Chapter 6: The Surrogate

An adult sea otter named Toola was already in the tank. Toola was a very special sea otter. She was one of the Aquarium’s surrogate moms and would ultimately raise thirteen abandoned pups over her lifetime.”

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By the 1st of August, at nearly eight weeks old, Otter 501 was 6.14 kilograms (13.5 pounds). She was now more than three times the weight of when she was first found. Caretakers would soon stop bottle–feeding. And, her pup coat was almost gone, except for a few straggling, wispy hairs. 

Not all food benefits sea otters the same

Her new coat was called juvenile velvet, which was as soft as it sounds. In fact, it was just very short adult fur that resembled the close-cropped grass on a putting green. She was also getting much better at grooming her own fur.

Three days later, 501 was put into one of the big rooftop, outdoor tanks again. She hadn’t been back since her taste of sunshine two weeks ago. But this time she was not alone.

An adult sea otter named Toola was already in the tank. Toola was a very special sea otter. She was one of the Aquarium’s surrogate and would ultimately raise thirteen abandoned pups over her lifetime.

The early SORAC rehabilitation program was vastly different from today. It started with one stranded pup and caretakers who had never raised an orphaned otter before. Where 501 had a specially designed ICU facility to receive her, this pup had a bathtub. Over time, as the Aquarium’s marine biologists gained more experience with abandoned sea otters, they gradually learned ways to improve sea otters’ chances of survival once released back into the wild.

In the early days, there was a great deal of direct human contact with the otters, much friendlier than the Darth Vader suits Otter 501 came to know. Little attempt was made to prevent association and bonding between caregivers and their fuzzy charges.

When it came time for pups to learn how to find food for themselves, human snorkelers took the pups for swims in Monterey Bay. So naturally, sea otters raised this way were not afraid of people. Unfortunately, back in the wild these friendly tendencies became a liability to both the otters and people. Hungry wild animals and boaters don’t mix.

How pups were reared before surrogates

SORAC began to understand that there was no substitute for the real thing. But they were dealing with abandoned pups. Their mothers weren’t around. So what could they do?

Toola arrived in 2001, seventeen years after the first otter came through the Aquarium’s doors. She had stranded because she was sick, and after her rescue, gave birth to a stillborn pup. Two days later, an abandoned male pup arrived at the Aquarium.

The SORAC staff decided to test an idea. Since Toola lost her pup and the male pup lost his mom, then why not put them together and see if they would accept each other. And it worked, perhaps better than anyone had imagined.

Ever since then, surrogate sea otter moms have been used to raise abandoned pups. This change doubled the survival rate of these pups back in the wild, compared with old methods of rearing.

See Toola with her first pup

As successful as the surrogate program has been, the introduction of Toola and 501 didn’t go so well. 501 spent the entire time grooming herself or carefully hiding in the fake giant kelp caretakers added to the tank. 501 didn’t eat at all. Neither sea otter attempted to interact with the other. This was a more important observation than it might sound.

Watch Otter 501 and Toola's first introduction

“I’d say most of the time the pups will initiate interactions,” Karl Mayer said, as he explained what made this encounter different than most he has seen. “The females tend to need a little bit of time to warm up, but the pups will tend to go over immediately and check out the female.”

Caretakers pulled 501 out of the tank after only an hour with Toola. They couldn’t risk stressing the two any more or, even worse, allowing time for something negative, such as aggressive behavior, to occur.

It had been fifty-five days since 501’s rescue when she was only three days old. For her, this was a long time ago.

She was only with her mother for a matter of hours, perhaps never seeing another sea otter besides her mom during that period. 501 may not have remembered what another sea otter looked like.

According to Karl, in an ideal situation, Toola would have grabbed 501, sniffed her, and immediately treated 501 like her own pup. Toola would then have carried 501 on her chest, shared food, and groomed her.

The maternal instinct may kick in right away but sometimes it may take as many as six or seven daily introductions. There are also times when it never happens.

It was important to monitor the otters closely during each introduction. “The most important factor we have to consider is whether the pup is actually adequately feeding itself,” Mayer said. “One of the maternal behaviors that we oftentimes see relatively early is food-sharing. When the female’s foraging, if the pup’s following the female and is really focused on her, it is less inclined to be focused on feeding itself.”

