When people think about animal behaviour, they often assume that everything animals do is purely focused on reproduction. Nature, however, is rarely that simple. One fascinating area of research is the study of homosexual behaviour in animals, which has now been documented in hundreds of species around the world.

Scientists have recorded same-sex courtship, pair bonding, mounting, parenting and social behaviours in a remarkable variety of animals. These observations have been made in mammals, birds, reptiles, fish and even insects. In fact, researchers estimate that homosexual behaviour has been observed in well over 1,500 animal species, with detailed scientific studies published on several hundred of them.

One of the most famous examples comes from penguins. Same-sex penguin pairs have been observed in zoos and in the wild, sometimes forming long-term partnerships. In some cases, these pairs have even incubated eggs and successfully raised chicks together. Their devotion to one another appears no different from that seen in heterosexual penguin pairs.

Dolphins provide another well-known example. Bottlenose dolphins form complex social bonds, and same-sex interactions are common among both males and females. Scientists believe these behaviours help strengthen social relationships within the pod, which can be important for co-operation and group stability.

Closer to the theme of pride, lions also display same-sex behaviours. Male lions have been observed forming close partnerships that include affection, grooming, nuzzling and mounting. These relationships can last for years and may help strengthen alliances between males that work together to defend territory and maintain their position within a pride.

Among birds, swans, gulls, albatrosses and many other species have all been documented forming same-sex pairs. Some female bird pairs even co-operate to build nests and raise young together. In the animal kingdom, family structures can be far more varied than many people realise.

As veterinarians and animal carers, what makes this subject particularly interesting is what it teaches us about the complexity of the natural world. Animals do not read textbooks. They do not follow social expectations. They simply behave according to the biology and social structures that have evolved within their species.

For many years, people assumed that homosexuality was uniquely human, but modern scientific observation has clearly shown otherwise. Homosexual behaviour occurs naturally across a wide range of species and has likely been present throughout evolutionary history.

This does not mean every animal displays these behaviours, nor does it mean scientists fully understand every reason why they occur. Nature is wonderfully complicated, and animal behaviour often serves multiple purposes beyond reproduction alone. Social bonding, alliance formation, conflict reduction and simple companionship may all play a role.

One of the great joys of studying animals is that they constantly challenge our assumptions. Just when we think we have nature neatly categorised, a penguin adopts a chick, two lions form a lifelong partnership, or a dolphin reminds us that the natural world is far richer and more diverse than we ever imagined.

• Lucy Richardson graduated from Edinburgh University in 2005. She started CedarTree Vets in August 2012 with her husband, Mark. They live at the practice with their two children, Ray and Stella, and their dog and two cats. She is also the FEI national head veterinarian for Bermuda