.

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A male Uca mjoebergi fiddler crab with his HUGE yellow claw (right) coupled with tiny feedling claw (left)

Males produce these huge claws and wave them around for two reasons: to  ward off male competitors and attract females.

Banana crabs dig burrows across the mudflats which they use as shelter when not out foraging and when the tide rises each day to inundate the area. It’s a lot of work digging a burrow so the crabs will defend them from cheeky neighboring crabs that may want to kick them out and take over their sweet pad. Males stop burrow takeovers by first waving at any potential rivals to stay away using their big claw as a signal of strength and willingness to inflict injury if necessary. If rivals do try to enter a male’s burrow, they will use their big claws as weapons and grapple until the eventual loser decides to retreat.

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Two male crabs fight for the resident male’s burrow (left), using their claws as weapons

Male burrows are also important real estate because this is where female fiddler crabs hang out after they have mated. Deep inside the burrow they extrude their eggs and wait for the embryos to develop on the underside of their abdomen before they pop up onto the mud flat and release their larval brood on the next spring tide.

Females are fussy about the burrows that they choose to rear their young. Because the burrows are only covered in water for a couple of nights each fortnight it is essential that her babies develop at the right speed so they don’t miss the chance to disperse into the ocean. Burrow width and depth directly influence temperature inside the shaft, such that narrow burrows are warmer and will speed up rate of embryo development in contrast to wider, cooler burrows.

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A female banana crab loaded with eggs. Photo credit: Tanya Detto, used with permission from Pat Backwell

So, just as a male waves to keep rival males away, he also waves to attract females as they wander over the mudflat in search of a mate. If the female likes the look of the male she’ll come in closer for a better look at his burrow by placing her feet at the entrance and checking out it’s suitability for raising her young. Often the female is not satisfied with the burrow conditions and will wander on in search of a better male and a better burrow, sometimes visiting up to 20 males before she finds one she likes.

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A male Uca mjoebergi fiddler crab (left) waves at an approaching female (right) to entice her to visit his burrow. Photo credit: Tanya Detto, used with permission from Pat Backwell

Normally as the female approaches, the male will go down his burrow and wait for the female to follow him down. If she does enter this is usually a sign that she has given him the go-ahead to mate and will move in for the next couple of weeks.

However, sometimes the male will hang back and awkwardly continue waving at the female next to his burrow while she waits for him to go down the burrow shaft. Usually the female gives up and moves on if the male doesn’t go down the shaft first – but sometimes she will enter the burrow anyway.

At this point, the male will dash down the burrow after the female and trap her inside the narrow shaft. If the female does get trapped below the male, there is a much higher chance that she will stay down, presumably mate with the male, and remain in the burrow to hatch her eggs. In a paper that we published this week in PlosOne, we argue that the alternative male behaviour of entering the burrow after the female has gone down is a form of coercion.

To figure out whether males that trap females had a higher chance of mating success than those that used the more traditional behaviour of going down the burrow first, we tracked females as they went in search of mates on the mud flat. Pat Backwell has been working on a population of fiddler crabs at East Point Reserve in Darwin for a number of years now, and knows just about everything there is to know about them.

When I was visiting in September 2014, Pat had two PhD students and two other postdocs working on fiddler crab behaviour. During the neap tides we would spend our days under umbrellas in the baking heat, armed with binoculars and notebooks as we watched the crabs running around the mud flat.

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Fiddler crab biologist Kecia Kerr watching her population of fiddler crabs

As females approached waving male crabs, I would observe whether the male entered the burrow before or after the female, and then checked whether the female stayed down with the male to mate. If the female did stay down for a few minutes – a pretty sure sign she would mate – I marked burrows with a little flag and covered it with a plastic enclosure so that after a few days I could dig up the female and see if she had extruded eggs.

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Lots of little yellow-clawed crabs with a few burrows marked out on the mudflat

We don’t yet know why females would take the risk of entering the burrow first if there was the possibility of getting trapped, but we found a convincing benefit for males: females that entered first were 3 times more likely to stay down and mate with the male, making it a pretty successful strategy for tricky males. 90% of the females we checked later had extruded eggs, suggesting that they did indeed mate after being trapped inside.

So who are these coercive males? We thought that males that “step aside” and wait for a female to enter the burrow first would be of poorer quality, because they may use coercion to mate with females that may not otherwise have chosen them. However, we found no relationship between male body size and the mating tactic he used, nor did we find that small (and perhaps naive females) were those that were ‘fooled’ by these coercive males. One thing we did not check was whether males that step aside have crappier burrows, making coercive mating a way of getting around the problem of not providing the female with a great home to develop her embryos. This is something to look at in future.

