Interindividual Behavioral Variation: Week 1

Introduction

Phenotypic and behavioral variation persists even within monomorphic populations of social organisms. In social invertebrate colonies, such individual behavioral differences—frequently conceptualized as animal "personality" or behavioral syndromes—can underpin ergonomic division of labor and collective foraging strategies.

In this study, we investigated whether consistent inter-individual variation in boldness and aggression correlates with body size in the social spider genus Stegodyphus. We simulated predatory threat and tactile competition to quantify behavioral responses.

Primary Hypothesis: Larger individuals exhibit greater physiological resilience, translating to increased boldness and higher aggressiveness, manifesting as a positive correlation between boldness and aggression.

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Materials and Methods

Materials

• 6 social spiders (Stegodyphus sp.)
• Petri dishes, soft camel-hair brushes, fine forceps
• 10 mL syringes for pneumatic puffs
• Digital stopwatches and blunt tactile probes
• Multi-color acrylic non-toxic paint for individual identification
• Millimeter graph paper for optical scale calibration

Methods

Cephalothorax Length & Somatic Size

Cephalothorax length was measured as a proxy for body size. Each spider was imaged in an open petri dish atop calibrated graph paper, and lengths were extracted in Fiji.

Figure 1: Stegodyphus specimen positioned on calibrated grid paper for cephalothorax measurement.

Boldness Assay (Anti-Predatory Huddling Recovery)

Boldness was quantified by measuring latency to recover from simulated avian predation (rapid air puffs from an air syringe). In response to aerodynamic disturbance, spiders contract into an immobile huddle. Latency to uncurl and resume locomotor exploration was recorded (cutoff threshold: 600 seconds):

$$\text{Boldness Score (s)}=600\text{ s}-\text{Latency to Uncurl (s)}$$

Higher scores denote greater boldness (rapid recovery), while lower scores represent shy phenotypes.

Aggression Assay

Tactile defense was scored by prodding the abdominal cuticle with a blunt probe, scored along an ordinal behavioral response scale:

Response	Score	Description
Huddle	1	Tight protective leg curl
Run	2	Rapid evasive displacement
Walk	3	Calm directional walking
Lurch	4	Forward lunging movement
No response	5	Neutral indifference
Leg Raise	6	Active threat display (front legs elevated)

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Data

ID	Cephalothorax Length (cm)	Boldness Time (s)	Boldness Score (s)	Aggression Score	Behavioural Response
1	0.222	60	540	1	Huddle
2	0.284	57	543	1	Huddle
3	0.334	23	577	1	Huddle
4	0.255	20	580	3	Walk
5	0.463	69	531	3	Walk
6	0.253	64	536	6	Leg raise

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Results

Boldness score vs. cephalothorax length	Aggression score vs. cephalothorax length
Boldness score vs. aggression score	Classification distribution (Bold $>400\text{s}$, Shy $\le 400\text{s}$)

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Discussion

Contrary to initial hypotheses:

1. Boldness vs. Body Size: Boldness exhibited a weak negative trend with cephalothorax length rather than positive scaling.
2. Aggression vs. Body Size: No discernible correlation was detected between body size and threat posture intensity.
3. Syndrome Coupling: Boldness scores exhibited a slight negative association with aggression.
4. Cohort Skew: All 6 tested spiders fell comfortably into the "bold" classification ($>400\text{ s}$), indicating that baseline communal laboratory handling or small cohort size ($N=6$) created an attenuated phenotypic range requiring longitudinal multi-week tracking.

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References

1. Pruitt, J.N. and Riechert, S.E. (2011). How within-group behavioural variation may influence social dynamics. Animal Behaviour, 82(1), pp. 141–148.
2. Sih, A., Bell, A. and Johnson, J.C. (2004). Behavioral syndromes: an ecological and evolutionary overview. Trends in Ecology & Evolution, 19(7), pp. 372–378.