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Understanding Female Aggression in Fruit Flies

00:00:13.19 Why do females fight?
00:00:15.21 For 150 years, since Darwin published his theory of sexual selection, scientists have
00:00:21.14 thought that females don't fight.
00:00:24.01 Because... they shouldn't.
00:00:27.02 If you think of aggression, you'll probably think of males being aggressive.
00:00:32.04 That's 'cause, across the entire animal kingdom, from humans to birds, even to fruit flies,
00:00:37.19 which I work on, males tend to be more aggressive than females, towards each other and even
00:00:44.06 towards inanimate objects, for, seemingly, very trivial reasons.
00:00:48.19 It's really hard to find videos of females doing the same sorts of questionable behavior.
00:00:54.10 So, why is this?
00:00:56.19 It's kind of a... a biological theory behind it, actually, which is that the sexes choose
00:01:02.07 really different paths to reproduction.
00:01:05.00 Which is that females have to be choosy,
00:01:08.11 and that's because females invest a lot in each offspring.
00:01:12.24 So, you go from the gametes stage, where females have to invest in these really large eggs,
00:01:19.05 whereas males invest in tiny sperm.
00:01:22.20 And then it goes on beyond that gamete stage as well.
00:01:25.00 You can think of females having to go through pregnancy, lactation, even beyond like this
00:01:31.12 long-tailed tit having to feel... feed 12 chicks by itself.
00:01:36.09 And, because females have to be choosy, because they invest so much in each offspring, it
00:01:41.23 means that, you know, they don't want to mate with every male around.
00:01:46.00 They want to mate with the best male.
00:01:49.05 Males, on the other hand, don't have the same sort of restrictions.
00:01:54.05 For them, the best option to have the most offspring is to mate with as many females
00:01:59.14 as possible.
00:02:00.14 But there's a bit of a problem here -- if females don't want to mate with every male,
00:02:05.12 but males want to mate with every female, it means that males have to compete for access
00:02:11.10 to those females.
00:02:13.11 But, females, because there should be in theory lots of available males, don't have to compete
00:02:20.01 for access to those males.
00:02:21.15 So, females shouldn't fight.
00:02:24.09 But if you actually look at the animal kingdom, you find that lots of females fight.
00:02:30.09 You find it in mammals -- so, female hyenas are actually more aggressive than male hyenas.
00:02:36.00 They'll fight for social dominance inside the pack, and more dominant females have a
00:02:41.16 better chance of raising their offspring.
00:02:43.23 You find it in birds, and females compete for access to nesting sites, so these are
00:02:49.19 hollows in tree trunks, and the better your nesting site the better your chance of raising
00:02:55.01 offspring and the better their chance of producing their own.
00:02:59.13 You even find it in insects.
00:03:01.18 So, here, this burying beetle female has had to compete with other females to get access
00:03:07.18 to food to feed her larvae, down here.
00:03:11.19 So, females do fight.
00:03:13.04 But they don't fight like males.
00:03:17.10 Females tend to fight for resources for their offspring.
00:03:20.00 So, there's this really interesting link between reproduction and female aggression that you
00:03:26.08 don't see in males.
00:03:28.24 So, it's something that's really specific to females, and I want to check out more what's
00:03:34.15 happening, what this specific link is
00:03:36.17 between reproduction and female aggression in fruit flies.
00:03:40.18 And fruit flies are a great system for this because we know females are aggressive.
00:03:45.00 They'll fight over access to food.
00:03:46.07 We also know that males are aggressive, so they'll fight over access to females,
00:03:50.24 they'll fight over... over access to food -- apple juice.
00:03:54.20 Males will even fight over access to headless females.
00:03:58.14 We also know that sex and reproduction has a really big influence on females.
00:04:05.03 So, sex completely changes female fruit flies.
00:04:09.02 It turns them into completely different animals.
00:04:11.24 So, before mating, females don't produce many eggs.
00:04:16.00 After mating, they start pumping out hundreds over their lifetime.
00:04:21.01 And these eggs are actually really costly to produce.
00:04:23.09 You require a lot of protein to build an egg.
00:04:26.18 The problem is that, for females in the wild, they live on rotting fruit.
00:04:30.10 There's not a lot of protein available, so there are kind of small patches where fermenting
00:04:35.11 yeast produces this protein, which females need to produce these eggs.
