It is well known that diving capacity for foraging in diving animals, such as seabirds and marine mammals, is related to body size; larger divers can make longer and deeper dives (for reviews, see Schreer & Kovacs, 1997; Watanuki & Burger, 1999; Schreer et al., 2001). This greater diving ability of larger diving animals has been explained by their larger capacity for oxygen storage and lower rate of oxygen consumption (Kooyman, 1989). Larger divers also have a greater anaerobic capacity (Hochachka & Somero, 1984). However, after the difference in body size was taken into account, species belonging to a taxonomic group that are on average small outperform species that are larger. For example, alcids achieve relatively longer dives than penguins (Watanuki & Burger, 1999), and phocids outperform cetaceans (Schreer & Kovacs, 1997). Intraspecific comparison also revealed that smaller Weddell seals (Leptonychotes weddellii) with a high metabolic rate and low oxygen store made longer dives relatively to the aerobic dive limit than larger seals with a low metabolic rate and high oxygen store (Burns, 1999). Unfortunately, we do not yet know what physiological mechanisms make it possible for smaller divers to dive relatively longer than larger ones, but Burns (1999) suggested that the physiological mechanism that allows smaller divers to dive longer may simply be more anaerobic metabolism pushing the physiological limits.
Despite the lack of knowledge about the physiological basis of diving differences, existing data indicate that there is higher mortality for smaller and younger animals in Weddell seals (Burns, 1999), suggesting selection pressure for diving ability could be stronger on smaller divers. Although there has been no interspecific comparison suggesting effects of body size on mortality in relation to diving ability, Schreer et al. (2001) reported that the dive limit constraint is stronger in smaller divers, which would affect mortality in smaller divers.
Diving behaviour was modelled creating optimal foraging models (Kramer, 1988; Ydenberg & Clark, 1989; Houston & Carbone, 1992; Carbone & Houston, 1996; Mori, 1998a, b, 1999; Walton et al., 1998; Thompson & Fedak, 2001). Mori (1998b) indeed discussed diving behaviour in relation to body size, but did not consider the fact that anaerobic respiration is closely linked to diving ability and that anaerobic capacity increase with body size.
In this paper, I expand upon the models presented by Mori (1998b, 1999) to investigate in detail the effect of body size on optimal foraging diving, taking into account anaerobic respiration. I also discuss selection pressure on diving ability in relation to body size.