Narine, 1
Shevin Narine
Bio 645
The influence of nepotism in eusocial insect colonies
Eusociality is the highest level of social organization within insect species and is marked
by clear levels of distinction between factions – each with their own important roles to play. Within
the context of Hamiltonian logic, it seems as if nepotism should be favored because specific
families would favor helping their relatives in order to control the dominant line of the nest. Thus,
in theory, we should observe significant levels of kin-dependent helping where members within a
caste will give biased help to their more closely related nestmates. In this paper, I will examine the
emergence of a eusociality in insect colonies, and whether or not nepotism is viable for the
members of said colony.
Eusociality is most frequently observed in Hymenoptera, a special branch of insects that
includes several ants, bees and wasps – the groups that have queens that perform the reproduction
function and groups of sterile workers (Hölldobler, 1990). It is worth mentioning that there are
varying degrees of observed eusociality in these species; it is only shown in some bees (Apidae
and Halictidate) and some wasps. Conversely, almost all ants are eusocial. The most distinct
features of a eusocial society include cooperative brood care, division of labor into reproductive
and nonreproductive groups, and overlapping generations of adults in a colony Mortensen et al,
2015). As mentioned above, eusocial societies are broken down into castes where each caste shows
differing levels of reproductive ability. Honeybees for instance are broken up in three castes.
Workers – small, non-reproductive females that perform brood care, hive maintenance (inclusive
of hive defense) and pollen and nectar collection. Each worker passes through the different tasks
in a specific order based on their age, a phenomenon known as age polytheism. Drones – males
Narine, 2
that are larger than the female workers and fulfill the role of reproduction with the queen. Queens
– the reproductive epicenter of the colony; she will mate with the drones (Mortensen et al, 2015).
Eusociality had to have emerged when it became beneficial for individuals to forgo their
individual gain and group together to form colonies. A study done by Martin A. Nowak, Corina E.
Tarnita, and Edward O. Wilson, put forth five steps for the emergence of eusociality within a
society. The first stage is the essential grouping of individuals – groups will have to form within a
freely mixing population. The easiest way for this to happen is when groups begin to assemble
around nest sites/food sources and create defensible nests. Additionally, groups of unrelated
individuals may band together through simple reciprocity or by mutualistic synergism. In the case
of Hymenoptera, a colony is started by an inseminated queen that will give birth to the workforce.
Hymenoptera species practice progressive provisioning in place of mass provisioning. Progressive
provisioning entails the queen building a nest, laying her eggs, and then guarding and feeding the
larvae until maturity (Nowak et al, 2010). The second stage involves the accumulation of traits
that benefit the shift towards eusociality. Traits begin to accumulate on the individual level by the
ancestor organism who recognizes and feels the gains of grouping together with other members of
the population. The diversification happens via adaptive radiation, a process by which species split
and develop into different niches (the most popular example outside of insects is Darwin’s
Finches). In the process of this adaptation, some castes gain abilities that the other ones losereproductive ability being the major example. The experimenters observed that solitary bees when
forced together, will adopt eusocial habits and begin to function as a colony. In Ceratina and
Lasioglossum, the species began to divide tasks into foraging, tunneling and guarding the nest.
This plays on the notion that there is behavioral groundwork in these species that will lend towards
a shift to eusociality once natural selection favors it. The third stage involves the evolution of the
Narine, 3
eusocial alleles – either by recombination or a mutation. Interestingly, eusocial-driving mutations
do not need to generate a novel trait, but rather cancel the old traits that promote individuality, i.e.
it requires that the queen and her offspring do not disperse and set up a new nest elsewhere, but
rather stay put in one location and raise larvae to maturity (Nowak et al, 2010). The experimental
evidence we have about the silencing of traits to favor eusociality comes in the form of a study
done by Abouheif and Wray on wing development silencing in the early ancestors of ants – this
process caused the wing formation to be silenced under the influence of an environmental factor
or diet and thus generated the wingless worker caste (Abouheif and Wray, 2002). It is important
to mention that these differing roles are not genetically determined, but rather they are the results
of collections of genes that are phenotypically programmable and flexible. This means that a queen
and her workers come from the same ‘stock’ that prescribes the division into castes, but they differ
in other genes (West-Eberhard, 2003). The realization of these novel traits in the colony leads to
the fourth stage of development; natural selection will target emergent eusocial traits generated by
the individuals interacting together in the colony. The fifth and final step in the emergence of
eusociality comes in the form of multi-level selection pushing colony life and social set-ups to new
levels of sophistication (Nowak et al, 2010).
