The cetaceans also recognised individual "names" in the form of signature "whistles", looked after youngsters that were not their own, and showed an ability to work with different species, such as humans. Lead scientist Susanne Shultz, an evolutionary biologist at the University of Manchester's School of Earth and Environmental Sciences, suggested it was only a lack of thumbs that held whales and dolphins back. "Dolphin and whale societies are at least as complex as what we have observed in primates," Dr Shultz said. "They are extremely playful, they learn from each other, have complex communication. "One problem for understanding just how smart they are is how difficult it is to observe them and to understand their marine world.

"Therefore, we have only a glimpse of what they are capable of. "As humans, our ability to socially interact and cultivate relationships has allowed us to colonise almost every ecosystem and environment on the planet. "We know whales and dolphins also have exceptionally large and anatomically sophisticated brains and therefore have created a similar marine-based culture. "That means the apparent co-evolution of brains, social structure, and behavioural richness of marine mammals provides a unique and striking parallel to the large brains and hyper-sociality of humans and other primates on land. "Unfortunately, they won't ever mimic our great metropolises and technologies because they didn't evolve opposable thumbs."

The researchers created a comprehensive database of brain size, social structures and cultural behaviours across cetacean species. They also identified key differences among cetaceans linked to brain size. In terms of sheer brain size, the sperm whale is tops on earth, with a brain six times larger than that of a human. Those cetaceans with larger brains exhibit greater complexity in social structures and behaviours, with species such as the killer and sperm whales leading the way. The group of species with the largest brain size relative to body size was the large whale-like dolphins such as the killer whale, the similar-looking false killer whale and the pilot whale, Dr Shultz said.

"Killer whales have cultural food preferences, have matriarchs that lead and teach other group members, and co-operatively hunt," she said. In terms of intra-species food preferences, certain killer whale populations, also known as orcas, prefer salmon whereas others prefer seals or other whales or sharks, depending on their group's culture. Other big-brained cetaceans also demonstrate sophisticated behaviours. Mother sperm whales organise babysitting duties using other members of their pod to protect their young while they hunt for food down deep. The distinctive vocalisations sperm whales use to communicate sometimes differ depending upon where they live, much like regional dialects in human language.

Bottlenose dolphins use sea sponges as tools to protect their beaks while foraging for food, and live in structured communities. Some of the largest cetaceans - filter-feeding baleen whales such as the blue whale, fin whale and humpback whale that eat tiny crustaceans called krill rather than fish or squid - were on the low end of relative brain size. They live fairly solitary lives, coming together only for breeding seasons and near rich food sources. Writing in the journal Nature Ecology & Evolution, the team argue that large brains such as those of humans, great apes and cetaceans are an evolutionary response to complex and information-rich social environments. Co-author Dr Michael Muthukrishna, from the London School of Economics, said: "This research isn't just about looking at the intelligence of whales and dolphins, it also has important anthropological ramifications as well.

"In order to move towards a more general theory of human behaviour, we need to understand what makes humans so different from other animals." PA, Reuters