Encyclopedia of Evolution.pdf - Online Reading Center

Two different hypothetical Bateman gradients. The graph on the top
represents the situation in most animal species, in which males provide
little care for the offspring, and can afford to produce many offspring. Males
often have no mates or offspring, but when they do, increasing the number
of mates increases reproductive success. Females nearly always have
offspring, but increasing the number of mates does not help to increase the
number of offspring very much. The situation is reversed in the graph on the
bottom, which represents the situation in broad-nosed pipefish, in which
the males care for the offspring.
Males are often violent toward offspring, though usually not
toward their own. Male lions will often kill the cubs of a
female he has just acquired. If the female expended time and
resources nurturing the cubs, this would reduce her ability to
nurture his offspring. One-third of all deaths of infant gorillas
are caused by a new dominant male killing them. Even though
infanticide is much less common among humans than gorillas
or lions, men tend to show less care toward stepchildren than
toward their own children. According to behavioral scientists
Martin Daly and Margo Wilson, “the truth about Cinderella”
is that there is an evolutionary reason why stepchildren receive
less care than biological children.
Reproductive systems of most animals can be roughly
classified into (1) monogamy, where one male and one female
are mated for life or at least for a long time; (2) polygamy,
which includes (2a) animal societies that have a dominant
reproductive individual and (2b) promiscuity, where individuals
mate with other individuals without forming even
temporary pair bonds. Animal reproductive systems form a
continuum among these categories:
Monogamy. Among the explanations that have been
offered for monogamy are:
• Expensive offspring. Monogamy is advantageous only
when neither the male nor the female could obtain greater
reproductive success by finding more mates. This may
happen when the offspring are expensive and require
considerable care during upbringing, as in humans and
some birds. Human offspring are notoriously expensive
to produce, vulnerable at birth and during infancy, and
dependent upon long years of training, particularly for
social interactions. Social interactions may have been
the primary stimulus for the evolution of language and
of large brains (see intelligence, evolution of; language,
evolution of). It would be uncommon without
organized society or extended family structures for a single
mother to raise children by herself, and even then it is
extremely difficult.
• Certainty of paternity. Within a monogamous couple, the
male can be certain that he is the father. In many animal
species, such as lions, the male will kill their mate’s offspring
if these offspring were sired by other males. This
may be in the interest of the male, but not of the female.
Monogamy might help to prevent infanticide.
Monogamy is an example of one of the behavior patterns
that humans call “virtues” and that turn out to provide an
evolutionary advantage. Lifelong and strict monogamy, however,
does not yield to this simple evolutionary explanation. In
Monogamous Animal Species
reproductive systems
Category Scientific name Common name
Invertebrates Lysiosquilla sulcata Mantis shrimp
Reticulitermes flavipes Termite
Birds Coragyps atratus Black vulture
Corvus monedula Jackdaw
Pygoscelis antarctica Chinstrap penguin
Aceros undulatus Wreathed hornbills
Cygnus columbianus
bewickii
Bewick’s swan
Mammals Modoqua kirkii Kirk’s dik-dik
(antelope)
Peromyscus californicus California mouse
Microtus ochrogaster Prairie vole
Phodopus campbelli Djungarian hamster
Hystrix indica Indian crested
porcupine