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It takes a while to get your eye in to watch fiddler crabs & it’s particularly hard to locate females, who lack the distinctive yellow claw of males

Coercive mating has been observed in a number of other animals including insects and birds. Male camel crickets in the USA, for example, use big spines on their hind legs to pin a female down and force her to mate. Check out this video to see how they do it. Although it would be logical to assume that large males with the longest leg spines are best able to restrain females, coercion is actually a small-male behaviour. In this species, like what we predicted for the fiddler crabs, coercion is only done by males that probably wouldn’t have had much hope in attracting females anyway.

Male hihi (stichbirds) in New Zealand also appear to sometimes forcefully copulate with females, during which they pin the female to the ground and mate face-to-face, a unique behaviour among birds.

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A male hihi on Tirtiri Matangi Island, NZ. Photo by Duncan Wright (Wikipedia Commons)

It’s really, really tricky to actually test the difference between female choice (i.e. she actually wants to stay down in the burrow) versus coercion (she is trapped and would leave if the male didn’t block her passage) in fiddler crabs. Given that mating happens underground we can’t witness any potential struggle between the pair, or if the female may even be using the struggle to test something about the male (like his strength or vigour). So, we may never know for sure if this alternative male mating tactic is truly coercion. What this study has shown is that male fiddler crabs will get up to all sorts of tricks to better their sex lives!

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One of the many amazing sunsets in Darwin

A new life in Singapore

I recently made one of those decisions which, although pretty normal for a lot of academics, meant uplifting my life in New Zealand and moving to Singapore. I finished my PhD almost 2 years ago now and have been lucky to get several postdoc positions, one at the University of Auckland (where I did my PhD) and a shorter stint at the Australian National University (playing with fiddler crabs in Darwin).

photos for blog(left to right) 1. Male Forsteropsalis harvestmen eating a fly, 2. Male Pantopsalis cheliferoides harvestmen, 3. male Uca elegans fiddler crabs jostling for a burrow

These opportunities allowed me to get publications from my thesis out, make some new collaborations and delve into new research areas. The time had come, however, to head off and do something different and so I jumped at the chance to take up a research fellowship at the National University of Singapore. Although exciting, this has meant that my husband and I had to make the decision whether to do this together or live apart for the next couple of years. We decided (after much discussion & ultimately sacrifice on his part) that he would come along too & so he quit his job, we packed up our house, put all our belongings in storage and off we went to Singapore!

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Strolling around Kent Ridge Park

So here we are, a few weeks into our new life living in a country the same size as my old city but with a LOT more people. The upside is that living in the tropics means there is a pretty incredible amount of wildlife to see, even in a little city park or on campus on my walk to work in the mornings. I’ve been having fun ticking off the birds on campus using this handy website & this great app of Singapore birds. I even found out today there is an app for the snakes of Singapore which is great as we have already spotted three snakes while out and about. I’ve also realized that birding is a great way to make friends – on the weekend we met a Singaporean bloke called Philip who gave us all the latest gossip on where to see an orange headed thrush (you take the track at Bukit Timah with the stairs, turn left, then go straight and you should see a bunch of other birders with their tripods). He also told us how to find this super cute baby Spotted Wood Owl at Pasir Ris Park which we spotted snoozing up high in a tree near the beach (turns out we need a better zoom lens).

OwlAnyway, before anyone mistakes me for biologist working on charismatic megafauna, I should mention that I’m actually here to work on jumping spiders (Salticidae). As anyone who has a love for jumping spiders will know, they are renowned for their incredible diversity in colour and their charming courtship behaviours.

jumping spidersI’m going to be exploring a few different questions, but the main aim is to trace the origin and evolution of UV-colour in male jumping spiders and to compare this to how females use male colour to make judgments on male mate quality. Daiqin Li’s group ­­has already done a fair amount of work in this area already, which is actually really exciting for me as I spent a lot of time during my PhD working out the very basics of giraffe weevil ecology. Here I can jump in and build on all the great stuff already done and hopefully get stuck in to some bigger picture research such as using phylogenies to map the evolution of colour & its use in mate choice.

In the meantime, here are a couple of pics we’ve taken when out exploring the local parks on the weekends – still need to get a macro lens so mostly larger animals so far!

photos for blog(left to right) Female laced woodpecker, Clouded monitor lizard, Painted bronzeback snake