Ontogeny of behavior and self sufficiency in free-ranging otters

J. SCOTT SHANNON

ABSTRACT

I chronicled the behavioral development of free-ranging otters (Lontra canadensis). Young otters attained proficiency in aquatic locomotion within 20 weeks of birth, but proficiency in the techniques of aquatic hunting required >20 additional weeks of learning. Otters became self sufficient at 9.5 months and fully independent of maternal care at 11 months. Males continued to live at their natal area long into adulthood; females did not disperse from their natal area unless expelled forcibly by an elder resident female. Juvenile mortality rate was 2/3. To optimize pup survivorship, no mother otter should be removed from her dependent young prior to the pups' ninth month of life.

INTRODUCTION

The behavioral development of wild lutrines has thus far only been documented for sea otters (Enhydra lutris) (Payne and Jameson 1984). Previously, I described the behavioral development of North American otters (Lontra canadensis) (Shannon 1989, 1991a, 1991b, 1992, 1994), but these data were preliminary. Here, I elaborate upon past findings and incorporate new observations that have brought my long-term study of the behavioral development of otters to comprehensive understanding.

METHODS

My study area was a small (0.7 km^2) shallow ocean roadstead on the far-northern Pacific seacoast of California. The harbor opened to the south and was bounded on the west and north by steep rocky shore cliffs. The otters' dens were located in the dense scrubby vegetation atop the shore cliffs. The otters were exclusively marine in their habits, and satisfied their requirement for fresh water by drinking from springs which flowed down the headland at the west of the harbor.

I studied the behavioral development of otter pups by direct naturalistic observation. Otters at my study site were habituated to human activity, so I could observe them during daytime hours at very close distances (1-100 m.). Because of the otters' diurnal habits, proximity, site affinity and ease of observation, I learned to recognize individual otters by sight. Each otter possessed unique facial, physical and behavioral characteristics which marked it as an individual, making reliable identification possible.

After 3 summers of preliminary observations, my study began formally in May 1986. I conducted my sessions during the 2-4 hours preceding sunset; daily whenever possible. To determine the period of ontogeny of a particular behavior, I noted the first occasion 1 pup was seen to display that behavior, and subsequently when all pups displayed it.

I could not observe actual parturition, so I had to estimate the pups' date of birth. Females of L. canadensis exhibited a post-partum estrus (Hamilton and Eadie 1964) which began approximately 10 days after parturition (Lee Roy Sevin, pers. comm). Because most estrous activity in my study population peaked during the first and second weeks of April, the pups were probably born around April 1, 1 week.

I determined the onset of self-sufficiency by observing the behavior of the mother, noting when she stopped providing food specifically for her young, and might even punish pups for taking food from her. I declared a young otter to be independent of maternal care when it was not seen in the company of its mother for 1 week.

RESULTS

From May 1986-August 1998, I conducted 3,319 sessions, and saw one or more otters at my study site during 2,869 (86%) of those sessions. During this period, 61 otters of known identity or birth lived here. The population ranged from a minimum of 6 individuals to a maximum of 18: 3-11 adults (1-3 females, 2-8 males), and 0-8 pups. The otters' home range comprised approximately 4 linear km. of marine coastline.

From 1986-1998, I chronicled the lives of 38 pups born in 15 litters out of 4 mothers: 6 litters (22 pups) from "Old Mama"=[F'2], 2 litters (5 pups) from F86A[F'2]="Mama Junior"=[F'3], 2 litters (4 pups) from F86B[F'2]="Scarnose"=[F'4], and 5 litters (7 pups) from F91A[F'3]="Little Mama"=[F'5]. Mean litter size was 38/15=2.5 pups. Of the 38 pups born here since 1986, 17 were females, 14 were males, and 7 died before I could determine their gender. The mortality rate for juveniles (age <22 months) was 26/38 (approx. 2/3). Nineteen otters died as dependent pups; 19 achieved independence, but 7 of these died as yearlings (age 13-24 months). Only 11/38 (approx. 3/10) otters survived to adulthood, and only 3 females lived on to become mothers themselves.

Adult otters here had a social organization characterized by sexual segregation. All adult females lived together in the maternal "Family" group, and all adult males comprised a seasonally-stable, cohabitating "Clan." No adult male has lived with the Family in a paternal role during the period of my study.

The pups were born in a natal den that was located away from the mother's usual place of residence. A mother made her pups become aquatic by dragging them into the water by the neck and forcing them to swim, soon before she brought them from the natal den to the main dens in the harbor at 9-10 weeks of age (first 2 weeks of June). The mother began providing small fishes to pups during week 11, and pups exhibited active fish-chasing behaviors from week 12 on. Pups first learned the skills of seizing and manipulating prey when their mother presented them with live fishes on land.

Pups attained efficient coordination of all basic swimming movements by the end of week 14, and might be taken on their first fishing excursions out of the nursery area as early as week 15. Pups possessed the complete adult repertoire of grooming and swimming behaviors by the age of 16 weeks, and could swim as proficiently as adults by the end of week 19.

The mother was the pups' sole care-provider and their primary agent of socialization. However, multiparous mothers allowed their elder daughter(s) to return to live as companions with her and her new pups after weaning began in week 14. After being accepted back by her mother, an elder daughter became a full-time, cohabitating member of the Family. Encounters with adult males first took place when the pups were 16-18 weeks old. The males almost always behaved amicably toward pups that solicited social interaction. However, pup-male interactions typically remained infrequent until after the young became independent.