Learn a lot of different sea otter behaviors

On the following day, the second introduction went about the same as the first. An hour passed without a positive interaction and 501 was removed.

The next day’s introduction offered the first glimmer of hope. 501 made an attempt to interact with Toola,

However, Toola, who had grown accustomed to sharing the tank with 501, was not pleased. This was too close. Toola spooked and unceremoniously pushed 501 away with her forepaws.

Luckily, 501 still seemed interested and, after the interaction, Toola appeared more curious. The caretakers, following the otters’ interactions closely, allowed this introduction to last a bit longer, hoping the ice might finally have been broken.

501 and Toola take a couple of goes before bonding

The Aquarium staff waited two days before the next introduction, allowing both animals ample time to resume their normal behaviors and stress levels. On the 8th of August, they put 501 in with Toola a fourth time.

This time, 501 solicited food, and to everyone’s pleasant surprise, Toola handed it right over. The two interacted with increasing familiarity and the signs of a real mother and pup bond started to become apparent. With such success, the caretakers decided that 501 was ready for her first overnight with another otter. She and Toola were left together as the sun set and the staff went home. 

About 24 hours after this critical introduction, 501 was taken from Toola to be weighed. Her weight had dropped 6.5% since the beginning of the introduction, indicating that 501 was still not eating enough.

Hear more from a scientist about 501 and Toola's introduction

A drop like this was expected as a result of 501’s incredible new experience with Toola.  However, these weigh-ins were important to help handlers measure her growth over the long term and provide an opportunity to see close-up if there were obvious signs of problems.

“After we pulled the pup, weighed her, and put her back in, Toola grabbed the pup and was swimming around with 501 on her chest,” recounted Mayer. “And so we’re seeing those early maternal behaviors—the food sharing by Toola, the towing around on the chest, and being a little more protective of the pup.” It looked as though 501 and Toola had forged a bond.

Toola tows 501 around

On August 12th, 501 weighed 6.8 kilograms (15 pounds). She was now above her pre-introduction weight.

501 was in the middle of her maximum growth phase, which begins to slow down around six months of age. During this phase, she was packing on an average of 71 grams (2.5 ounces) per day.

As time passed, 501 was weighed less frequently. The tanks still had to be cleaned every three or four days. So both sea otters had to be pulled from the tank.

It takes a lot of work to care for sea otters

“Typically, Toola’s response typically to being netted was that she would generally ditch the pup and try to avoid being captured herself,” Mayer said. “And that’s pretty much what she did with 501. Toola was by far the most adept of any of the otters that we’ve worked with at avoiding being captured by a net. And I think she sort of perceived it as a game. You know, she relished it. She’d avoid capture, avoid capture, avoid capture. And at some point she’d give up.” 

Mayer was oftentimes the only one who could net Toola.

Within a few days, 501 had also become a master of deception. During periodic tank cleanings when the handlers tried to net 501, she dove with Toola to escape and even hid under the haul-out. 501, as she began taking cues from Toola, was developing wild ways.

See 501's weight come back up

Though the pair didn’t always do things together in the tank, they did seem to have a keen awareness of one another. No matter what either otter was doing, when handlers came by, Toola would dive and 501 would follow.

See more of 501's development
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Special Features

This Page All
  • The Old Way

    Video Clip: The Old Way of RearingHow pups were reared before surrogates

  • Toola's First Pup

    Video Clip: Toola’s First PupSee Toola with her first pup

  • 1st Introduction

    Video Clip: 1st IntroductionWatch Otter 501 and Toola's first introduction

  • Otter Behavior

    Descriptions of Otter BehaviorsLearn a lot of different sea otter behaviors

  • ITF: Introduction

    In the Field: Introductions501 and Toola take a couple of goes before bonding

  • AAS: KM Intros

    Ask a Scientist: Karl Mayer – Introducing 501 to ToolaHear more from a scientist about 501 and Toola's introduction