00:04:42.11 Mating also changes females’ receptivity to remating.
00:04:46.09 Before they mate, females are generally pretty keen; after mating... after mating
00:04:51.06 females are not receptive to any other males.
00:04:55.21 Sex also changes a whole lot of other aspects of female fly behavior.
00:04:59.14 It changes their sleeping patterns -- mated females sleep a lot less than virgins.
00:05:05.03 It also changes things like their immune system, gene expression, their feeding behavior.
00:05:10.11 So, it seems like mating is a really good place to start looking to test the link between
00:05:17.20 reproduction and female aggression in flies.
00:05:20.11 So, how do you make female flies fight?
00:05:25.17 So, the first day, what you can do is... basically, with fruit flies, you can collect a whole
00:05:31.08 bunch of eggs, then ten days later these eggs will emerge into fully formed flies.
00:05:37.15 So, you've got a sort of a seven-hour window from when they emerge as flies until they
00:05:43.11 start mating.
00:05:44.11 So, in that seven-hour window, you can be pretty sure they're virgins.
00:05:47.16 So, I collected a whole bunch of virgin females and I kept them in these individual vials.
00:05:52.10 So, here, they've got some regular fly food, down there,
00:05:57.03 which doesn't have a lot of protein in it.
00:05:59.22 Then, after I've let them recover for a day or so, I paint the flies so that I can identify
00:06:05.14 them later.
00:06:06.14 So, I'll give them a nice little red or... or yellow spot.
00:06:09.10 The next day, I will either mate them or I'll keep them as virgins and not mate them, so
00:06:17.15 that I can control their mating status.
00:06:21.09 And then, the final day, I'll fight them.
00:06:23.20 So, on the day of fighting, I'll start by putting them, in the morning, into what I
00:06:29.02 call a starvation vial.
00:06:30.22 So, essentially, what it is is a vial that just has some damp cotton wool in it.
00:06:35.10 So, they don't have access to food, but they do have access to water.
00:06:38.19 I leave them in there for two hours, so it's not too traumatic for them, and then it just
00:06:44.13 makes them a little bit more motivated, a little bit more hungry.
00:06:47.14 And then I put them into a contest arena, which is actually just a small plastic cap
00:06:52.18 that has sort of the regular fly food in it, that they've been living on for their lives,
00:06:58.15 and the small red dot, here, which is made up of live yeast and is really high in protein.
00:07:04.18 So, then, I'll pop the flies in there and then I'll record them for 30 minutes.
00:07:10.13 So, for those of you who've always wanted to know what a fly fight looks like, here
00:07:15.11 we have an actual video.
00:07:16.14 So, this orange stuff is the regular fly food.
00:07:19.18 Here, you can see one of them is painted yellow, one of them is painted red, and they're eating
00:07:24.16 a little yeast paste in there, which you can't really see on this video.
00:07:31.08 So, here we have the flies starting to headbutt each other and starting also have a go with
00:07:36.18 their legs as well, which you can see.
00:07:38.10 Red's getting a bit angry, here.
00:07:40.04 And Yellow is having a little bit of a go at Red as well.
00:07:44.18 Now, there are a couple of different types of female fly fighting behavior which are
00:07:50.01 really specific to females.
00:07:51.14 So, one is this kind of fiddling around with their legs, which we call fencing.
00:07:56.04 The one that's sort of the highest intensity of female aggression is called headbutting.
00:08:00.20 So, here we're going to have a look in a bit more detail at flies headbutting.
00:08:08.11 So, we have Yellow chasing Red off and, just to slow it down a bit for those who haven't
00:08:16.07 spent hours watching fly videos...
00:08:17.22 Yellow coming around, having a go and headbutting Red off the food cap.
00:08:27.04 So, that's what fly aggression looks like.
00:08:30.24 So, the first question we want to ask was essentially,
00:08:35.05 do mated females fight more than virgin?
00:08:38.07 Does mating increase female aggression in fruit flies?
00:08:41.09 So, for these graphs, basically what's going to be happening for all of the graphs is...
00:08:46.03 on the y-axis, we're going to have the total fight duration, so, the total amount of time
00:08:51.07 female spend fighting in an observation period.
00:08:54.21 And in this particular experiment we took two virgin females, down here in blue, and
00:08:59.06 fought them against each other.
00:09:01.05 We took one mated female and one virgin female, and fought them against each other.
00:09:06.04 And then we had two mated females fighting against each other.
