A few weeks ago I wrote about the intense rivalry that can occur among some brothers and sisters in the animal kingdom, but this family conflict isn’t restricted to siblings – the parents are at it as well. It’s a story of extreme selfishness, violence and even murder and it all boils down to one thing, the Holy Grail for our furry, feathered, and slimy friends – making babies.
When it comes to continuing your family legacy, it really is a dog-eat-dog world in the animal kingdom. Conflict arises over reproduction in many species and can begin right at the very beginning, from the moment you spot your desired mate. Whilst some species invest a lot of time and energy into attracting a mate – producing beautiful plumage, elaborate antlers, or performing a wonderful dance or call – other species don’t bother with the formalities of courtship – there’s no elaborate ritual to show-off their ‘sex-appeal’ to try and win over a potential partner.
The bed bug is an excellent example of this. They use a particularly romantic mode of copulation called traumatic insemination (I know – even the sound of it brings up images of candlelit dinners and love songs). Traumatic insemination in bed bugs occurs when the male pierces the female’s side (or abdominal wall) with his genitalia and inseminates directly into her body cavity, where the sperm then makes its way to her ovary to fertilise her eggs. It is important to point out that female bed bugs have perfectly adequate genitals for copulation, but the males have apparently decided to ignore this fact. So, why do they take such a nasty approach?
Well it’s all in the name of sperm competition. Not only do males have to compete to find a mate but they must continue the battle after copulation. This is due to the polygamous nature of bed bugs (and many other animals), meaning they mate with more than one individual. If a female mates with one male, all her eggs will be fertilised by his sperm, meaning he will father more offspring. However, if the female mates with two or three males, all their sperm must compete to fertilise as many eggs as possible. In the case of the bed bugs, damaging the female in this way has been shown to reduce her life expectancy significantly. She probably won’t mate again, and as evidence has shown the sperm of the last male to mate with her has precedence, performing this harmful act is beneficial to the male – in the absence of competition from other males, his sperm will fertilise all of her eggs and so he will become the father of lots and lots of babies.
Bed bugs, unfortunately, aren’t the only male insects that harm the female in the name of sperm competition. The genitalia of male bean weevils is covered in spines that damage the female’s genitals during copulation. Mating with another male can reduce the life expectancy of the female which means she usually won’t bother trying again – reducing the sperm competition. In this example however, the female has evolved to fight back. Females kick the male during copulation, making the act shorter and reducing the damage they receive. We have an evolutionary arms race again – the female will keep trying to prevent the male harming her, whilst the male will find new ways to make sure he prevents the female from mating with anyone else.
Some male insects aren’t so violent in their quest for fatherhood. The male fruit fly transfers proteins alongside its sperm that kills the sperm of rival males – effective and much less gruesome for the female.
I’m afraid to say the battle over reproduction doesn’t end after the act itself. The fight to produce as many offspring as possible takes an even more sinister turn in lions after the birth of their cubs. Prides are made up of an uneven mix of adult females and male, with females dominating the numbers. One male will be in charge for about 2-3 years, meaning they get the pick of mating partners. However, new males will challenge the pride leader for his place (think Mufasa and Scar in the Lion King). If they successfully defeat the leader and take his place, they are faced with a problem – their pride is full of lionesses looking after the cubs of another male. And females who have cubs that depend on them will not mate with another male for some time – not ideal for the male leader who only has around 2 years to make as many babies as he can.
They use a horrible strategy to solve this problem – when a new male take overs they will kill any dependant cubs. This means the females will be ready to mate sooner, allowing them to start making babies as soon as possible.
And it’s not just males who use this strategy, called infanticide. Female Wattled Jacanas, wading birds found in South America (among other places) mate with several different males – each female has several males that care for their separate broods. A female will control a certain area, but if for some reason she is removed – either she dies or some pesky scientist removes her for an experiment – a neighbouring female may take over her patch. These replacement females will remove all the old female’s broods and start afresh.
There are many more examples that I think I’ll save for another day, but it is clear that in the animal kingdom conflict over reproduction is intense. This is because males and females often have different reproductive goals. For the female, mating with as many different males as possible may mean she produces a stronger set of offspring – she has the advantage that all the offspring she gives birth to or eggs she lays will share half of her DNA. But males don’t have this luxury and so have to fight to make sure they father as many of the female’s offspring as possible. And boy, are they willing to fight dirty.
Stutt and Siva-Jothy (2001) Traumatic insemination and sexual conflict in the bed bug Cimex lectularius. PNAS, 98 (10); 5683–5687.
Crudgington and Siva-Jothy (2000) Genital damage, kicking and early death. Nature, 407.
Guess what I know? 