The Young and the Restless

The story of two giant otters in Peru’s Manu National Park

Cocha Otorongo, Manu National Park, Peru, 1993, 5:15 am. The otters stir in their den. A series of soft cooing sounds, followed by the characteristic “Let’s go” hum, indicates the family is ready to start the day. A moment later, Isla’s parents emerge. Together they visit the nearby latrine, their broad, flattened tails held high, before thoroughly spreading their scat. The circling movements of their forepaws and simultaneous shuffling of their hind legs combine in a comical scent-marking dance. Next, three-year old Isla appears at the entrance of the den. Unlike her parents, she pauses only briefly on the latrine, and is followed in rapid succession by her siblings, all of whom eagerly rush into the water. Their father does the work for them, waddling once more over the latrine to mix their scat. He is the last to leave the den site. The group sets off along the shoreline, just as a gossamer mist lifts from the surface of the water.

Photo by Frank Hajek A giant otter with a fish in Manu National Park.

Cocha Cashu, Manu National Park, Peru, 1993, 5:40 am. Dedo awakens. He will not hunt on the lake today. For a while now he has been feeling restless; the urge is upon him to find a mate and raise cubs of his own, in his own territory. It is the end of the dry season, water levels are at their lowest, and fish are readily accessible. After two carefree years with his family, the time has come to leave the only home he’s known. He slips out of the hollow amongst the tree roots where he has spent the night and enters the water. As he heads toward the far end of the lake, he sees his family. They are fishing along the shore and don’t notice him. He swims past them quietly and purposefully, and without looking back, enters the channel that will lead him to the Manu River.

Between 1999 and 2006, my husband, Frank, and I spent many months in the lush rainforests of southeastern Peru, monitoring and helping to protect populations of the endangered and charismatic giant otter (Pteronura brasiliensis) as part of a long-term and ongoing conservation program initiated by the Frankfurt Zoological Society in 1991. This is the story of Isla and Dedo, two young otters inhabiting the jewel that is Manu National Park, whose life histories became as familiar to us as the lives of favorite characters in a television soap opera.

The 16,921 square-kilometer Manu National Park lies in Peru’s Madre de Dios region, at the foothills of the tropical Andes. Created in 1973 and celebrated for its biodiversity, the Park encompasses the entire watershed of the meandering Manu River, which experiences strong floods during the wet season from November to April. As the river current erodes its banks, some meanders are cut off to form oxbow lakes, known locally as “cochas.” These tranquil lakes are nutrient rich and are the favored habitat of giant otters. Manu harbours the single largest giant otter population in Madre de Dios, estimated at 100-130 individuals.

The giant otter, known in Peru as the lobo de rio or “river wolf,” is an apex carnivore formerly occurring throughout the lowland rainforests and wetlands of South America (with the exception of Chile). Between the 1940s and mid-1970s, the species was widely hunted for its dense, lustrous pelt; over 24,000 skins were officially exported from Peru alone. As a result, the giant otter was extirpated, or virtually so, from much of its southern and easterly range, including Uruguay, Argentina, and Paraguay. In 1975, the otters were listed as threatened under CITES, which lead to a collapse of the international market in otter pelts. However, the species currently faces new threats.

As one of the few remaining colonization frontiers of the Amazon, the Madre de Dios region has seen considerable mining, logging and agricultural expansion over the last two decades. The most important economic activity in the region by far is gold mining, more than half of it informal and illegal, and all of it destructive.

Gold occurs as dust in the sediments of rivers and their floodplains, and is extracted with the aid of mercury after tree and vegetation cover has been removed. Driven by high gold prices and new road access, the extent of gold mining in Madre de Dios increased from less than 10,000 hectares in 1999 to more than 50,000 hectares in 2012. Madre de Dios generates 70 percent of Peru’s artisanal gold production and Peru’s mercury imports have increased exponentially, resulting in the release of an estimated 30-40 tons of mercury into Madre de Dios watersheds each year. A 1997 study in Manu found that mercury levels in 68 percent of fish exceeded the tolerable level for the European otter (Lutra lutra), and a more recent study found high levels of mercury in 40 percent of Madre de Dios human residents.

As the human population of Madre de Dios increases (the average growth rate between 2002 and 2012 was 3 percent, the highest in the country), so does the potential for conflict between giant otters and people, especially fishermen. An adult giant otter consumes up to four kilograms of fish per day and some fishermen perceive there to be a high degree of competition, especially when fishing for subsistence. Whether competition is real or not remains to be seen, but people often express resentment of the otters and the situation is exacerbated by the belief that giant otters are more numerous than is actually the case.

The giant otter is aptly named. Weighing as much as 32 kilograms and measuring up to two meters in length, they can be as long as humans are tall. Each giant otter is identifiable from birth by its throat marking, as unique as your fingerprint. Isla, for example, was named for the dark island surrounded by a sea of white on her throat, while Dedo means finger in Spanish.