00:09:09.24 So, if you look at the two virgins first,
00:09:12.20 you find that they don't spend a lot of time fighting.
00:09:16.16 They're not really very aggressive.
00:09:19.19 If you change it and put in one mated female, all of a sudden you get this increase in female
00:09:25.01 aggression.
00:09:27.15 What happens if you have two mated females?
00:09:30.18 You get a further increase on that mated-and-virgin pair.
00:09:35.23 What's really interesting is that, between the red and the orange, there's not actually
00:09:40.00 a significant difference.
00:09:42.01 But between those two and the blue, there is a big difference.
00:09:46.14 So, mating makes females a lot more aggressive, even if there's just one of them in there.
00:09:53.23 So, that's pretty cool, that you see this direct link between mating and increased aggression.
00:09:59.09 But what is it about mating that makes females more aggressive?
00:10:02.12 So, the first sort of logical question was, is it because of eggs?
00:10:07.22 As soon as females have mated, they start producing a whole lot of eggs.
00:10:13.03 And you can imagine, if you're eating for 2 or 200, that you have to get a lot more
00:10:18.02 food, that your body tells you to collect more nutrients.
00:10:22.01 But, just because you need more food doesn't mean that there's more available in the environment.
00:10:27.13 You know, there could very well be other females who are in exactly the same situation.
00:10:31.15 So, to get that food, you need to get aggressive.
00:10:35.03 So, what we did was we tested females that couldn't produce any eggs, so these are completely
00:10:42.02 sterile females, and we said, okay, if it's sort of these costs of egg production that
00:10:47.16 are driving female aggression, these sterile females shouldn't show that increase in aggression
00:10:54.01 after mating, because they don't have these costs.
00:10:57.06 So, our prediction was that you'll see a bigger difference in terms of levels of aggression
00:11:02.14 between mated fertile flies and virgin fertile flies than
00:11:07.04 between mated and virgin sterile females.
00:11:10.24 So, here we have... we have our two sterile treatments down here, whoo... and our two
00:11:17.18 fertile treat... treatments down here, and we had two pairs.
00:11:21.16 So, we had... in this case, we had females fight against someone from their own mating
00:11:27.00 status, so you have two mated females or you
00:11:29.07 have two virgin females fighting against each other.
00:11:32.04 So, if we have a look at the virgins, first, for both the sterile and the fertile females,
00:11:37.14 there wasn't a lot of fighting going on.
00:11:39.04 And there's no difference between these two, which we wouldn't expect.
00:11:42.17 Then, if we look at fertile mated females, we see that same dramatic increase in female
00:11:48.11 aggression that we saw in the last graph.
00:11:50.24 So, mating increases female aggression there.
00:11:53.14 But, what happens with mated sterile females?
00:11:56.15 So, here, you still see an increase in female aggression after mating with sterile females.
00:12:03.23 But what we're really interested in is, is the difference between these two bars smaller
00:12:10.04 than the difference between these two bars?
00:12:12.24 And the answer is that there is no difference.
00:12:17.05 So, egg production doesn't actually change how much more aggressive you get after mating.
00:12:23.23 So, it's not eggs.
00:12:25.17 It's not the costs of egg production driving this increase in female aggression, which
00:12:31.11 kind of blew us away a little bit because it seemed like such an easy, logical answer.
00:12:38.03 So, we thought, okay, let's take a few steps back, you know?
00:12:41.23 What happens before egg production?
00:12:43.07 What else happens during mating?
00:12:45.00 The answer is that males transfer a whole lot of stuff to females.
00:12:48.22 So, we said, alright, let's take a look at that stuff.
00:12:51.22 Let's take a look at that male ejaculate.
00:12:53.10 And the male ejaculate, at least in fruit flies, it's a bit of a cocktail.
00:12:58.13 So, you get sperm.
00:13:01.10 You also get the seminal fluid, which contains a whole lot of proteins that can actually
00:13:05.18 induce changes in female behavior.
00:13:08.12 You also have what I like to call the zest of life.
00:13:12.09 So, we decided that the first obvious thing to check was sperm.
00:13:16.20 You know, it's crucial for fertilization, at least in this species, and it's needed
00:13:22.11 for a lot of those post mating behavioral responses I talked about earlier, such as
00:13:27.11 increasing egg production, reducing receptivity, immune changes, that sort of thing.
