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Hand-Rearing 3 Neonatal Pacific Walrus (Obobenus Rosmarus Divergens)

By William R. Winhall

Originally published in Soundings Volume 41, Number 4 — Forth Quarter 2016

SeaWorld San Diego began documenting formulas and hand rearing guidelines for walrus calves in 1987. The walrus guidelines became a living manuscript where subtle changes in the formula and hand rearing procedures were made after many neonate trials. This paper reviews growth rates, caloric consumption, formula ingredients and diet changes, as well as basic training and enrichment of three walrus calves.

In 1982 F. H. Fay stated that the available data on wild walrus calves suggests that they are dependent on mom’s milk for at least the first 5 months after birth. Some calves may begin to eat invertebrates infrequently by the time they are 6 months old, and a few are proficient at benthic feeding by the end of their first year.

According to F. H. Fay (1982) various artificial milk formulas have been utilized with walrus in the past using fish, clams, water, heavy whipping cream and supplementary vitamins in proportions of 5:3:4, having 60% less fat, 30% more water and 100 times more carbohydrate (76% water, 12% fat, 7 % protein and 2.6% sugar) than natural walrus milk. The gross energy value per unit volume was about 50% that of natural walrus milk. The gross energy value of natural walrus milk is about 3.24 kcal/mL (M. Kleiber, 1961). Net energy available from the artificial milk diet is appreciably lower than would be available from an equivalent volume of natural walrus milk. With this in mind SeaWorld has tried to produce a more nutritionally balanced diet for appropriate neonatal growth with its walrus calf formula.

Wild Arctic Team and walrus calf

photo credit: SeaWorld Photographer

Neonate walrus in wadding pool at seal holding

photo credit: SeaWorld Photographer

Walrus calf feeding at seal holding

photo credit: William Winhall

Dana Wynn working bottle with neonate walrus

photo credit: Mitzi Synnott

3 walrus calves on exhibit

photo credit: SeaWorld Photographer

Neonate walrus with author

photo credit: Dana Wynn

Walrus calf #1 was born on May 1, 2003. This 50 kilogram female calf was hand raised by SeaWorld Animal Care staff due to maternal neglect. The second calf (walrus calf #2) in this study was a 61 kilogram female walrus rescued from Barrow Alaska on July 11, 2004. Initially this neonate was cared for by Alaska SeaLife Center, and then transferred to SeaWorld San Diego on July 17, 2004. The third calf (walrus calf #3) was a 47 kilogram female rescued from Kivalina Alaska on June 28, 2005. She was brought to Alaska SeaLife Center, and then transferred to SeaWorld San Diego on July 7, 2005.

The priority for all three neonates was to address their (dehydration) hydration status by first replacing any fluid deficit within 24 to 48 hours, while simultaneously meeting the animals’ daily fluid maintenance requirements (Odobenus rosmarus Formula 7/95). The neonates received subcutaneous fluids and/or electrolyte solution (Hydralyte) and/or dilute formula initially via stomach tube (gastric intubation), gradually increasing the volume, number of feeds and concentration of bottle fed formula over time. Weather conditions and physical activity dictated whether the neonates were kept indoors for the first few days after rescue (protecting them them from being chilled or over heated).

After the animals’ state of dehydration and daily fluid requirements were met, the focus gradually shifted to acclimation to bottle feeding and meeting the nutritional supplementation with formula while maintaining daily fluid requirements. Fish oil was incrementally added to the 100% formula to increase the caloric concentration from 1.67 kcal/mL to 1.94 kilocalories/milliliter of formula. This was accomplished by adding up to 150 mLs of oil to 2.4 L of formula. At roughly 3 months of age the formula was changed for the last time to a maintenance formula (Maintenance Formula Recipe). The calves’ weights were recorded at the same time each morning for the first month of managed care. Observations of defecation, urination, respiration, body temperature, and unusual behavior were also carefully monitored.

The SeaWorld team spent 24 hours/day with the calves upon their arrival, which not only allowed the staff more time to get the calves to accept the bottle and artificial milk formula, but also helped the neonates’ adjust to their new surrounding environment. The formula was offered in 1824 mL or 2 quart calf nursing bottle with a lamb or calf nipple, and warmed to body temperature (98 degrees Fahrenheit). Initially seven feedings were offered within a period of 12 to 16 hours each day (~ every 2.5 hours). The number of feedings decreased to 6 then 5, respectfully by the second or third month, and by six months only 4 feeds per day were needed to meet the calves’ caloric and fluid requirement (Feeding Guidelines). The volume fed was gradually increased from approximately 250 mL per feed to 1500mL per feed from 2 liters (supplemented with additional fluids) per day to 6.5 liters per day, and concurrently the formula concentration was also increased from the dilute formula at roughly 0.92 kcal/mL to the more concentrated formula at 1.94 kcal/mL during the hand rearing period.

