Season length, body size, and social polymorphism: size clines but not saw tooth clines in sweat bees

Abstract

Abstract. 1. Annual insects are predicted to grow larger where the growing season is longer. However, transitions from one to two generations per year can occur when the season becomes sufficiently long, and are predicted to result in a sharp decrease in body size because available development time is halved. The potential for resulting saw-tooth clines has been investigated only in solitary taxa with free-living larvae.2. Size clines were investigated in two socially polymorphic sweat bees (Halictidae): transitions between solitary and social nesting occur along gradients of increasing season length, characterised by the absence or presence of workers and offspring that are individually mass provisioned by adults. How the body size changes with season length was examined, and whether transitions in social phenotype generate saw-tooth size clines. We measured Lasioglossum calceatum and Halictus rubicundus nest foundresses originating from more than 1000 km of latitude, encompassing the transition between social and solitary nesting.3. Using satellite-collected temperature data to estimate season length, it was shown that both species were largest where the season was longest. Body size increased linearly with season length in L. calceatum and non-linearly in H. rubicundus but the existence of saw-tooth clines was not supported.4. The present results suggest that because the amount of food consumed by offspring during development is determined by adults, environmental and social influences on the provisioning strategies of adult bees may be more important factors than available feeding time in determining offspring body size in socially polymorphic sweat bees.

Environmental barriers to sociality in an obligate eusocial sweat bee

“…Theoretically, faster development or the rearing of smaller offspring in response to cooler conditions could compress the life cycle and enable eusociality in sweat bees to persist further north (Nylin and Gotthard 1998 ; Gotthard et al 2000 ; Inagawa et al 2001 ). However, although foundresses from more northerly populations of sweat bees tend to be smaller, there is no evidence for increased growth rates during immature development (Field et al 2012 ; Davison and Field 2017 , 2018 ). Most intraspecific variation in development time is apparently driven by temperature, such that development is prolonged at lower temperatures (Weissel et al 2006 ; Field et al 2012 ); moreover, sweat bees can exert only limited control over temperatures experienced by developing brood, such as by locating nests in exposed, south-facing ground (Potts and Wilmer 1997 ; Hirata and Higashi 2008 ).…”

“…If Cromarty-transplanted foundresses experienced fewer or shorter suitable foraging windows, or fewer resources overall, they may have provisioned an equal number of smaller offspring (Richards and Packer 1996 ; Richards 2004 , but see Richards et al 2015 ). Indeed, adult sweat bees do tend to be smaller in more northerly environments (Field et al 2012 ; Davison and Field 2017 ).…”

Limited social plasticity in the socially polymorphic sweat bee Lasioglossum calceatum

“… a Mean annual land surface temperature 1981–2006 (data from Hay et al 2006) b Estimate of the time available for nesting during the active season, calculated from the mean number of days during the year between 1981 and 2006 on which the land surface temperature exceeds 16 °C (see Davison and Field 2017 for methodology)…”

Frog body size responses to precipitation shift from resource‐driven to desiccation‐resistant as temperatures warm

“…This lack of uniform responses suggests that the negative impact of temperature on size is not acting alone or is not always the main factor dictating size as climate changes. For example, in temperate regions, warmer temperatures also bring longer growing seasons, which may act to increase body size (Chown & Klok, 2003;Davison & Field, 2017;Eastman et al, 2012).…”

“…This lack of uniform responses suggests that the negative impact of temperature on size is not acting alone or is not always the main factor dictating size as climate changes. For example, in temperate regions, warmer temperatures also bring longer growing seasons, which may act to increase body size (Chown & Klok, 2003 ; Davison & Field, 2017 ; Eastman et al, 2012 ). Additionally, changes in precipitation are expected to alter primary productivity and water availability, both of which can impact body size (Dubos et al, 2018 ; Kelly et al, 2018 ; Morales‐Castilla et al, 2012 ; Yom‐Tov & Geffen, 2006 ).…”