00:13:33.18 And sperm's really interesting because it doesn't just act on its own.
00:13:37.08 It actually interacts with these proteins that are involved in the seminal fluid.
00:13:42.21 So, it seems like a good candidate, both sort of by itself but also as sort of a vehicle
00:13:49.20 for other proteins to induce these changes in females.
00:13:53.21 And this picture here is actually the female reproductive tract, these tubes, and the green
00:13:58.16 and red dots are sperm from two different males.
00:14:02.04 So, it kind of shows...
00:14:04.19 I like it because it illustrates that a lot can happen in females after mating, particularly
00:14:09.16 related to the male ejaculate.
00:14:12.00 So, we predicted that sperm would be necessary for increased female aggression after mating.
00:14:19.01 So, in this situation, we have our blue is our virgin, which were, again, not very aggressive,
00:14:27.04 then we have our orange, which was our control males... our control females, who mated to
00:14:32.03 males that gave them everything in terms of an ejaculate, who transferred sperm.
00:14:38.17 So, here, again, there was that same sort of increase in aggression after mating.
00:14:45.15 But then, for our experimental treatment, we had females who were mated to males that
00:14:50.15 didn't produce sperm.
00:14:52.00 So, they produced the rest of the ejaculate, they just didn't produce sperm.
00:14:56.09 So, we predicted that, if sperm is involved, you should see a bar that's pretty similar
00:15:03.08 to virgins, which is exactly what we found.
00:15:07.18 We found that, if you don't get sperm, you're essentially fighting like a virgin.
00:15:14.14 So, sperm is really crucial for this increase in female aggression after mating.
00:15:20.22 But, as I discussed before, that might not be the end of the story.
00:15:27.12 Because sperm also interacts with all of these other proteins and has a lot going on, we
00:15:32.14 didn't want to just stop here and say, okay, it's just sperm.
00:15:35.21 So we said, let's have a look at some of those proteins.
00:15:39.24 Because what about the rest of the ejaculate?
00:15:41.22 The rest of that cocktail is still there.
00:15:44.14 So, the one that we really wanted to look at is
00:15:47.08 this nasty little protein called sex peptide.
00:15:50.09 So, this is it here, in this beautifully colored picture, and it's 36 amino acids long, so
00:15:55.06 it's pretty tiny, but it's really powerful.
00:15:58.12 So, all of those responses I've told you about before -- increasing egg production, reducing
00:16:04.18 receptivity, immune changes, gene expression, even reducing female lifespan -- those are
00:16:12.17 all caused by sex peptide.
00:16:15.01 And not only does sex peptide have dramatic effects on females, it also comes into the
00:16:23.03 females bound to sperm.
00:16:25.09 If you don't have sperm, a lot of the effects of sex peptide are actually really diluted,
00:16:29.20 because... but they work together.
00:16:32.03 So, we thought, well, it seems pretty likely that sex peptide could be involved in increasing
00:16:38.15 female aggression after mating.
00:16:40.20 So, again, we have our virgin females in blue.
00:16:46.16 Again, not very fighty.
00:16:48.13 Then, we have our control females, who were mated to males that gave them the entire ejaculate.
00:16:54.11 Again, showing that same increase in female aggression.
00:16:58.16 But this time, instead of removing sperm, we mated females to males that did not produce
00:17:04.15 sex peptide.
00:17:06.02 They produced the entire ejaculate except for sex peptide.
00:17:09.14 So, here we see that, actually, it's kind of an intermediate response.
00:17:15.23 So, you see that females that mated but didn't get sex peptide, they don't show that full
00:17:21.24 increase in mating that we see when you get sex peptide,
00:17:26.24 but they're not quite like virgins either.
00:17:29.07 So, this tiny little peptide, this 36-amino-acid-long protein, is at least partially responsible
00:17:35.23 for increasing female aggression after mating.
00:17:38.24 So, sex peptide is also pretty important.
00:17:42.19 So, these results show that males can actually alter female behavior towards other females
00:17:50.13 through their ejaculate, which I find pretty creepy.
00:17:56.08 But, I mean, that raises a whole lot of other questions, like, why would males do this?
00:18:02.10 So, the first option is that females actually use it as a cue.
00:18:09.06 So, females mate, they get a whole lot of sperm and they say, okay, now I've got this
00:18:14.12 sperm, I have this sex peptide, that's a sign that actually I'm gonna really soon be producing
00:18:18.23 a whole lot of eggs.
