Increased Risk Proneness or Social Withdrawal? The Effects of Shortened Life Expectancy on the Expression of Rescue Behavior in Workers of the ant Formica cinerea (Hymenoptera: Formicidae)
Increased Risk Proneness or Social Withdrawal? The Effects of Shortened Life Expectancy on the Expression of Rescue Behavior in Workers of the ant Formica cinerea (Hymenoptera: Formicidae)
Krzysztof Miler 0 1
Ewa J. Godzińska 0
Beata Symonowicz 0
0 Laboratory of Ethology, Department of Neurophysiology, Nencki Institute of Experimental Biology of the Polish Academy of Sciences , Pasteur St. 3, 02-093 Warszawa , Poland
1 Institute of Environmental Sciences, Jagiellonian University , Gronostajowa St. 7, 30-387 Kraków , Poland
In social insects behavioral consequences of shortened life expectancy include, among others, increased risk proneness and social withdrawal. We investigated the impact of experimental shortening of life expectancy of foragers of the ant Formica cinerea achieved by their exposure to carbon dioxide on the expression of rescue behavior, risky pro-social behavior, tested by means of two bioassays during which a single worker (rescuer) was confronted with a nestmate (victim) attacked by a predator (antlion larva capture bioassay) or immobilized by an artificial snare (entrapment bioassay). Efficacy of carbon dioxide poisoning in shortening life expectancy was confirmed by the analysis of ant mortality. Rescue behavior observed during behavioral tests involved digging around the victim, transport of the sand covering the victim, pulling the limbs/antennae/mandibles of the victim, direct attack on the antlion (in antlion larva capture tests), and snare biting (in entrapment tests). The rate of occurrence of rescue behavior was lower in ants with shortened life expectancy, but that effect was significant only in the case of the entrapment bioassay. Similarly, only in the case of the entrapment bioassay ants with shortened life expectancy displayed rescue behavior after a longer latency and devoted less time to that behavior than ants from the control groups. Our results demonstrated that in ant workers shortened life expectancy may lead to reduced propensity for rescue behavior, most probably as an element of the social withdrawal syndrome that had already been described in several studies on behavior of moribund ants and honeybees.
Introduction
Risk proneness and individual life expectancy of social insect workers are strongly
connected. Such workers as a rule engage in intranidal tasks at the start of their
adult life and then switch to more risky extranidal activities as they age
(Wilson
1971; Hölldobler and Wilson 1990, 2009)
. Delayed performance of risky tasks
enhances mean worker longevity, which is beneficial for the colony
(Jeanne 1986;
O’Donnell and Jeanne 1995; Woyciechowski and Kozłowski 1998; Tofilski 2002,
2009)
. Many studies provided evidence that participation in risky tasks involved in
foraging or colony defense is indeed delayed until late in worker life
(for examples
in honeybees, ants, and wasps see Winston and Katz 1981; Schmid-Hempel and
Schmid-Hempel 1984; O’Donnell and Jeanne 1992)
. Other studies demonstrated
that experimental shortening of worker life expectancy accelerates the transition
from intranidal tasks to foraging in ants (
Moroń et al. 2008
) and honeybees
(Woyciechowski and Moroń 2009)
. Such acceleration of worker behavioral
development was observed irrespectively of experimental treatment applied to reduce
worker life expectancy: poisoning with carbon dioxide, physical harm, or parasitic
infection (
Moroń et al. 2008
; Woyciechowski and Moroń 2009). Ants with life
expectancy shortened either by poisoning with carbon dioxide, or by
removal of propodeal spines, were also more prone to engage in foraging under
more risky conditions (Moroń et al. 2012).
Decreased life expectancy impacts not only risk proneness, but also other aspects
of worker behavior. In particular, injured and/or infected ants were often observed
to reduce contacts with nestmate workers and/or brood by avoidance of interactions,
loss of attraction to social cues and spatial separation (staying away from brood
chambers or even leaving the nest altogether)
(Hölldobler and Wilson 1990;
Ugelvig and Cremer 2007; Aubert and Richard 2008; Heinze and Walter 2010;
Bos et al. 2011; Diez et al. 2015; Leclerc and Detrain 2017)
. Similar reduction of
caregiving behavior and/or altruistic self-removal of health-compromised workers
was also documented in the honeybees
(Wang and Moeller 1970; Shimanuki et al.
1994; Kralj and Fuchs 2006; Rueppell et al. 2010)
. In natural conditions shortened
life expectancy often occurs due to transmittable sickness. Therefore, it was argued
that social withdrawal of moribund individuals may benefit the colony by limiting
disease transmission
(Evans 1982; Heinze and Walter 2010; Rueppell et al. 2010;
Bos et al. 2011)
. However, it should also be noted that infected ant workers
frequently remain not attacked and even not avoided by their nestmates who
respond to them by hygienic behavior
(Ugelvig and Cremer 2007; Aubert and
Richard 2008; Heinze and (...truncated)