Research Abstract

J. SCOTT SHANNON

Since 1983, I have studied a population of otters (Lontra canadensis) at a marine roadstead on the Pacific coast of northern California. Over this 25-year period, I have observed 89 free-ranging individuals of known identity or birth whose lives spanned 7 otter generations. As of the end of December, 2007, I conducted 6,126 formal sessions, recorded >12,000 otter-hours of direct observations, and saw at least 1 otter here 4,796 times. From 1983-2007, 3-18 otters shared a home range comprising 4 linear km. of marine coastline. My subjects were observable at very close distances (1-100 m.). Individuals were identified reliably by carefully noting each otter's unique combination of facial, physical and behavioral characteristics.

The otters at my study site formed 2 distinct social groups: a "Family" comprising the adult female(s) and offspring, and a male "Clan." The Family usually consisted of a dominant "matriarch" and her pups of the year, and also typically included at least 1 elder daughter as a full-time cohabitant. Elder daughters provided socialization to the matriarch's pups and helped defend the Family's home territory. In most instances, though, the satellite females were not true "nest helpers," because they did not directly provision their mother's pups with food, nor did they assume active parental care in the matriarch's absence. In 2000, however, I observed an aunt display unmistakable parental behavior toward a niece whose mother had stopped caring for her. In 2002, another aunt provided maternal care to a nephew, and a matriarch with no pups of her own provided true maternal care to a grandson; in the latter case, the grandmother was often more attentive to the grandson than was his own mother. When an elder daughter of the matriarch became a mother herself, the matriarch and her daughter might form a "maternal alliance," combining their respective families into a stable, cohabitating 3-generation "superfamily." In 2002, a superfamily was formed in which a 7-year-old daughter allowed her pupless 11-year-old mother and pupless 7-year-old littermate sister to live with her and her 2 male pups. Despite these situations of apparent co-operation among matrilineally-related adult females, I have also observed 3 instances in which a matriarch's adult daughters committed deliberate acts of infanticide against their mother's infant young, and in 3 other instances, I observed a mother abandon or neglect her own pup(s) to die. Infanticides took place during periods of both high and low population densities.

Despite the fact that adult males and adult females here inhabited the exact same habitat and exploited the exact same food sources, the 2 sexes displayed completely opposite expressions of sociality and territoriality. Males at my study site were highly gregarious, forming a seasonally-stable, cohabitating, socially-egalitarian "Clan." The Clan comprised all of the population's 2-8 adult males: the local fathers, sons, matrilineal brothers, unrelated immigrant males and yearlings of both sexes. No adult females were observed to share the society of the males until summer 1997, when 2 elder uncles accepted their 2 nulliparous adult nieces into their company. Adult females, on the other hand, expelled all unrelated females from the home territory with violent aggression. Although the 2 mothers of the superfamily were usually tolerant of each other's dependent female pups, an adult female usually aggressively expelled any independent yearling female that was not her own daughter. Expulsion attacks by territorial adult females were astonishingly fierce; even a grandmother expelled her yearling granddaughter with uninhibited ferocity. In the severest territorial attack, the eldest daughter of the matriarch killed the yearling daughter of the matriarch in the presence of their common mother. Although territory literally meant life or death to the resident females, males here displayed no intrasexual territoriality whatsoever, except for brief fights during the females' estrus. The mortal violence that females can display against one another has no reasonable explanation, given the evident abundance of the habitat in which they live.