00:18:19.23 That means that I'm gonna need to get more food, so I need to get aggressive.
00:18:24.10 So, maybe it's in the females’ benefit.
00:18:27.03 You know, maybe they're able to detect that all of a sudden they've got this huge amount
00:18:31.23 of sperm that they can use to change their behavior.
00:18:37.11 The other slightly more disturbing possibility is that
00:18:41.02 maybe it's actually a form of male manipulation.
00:18:44.03 Maybe it's not in the female's best interest.
00:18:46.10 So, this is a possibility because you could imagine, you know, a female mates to a male,
00:18:52.08 has some eggs with his sperm, they both then go their separate ways, and she goes on to
00:18:57.23 mate with another male.
00:18:59.23 And, sort of, how much she wants to invest, in terms of time, in terms of energy, could
00:19:07.09 be dependent on the male she's mating with.
00:19:09.18 So, you can imagine if you have, you know, a bit of a dud male, you don't want to invest
00:19:14.12 a lot of your time and energy in producing his offspring, whereas if you had a really
00:19:19.17 good male it's actually really beneficial for you to
00:19:22.07 invest energy in fighting for his offspring.
00:19:28.07 But that first male is not gonna see it like that.
00:19:32.03 The first male wants the female to invest as much as possible in his offspring.
00:19:36.21 So, if he can make her more aggressive, it elevates her energy level, but it could have
00:19:43.05 flow-on effects for her.
00:19:45.03 Maybe she has less energy later on in life.
00:19:47.16 Maybe aggression results in injury.
00:19:50.13 Maybe it shortens her lifespan.
00:19:52.09 So, it could be that, actually, males manipulating females into fighting is actually a form of
00:19:59.12 sexual conflict.
00:20:00.19 So, as well as sort of these, you know, broad questions about what the evolutionary purpose
00:20:07.16 is, there are also a whole lot of questions that this kind of research leads to, like,
00:20:14.09 how do sperm and sex peptide do it?
00:20:17.14 How do they transmit these complicated messages to... to result in female behavior?
00:20:24.09 Do females just detect that their sperm or storage organs are full?
00:20:27.23 Are their receptors involved to pass neu... to pass messages up to the brain?
00:20:33.23 What about other seminal fluid proteins?
00:20:35.11 There are another 150 that trans... get transferred in the ejaculate and we've seen that sex peptide
00:20:41.04 isn't fully responsible, so maybe some other proteins are involved.
00:20:45.24 You also have the question, sort of, are more aggressive females actually more successful?
00:20:52.16 If you increase your aggression, maybe you succeed initially, but you can imagine that...
00:20:57.10 that comes with a high cost.
00:20:59.05 If you're spending your entire time fighting off other females, maybe you have less time
00:21:03.09 to eat, maybe you have less time to mate, less time to lay eggs.
00:21:07.04 So, is there actually a benefit to being aggressive?
00:21:10.14 I'm trying to answer that question about, who is... who is this benefiting?
00:21:15.14 And also some really tricky questions like, does it depend who you mate with?
00:21:22.01 Does this effect change?
00:21:24.05 Do different males induce different levels of aggression in females?
00:21:29.05 And, then, the flow-on effects from that.
00:21:32.09 If you're a female, you not only have to worry about how many resources are available in
00:21:36.14 the environment, or, you know, your mating status, you also have to think about the mating
00:21:41.23 status of other females in the environment and who they've mated to.
00:21:45.11 So, it has some serious evolutionary and ecological consequences.
00:21:50.02 And then, of course, there's the question about, could this happen in other species?
00:21:56.00 So, in a lot of insects, there are really similar proteins, really similar reproductive
00:22:01.02 systems, that suggest that maybe you could see the same increase in aggression after
00:22:05.19 mating in females.
00:22:07.16 Potentially, you could even see it in a broader range of animals, so from birds, reptiles,
00:22:13.22 mammals... all of these have sperm and seminal fluid proteins,
00:22:17.16 as well as instances of female aggression.
00:22:20.05 So, could there be a link there?
00:22:21.24 These are obviously very different life history strategies, so it's a big jump, but, could
00:22:28.05 this be a potential mechanism to link reproduction and female aggression across species?
00:22:35.11 And the million-dollar question, of course, is, what about one other species?
00:22:40.22 Could it happen in humans?
00:22:44.18 And, with that, I'd like to thank my co-authors, and my funders, and iBiology.
00:22:49.18 Thank you.

