Biology Reference
In-Depth Information
just the mother; (ii) they have sterile castes, and (iii) they have overlap of generations so
that mother, adult offspring and young offspring are all alive at the same time. More
recently, Crespi and Yanega (1995) have argued that this definition is too vague from an
evolutionary perspective and can include species which are better characterized as
cooperative breeders. They suggest that the key factor which should define eusociality is
the presence of specialized castes, where different groups of individuals become
irreversibly behaviourally distinct at some point prior to reproductive maturity.
Individuals of one caste have higher rates of reproduction (breeders) and they are
helped by at least one or more other caste members (helpers). This definition makes no
use of overlapping generations, the presence or absence of which need not be linked to
the level of altruism in or complexity of a society.
Crespi and Yanega (1995) and Boomsma (2007, 2009) also stressed the special
importance of whether eusociality is obligate. The key factor here is whether castes are
permanently fixed or can be switched, and whether full reproduction is still possible for
at least some of the helpers. If individuals of a caste retain, throughout their life, the
ability to perform the full range of behaviours open to all the castes (including
reproduction), then this is termed totipotency, literally
total potential
. In obligately
eusocial societies, totipotency has been lost, so castes are permanently fixed. This is
special from an evolutionary perspective because it leads to a complete mutual
dependence, with the breeding caste dependent on the help of the helping caste
(or castes), and the helping caste being dependent on the presence of some breeders to
help. This contrasts with cooperative breeding or facultatively eusocial species, where at
least some of the workers are also able to breed if the opportunity arises.
Up until the mid 1970s it was thought that eusociality only occurred in the social
Hymenopetra (ants, bees and wasps) and the termites. Since then a number of other
eusocial species have been found, including other insects (aphids, gall-forming thrips,
Austrolplatypus
ambrosia beetles; Aoki, 1977; Crespi, 1992; Kent & Simpson, 1992),
sponge-swelling shrimps (Duffy, 1996) and even two mammals, the naked and
Damaraland mole-rats (Jarvis, 1981). The number of times that eusociality is thought
to have evolved depends upon how exactly it is defined (and the quality of phylogenetic
trees!). For example, in the social Hymenoptera, while it is sometimes argued that
eusociality has evolved up to 11 times, only three to five of these represent obligate
eusociality. These are once in the ants (Brady
et al
., 2006), once or twice in the bees
(Cameron & Mardulyn, 2001; Danforth
et al
., 2006), once in the vespine wasps (Hines
et al
., 2007) and possibly once in the polistine wasps (Boomsma, 2009). Amongst the
other cases, obligate eusociality is limited to the termites (one to three origins; Inward
et al
., 2007; Boomsma, 2009) and possibly the aphids, thrips or Ambrosa beetles
(Boomsma, 2009). However, in these last three cases, eusociality has not led to the
successful radiations that have occured in the eusocial hymenoptera and termites.
Eusocial insects
have castes
Obligate
eusociality
involves
permanent castes
Eusociality has
been observed in
several taxa …
… but obligate
eusociality has
only been
confirmed in the
eusocial
hymenoptera and
the termites
The importance of social insects
The social insects are not only important because of their central role in the attempts of
evolutionary theorists to understand the origin of altruism, but they are also extremely
impressive in terms of their natural history. In a persuasive piece of salesmanship
E.O. Wilson (1975) advertises that there are more than 12 000 species of social insects