The emergence of eusociality within a population, in theory, should push colonies toward
a family structure where more help is directed towards more closely related kin. Hamiltonian logic
postulates that animals should be directly helpful towards relatives so that the indirect fitness
gained can justify the cost of helping. How could this nepotism possibly benefit members of a
eusocial society? Well, a family member in the working class helping their reproductive class
relative more than a less related relative could help that family’s chances of becoming the dominant
line within the colony, thereby increasing the overall fitness of the individual that initially helped
Narine, 4
out (so long as the cost of this helping is offset by the benefit). An article by Jacobus Boomsma
and Patrizia d’Ettorre examined the role of nepotism within a eusocial population of insects, and
whether or not Hamilton’s theory overestimates the effect nepotism has in a eusocial colony. The
researchers first examined Hamilton’s rule under certain revisions that suit the push towards
eusociality. They take the standard rule, br > c, and modify the terms to co , personal reproduction
currency, rn, non-offspring nestmate of some level of relatedness, and ro , relatedness to own
offspring. Therefore, as all individuals within the colony reproduce (which we know is not the
case for eusocial insects), we can think of the modified rule to function the same way as the
‘regular’ rule, where kin-dependent helping will be favored so long as the benefit times the
relatedness outweighs the cost. However, given the special nature of reproduction in a eusocial
society, the inequality must now read, brn+ > crn, which means that the benefit times the relatedness
to a specific nest-mate of higher relatedness, must outweigh the cost of not helping a non-offspring
nestmate of some lesser relatedness. This essentially boils down to nepotism in the context of help
directed towards highly related family members; which would only evolve in a eusocial society if
the benefit of helping this particular higher-related nest-mate outweighs the cost of not helping
another nest-mate of average relatedness. Thus, the experimenters laid out the theoretical condition
for kin-dependent helping to be favored in a eusocial society (Boomsma and d’Ettorre, 2013).
Boomsma and d’Ettorre then examined a major factor in determining whether or not the
members of the colony would be able to practice nepotism – the diminishing of recognition cues
between nest-mates. First, they highlighted a study done in 1992 by FLW Ratnieks and HK Reeves,
on the effects cue recognition and its role in generating nepotism within a colony. Ratnieks and
Reeve developed the hypothesis that recognition cue variation among nest-mates from different
patrilines would become eroded if nepotism was a strong force within a eusocial colony.
Narine, 5
Eventually, this would favor carriers of the same gene and unbalance the colony’s composition,
leading to a dominant patriline that was generated because of precise kin-dependent helping
(Ratnieks and Reeve, 1992). However, Boomsma and d’Ettorre have given an insight as to why
this might not be the case. First, they reference the split in ability of the eusocial castes; the majority
of nest-mates are unable to reproduce and must fulfill their role by taking care of the colony’s
offspring to ensure its prolonged survival. Eusocial colonies have a large population but only a
small mating caste and as a result, clutch sizes have to get larger in order to compensate for this.
Secondly, the experimenters suggest that large clutch sizes create a push towards dissociative
outbreeding, which could only function if all of the larvae possess a similar recognition smell. This
becomes necessary so that all workers, no matter their patriline, can recognize and tend to any
individual nest-mate, regardless of patriline. Thus, selection for odor mixing via trophallaxis,
mutual exchange of food between larvae and adults, generates a ‘Supercolony’ that all possess
similar recognition cues and as a result, kin-depending helping based on being closely related to a
nest-mate becomes quite difficult to initiate. The erosion of cue diversity would decrease the left
side of the inequality, bn+rn+ > cnrn – because the benefit to the actor is reduced since they might
be helping to raise a nest-mate that is a non-relative - thus showing that nepotism would not pay
off in the eusocial colony (Boomsma and d’Ettorre, 2013).