Figure 1.  Walrus calf caloric intake during bottle feeding only (kcal/day)

A large drained holding pool (24 foot diameter by 4 feet deep) was utilized to house the rescued calves during the first 30 days of the quarantine period. Following quarantine, the calves and a shallow plastic wading pool were moved to a (seal) dry holding area. Later a back area (seal) pool and dry holding area were utilized, eventually the calves graduated to an (walrus) off-exhibit dry haul out and pool area. As the walrus calves gained strength and became better swimmers they moved back and forth from the off-exhibit area to the main walrus exhibit.

After 3 to 4 months of bottle feedings the SeaWorld team began introducing freshly thawed fish (capelin and herring) and invertebrate (squid and clam) pieces under a pile of shaved ice or into the small plastic wading pool full of seawater. Once the walrus calves began playing with and eating the fish and invertebrates, the bottle feeding volume and calories were decreased proportionately (one kilogram of the mixed food equals approximately one liter of fluid and 1200 kilocalories). The calves were allowed increasing allotments of time in larger holding and exhibit pool areas, and with other walruses. This greater amount of activity and more natural social interactions resulted in more natural walrus behavior. The walrus calves were weaned at 10, 7 and 9 months respectively.

The training program for the walrus calves began with simple management and husbandry behaviors, such as: tactile, come/follow and body exams while the neonates nursed from the bottle. The names of the walruses were used to call them over for their feeding session, and repeated as the calves were being bottle-fed. To train the calves to station, they were fed through a mesh fence while targeting on a section of the fence so another trainer could walk through the gate into the holding area. The bottle was then transferred to the other trainer as the feeding and training session continued. These steps were reversed for exiting the holding area. After repeating the stationing procedure and reiterating the word “target” during the procedure, the calf would station on the target as a team member entered and exited the area. The walruses were led and bottle fed outside their holding area on to an elevated scale, and later conditioned to feed and station in a transport unit. The calves soon became comfortable following the trainer out of their holding area and spending more time inside the transport unit. The calves could then be transported from one animal area to another. One of the walruses was so well conditioned to the transport unit that she was transported to the television studio of the Tonight Show with Jay Leno and made a successful appearance.

As the calves began eating more fish, their training program progressed and the team regularly worked tactile, hand target, lay down, name recognition, come, stay, full body exam, mouth inspection, blood sampling set up, stay, station, movement from location A to location B, into the water, enter and exit transport unit and cooperative gating.

The calves were offered enrichment soon after arriving at SeaWorld.  The approved enrichment included running freshwater, ice piles, snow, Boomer ® products and other durable plastic objects. It has been the author’s experience, as well as the Oregon Zoo’s Conservation Manager David Shepherdson’s, that offering enrichment to young animals like walruses keeps them active and interested in exploratory behaviors as they develop into adults.  Some of the older animals with little experience with enrichment seem less interested in enrichment devices and other natural activities. This is supported not only by David Shepherdson’s Ph.D. work where he states differences in environmental complexity during the development phase of an animal’s life can have far reaching physiological and behavioral effects. These include changes in behavior, physiology and brain morphology (Shepherdson, D. J. and Carlstead K. C. 1995), but other researchers as well. For example, it was noted environmental enrichment during early stages of brain development in mice enhanced brain plasticity and produced persistent changes in the brain that improve behavioral function and protect mice against the later development of stereotypic behavior (Baroncelli, L., et al. 2009, Jones, M. A., et al. 2011).

Kovacs et. al. reported walrus calf growth rate in the wild at 0.412 kg/day (Kovacs and Lavigne, 1992). According to Fay, calves in the wild gained weight during the first 4 to 9 months at rates ranging from 0.27 to 0.59 kg/day (mean of 0.42 kg/day), and their body weight doubled in approximately 6 months (Fay, 1982). Calf #1, born at and hand-reared by SeaWorld, had a growth rate of .51 kg/day. Calf #2, the rescued walrus from Barrow Alaska, had a growth rate of .53 kg/day and calf #3 rescued from Kivalina Alaska had a growth rate of .46 kg/day (Figure 2). The hand-reared calves ate fish consistently at approximately 8 months, 3 months and 4 months of age (or from rescue), respectfully. The hand-reared walruses were completely weaned off formula at approximately 10 months (309 days- calf #1), 7 months (218 days- calf #2) and 9 months (263 days- calf #3) of age or from date of rescue for latter two.

