Serotonin and aggressive motivation in crustaceans: altering the decision to retreat

Abstract

In crustaceans, as in most animal species, the amine serotonin has been suggested to serve important roles in aggression. Here we show that injection of serotonin into the hemolymph of subordinate, freely moving animals results in a renewed willingness of these animals to engage the dominants in further agonistic encounters. By multivariate statistical analysis, we demonstrate that this reversal results principally from a reduction in the likelihood of retreat and an increase in the duration of fighting. Serotonin infusion does not alter other aspects of fighting behavior, including which animal initiates an encounter, how quickly fighting escalates, or which animal eventually retreats. Preliminary studies suggest that serotonin uptake plays an important role in this behavioral reversal.

Figure 1

Serotonin effects on crayfish fighting: Univariate analyses. Mean ± SE of intensity (maximum level reached during each interaction) and duration from 1453 agonistic interactions are summarized for infusions of 5HT (Upper) and OA (Lower). Dominance is established within the first 30 min of the experiment (preinjection period) and escalated fighting subsides. No increases in fighting behavior are observed during saline infusion (0.125 M NaCl, hatched period). The infusion of 5HT in saline into the subordinate animal (Upper, gray area) is accompanied by a constant rise in the duration and intensity of agonistic encounters. After the 5HT infusion is turned off, the levels of fighting return to those of the preinjection period within 30 minutes. Infusion of OA into subordinate animals (Lower, gray area) does not significantly alter fighting.

Figure 2

Serotonin effects on crayfish fighting: Multivariate analysis. When 5HT is infused into the hemolymph of crayfish, all behavioral characteristics of fighting change significantly. Measures of intensity and duration are highly correlated in escalated fights and are dependent on the identity of the animal initiating the encounter or retreating. Therefore, a multivariate approach was used to identify the behavioral aspects most altered as a result of 5HT infusion. For this analysis, interactions were assigned to one of six experimental groups: preinjection (PRE, gray), control injection (C, blue), first (S1, red) or second (S2, red) half hour of 5HT infusion, and first (P1, yellow) or second (P2, yellow) half hour of post injection. Discriminant function analysis identified the behavioral characteristics that were most useful in distinguishing among the treatment groups. The results are summarized as a canonical centroid plot, in which each measured variable (e.g., duration or intensity of interactions) is indicated as a vector whose length represents its relative importance. (A) Significant separation is provided by the duration (dur) of the interactions with little additional information contributed by the intensity (int), which animal approaches (appr) or which retreats (retr). (B) The agonistic interactions for each treatment group are summarized as a multivariate mean (centroid) with 95% confidence limits. The duration vector is not shown at its full length in this graph. The overlap of the preinjection (PRE), control injection (C), and second postinjection (P2) periods indicates a similarity in their behavioral characteristics. With 5HT infusion, the fights move from the center of the graph toward longer durations, with only minor contributions from changes in intensity.

Figure 3

Changes in mean fight duration with 5HT and Prozac treatment. Each pair of bars represents the mean fight duration during the first and second 30 min of treatment with the agent. Compared with the average duration of fights in control (preinfusion) periods (average, 41 s), the length of encounters increases in the presence of 5HT (left two bars). Acute Prozac (final hemolymph concentration 1–2 × 10⁻⁵ M) results in a slight decrease in fight duration (middle two bars). The infusion of Prozac together with 5HT led to a significant reduction in the 5HT-mediated increases in fight duration (right two bars). As anticipated, similar results were seen in measurements of fight intensity (not shown).