  • 4th Time

    Video Clip: 4th IntroductionToola tows 501 around

  • Tank Cleaning

    Video Clip: Tank CleaningIt takes a lot of work to care for sea otters

  • Growth Chart 3

    Growth of Otter 501 to 66-days OldSee 501's weight come back up

Chapter 1: The Ocean is Cold but Mom is Warm

Chapter 2: Less than Ideal Rescue

Chapter 3: Emergency Treatment

Chapter 4: Species in Danger

Chapter 5: No Substitute for Mom’s Cooking

Chapter 6: The Surrogate

Chapter 7: Furry New Friends

Chapter 8: Release

Chapter 9: A Wild Sea Otter

Science Illustrators Present: Behavior and Prey

Video Clip: The Old Way of Rearing

The process for raising abandoned sea otter pups has come a long way. In the old days, humans were the fill-ins for the missing sea otter moms—even when it came to showing pups how to find food in Monterey Bay.

Video Clip: Toola’s First Pup

Toola is the mother of the entire surrogate program. This video from 2001, shows the very first pup Toola raised in captivity.

Video Clip: 1st Introduction

After Toola was fed, the next thing that dropped into her rooftop tank was 501. There was reluctance to leave the pet carrier, but a few degrees more tilt plopped her in. 501 then made a beeline to hide under the fake giant kelp.

Descriptions of Otter Behaviors

If you watch a sea otter long enough, you might find it hard to keep track of the many different types of behavior. Here’s a handy, but long, list that was first published by J. M. Packard and C. A. Ribic in 1982. The list describes behaviors broken down into four categories: locomotion; feeding; grooming and resting; and interaction.


Diving From a belly-down position, the otter submerges head then feet (this dive usually used while feeding). Low intensity: arching of the back is minimal. High intensity: otter leaps out of water with arched torso clearly visible.
Folding dive From a belly-up position, the rear feet and shoulders move toward the center of the body and the otter sinks backward into the water.
Porpoising As the otter swims just below the surface, the arched back repeatedly appears on the surface; general movement is in the forward direction (contrasted with a feeding dive). Low intensity: back just breaks the water surface. High intensity: the otter repeatedly leaps out of the water with back arched in an inverted U.
Rowing Floating belly up, otter folds ventrally in a V shape then straightens: may be repeated; otter does not submerge.
Sculling Belly up, the otter moves along the surface propelled by movement of the tail and (or) feet.
Sidestroking The otter moves along the surface on its side; one foot may be waved above surface and head may be oriented toward an object.
Swimming Belly down; the head and back are visible moving along the surface.
Underwater swimming Body is totally submerged; the otter reappears at a distance at a location in line with previous direction of movement.


Eating While floating on the back, the forepaws are brought repeatedly to the mouth; object may be shoved into the mouth or pieces bitten off.
Periscoping Only the shoulders and head are visible above the water as the otter takes a few seconds to look around: usually precedes a high intensity dive.
Pounding Rapid pounding movements are made onto the chest with or without an object held between the forepaws: a hard object may be balanced on the chest as the otter floats on its back; observer can often hear pounding.
Submerging Body is totally submerged; the otter reappears at a short distance not in line with the previous direction of movement.

Grooming and Resting

Dunking While floating on the back, the otter briefly dips the head in and out of the water; the chin is pushed forward, and the back of the head moves dorsally.
Floating Otter floats on the surface, belly up, rear feet up, no sculling, feeding, or grooming movements. Low intensity: body motionless. High intensity: slight movement of paws, head, or feet.
Hanging Belly down with both rear and head submerged; the arched back remains visible at the surface but motionless for a few seconds as the otter apparently grooms its belly.
Logrolling From a belly-up position, the otter rotates to the side like a rolling log: differs from rocking in that feet and paws are submerged.
Looking Belly up or on its side, the otter turns its head in various directions. Low intensity: slow, occasional head turns. High intensity: rapid agitated movements of head from side to side.
Nibbling or licking Mouth contact is made with some part of the otter’s own body, in a nibbling or licking movement; commonly directed toward paws, belly, feet, tail.
Rocking From a belly-up position, the otter does a side roll with torso arched such that the feet and paws remain out of the water. Low intensity: otter rocks 180° from side to side. High intensity: otter rolls 360°.
Rubbing Rear feet rub some area of otter’s own body. Low intensity: both rear feet are rubbed slowly against each other in a “hand-washing” movement. High intensity: rapid scratching movement of one foot directed toward back, neck, or side of body.
Shaking The head is rotated rapidly from side to side in a typical shaking movement; water flies; the muzzle may be outstretched.
Somersaulting Full 360° forward roll with the head tucked close to the belly; often only the curved back is visible until the head reappers at the end of the roll.
Stroking Front paws repeatedly stroke some area of the otter’s own body; may vary in intensity (rapidity of strokes): commonly directed toward chest, head, rear feet, belly, tail, flank, back.
Tuckrolling Head is brought toward chest but bent over to side while otter does a 360° tide roll: intermediate between a somersault and a logroll.