Figure 2. Walrus calf weight during hand-rearing phase (kg)

The gross energy value of walrus milk based on mean composition of samples collected by Kleiber (1961) is about 3,240kcal/L, consisting of 30.2 % fat, 7.8 % protein and trace amounts (less than .025%) of carbohydrate.  The full strength formula* initially given to the calves in this study was 1,670 kcal/L containing 14.10% fat, 7.51% protein and 2.62 % carbohydrate. With the gradual addition of fish oil, this walrus formula reached an energy value of 1,940 kcal/L containing 17.81% fat, 7.70% protein and 0.53% carbohydrate (Odobenus rosmarus Formula 7/95 & Formula Guideline).

The rescued calves gained 0.61 to 0.75 kg/day during the first 4 months and 0.46 to 0.53 kg/day at weaning (Figure 2), and initially required roughly 120 - 130 kcals/kg/day of the walrus formula.* As the calves developed and consumed less of the milk replacement product and consumed more of the natural ingredients (fish, clam and squid) in the maintenance formula**, their energy demand appeared to decrease. Collectively, the daily caloric requirements of all three calves’ decreased to approximately 90 kcals/kg/day after 4-6 months of bottle feedings due to more efficient utilization of the more natural ingredients (Figure 3 & 1).

Figure 3.  Walrus 1 weight gain (kg) and caloric intake (kcal/day)

Behavioral conditioning began immediately upon rescue. The human acclimation and hand-rearing process used trainer attention and bottle feeds as reinforcement. These positive rewards helped condition simple management and husbandry behaviors with the calves. Enrichment at an early age is key to normal calf development and prevention of undesirable habits. The walruses in the study spent more time playing and demonstrating “natural behavior” with the organic and synthetic environment enrichment objects, and spent less time involved in stereotypic behavior and other undesirable acts.

An important goal for walrus calves and their husbandry training program is to allow the calves to be young. The trainer must realize where the walrus calves are developmentally, being creative and patient, and keeping interactions with them short, fun, mutually rewarding and age appropriate.

The three neonate walruses in this study, as well as other previously hand-reared walrus calves have done well on the aforementioned formula and hand raising guidelines. The growth rates and caloric requirements were fairly consistent with all three calves in this study, growing faster (0.51 kg/day, 0.53 kg/day and 0.46 kg/day) and weaning earlier (10 months, 7 months and 9 months) than reported for walruses in the wild which averaged growth of 0.42 kg/day and weaned at ~2 years of age (Fay, F. H. 1982).

A special acknowledgement goes out to the San Diego Wild Arctic team, Mammal Department, SeaWorld’s veterinary staff, Alaska SeaLife Center and all those who contributed to the welfare of the animals at SeaWorld.


This formula contains 14.1% fat, 7.5% protein and 2.6% carbohydrate. Assuming there are 9 kcal/gm of fat, 4 kcal/gm of protein, and 4 kcal/gm of carbohydrate, the caloric density will be ?1.67 kcal/ml (formula analyzed).

The daily requirement for a walrus pup is ?120 kcal/kg body weight (This is a target number that works for calculations).

Example: A 62.5 kg (137.5lb) orphaned neonate walrus
    62.5 kg walrus x 120 kcal/kg neonate/day = 7500 kcal/day
    7500 kcal/day ÷ 1.67 kcal/ml = 4491 ml/day
    4491 ml/day ÷ 6 feedings/day = 748.5 or about 750 ml/feeding 

NOTE:  Assess the animal's hydration, as the walrus calf may need to be rehydrated. If a calf is without food for a prolonged period of time it may develop a fluid deficit due to dehydration. It is important to replace the animal's fluid deficit within 24 hours to 48 hours. The walrus calf's fluid requirements consist of the deficit plus the daily maintenance. Initially the calf is given 50% diluted walrus formula (1/2 the kcals/ml), gradually increase the concentration of the formula over a period of a few days as the neonate demonstrates the ability to digest and assimilate it. In cases of extreme dehydration, the walrus may not be able to digest or "handle" even the dilute formula. In these cases some rehydration with a balanced electrolyte solution (e.g. Revive®) and/or water may be necessary before utilizing the formula. Consult with the veterinarians. The initial volume of fluids and formula, given over a 24-hour period, will be the volume calculated above plus the amount planned to make up for dehydration loss (deficit). The total volume consumed will be reduced as the walrus becomes rehydrated. The optimal formula volume will depend upon the calf's average daily weight gain.