This Talk
Speaker: Eleanor Bath
Audience:
  • Educators of H. School / Intro Undergrad
  • Student
  • Educators of Adv. Undergrad / Grad
  • Researcher
  • Educators
Recorded: May 2017
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Talk Overview

Why do females fight? For over a century, biologists thought that female aggression was uncommon in the animal kingdom. In this lecture, Dr. Eleanor Bath dispels that notion and shows that female aggression in fruit flies increases after mating. More specifically, she shows that semen and a small protein in the male ejaculate lead to increased female aggression in Drosophila melanogaster.

This talk is part of the Young Scientist Seminars, a video series produced that features young scientists giving talks about their research and discoveries.

Speaker Bio

Eleanor Bath

Eleanor Bath

Dr. Eleanor Bath completed her undergraduate degree at the University of New South Wales, Australia, with majors in biology, international relations, and history. She was awarded a Rhodes Scholarship to undertake a PhD at the University of Oxford, UK in 2012. During her PhD, she studied female aggression in flies, with Dr Stuart Wigby. Eleanor… Continue Reading

Playlist: Circuits, Learning and Behavior

Related Resources

Bath, E., et al. (2017) Sperm and Sex Peptide Stimulate Aggression in Female Drosophila. Nature Ecology & Evolution 1, Article number: 0154.

Reader Interactions

Comments

  1. Dr. Bonzo says

    This is fascinating work. Kudos.
    Perhaps not appropriate for iBiology, but i’m always interensted in ‘n’ for studies of this sort – i’ll have to check the pubs…
    Even though it’s ‘just flies’, i wonder if there’s any PETA blowback for invertebrate ‘dog fighting’?
    i imagine you’re aware of the dangers of over-anthropomorphizing…
    Evolutionary and genetic influences on aggression always lead me back to the opening tableau of Kubrick’s “2001 – a Space Odyssey”

    Reply

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