Nature's weirdest mating practices
The pressure, it seems, is proving too much for Yang Guang and Tian Tian, the giant pandas at Edinburgh Zoo.
The pair have been brought together to mate five times and, so far, have been unsuccessful.
With the zoo's experts struggling to create the perfect conditions for the panda couple to "get together", BBC Nature looks at the animal kingdom's strangest and most complicated mating rituals.
Bears' many neccessities
Female pandas ovulate just once a year and there is a 36 hour window during which they can get pregnant.
Pandas in captivity often fail to mate within that time. And although there have been successful captive breeding programmes with giant pandas, most of them have relied on artificial insemination.
One issue could be Yang Guang's lack of motivation. In the wild, males gather around a female and compete for a chance to mate, at which point levels of the sex hormone testosterone increase in their blood. In zoos, this lack of competition could be problematic.
Nutrition is also crucial.
A study published last year in the European Journal of Wildlife Research revealed that pandas conserve their energy by keeping testosterone levels relatively low until they have found a female.
This is linked to the fact that the bamboo the animals eat gives them relatively little energy. It is so nutritionally poor that the bears have to consume about 20kg per day to take in enough calories.
This direct link between breeding and nutrition makes the destruction of their forest habitat even more problematic for the bears.
Panda expert Ronald Swaisgood, director of animal ecology at San Diego Zoo, California, US, told BBC Nature last year that - left to their own devices, in the perfect bamboo-rich habitat - the animals would "do just fine".
The trouble with captivity
It is not just pandas that struggle to breed in captivity.
At London Zoo in the UK, keepers recently discovered that their pair of aardvarks were both female. Aardvarks are notoriously difficult to sex, because their genitalia are internal.
But John Ellis from London Zoo says that the animals "get along very well indeed".
The Zoological Society of London says it is currently working with the studbook holder for aardvarks in the European breeding programme, "so that we can introduce a male to our pair of females".
Anatomically 'choosy' ducks
The humble duck has a very complicated sex life.
Both male and female ducks have unusual genitalia: males have corkscrew-shaped penises while females have labyrinthine vaginal tracts with a number of "dead ends" that do not lead to the uterus.
While male ducks attempt, often very aggresively to mate with females, the females have evolved the ability to close off their real reproductive tract to the sperm of unwanted partners.
Female mallards also appear to be able to tell if their partner is likely to be healthy and disease-free.
Research has also shown male mallards have antibacterial semen, and the more antbacterial this fluid is, the brighter the colour of the male duck's beak.
Females probably use bill colour as a visual cue when choosing a mate, to avoid sexually transmitted diseases (STDs).
Shouting and dancing
Elaborate displays are often a crucial part of the mating ritual in nature.
The mating season turns usually quiet male koalas into bellowing beasts. And when the males begin to call, females appear to move towards the male with the most impressive voice.
A koala's vocal tract anatomy is so unusually specialised, in fact, that they are able to make sounds that make them sound far larger than they are
While courtship dances are common in the bird world, great bustards - one of the largest flying bird species - have developed a trick to make their elaborate mating displays even more alluring to females.
An observational study recently revealed that the large birds lift and point their bottoms towards the Sun, making their bright white tail feathers more conspicuous.
The behaviour makes them more visible to females, say scientists.
There are many weird and somewhat gruesome mating habits in the world of insects and spiders.
Preying mantis females are notorious for their habit of tucking into their partner while he continues his attempt to copulate with her.
There are also many spider species where the female is known to devour the male after sex.
In black widows, the species most infamous for this practice, this appears to be more about nutritional necessity than a strange sexual habit.
A study published in 2011 in the journal Animal Behaviour, showed that male black widow spiders had a trick to avoid being eaten.
The males, it seems are able to sniff out a well-fed female simply by walking on her web and they use this technique to avoid the hungry females, which are much more likely to cannibalise them.
Sneaky sex tactics
In the aquatic world, becoming a father can be a game of wits.
A recent study published in the journal Ethology found that smaller zebrafish were able to "sneak" in between females and their larger male competition in order for a chance to fertilise the females' eggs.
Like many fish, male zebrafish fertilise eggs after they are laid, a process of external fertilisation known as spawning.