The veterinarians will assess the condition of the neonate and decide if the calf requires fluids and/or calories (dehydrated, depressed, lethargic), and will assist in the initial treatment (tubing, subcutaneous fluids, etc.).


The MultiMilk® formula consists of 6 parts water to 5 parts artificial milk powder by volume, and is made as follows:

  1. Blend 1800 ml (1822 gm) of water with 1500 ml (530 gm) of MultiMilk® or Zoologic Milk Matrix® 30/55 (non-packed) powder.
  2. Slowly add MultiMilk® or Zoologic Milk Matrix® 30/55 to water while blending.
  3. Grind (mortar and pestle) 1 Mazuri? #5 SeaWorld marine mammal multi-vitamin (16,000 IU vitamin A, 250 IU vitamin E, 250 mg vitamin C, 200 mg thiamin mononitrate, 15 mg riboflavin, 15 mg pyridoxine, 15 mg pantothenic acid, 500 mcg folic acid, and 250 mcg biotin), 455 mg (7 grains) dicalcium phosphate, 9 gms NaCl and 250 mg taurine, then add together with an additional 500 units of vitamin E to blended formula.
  4. Blend the mixture until it has a smooth and even consistency (makes approximately 2.4 liters).


For the first day or two, the formula is usually diluted by adding 3,000 ml of water instead of 1,800 ml. Initially the formula is diluted in an effort to help rehydrate the animal and make it easier to digest. This diluted formula has an energy content of ?0.92 kcal/ml. Warm the formula to body temperature before feeding.

Thereafter, the undiluted MultiMilk® formula is used. In some cases a more gradual change in formula concentration may be required.

At week 4 add 50 ml of fish oil- salmon and menhaden oil have both been used (*or a heavy whipping cream formula may be substituted- see next page for procedures) and blend into 2.4 liters of the formula (remember that 1 ml of fish oil equals approximately 9 kcal). This will increase the caloric content of the now 2450 ml formula to ≈1.76 kcal/ml .

At approximately week 6, increase the amount of fish oil blended into the 2.4 liters of formula from 50 ml to 100 ml.  This 2500 ml formula will have a caloric content of ≈1.85 kcal/ml (formula analyzed).

By week 8, add 150 ml of fish oil to the 2.4 liters of formula. This makes for a formula with a caloric content of ≈1.94 kcal/ml.  Later, at about 2 or 3 months, the formula is changed to the maintenance formula (see below). 

If it is decided that heavy whipping cream is to be used instead of fish oil, then at approximately week 4 add 114 ml (4 ounces) of heavy whipping cream and 14 ml (.5 ounces) of cod liver oil to 1710 ml (60 ounces) of 6:5 formula in a 64 ounce (1.8 liter) calf nursing bottle. Shake well before feeding (do not use blender to mix formula with heavy whipping cream). The resulting formula has a caloric value of ?1.56 kcal/ml.

At about week 6 add 228 ml (8 ounces) of heavy whipping cream and 14 ml (.5 ounces) of cod liver oil to 1596 ml (56 ounces) of the formula in the 64 ounce bottle. Cod liver oil is never used as a substitute for fish oil as prescribed in the previous formula. Shake well before feeding.  The resulting formula has a caloric value of ?1.70 kcal/ml.

Add 456 ml (16 ounces) of heavy whipping cream and 14 ml (.5 ounces) of cod liver oil to 1368 ml (48 ounces) of the formula in the 64-ounce bottle at approximately week 8. Shake the contents of the 64 ounce bottle well before feeding. This formula will have a caloric value of ≈1.97 kcal/ml.

By 2-3 months the neonate formula is changed one last time to a Maintenance Formula** of 568 gm (20 ounces) ground freshly defrosted herring, 114 gm (4 ounces) ground freshly defrosted clam, 1800 ml (56 ounces) water, 150 ml (5.25 ounces) fish/salmon oil, and 400 ml (14 ounces) Zoologic Milk Matrix® 30/55 (or MultiMilk®) powder [or if using the heavy whipping cream formula rather than the formula with fish oil, add 684 ml (24 ounces) water, 228 ml (8 ounces) heavy whipping cream, and 342 ml (12 ounces) MultiMilk® powder].