Begging Otter remains near the side or head of a feeding otter; head is oriented toward the feeding otter: paws may or may not make contact.
Biting Otter closes jaw briefly on body of opponent: more intense and not as prolonged as grasping; may be repeated.
Chasing Rapid swimming with one otter behind another.
Chinning The otter swims slowly toward another and places chin on chest, belly, or near the head of another otter.
Clasping Female uses front arms to hold pup to her chest: the pup is usually clasped around the chest, neck, or head, and becomes limp.
Gaping Otter holds mouth open, usually oriented toward partner’s head. Low intensity: brief duration, quality of a threat; may lunge without making contact. High intensity: prolonged interaction. each otter parrying lunges made by the other.
Giving Holding food or another object, otter moves paws toward another otter; the object is relinquished when the other otter takes it.
Grabbing Jaws are closed on the body of another otter, maintaining prolonged contact. Female grabs pup by neck. Male grabs female by nose.
Interfering Otter attempts to move body between two other interacting otters.
Lunging Sudden forward body movement toward another otter. Low intensity: no contact. High intensity: lunge and nip, mouth contact.
Mutual porpoising Porpoising as described under Locomotion, synchronously or in close sequence with a partner moving in same direction.
Nosing Muzzle contact made with another otter; difficult to distinguish between nibbling and sniffing as the two movements are often intermixed.
Pawing With one forepaw, the otter reaches out to contact its partner: may be shoving or patting movement.
Riding The otter places its body on the belly of another otter by swimming up slowly or by rolling sideways onto its partner: the other otter may move away or remain stationary. Low intensity: front half of body covers head and front half of partner’s body. High intensity: full body contact.
Shoving Otter forcefully pushes another otter away with forepaws.
Splashing Belly up, otter moves partly submerged front paws away from body towards another animal, making water splash.
Suckling Pup has mouth in area of female’s nipples. Low intensity: suckling interrupted. High intensity: continuous contact with nipples.
Tumbling Two otters toll over and over each other; the arched backs are usually visible, with an occasional glimpse of feet, tails, or heads. The body contact is highly variable: sometimes it appears they are wrestling, at other times making jaw contact.
Wrestling In a vertical position, two otters actively grasp each other with forearms around the head and shoulders, then twist to break the hold.

In the Field: Introductions

Sea otter biologist Karl Mayer, gives a play-by-play description of 501 and Toola’s first introduction. This behavior was similar to the second and third introductions.

Ask a Scientist: Karl Mayer – Introducing 501 to Toola

Before 501 was introduced to Toola, she was only with humans. Does this time away from another sea otter cause problems when the pup and surrogate are eventually introduced?

Karl Mayer: The short answer is no: any potential negatives that may occur during the relatively short stabilization period (between stranding and introduction to the surrogate mother) seem to be superseded by the bond that forms between pup and surrogate mother. Certain critical developmental milestones need to occur before pups can be introduced to the surrogate: their pup coats need to be mostly shed and fur in good condition; they need to be able to dive, retrieve and consume solid food on their own; and they need to be relatively self-sufficient at grooming. We try to accelerate development of these critical behaviors during the stabilization period in order to introduce pups to a surrogate mother as soon as possible. But really, 8 weeks of age seems to consistently be the earliest that pups are able to acquire prerequisite skills.

