Predator-induced maternal effects determine adaptive antipredator behaviors via egg composition

Abstract

In high-risk environments with frequent predator encounters, efficient antipredator behavior is key to survival. Parental effects are a powerful mechanism to prepare offspring for coping with such environments, yet clear evidence for adaptive parental effects on offspring antipredator behaviors is missing. Rapid escape reflexes, or "C-start reflexes," are a key adaptation in fish and amphibians to escape predator strikes. We hypothesized that mothers living in high-risk environments might induce faster C-start reflexes in offspring by modifying egg composition. Here, we show that offspring of the cichlid fish Neolamprologus pulcher developed faster C-start reflexes and were more risk averse if their parents had been exposed to cues of their most dangerous natural predator during egg production. This effect was mediated by differences in egg composition. Eggs of predator-exposed mothers were heavier with higher net protein content, and the resulting offspring were heavier and had lower igf-1 gene expression than control offspring shortly after hatching. Thus, changes in egg composition can relay multiple putative pathways by which mothers can influence adaptive antipredator behaviors such as faster escape reflexes.

Keywords: C-start response; antipredator response; developmental plasticity; egg size; maternal effects.

Fig. 1.

Clutch characteristics of unfertilized eggs from the two parental treatments in experiment 1, and clutch size and gene expression of igf-1 in experiment 2. (A) Fresh egg weights measured right before amino acid quantification. Open circles: mean of five unfertilized eggs obtained per brood; solid circles and error bars: means ± SE across broods; N_control = 6 (two data points overlap) and N_predator = 5 for each treatment. (B) Total amino acids present in eggs. Open circles: mean of three eggs per brood; solid circles and error bars: means ± SE across broods. (C) Relative expression of igf-1 gene expression at day 3 (with respect to the reference gene rpl13a) and (D) day 10 of experiment 2. In the statistical models, igf-1 values on day 3 were subjected to a negative power transformation using the boxcox function, whereas those on day 10 were subjected to a positive power transformation. Here, the means (solid circles) and SE and individual data points (open circles) are back-transformed to their original scale. (A–D) Blue: control treatment; green: predator treatment; bars show mean ± SE; ***P < 0.001, **P < 0.01, *P < 0.05.

Fig. 2.

Behaviors assayed in the predator escape test in experiment 2. (A) Time until fish responded to the dropping marble by a C-start reflex. Time point “0” represents the instant the marble touches the water; most fish showed a negative latency, meaning that they responded to the marble before time point 0; open circles: single data points; solid circles and error bars: mean ± SE (see Movie S1). (B) Number of times the test fish visited the feeding plate during 15 min after the marble had been dropped. Open circles: single data points; box plots show medians, interquartile ranges; whiskers represent values in between the upper quartile and maximum value. (A and B) Blue: control treatment; green: predator treatment; *P < 0.05.

Fig. 3.

Experimental timeline and design for the predator escape test. (A) Timeline of experiments 1 (in blue) and 2 (in green) from exposure of parents to predator cues to the escape test. (B) Predator escape test (performed at day 90): a 200-L tank was virtually subdivided along its long axis into three vertical zones: a “shelter zone” with the shelter placed 15 cm away from the shorter edge of the tank, a 25-cm feeding zone with a feeding Petri dish placed at the edge of the boundary away from the shelter, and a 20-cm zone where a marble was dropped in a distance of 10 cm from the petri dish. White sand was used to provide sufficient contrast between a small focal fish and its background on the video recordings. A wooden “marble holder” was fitted and a needle was inserted to hold it in place. The needle, in turn, was attached to a string that could be pulled by the observer. The gray curtain separated the observer from the experimental setup, and the experiment was carried out as described in Escape test. After an acclimatization period, a few pieces of krill were placed on the feeding dish, and the observer waited for the fish to come to feed. The cameras were switched on, and the observer waited for the fish to take up the start position. As soon as the focal fish was spotted feeding on the dish, the needle, supporting the marble, was immediately pulled (within 1 to 2 s), and video recording was continued for 15 min after the marble drop.