A horsefly of some sort drones around my head. There’s something cloying about the way it flies; almost, but not quite, brushing my skin. I sit still, hoping it will land on my hair so I can swat it into permanent silence. A toucan loops from one side of the lake to a tall tree on the other, its flight stitching across a fleecy sky. Nearby, red-capped cardinals flit among overhanging branches and a hoatzin, the clumsiest of rain forest birds, fusses peevishly at my presence.

It’s July and I’m sitting in a wooden canoe on the lake adjacent to the Cocha Cashu Biological Station in the heart of Manu National Park, Peru. I’m waiting for the giant otters. Earlier in the morning, I watched a pair emerge from their den on the shore of the lake and begin the day’s hunting. They foraged briefly in the shallows before heading purposefully into deeper water. Eventually, I saw them disappear into the grassy vegetation at the end of the lake, but I’m pretty sure they’ll be back.

There have been exciting rumors recently and I’m keen to confirm whether they are true. Some weeks earlier I received an e-mail with a photo showing one of the otter pair carrying something in its mouth. It looked very much like a cub. If so, this would mean the birth of a new giant otter litter on the lake for the first time in four years. I’m also anxious to identify the breeding pair.

As I wait, I listen for the otters as much as I watch for them. They may be giants (measuring as long as you are tall) but otter heads are small and brown, not unlike the color of the water, and are easily missed. Otters are also noisy, especially when chasing fish. Their loud exhalations after surfacing from a dive are an instant giveaway. And it’s these I hear now.

Picking up my binoculars I scan the edge of the aquatic vegetation. There they are. One has a fish in its jaws, and the cracking of the skull and bones is audible even at this distance. Soon they head in my direction. I push the boat away from the shore so I don’t startle them. I want them to see me and to come and investigate. Sure enough, they change direction and swim steadily towards me. I lower my paddle and make sure the lens cap is off my camera. When they periscope, showing their distinctive white throat markings, I’m ready and take several photos. Later, when I get back to the station, I should be able to identify them.

Satisfied, I retreat and make sure the otters see me doing so. Then I wait again (there’s a lot of waiting involved in otter watching!). The pair swims directly towards the den. One stays outside while the other enters. I hear a hum, rising and falling, and I know that any moment now I will find out if the pair has cubs. I grip my binocs and stare until my eyes burn. Suddenly they’re all in the water just in front of the den. How many? One, two… three, four, five… six! Six otters! That means four cubs! Fantastic! I’m ecstatic.

The young appear to be about two months old. They can swim, but not efficiently and every now and then I see one of the adults briefly grasp one by the neck to bring it to the safety of the shallows. When Mom or Dad (I don’t know yet which is which) offers a small fish, the cubs are intrigued and mouth it, not quite sure what to do.

The family gradually makes its way to a large tree that has fallen in the water. If I’m lucky, the breeding pair will haul out on the trunk and I will be able to sex them. First a cub climbs up. It’s adorable, enchanting. Then an adult sniffs the tree. I hold my breath. The otter looks around - I note the shape of its throat marking (large, with three teardrops) - then it clambers up and lies down alongside the cub. But not before I get a fine view of a set of testicles. So this one’s the male! Which of course means the other adult is the mother. Now all that remains is to find out who they are.

I row back to the station as fast as I can. Grabbing my camera, I stride to my laptop and start it up, eager to locate the folder of photos of the Cocha Cashu otters. I turn on my camera and start comparing. My suspicions are confirmed and I whoop in delight. The mother is an otter named Footsie, born on this same lake in 2011, sole survivor of a late litter. Her mother disappeared the following year, aged 13. The young female and her father remained and no cubs were observed during 2012, 2013, and 2014. But the male I photographed today is not Footsie’s father; he has most likely died due to old age (13 years). It’s a new male, who we later name Inkani, Matsigenka for ‘rain’. Footsie was only able to breed once she’d reached the right age (over three years old), and with the arrival of a new male (since incest has never been recorded). This explained the long absence of litters on the lake. And not only is there once again a family of giant otters on Cocha Cashu, it’s also the twelfth case of female territory inheritance documented in Manu National Park.

I close my laptop with a pleasing clunk; a wonderful morning’s work. To me, a lake without otter cubs is only half a lake. And Cocha Cashu is good otter territory. If we look after this family well, there’s no reason it can’t be as successful as a previous resident otter group, which managed to produce no fewer than 21 cubs over a nine-year period. Fingers crossed!