In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies ’mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.
These galaxies may constitute an answer to the long-standing puzzle of the missing
baryonic mass in the universe. Baryons—subatomic particles亚原子粒子 that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy光谱学, suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually
be discovered in intergalactic space星系际的空间 or as some large population of galaxies that are difficult to detect.
Until recently, zoologists believed that all species of phocids (true seals), a pinnipped family, use a different maternal strategy than do otariids (fur seals and
sea lions), another pinniped family. Mother otariids use a foraging strategy. They acquire moderate energy stores in the form of blubber before arriving at breeding sites and then fast for 5 to 11 days after birth. Throughout the rest of the lactation (milk production) period, which lasts from 4 months to 3 years depending on the species, mother otariids alternately forage at sea, where they replenish their fat stores, and nurse their young at breeding sites. Zoologists had assumed that females of all phocids species, by contrast,
use a fasting strategy in which mother phocids, having accumulated large energy
stores before they arrive at breeding sites, fast throughout the entire lactation period, which lasts from 4 to 50 days depending on the species. However, recent studies on
harbor seals, a phocids species, found that lactating females commenced foraging
approximately 6 days after giving birth and on average made 7 foraging trips during the remainder of their 24-day lactation period.
The maternal strategy evolved by harbor seals may have to do with their small size and the large proportion of their fat stores depleted in lactation. Harbor seals are small compared with other phocids species such as grey seals, all of which are known to fast for the entire lactation period. Studies show that mother seals of these species use respectively 84 percent, 58 percent, and 33 percent of their fat stores during lactation. By comparison, harbor seals use 80 percent of their fat stores in just the first 19 days of lactation, even though they occasionally feed during this period. Since such a large proportion of their fat stores is exhausted despite feeding, mother harbor seals clearly cannot support all of lactation using only energy stored before giving birth. Though smaller
than many other phocids, harbor seals are similar in size to most otariids. In addition,
there is already some evidence suggesting that the ringed seal, a phocids species that
is similar in size to the harbor seal, may also use a maternal foraging strategy.
There is no consensus among researchers regarding what qualifies a substance as a pheromone. While most agree on a basic definition of pheromones as chemicals released by one individual of a species which, when detected by another individual of the same species, elicit a specific behavioral or physiological response, some researchers also specify that the response to pheromones must be unconscious. In addition, the distinction
between pheromones and odorants—chemicals that are consciously detected as odors---can be blurry, and some researchers classify pheromones as atype of odorant. Evidence that pheromone responses may not involve conscious odor perception comes from the finding that in many species, pheromones are processed by the vomeronasal (or accessory olfactory) system, which uses a special structure in the nose, the vomeronasal organ (VNO),to receive chemical signals. The neural connections between the VNO and the brain are separate from those of the main olfactory system, whose processing of
odorants triggers sensations of smell. But while the VNO does process many animal
pheromone signals, not all animal pheromones work through the VNO. Conversely,
not all chemical signals transmitted via the VNO quality as pheromones. For example,
garter snakes detect a chemical signal from earthworms—one of their favorite foods—via
the VNO, and they use this signal to track their prey.