State of Satiation Partially Regulates the Dynamics of Vertical Migration

Vertical migrations into shallower waters at night are beneficial for migrators as they reduce predation risk and allow migrators to encounter a higher density of prey. Nevertheless, ocean acoustics data and trawl data have shown that a portion of some vertically migrating populations remain at depth and do not migrate. One hypothesis for this phenomenon is the Hunger-Satiation hypothesis, which in part states that the non-migrating portion of the migrating species-assemblage refrains from migrating if they have full or partially full stomachs from daytime or nocturnal feeding. However, stomach fullness of the non-migrating subpopulation compared to the migrating portion has rarely been studied, due to the difficulty in obtaining sufficient samples. The stomach fullness levels of numerically abundant crustacean and fish species with well-known depth distributions were quantified in the present study. Animals were captured during night trawls from discrete-depth intervals between 0 and 1,500 m. Stomach fullness indices were assigned from 0 to 5 and compared between migratory taxa caught in shallow and deep waters. Data from the crustaceansAcanthephyra purpurea, Gardinerosergia splendens, Plesionika richardi, andSystellaspsis debilis, as well as the fishesLampanyctus alatus, Lepidophanes guentheri, andNotolychnus valdiviae, provided support for the Hunger-Satiation hypothesis, while data from the crustaceansGennadas capensisandGennadas valensand the fishBenthosema suborbitaledid not. These findings suggest that stomach vacancy may be just one of several factors regulating the dynamics of vertical migration in those species whose behavioral plasticity suggests daily “choices” in whether or not to vertically migrate.

The Phantom Satiation Hypothesis of Bariatric Surgery

The excitation of vagal mechanoreceptors located in the stomach wall directly contributes to satiation. Thus, a loss of gastric innervation would normally be expected to result in abrogated satiation, hyperphagia, and unwanted weight gain. While Roux-en-Y-gastric bypass (RYGB) inevitably results in gastric denervation, paradoxically, bypassed subjects continue to experience satiation. Inspired by the literature in neurology on phantom limbs, I propose a new hypothesis in which damage to the stomach innervation during RYGB, including its vagal supply, leads to large-scale maladaptive changes in viscerosensory nerves and connected brain circuits. As a result, satiation may continue to arise, sometimes at exaggerated levels, even in subjects with a denervated or truncated stomach. The same maladaptive changes may also contribute to dysautonomia, unexplained pain, and new emotional responses to eating. I further revisit the metabolic benefits of bariatric surgery, with an emphasis on RYGB, in the light of this phantom satiation hypothesis.

A test of the predator satiation hypothesis, acorn predator size, and acorn preference

Mast seeding is hypothesized to satiate seed predators with heavy production and reduce populations with crop failure, thereby increasing seed survival. Preference for red or white oak acorns could influence recruitment among oak species. We tested the predator satiation hypothesis, acorn preference, and predator size by concurrently measuring acorn production, mouse abundance, and white versus red oak acorn removal rates in exclosures allowing access by mice (HW), squirrels and smaller-sized vertebrates (WW), or all-sized vertebrates (C) for 12 years. Annual removal rate varied, but virtually all acorns were eventually removed from all exclosure types all years except one. Acorns were removed more slowly from HW than from WW or C exclosures, indicating that large vertebrates were not major acorn consumers, locally. Red and white oak acorn removal rates were similar except in two years, when red oak acorns were removed more rapidly. Removal slowed with increasing acorn crops, suggesting that heavy crops can “swamp” predators. Removal rate was negatively correlated with crop size the previous fall. A positive trend between mouse abundance and crop size the previous fall was evident; abundance decreased sharply the year following crop failures but not after moderate or heavy crops, suggesting that poor crops can dampen acorn predation the following year.

Seedling recruitment failure following dipterocarp mast fruiting

Many rain-forest trees in South-East Asia, including the dominant canopy family Dipterocarpaceae, reproduce in gregarious mast-fruiting events once every 2–11 y (Ashton et al. 1988). The dominant hypothesis for the evolution of masting is that predators are incapable of consuming mast seed crops, so that natural selection has favoured parent trees that fruit in synchrony (Janzen 1974, 1976). Mast flowering and fruiting are visually spectacular and quantified in harvest records for dipterocarp species producing large, oil-rich tengkawang seeds (Curran et al. 1999). Seedling recruitment following a mast is less obvious and has no immediate commercial value. However, a number of pulses of recruitment have been documented (Ashton et al. 1988, Chan 1980, Fox 1972, Liew & Wong 1973). These, together with general acceptance of the satiation hypothesis, have led to the widespread assumption that masts reliably increase seedling density and generate distinct seedling cohorts (Whitmore 1998). Indeed, foresters in Malaysia and Indonesia often recommend harvesting only after a mast, to ensure high densities of seedling regeneration.

Chemical defences of fruits and mast-fruiting of dipterocarps

Mast-fruiting is the intermittent and synchronous production of large fruits by a population of plants at long intervals (Herrera et al. 1998, Kelly 1994). Several hypotheses have been proposed concerning the adaptive advantages of mast-fruiting (Janzen 1971, 1974; Kelly 1994), and some field observations have provided evidence for these hypotheses (Norton & Kelly 1988, Shibata et al. 1998, Sork 1993). The predator-satiation hypothesis is one well-known explanation for reproductive synchrony in plants and animals (Janzen 1971, 1974; Kelly 1994). This hypothesis claims that mast fruiting at irregular intervals of several years is an effective means of satiating vertebrate fruit predators: low seed production can only support low densities of predators during the periods between mast-fruiting events, but more fruits are produced than predators can consume in masting years (Janzen 1971, Kelly 1994). Thus, it may be said that mast-fruiting is a defence strategy of plants against post-dispersal vertebrate fruit predators.