Biology Reference
In-Depth Information
Characteristic
Fortress Defenders
Life Insurers
Table 13.2 The
two types of social
insects, as divided
by whether the
evolution of
eusociality was
driven by life
insurance or
fortress defence
(adapted from
Queller &
Strassmann, 1998).
Taxa
Thrips, aphids, beetles, termites
(and sponge-dwelling shrimps)
Ants, bees, wasps
Main advantage
of social living
Valuable, defensible resource
Overlap of adult lifetimes
to provide extended care
to young
Food
Inside nest or protected site
Outside nest
Juveniles
Active, feed selves and may work
Helpless, need to be fed
and do not work
Non-social ancestors
Not necessarily parental
Highly parental
First specialized caste
to evolve
Soldiers
Foragers
Colony size
Usually small
Often large
Ecological success
Usually limited
Excessive
Dave Queller and Joan Strassmann (1998) have argued that the social insects can be
divided into two types, depending upon whether the primary ecological benefit of
sociality was life insurance or fortress defence. These types and their defining
characteristics are given in Table 13.2.
Food distribution
Several researchers have argued that a major factor driving the evolution of cooperation
and eusociality in mole-rats is the distribution of food ( Jarvis et al ., 1994). Mole-rats are
small African rodents that spend their lives underground, digging tunnels in search of
subterranean storage organs of plants (roots and tubers), which are their source of
food. The mole-rats include solitary species, group living species (with maximum group
sizes of approximately 15) and two eusocial species, the naked mole-rat and the
Damaraland mole-rat (with maximum group sizes of approximately 300 and 40
respectively; Fig. 13.9). In these eusocial species, only one pair in the group breeds. The
other females have undeveloped ovaries and the males, while they may have active
sperm, are apparently non-breeders.
A possible role of food distribution was suggested by the distribution of species across
different habitats. The solitary species inhabit mesic areas with a well-balanced supply of
water that leads to a more even distribution of food, and damper soil, that is easier to dig.
In contrast, the eusocial species live in arid areas where rainfall is low and unpredictable.
In the latter kind of habitat, food resources are more patchy, making foraging more risky,
and the harder soil means that the energetic cost of digging is higher. Furthermore,
when food is found, it is usually enough to feed a colony for a long time. This has led to
the suggestion that a patchy and hard to find food source, that is plentiful when found,
Search WWH ::




Custom Search