Social polymorphism in the sweat bee Lasioglossum (Evylaeus) calceatum
Insect. Soc. (2016) 63:327–338
DOI 10.1007/s00040-016-0473-3
Insectes Sociaux
RESEARCH ARTICLE
Social polymorphism in the sweat bee Lasioglossum (Evylaeus)
calceatum
P. J. Davison1 • J. Field1
Received: 13 August 2015 / Revised: 4 February 2016 / Accepted: 19 February 2016 / Published online: 10 May 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Temperate-zone socially polymorphic sweat bees
(Hymenoptera: Halictidae) are ideal model systems for
elucidating the origins of eusociality, a major evolutionary
transition. Bees express either social or solitary behaviour in
different parts of their range, and social phenotype typically
correlates with season length. Despite their obvious utility,
however, socially polymorphic sweat bees have received
relatively little attention with respect to understanding the
origins of eusociality. Lasioglossum (Evylaeus) calceatum
is a widespread sweat bee that is thought to be socially
polymorphic, with important potential as an experimental
model species. We first determined the social phenotype of
L. calceatum at three sites located at different latitudes
within the UK. We then investigated sociality in detail
across two years at the southernmost site. We found that L.
calceatum exhibits latitudinal social polymorphism within
the UK; bees were solitary at our two northern sites but the
majority of nests were social at our southern site. Sociality
in the south was characterised by a relatively small mean of
two and 3.5 workers per nest in each year, respectively, and
a small to medium mean caste-size dimorphism of 6.6 %.
Foundresses were smaller in our more northern and high
altitude populations. Sociality is clearly less specialised
Electronic supplementary material The online version of this
article (doi:10.1007/s00040-016-0473-3) contains supplementary
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& P. J. Davison
1
School of Life Sciences, University of Sussex, John Maynard
Smith Building, Brighton BN1 9QG, UK
2
Present Address: Carl Icahn Laboratory, Lewis-Sigler
Institute for Integrative Genomics, Princeton University,
Washington Road, Princeton, NJ 08544, USA
than in some closely related obligately social species but
probably more specialied than other polymorphic sweat
bees. Our research provides a starting point for future
experimental work to investigate mechanisms underlying
social polymorphism in L. calceatum.
Keywords Sweat bees Halictinae Lasioglossum
Eusociality Caste-size dimorphism Workers
Introduction
Understanding why an individual gives up its own reproduction to help others is a central goal of evolutionary
biology (Maynard Smith and Szathmáry 1995). Despite this,
accounting for how the decision to help is made at the
individual level has received comparatively little attention
(Field et al. 2012). Primitively eusocial sweat bees (Hymenoptera: Halictidae) lack fixed castes and vary greatly in
their social organisation, making them model organisms for
studying the origins of eusociality (Schwarz et al. 2007). Of
particular interest is social polymorphism, where both
solitary and social phenotypes are expressed within the
same species (Soucy and Danforth 2002). In social nests, at
least some first brood offspring are workers that help rear a
second brood of reproductives. In contrast all offspring in
solitary nests are reproductives, which tend to occur where
the season is probably too short to complete the social life
cycle (Hirata and Higashi 2008; Kocher et al. 2014).
Socially polymorphic sweat bee lineages therefore offer
unique opportunities to understand the mechanisms underlying the origin of eusociality, because it is possible to
directly investigate the environmental and genetic processes
mediating the decision to become a worker or a reproductive
(Field et al. 2010, 2012).
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The Palearctic sweat bee Lasioglossum (Evylaeus) calceatum Scopoli is socially polymorphic (Sakagami and
Munakata 1972; Field 1996). Originally this species was
thought to be only primitively eusocial (e.g. Bonelli 1965,
1968). Then, more than 40 years ago, Sakagami and Munakata (1972) discovered that L. calceatum was socially
polymorphic in Japan; nests were found to be solitary at more
than 1000 masl on the summit of Mt Yokotsu, but a social life
cycle was inferred in the surrounding lowlands. Similarly,
Field (1996) reported solitary nests from Dartmoor, an upland
area in the southern United Kingdom (UK). Since these
studies, however, there has been no attempt to further
understand the underlying causes of social polymorphism
within L. calceatum. Moreover, details of the social life cycle
and the degree of social complexity relative to other primitively eusocial sweat bees remain poorly understood from the
wild (Plateaux-Quénu 1992; Pesenko et al. 2000).
From studies to date, the life cycle of L. calceatum can be
summarised as follows (Bonelli 1965, 1968; Sakagami and
Munakata 1972; Plateaux-Quénu 1992 and references
therein). Mated females (foundresses) emerge from hibernation in spring and initiate a subterranean nest.
Foundresses mass provision a first brood (B1) of &4–6
offspring including both females and males, providing each
with a ball of pollen and nectar in a cluster of separate,
sealed brood cells. In solitary nests offspring emerge, mate,
and females enter directly into hibernation. In social nests,
however, B1 females are typically slightly smaller than their
mothers and are thought to become workers that help provision a second brood (B2) of reproductives. This
conclusion is supported by field data from Europe and
Japan; summer caught females are reported to be mostly
unmated and to have undeveloped ovaries (Bonelli 1965;
Sakagami and Munakata 1972, but see Plateaux-Quénu
1992 who reports a greater proportion of mated summer
females). B2 offspring emerge at the end of summer to
mate, and females enter hibernation before emerging as
foundresses the following spring. Males are produced in
both broods but die before winter and play no role in nesting. Nevertheless, B1 sweat bee offspring may also assume
replacement queen status, lay eggs in nearby nests, found a
nest directly or enter hibernation to become a foundress in
the following year (Yanega 1988; Yagi and Hasegawa 2012;
Brand and Chapuisat 2016). It is currently unknown to what
extent these behaviours occur in L. calceatum.
More advanced halictine sociality is generally associated
with traits such as larger colony size, and a greater degree of
caste-size dimorphism between workers and foundresses
(Packer and Knerer 1985). A population level comparison
between foundresses and presumed B1 females in Japanese
L. calceatum found a size difference of 3.5–5.5 % (Sakagami and Munakata 1972), whereas in France foundresses
have been reported as being up to 13 % larger than workers
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P. J. Davison, J. Field
(see Plateaux-Quénu 1992). It is unclear whether the latter
figure was als (...truncated)