Photo by Frank Hajek Giant otters are identifiable from birth by their throat markings, which are as unique as human fingerprints

A typical giant otter population consists of close-knit families with exclusive territories, plus dispersing individuals that have left the groups into which they were born, normally on reaching sexual maturity. Each family of up to 16 members consists of a monogamous breeding pair and their offspring of several years. The dominant pair produces a litter once a year during the dry season and all sub-adults and adults in the group help out with feeding, teaching hunting skills, and babysitting in the den.

Isla was born in 1990 on Cocha Otorongo, one of over 30 oxbow lake ”beads” along the string that is the Manu River. Dedo was born a year later in Cocha Cashu. For the first two years, Isla and Dedo did what all young otters do: they played and squabbled with their litter mates, learned to swim, begged food from their elders with ear-piercing cries, and chased and captured their own prey. They also learned to defend themselves against predators, such as black caiman, by presenting a united front.

Every giant otter is a potential breeder. To achieve this, each must find a mate and a territory with sufficient resources. There are three routes to breeding for a young adult: leave the birth territory and form a new group elsewhere; disperse and fill a vacant breeding position in another group; or remain on the birth lake and take advantage of the accumulated knowledge of older members while awaiting an opportunity to take over the breeding position from a parent or older sibling. (Giant otters will only assume breeding positions with non-related partners.)

In Manu, we found that males invariably leave their original home ranges, either to integrate into another group if they were able to claim dominant breeding status, or to establish a new group elsewhere. One dispersing male wandered at least 196 kilometers down and back up the Manu River in just seven months. Females, on the other hand, either opt to bide their time within the natal family in hopes of inheriting the dominant position from their mother or sister, or leave to form new breeding groups elsewhere.

Returning to our young otters, when Dedo turned two, true to form, he left his family and birth lake and set off on his own. Being a disperser means having to hunt in less-than-ideal areas, since good quality habitat is likely to be occupied by resident otter groups. Driven by the urge to breed, these otters tend to spend more time on the move, stopping for only a few days, if at all, in any one location. They also depend more on rivers than lakes for hunting, which is a challenge — visibility is lower in the river, and currents make the work of hunting more energy intensive.

Dispersing from their birth territory is also dangerous. Lone otters in unfamiliar terrain are less likely to detect predators, and they must spend more time on the look-out for danger. More importantly, otters-on-the-move are more likely to come into contact with people and their activities. After leaving his birth lake, Dedo was not seen for five years and we feared he had come to an untimely end.

Meanwhile, on Cocha Otorongo, when Isla was about two years old, her parents disappeared (they probably died) and their positions were occupied by her older sister, Triangel, and an immigrant male, Nuevo. Isla was faced with a difficult choice: Should she stay within the safety of her family and the territory she knew so well and forego, for the time being, the opportunity to have cubs of her own? Or should she search for a mate and territory elsewhere? Our records show that she stayed. And struck it lucky.

Photo by Frank Hajek A giant otter family.

When Isla turned three she took over from Triangel, pairing up with Nuevo (who, you will recall, was not related to her). Triangel only bore one litter during her brief tenure. We don’t know if Triangel lost her reproductive capacity, or simply stopped having cubs after conceding her position to Isla. In any case, Isla and Triangel continued to live together in apparent harmony, with Triangel remaining a member of the group for three more years before disappearing.

Isla never left Otorongo. In 1998, a new male called Hueco, from Cocha Maisal, replaced Nuevo and became Isla’s partner; she had her last litter when she was nine years old. In 2001, Isla ceded her breeding position to her daughter Microbio, but, like Triangel before her, remained within the group as a non-reproductive member. Three years later, Microbio disappeared and her sister Batman became the breeding female in her place, with yet another new male, Diablo, from Cocha Salvador. In other words, between 1991 and 2004, the Otorongo group experienced four breeding female changes, with sisters or daughters taking over the breeding position, and three immigrant males entering the group as new partners.

It seems, then, that females run the show in giant otter society. They are the ones that hold on to the good territories, passing them down through the female line. In fact, the tendency of successive generations of female otters to inherit allowed this family to persist in Cocha Otorongo for two decades.

To our delight, in 1998 Dedo reappeared after his long absence and was photographed on Cocha Salvador, about 60 river kilometers from his birth lake; by October 1999, he had fathered four cubs there. Dedo eventually became the most successful male in Manu, producing at least 25 offspring over his 15-year lifetime.

Isla and Dedo were the lucky ones. By age of dispersal, approximately 50 percent of offspring in the Manu population have died, most likely due to natural causes. And many more are likely to die when they finally leave the nest. As can be the case with humans, being young and restless is a risky business for giant otters.

A decade has passed since I last monitored Manu’s giant otter population. I now have two children of my own and can no longer spend months in the field as I used to. However, I still receive news of Manu’s otters and last year I had the opportunity to visit Dedo’s birth lake, in my role as the Communications Coordinator for the Cocha Cashu Biological Research Station, managed by San Diego Zoo Global. I spent many happy hours observing the resident giant otter pair and their two cubs, my pleasure greatly enhanced by the knowledge that the breeding female is none other than Isla’s five-year-old granddaughter. Yesterday, I heard from my colleagues at the Station that the family has a new litter of cubs. And so the saga continues.

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