Although the mother appeared to cease lactation around week 18, and her mammae were flat by week 23-24, suckling for comfort might continue past the pups' sixth month. First fishes (sanddabs) were captured early during week 17. The first small free-swimming fishes were caught during week 19. Despite these early fish captures, however, the pups' overall foraging effectiveness remained functionally nil for 3 more months. At 6 months (26 weeks), pups had become very active, agile fish chasers, but they remained observably ineffective at actually capturing fishes, and still depended entirely on their mother for food. During the pups' seventh month, however, multiple solo prey captures became daily occurrences. By the end of week 36, pups became observably effective at capturing small fishes (<25 cm.), but they continued to depend on their mother to provide the balance of their food.

Pups attained basic self-sufficiency during weeks 38-42 (circa 9.5 months). The mother usually stopped providing food for the exclusive use of pups around week 38. After week 42, a mother might bite a pup that took food from her, but she would continue to share her food with her offspring until they became independent. After they became self-sufficient, pups foraged frequently on their own. Although the young could satisfy their immediate life requirements adequately by this time, they did not become observably effective in procuring food or utilizing their habitat until after they were abandoned by their mother at 48 weeks (11 months; end of February).

No mother was ever seen to force her own independent young to disperse, and no yearling of either sex was known to have dispersed from its home area voluntarily. After independence, most yearlings spent the following 2-3 months living solitarily. By the end of May (month 14), yearlings of both sexes usually joined the male Clan. Most yearling females were members of the male Clan from May-July of their second year. In her 16th month (July), a yearling female either returned to live with her newly-maternal mother, remained with the Clan, or was expelled aggressively (and permanently) from her home area by a territorial elder sister. One yearling female (f91B[F'2]) was killed by her elder sister (F86B[F'2]) during a territorial expulsion attack.

Males continued to base their activities at their home area after attaining adulthood, and most remained residents at their birthplace for their entire lives. No male had ever remained a member of the Family group during his second year of life until 1997-1998, when yearling male M96[F'5] returned to live with his mother and elder sisters, rather than joining the society of his elder uncles. M96[F'5] attained adulthood in February 1998 at the age of 22 months. This was the first time I observed an adult son living with his mother and elder sisters. After spring 1998, however, M96[F'5] was expelled from the Family, and he permanently joined the other males in the resident Clan.

DISCUSSION

My study site was a rich habitat where food sources were highly predictable. I believed it plausible that the onset of self-sufficiency could take place later in habitats where the location and availability of food sources were more variable.

The observation that pups were born away from the mother's usual place of residence replicated an original finding by Woolington (1984).

Otter pups were taught to swim by forced introduction, confirming Liers (1951).

Lariviere and Walton (1998), citing preliminary results of Shannon (1991a), stated that otter pups were weaned at 12 weeks, and that mothers provided food to pups until weeks 37-38. In actuality, Shannon (1991a) reported that suckling was last observed during week 14, and in Shannon (1989), it was noted that mothers still had distended mammae in week 18, suggesting that lactation and suckling might still be taking place. Additionally, although the mother stopped providing food for the pups' exclusive use during weeks 37-38, the mother would continue to provide shared food to her pups until the time she abandoned them at 48 weeks.

Elder daughters that lived with their mother and her young performed socialization and supervisory roles, but they were not true alloparents, because they did not directly provision their mother's pups with food, nor would they assume maternal care in the mother's absence.

Otter pups became proficient swimmers only 20 weeks after birth, but they spent most of their first year of life learning to become proficient hunters by a slow process of individual trial and error. Although the young essentially taught themselves the techniques of capturing aquatic prey, the experience and example of the mother was crucial to their learning the logistics of optimal foraging and habitat utilization. A mother's knowledge of local sources of food and shelter should be especially needed in habitats where such resources were widely dispersed or limited in availability.

MANAGEMENT IMPLICATIONS

To remove a mother otter before her young attained self-sufficiency could only have a negative effect on pup survivorship and population recruitment. Therefore, female otters should not be killed or translocated until after their pups are 9.5 months old (mid-January). Additionally, because adult females usually give birth around the beginning of April, removal should not take place after this time.

ACKNOWLEDGEMENTS

Grateful thanks to my assistant, Sara Moore, for her many years of dedicated observations and support. Thanks also to A. S. Mossman and J. F. Waters for their helpful comments on this manuscript.

LITERATURE CITED

Hamilton, W. J. and W. R. Eadie. 1964. Reproduction in the otter, Lutra canadensis. Journal of Mammalogy, 45: 242-252.

Lariviere, S. and L. R. Walton. 1998. Lontra canadensis. Mammalian Species, No. 587, American Society of Mammalogists, 8pp.

Liers, E. E. 1951. Notes on the river otter (Lutra canadensis). Journal of Mammalogy, 32: 1-9.

Payne, S. F. and R. J. Jameson. 1984. Early behavioral development of the sea otter, Enhydra lutris. Journal of Mammalogy, 65: 527-531.

Shannon, J. S. 1989. Social organization and behavioral ontogeny of otters (Lutra canadensis) in a coastal habitat in northern California. I.U.C.N. Otter Specialist Group Bulletin 4: 8-13.

_____. 1991a. Progress on Californian otter research. I.U.C.N. Otter Specialist Group Bulletin 6: 24-31.

_____. 1991b. Social organization, reproductive behavior, and pup development in a coastal population of otters [Lontra (=Lutra) canadensis]. Pp. 151-152 in: Reuther, C. and Roechert, R. (eds): Proceedings V. International Otter Colloquium. - Habitat 6, Hankensbuettel, 344pp.

_____. 1992. Progress on Californian otter research: 1991. I.U.C.N. Otter Specialist Group Bulletin 7: 29-32.

_____. 1994. Onset of self sufficiency in free-ranging otters. Abstract: First Annual Conference, The Wildlife Society, p.85.

Woolington, J. D. 1984. Habitat use and movements of river otters at Kelp Bay, Baranof Island, Alaska. M. S. Thesis. University of Alaska, Fairbanks, 147pp.

 


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