Their suggestions on the action of recognition cue diversity were supported by a study done
by Tom Wenseleers that highlighted the patriline composition of colonies, their genetic makeup
and how the recognition cue variation in the colony is set up. In his study, Wenseleers uses an
experiment done by Goodisman et. al on the eastern yellowjacket wasp, Vespula mcaulifrons,
patrline and how the colony composition is generated. Yellowjacket queens mate with five to nine
males and hence, multiple patrilines are generated within a single colony. Goodisman postulates
Narine, 6
that for nepotism to be present within the yellowjacket colony, workers should favor helping their
more related sister relatives from the same patriline (with an r factor = 0.75), over their halfsiblings from another patriline (with an r factor = 0.25). Thus, the colony’s queen brood and worker
patriline should differ greatly (Goodisman et al, 2007). The experimenters tested this theory by
genotyping 581 workers and 397 queens from eleven different colonies. They went on to determine
that the patriline distributions of the queen brood do NOT differ from those present in the worker
brood. This meant that no particular patriline had a significant advantage over the other. The
experimenters coupled this with the assumption that nepotism expressed by different patrilines
would not ‘cancel’ each other out and therefore concluded that nepotism is absent. Based on this,
they derived that there is no weighting (i.e. kin-dependent helping) of the queen patriline – any
larvae reared as a queen happens simply due to chance and not due to the assistance or help of a
closely related worker relative (Goodisman et. al 2007). Additionally, Wenseleers cited work from,
Dani et al. 2004 to help provide a window into why nepotism might be selected against in these
eusocial colonies; the fact that cuticular hydrocarbons from Vespinae wasps fail to accurately
distinguish among different patrilines. This means that nestmates could not accurately discern who
is a more related relative and who is a half-related relative. Thus, in a system where discrimination
is difficult and frequent mistakes by workers may occur, nepotism becomes selected against (Dani
et al, 2004).
Based on the evidence provided, it appears that nepotism in the form of kin-dependent
helping is not present in eusocial colonies because it is selected against. The colony gains much
more in a situation where workers can combine their efforts to support and raise all of their nestmates, regardless of the degree of relation. This is supported by the fact that eusocial species do
not show biased patrilines within the colony and the fact that recognition cue erosion assists the
Narine, 7
aforementioned workers with helping all members by preventing biased recognition. Therefore,
based on the information referenced above, we can conclude that nepotism is not favored within a
eusocial insect colony and thus exerts no influence.
Narine, 8
Bibliography
Abouheif E., Wray G.A. 2002. Evolution of the gene network underlying wing polyphenism in
ants. Science. 2002; 297:249–252.
Boomsma, J. and d’Ettorre, P. 2013. Nice to kin and nasty to non-kin: revisiting Hamilton’s early
insights on eusociality. Biology Letters 9: 20130444.
Dani F.R., Foster K.R., Zacchi F. et al. 2004. Can cuticular lipids provide sufficient information
for within-colony nepotism in wasps? Proceedings of the Royal Society of London. Series B,
Biological Sciences, 271: 745–753.
Goodisman M.A.D., Kovacs J.L., Hoffman E.A. 2007. Lack of conflict during queen production
in the social wasp Vespula maculifrons. Molecular Ecology, 16, 2589–2595.
Holldobler, B and Wilson, E.O. 1990. The Ants. Belknap Press of Harvard University: Cambridge.
Mortensen, A.N., Smith, B., and Ellis, J.D. 2015. The social organization of honeybees. ENY-166:
45-48.
Nowak, M.A., Tarnita, C.E., Wilson, E.O. 2010. The evolution of eusociality. Nature 2010 August
26; 466(7310): 1057-1062.
Ratnieks F.L.W., Reeve H.K. 1992. Conflict in single-queen hymenopteran societies: the structure
of conflict and processes that reduce conflict in advanced eusocial species. Journal of Theoretical
Biology, 158: 33–65.
Wenseleers, T. 2007. Nepotism absent in insect societies - or is it? Molecular Ecology (2007) 16:
3063-3065.
West-Eberhard, M.J. 2003. Developmental Plasticity and Evolution. Oxford Univ. Press; 2003.