Over the last 25 years, by far the most noticeable pattern in this population's social organization has been that adult males and adult females lead largely sexually-segregated lives. The strength of this behavioral segregation of the sexes has been, at times, truly remarkable. For example, in the early years of my study, a period of 70 mon. once elapsed between instances when I saw an adult male and an adult female simply forage together, and 58 mon. elapsed between episodes of reciprocal play between opposite-sex adults. Typically, the primary enforcers of sexual segregation were the adult males. Avoidance was the principal means for enforcing the segregation of the sexes, although occasionally the males would attack females and drive them away. In general, the only prolonged social interaction that adult females and adult males engaged in was copulation, during the females' annual estrus from late-March to late-April. After all the adult females died in 1992, however, a new matriarchy was established in 1994 by a female who had lived the entire second year of her life with the male Clan. Although this female was shunned by her male companions after she attained sexual maturity, when this female subsequently attained motherhood, the frequency of amicable interactions between the Family and Clan increased noticeably, to the extent that, in mid-1995, I concluded that the rigid regime of sexual segregation I documented here from 1988-1992 was no longer in effect. The Family and Clan still maintained essentially separate and independent existences, but free interaction between the sexes was observed frequently. In 1997, all of the otters here were direct matrilineal relatives, and amicable intersexual interactions reached an all-time high, and for a brief unprecedented period, all of the resident adults formed a single unified social group. Then in 1998, 2 yearling males unrelated to the resident females joined the population, and this resulted in a re-emergence of segregation and agonism between the sexes. In contrast to previous manifestations of sexual segregation, the females became the principal enforcers of the social distance, and the females' aggressive attacks were directed exclusively against the new unrelated males. After a few months, however, the females ceased attacking the new males, and in 1999, amicable interactions between the sexes returned to the level observed in 1995. Thereafter, the 2 young males often sought out the adult females for reciprocal play. Only the eldest male - the last male who grew to adulthood under the previous strict regime of sexual segregation - continued to avoid social interaction with the females.

From 1986-2004, I studied the maternal and developmental behavior of 4 generations of otters. I chronicled the ontogeny of 6 litters (22 pups) by one mother, 4 litters (9 pups) by that mother's daughters, 9 litters (11 pups) by her granddaughter, 1 litter (1 pup) by a great-granddaughter, and 2 litters (5 pups) by another great- granddaughter. At my study site, otters achieved proficient aquatic locomotion only 9 wks. after leaving the natal nest, but proficiency in aquatic hunting required >9 mon. of additional trial-and-error learning. Basic self-sufficiency in food procurement was attained at 37-42 wks., but the young did not achieve optimal utilization of food sources and habitat until after the mother ceased all care for them and abandoned them at 48 wks. Yearlings did not disperse from their home area voluntarily. After 3 mon. of independence, yearlings of both sexes joined the Clan. Yearling females were accepted as fully co-equal members of the male Clan. In her 16th mon., a yearling female either returned to her mother, remained with the Clan, or was expelled aggressively from the Clan by an elder resident adult female. A female could remain a member of the Clan until she reached sexual maturity. After her first estrus, however, most males would shun her socially, and sometimes aggressively attack their former Clan-mate without apparent provocation. The 2 sisters born in 1986 were sexually mature at 24 mon., and they mated every year thereafter, but they remained nulliparous until 48 mon. and 60 mon. respectively. The first daughter to give birth was accepted into a maternal alliance with her mother. When the lower-ranking daughter became a primipara, however, her mother expelled her aggressively from the home territory. The sole surviving female of the third generation grew to adulthood in the absence of a dominant mother, and she first gave birth at the normal age of 36 months. The two sisters born to that female in 1995 were also sexually mature at 24 mon. and they mated every year thereafter, but they remained nulliparae until 60 mon. and 84 mon. respectively. I interpreted these data as clear evidence that the presence of a dominant matriarch inhibits reproduction in her subordinate adult daughters.

Males born here continued to base their activities at their home area well into adulthood, and most remained residents here for their entire life. Infrequently, males not born here joined the local Clan, indicating that some males do disperse permanently from their home area. Older males were noticeably less gregarious than young males, and 1 old male was expelled aggressively from the Clan at approximately 11 years of age; this exiled male subsequently died as a direct result of his social isolation. Until recently, the eldest female I observed here was the former matriarch "Old Mama;" I estimated she was 11-12 years old when she terminated her own life in July, 1992. The eldest male was "Beady Eyes," who joined the population as a young adult in 1987 and died in November 1996 at approximately 12 years of age. The male with the longest life span who I followed through his entire life was "Ninety" who died in January, 2002, at the age of 11 yrs., 10 mon. due to injuries suffered in a falling accident 8 months previously. The matriarch, "Little Mama," died of non-natural causes in January, 2005 at the age of 13 yrs, 10 mon. - the longest life span of any otter I've observed since birth, and the oldest wild otter of this species ever documented. Overall, on the basis of the dental erosion observed in Little Mama, I estimated that the maximum life span of an otter born in this habitat would be year-class 15.


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