Indiscriminate oophagy by ant larvae: an explanation for brood serial organization?

Abstract

SummaryLast instar larvae of Lasius niger under standard laboratory conditions and abundant food supply feed on conspecific eggs and, if forced to a choice, show a significant statistical preference for trophic versus fertilized eggs (approx. 60 % of the trials observed). On the other hand, they are unable to discriminate between kin and non-kin eggs, both fertilized and trophic. Fertilized eggs killed by freezing and trophic eggs handled in the same way are also selected in a random manner.Last instar larvae of Messor semirufus regularly failed to discriminate in a significant way between trophic and fertilized and between kin and non-kin eggs.Substantial experimental evidence confirms that larvae of both these species -as already suggested in the literature for other ants-rely on trophic eggs as an essential component of their diet in order to be able to develop. Additionally, L. n~ger last instar larvae appear to be able to attack and pierce the egg chorion with their own mandibles, while this capacity is at least very reduced or very rare in M. semirufus. First and second instar larvae of both species never succeeded in piercing the egg membrane alone.These findings imply that the "right" (i.e. trophic) eggs should be presented to the larvae by the workers (i.e. worker discrimination should be assumed) and, at least in the case of young Lasius larvae, and probably for all stages of Messor larvae, the egg membrane must be pierced by the workers in order to allow the larvae to feed. The following evolutionary sequence is suggested to explain the origin of trophic eggs in ants: 1) larval oophagy (obligatory at least for the species founding new colonies in an independent claustral manner), 2) generalized facultative or obligatory larval oophagy for larvae of all colonial stages, 3) production of trophic eggs by the queen(s) and/or workers in order to avoid cannibalism of nestmates.Lack of discrimination or weak discrimination capacity between trophic and fertilized eggs and the consequent larval cannibalism in form of oophagy, as demonstrated in this paper, is suggested to explain age segregation among eggs and larvae from workers, a widespread phenomenon in ants which must have been selected to avoid the oophagy of viable eggs.

How to fool a virgin: Artificial dealation triggers oviposition in virgin Lasius niger queens

“…Three lines of evidence suggest that a considerable number of eggs laid by dealate virgin queens were viable. First, we observed no trophic eggs, which, in L. niger appear to be round and whitish (Baroni Urbani, 1991). All eggs observed using a microscope had the shape of an ellipse and were embryonate.…”

The possible role of ant larvae in the defence against social parasites

“…Any female brood of the host queen present in the colony at the time of parasite takeover may also benefit from larval defence, as female F. fusca larvae are more likely to develop into new queenswith direct fitness returns in its wake-in the absence of the queen [65]. Larvae as a secondary line of defence may, however, not work for species that (unlike F. fusca) maintain their eggs and larvae separately [34,68], or which have sterile workers incapable of producing male offspring, and thus unable to gain direct fitness in the absence of a queen [69]. In addition, some parasite species may inhibit their host from reproducing (complete or partial parasitic castration) [67,70,71].…”

“…In ants, queens and workers can produce trophic eggs destined for consumption [48]. These eggs are not viable, and are preferred by larvae over viable eggs [34]. To estimate whether differences in the rates of egg consumption could be caused by differential production of trophic eggs across species, we measured egg hatching success by placing clean, freshly laid eggs (3-5 colonies per species, 4-7 queens each, n ¼ 36-65 eggs per species, table 2) on a Petri dish with small sponge cloth strips for moisture.…”

“…To test whether larvae could play a role in nest defence against social parasites, we compared the rates at which larvae of the ant Formica fusca, a common host of temporary social parasites [33], consumed eggs laid by a queen that was a social parasite, a non-parasite, a nest-mate, or a conspecific non-nest-mate. For each species, we documented hatching success of eggs and egg size, as these traits may also influence a larva's decision to engage in egg consumption [34,35].…”

Brood recognition and discrimination in ants

“…Information about brood traits may also be useful in the context of brood cannibalism, i.e., when adult colony members consume brood, feed brood to larvae during periods of resource shortage (Crespi 1992), or when larvae selfishly consume other brood (Baroni Urbani 1991;Rüger et al 2007;Schultner et al 2013Schultner et al , 2014. According to inclusive fitness theory, least related brood should be consumed first, along with early brood stages that have not yet consumed large amounts of colony resources (Elgar and Crespi 1992).…”