The Zoologic Milk Matrix® Maintenance formula** is blended as follows:

  1. Lightly blend 4 ounces (114 gm) frozen clam with 250 ml of water until smooth.
  2. Lightly blend 20 ounces (568 gm) frozen herring fillet with 550 ml water and the clam gruel until smooth.
  3. Slowly add 200 gm MultiMilk® or Zoologic Milk Matrix® 30/55 to 1000 ml water and the herring and clam gruel while blending mix.
  4. Slowly add rest (220 gm) of Zoologic Milk Matrix® 30/55 with 250 ml fish oil (Vitashine) and grounded vitamins/nutrients.
    a. grind (mortar and pestle) 455 mg (7 grains) dicalcium phosphate and 250 mg taurine, then add together with an additional 500 units of vitamin E to blended formula (makes approximately 2600 ml of Maintenance Formula with a caloric value of »1.74 kcal/ml (formula analyzed)).
  5. Add1/2 Mazuriâ #5 SeaWorld marine mammal multi-vitamin (16,000 IU vitamin A, 250 IU vitamin E, 250 mg vitamin C, 200 mg thiamin mononitrate, 15 mg riboflavin, 15 mg pyridoxine, 15 mg pantothenic acid, 500 mcg folic acid, and 250 mcg biotin) just before bottle feeding.

Blend the herring, clam, fish oil, water and MultiMilk® powder, with the ground Mazuri? SeaWorld marine mammal multi-vitamin (the Mazuri vitamin should be added right before the feeding when the formula contains fish; the thiaminase in herring can make the diet thiamine deficient if the formula is allowed to sit too long with the vitamin in it), 455 mg (7 grains) dicalcium phosphate, 250 mg taurine, and 500 units of vitamin E. Blend to even consistency. [if using the heavy whipping cream formula add the heavy whipping cream to formula last and do not mix in blender]. Warm and shake contents in the 64 ounce (1.8 liter) calf nursing bottle well before feeding. This makes ?2600 ml of formula with a caloric value of ?1.74 kcal/ml (formula analyzed).

NOTE: MultiMilk®, fish oil and heavy whipping cream may not be assimilated completely by the walrus, and therefore their digestible energy values may be diminished.


  1. Clean, disinfect, and dry equipment (nipples, bottles, and blender) immediately after use.  Wash your hands and keep the neonate's living area clean and disinfected (eg. chlorhexidine, quaternary ammonia compounds, and dilute beach). Hygiene is critical.
  2. Mix formula well before using, refrigerate and discard leftover formula after 24 hours. If adding heavy whipping cream to neonate formula, shake it, do not blend it.
  3. If emulsification of fish oil is inadequate and the oil separates from the formula, then double check fish oil volume. Separation should not be a problem with this formula (the addition of lecithin will improve emulsification).
  4. Weigh the neonate every morning before the first feed for at least the first month (zero scale). Later, a weight may only be necessary once or twice a week. The calf should gain ≈0.6 kg/day. This amount varies from animal to animal and with changes in the formula composition.
  5. Note presence of defecation (normally a pasty consistency and yellow in coloration), urination, and any unusual behavior. Contact veterinarians if the items noted are a concern.
  6. Clean and disinfect umbilicus if necessary, consult veterinarian for recommendations.  Observe and record respiratory rate, including effort, and rectal temperature in the first week to establish the individual’s range.
  7. The neonate may need to be kept indoors for at least the first few days. Weather conditions and the calf’s physical fitness will dictate the best approach. Caretakers should be alert to signs of the neonate being chilled (cold flippers, shivering, uncomfortable behavior) or showing evidence of heat stress (rapid breathing, warm/hot flippers, lethargic behavior).
  8. Warm the formula in 64 oz (1824 ml) calf nursing bottles to approximately body temperature. This seems to make the formula more palatable. Initially the calf may more readily nurse from a lamb nipple. If this is the case, you may switch to a calf nipple (most walrus readily take to bottle feedings). If the walrus is unwilling to accept the formula from the bottle, they will often welcome water from the bottle. Spend time with the calf to help meet its social/physical needs.
  9. Initially administer 6 or 7 feeds within roughly 12 to 16 hours of each day, then the number of feedings may be reduced to 5 or 6/day starting the second or third month (maintaining or increasing the volume per feed), and finally the number of feedings may be decreased to 4 per day by about the sixth month.
  10. If bottle feedings are not adequately meeting the neonates requirements (a weak and dehydrated calf), the veterinarians will decide if the calf requires fluids and/or calories and assist in at least the initial tube feedings. Any difficulties with tubings (excessive neonate struggling, severe blockage of the tube, laborious breathing through mouth, increase in respirations , contact veterinarians immediately.
  11. Actual volume of formula or calories consumed by any specific walrus may vary. Use good judgement (animal's growth rate, age, behavior, appetite, appearance) and consult with veterinarians.
  12. At about 4 months of age, begin introducing freshly defrosted fish and invertebrate feedings (pieces and whole organisms, in the pool or in ice pile). By gradually increasing the amount of fish and invertebrate, this additional consumption of capelin, herring, clams, and squid by the pup will increases the total caloric intake for the walrus by roughly 1500 kcal/kg, 1750 kcal/kg, 800 kcal/kg, and 700 kcal/kg of fish respectively, therefore decreasing the number of necessary bottle feedings/amount of formula (when increasing fish/solid food intake, be sensitive to the fact that the animal’s fluid requirements must still be met).
  13. Introduce and then gradually increase the time the calf spends with other marine mammals (seals, small walruses, etc.) in a larger pool and deck area. This is often associated with an increase in exercise and more natural activity.
  14. The walrus can be weaned by approximately 8 to 12 months.
  15. Any questions, concerns or worries, please consult with the veterinarians.