Once pups are in with a surrogate mother, they seem to revert to being wild pretty quickly. The pups are cued in to the females for stimulation and to trigger all their reactions. We might have been bottle-feeding an animal for the previous 6 weeks, but within 3 days of going in with the surrogate, the pup is diving with the female to avoid being netted, hiding under the haul-out, and otherwise exhibiting appropriate flight responses in the presence of people. Simultaneously, changing both the social context (a female sea otter companion as opposed to human caregivers), as well as the environmental context (larger outdoor tank vs. ICU) seems to override any habituation to people that may have occurred during the first several weeks.

How long does it take for the pair to bond?

KM: In some cases, a female may accept a pup immediately and begin to display maternal behavior, and in others, it may take six or seven daily introductions before we start to see any progress. In about 10% of cases, the female never accepted a given pup, and we had to try with a different surrogate mother. The relative strength of the mother—pup bond also seems to vary: the bond between an individual mother and pup may get stronger or weaker overtime; and females also seem to have stronger bonds with some pups than with others.

As an example, one of the pups we’re working with right now, female 595, is in with a surrogate mother (Mae) that is not behaving maternally. The two are pretty compatible in that we are not seeing stress on the part of either otter, but because Mae is not sharing food with the pup, we need to be completely confident that 595 is foraging adequately. We make this determination by pulling the pup out of the tank and weighing it every 24 hours. 595’s weight dropped about 10% in the first 24 hours, but subsequently climbed back up, so we’re going to leave the two together. The advantage of doing so is that we might see a maternal bond develop over time. In Mae’s case, we know from experience that she might not behave maternally at first, but may do so subsequently.

How about with 501 and Toola?

KM: In Toola’s case, though, we really wanted to see some of those maternal behaviors before we were going to leave 501 in with Toola. Toola tends to take a little longer to accept a pup, but once she does, she is pretty consistently maternal. During the first introduction, there were no interactions between the two. They were doing their own thing, so we pulled the pup pretty early because the one thing we don’t want to do is wait until something negative starts happening, or one of the animals gets stressed. If Toola began pacing in the presence of the pup, for example, removing the pup would alleviate the stress. But the problem is, the next time the pup goes into the tank, Toola’s stress reaction might be triggered more rapidly.

The second introduction, also an hour long, was similar to the first. During the third introduction, 501 was more active and curious. She moved back and forth between the haul-out, the kelp and the sand pool that we had in there. She did make one attempt to interact with Toola, but Toola got spooked and immediately pushed 501 away. There were a few more instances where 501 looked as if she wanted to interact, but turned away before she actually got over to Toola. Toola also seemed to be a little bit more curious about 501 in the tank, so those were possibly tentative signs of progress.

Were these all an hour long?

KM: The first three introductions took place on consecutive days, and lasted an hour each time. The fourth introduction took place on August 8, at which point, we were confident enough in the behaviors we were seeing—soliciting of food by 501, and food sharing by Toola—to leave the two together overnight. We pulled 501 the next morning at 9 am, basically 24 hours after the introduction. Her weight had dropped 6.5% from the day before; however, initial weight loss is to be expected during introductions, as pups tend to increase their activity level in the presence of the female. After weighing 501 and returning her to the tank, Toola grabbed the pup and swam with 501 on her chest. Towing the pup on the chest, being a little more protective, and food sharing, are all early maternal behaviors, and positive indicators that a bond was forming.

As the day progressed, we began to see more instances in which 501 dove and retrieved food on her own. In the afternoon, 501 rested on Toola’s chest, and Toola was observed grooming the pup. At one point, while being groomed by Toola, 501 was oriented in the nursing position; however, we couldn’t tell from the remote camera whether she was attempting to suckle or not. At that point, we were confident that a bond had formed between the two, and that they could be left together through 501’s dependency. What remained was to continue to monitor 501’s weight progress and behavioral development, and see how the dependency played out.


Video Clip: 4th Introduction

After three unsuccessful introductions, 501 and Toola bond during the fourth try.

Video Clip: Tank Cleaning

Every few days, 501 and Toola’s rooftop tank had to be cleaned. During this time, 501 got weighed and had a quick checkup. This time-lapse video gives a good picture of the size of 501’s temporary home and all the things inside.

Growth of Otter 501 to 66-days Old

Otter 501’s growth to 66-days old is shown here. The fluctuations at the end of the graph occur around the time of 501 and Toola’s introductions.


Otter 501′s Development to 68-days Old