Behav Ecol Sociobiol (1994) 34:95-104 Behavioral Ecology and Sociobiology © Springer-Verlag 1994 Evidence of a maternal foraging cycle resembling that of otariid seals in a small phocid, the harbor seal D.J. Boness 1, W.D. Bowen 2, O.T. Oftedal 1 1 Department of Zoological Research, National Zoological Park, Smithsonian Institution, Washington, DC 20008 USA 2 Marine Fish Division, Bedford Institute of Oceanography, Department of Fisheries and Oceans, Dartmouth, N.S. B2Y 4A2 Canada Received: 26 March 1993 / Accepted after revision: 2 November 1993 Summary. Lactation strategies in the two largest families of seals have been characterized as a phylogenetic di- chotomy, with sea lions and fur seals (Otariidae) exhibit- ing foraging cycles and true seals (Phocidae) a strategy of fasting. We show that a lactating phocid, the harbor seal, Phoca vitulina, has a foraging cycle similar to that of otariids. Time-depth recorders attached to lactating har- bor seal mothers revealed that 9 of 11 females began bouts of diving, averaging 12-40 m, by mid-lactation (12 days). During the remainder of lactation, females made an average of seven diving trips, lasting about 7 h. They returned to the rookery during the interval between suc- cessive bouts to nurse their pups. Diving was more fre- quent during daylight than at night and diving bouts increased in duration as lactation progressed. The diving behavior of females that had weaned their pups and pre- viously collected data from stomach lavage, suggest that the bouts of diving represent successful foraging. We pro- pose that the lactation strategy of the harbor seal is inter- mediate to that of the otariids and other phocids studied. The harbor seal has a foraging cycle like the otariids, but typically resembles other phocids in length of lactation, rate of mass gain in pups, and in milk fat content. As harbor seals are among the smallest phocids, and only slightly larger than most otariids, it seems likely that ma- ternal size constrains the amount of stored energy harbor seal females can bring to the rookery, forcing them to start feeding during the lactation period. Key words: Maternal strategy - Lactation - Foraging - Pinnipeds Introduction In polygynous mammals, males maximize their repro- ductive success by monopolizing females. In contrast, fe- males, as the primary care-givers, maximize their success Correspondence to: D.J. Boness by selecting good mates and by ensuring survival of their young to nutritional independence (Trivers 1972; Clut- ton-Brock 1991). The suite of characteristics associated with successful maternal care (commonly referred to as the maternal or lactation strategy) should reflect phylo- genetic constraints and ecological pressures (Robbins and Robbins 1979; Loudon and Kay 1984; White and Luick 1984). A major ecological and physiological constraint that faces all pinnipeds (true seals, sea lions, fur seals and walrus) and plays a major part in shaping their maternal strategies is the duality of their marine food supplies and the terrestrial (or ice-based) sites where parturition and maternal care occur (Bartholomew 1970; Stirling 1975). Maternal strategies in the Order Pinnipedia have been categorized as dichotomous, with the two major families exhibiting different patterns. These patterns have been referred to as the "fasting strategy" and the "foraging cycle strategy." The fasting pattern is typical of the Phocidae (true seals) and the foraging cycle appears to be characteristic of the Otariidae (fur seals and sea lions) (reviewed in Bonner 1984; Gentry and Kooyman 1986; Oftedal et al. 1987; Costa 1991). The major features of these strategies are presented in the reviews cited above. The fasting strategy of the phocids is characterized by: (1) maternal arrival at the breeding site with large energy stores in the form of blub- ber to support maintenance and lactation costs; (2) a short lactation period (4-50 days); (3) extremely high-fat milk in mid to late lactation (40-60% fat); and (4) rapid fattening of pups. In contrast, the foraging cycle strategy of otariids is characterized by: (1) moderate maternal en- ergy stores used to sustain a short perinatal fast, followed by feeding trips at sea alternating with visits to land in order to nurse pups; (2) a long lactation period (4 months-2 years); (3) milk that is generally lower in fat (19-50%); and (4) slow rates of mass gain in pups. Although most of the available evidence supports this hypothesized phylogenetic dichotomy, the only phocids that have been studied to date are of larger, body size, especially in relation to the otariids. Little is known