To avoid any difficulties with the administering oral fluids, the stomach tube should be lubricated and gently pushed down the animal's throat. The length of the tube should be measured and marked (the distance from the animal's nose to their lower abdomen) to help make certain that the tube will be long enough to reach the stomach and not accidentally end up in the lungs (shorter distance). The gastric tube should be of the appropriate width for the mammal (wide enough to ensure that the tube will not enter the lungs). After the tube has been inserted into the mammal for the measured distance, the keeper should suck on the end of the tube to establish if the tube is truly in the stomach. If there is resistance and suction is created then it can reasonably be assured that the tube is in the stomach. If there is no resistance and air can be taken in freely, then the tube may be in the lungs. It is also advisable to wait for the animal to breath through its nose before pouring the fluids down the stomach tube. The second person doing the feed will double check that the tube is in the stomach.


We first warm the formula, then using a variety of nipples of different sizes, shapes or styles (human, lamb, calf nipple), squirt small amounts into the neonates mouth. To help initiate sucking put some formula on one's arm or hand and/or corner of a towel and offer this to the animal. Try to feed the walrus calf from the bottle before and after each tubing. Often the act of tube feeding can stimulate a sucking response by the calf. Mix formula well before using, and discard leftover formula after 24hr.

Most walrus calves in good condition accept the bottle in 24 to 96 hours of beginning the conditioning process.


  1. There are approximately 30 ml or 30 gm in one ounce.
  2. There are about 65 mg in a grain.
  3. Approximately 2.2 pounds are in one kg.


Baroncelli, L., Braschi, C., Spolidoro, M., Begenisic, T., Sale, A., and Maffei, L. 2009 Nurturing brain plasticity: impact of environmental enrichment.  Cell Death and Diffenentiation 17, 1092 – 1103.

Fay, F. H. 1982. Ecology and Biology of the Pacific Walrus, Odobenus rosmarus divergens Illiger. U. S. Fish and Wildlife Service, North American Fauna. No. 74 pp 138 - 145

Kleiber, M. 1961 The Fire of Life: an Introduction to Animal Energetics. John Wiley & Sons, New York. 454 pp

Kovacs, K. M., and Lavigne, D. M. 1992. Maternal Investment in Otariid Seals and Walruses. Can. J. Zool. 70:1953-1964

Jones, M. A., Mason, G and Pillay, N. 2011. Early environmental enrichment protects captive-born striped mice against the later development of stereotypic behavior. Applied Animal Behaviour Science V 135 pp. 138-145

Shepherdson, D. J. and Carlstead K. C. 1995. Understanding the relationship between environment and reproduction in captive animals: The role of environmental enrichment. The 2nd International Conference on Environmental Enrichment, Copenhagen.