Eavesdropping Predators

Many social interactions between insects involve the use of plant-borne vibrations. Many insects and spiders have evolved the ability to generate patterns of vibrations allowing them to communicate with potential mates or members of the same social group. It is now being researched that some spiders are able to forage through this form of communication. The predators use the vibrations that prey, such as the leafhopper, give off to track them down and consume the individual.

When leafhoppers and other such species send out vibrations along the leaf they increase the likelihood that a predator would pick up on these vibrations, managing to find the individual whom is sending out the vibrations. It has been known already that predators can exploit sight sound and smells of their prey but the fact that predators can pick up on vibration signals is a recent discovery.

By using molecular diagnostic methods with leafhopper specific primers it has been found that one species of spider is a frequent predator of the leafhopper. Predation was found to be much higher in sexually mature leafhoppers compared to the young leafhopper forms. These vibrations were once believed to be a private communication channel allowing a male to signal a female yet some predators can use these signals to track down the prey and eat it. The use of vibrations as a way to call a mate is why it is believed that younger forms of leafhoppers are not preyed on as greatly, as they do not give off the vibrations.  It was also found that males were consumed far more than females, which is believed to occur as the males are the individuals that give off the vibrations calling the females in, putting the males at a higher risk.

                                                                                    

An important question in the study of predator eavesdropping is how a particular signal in the environment becomes a foraging cue. Due to sexual selection there is an amazing amount of variety in the type of signals insect species use. For specialists the signals given off that guide the individuals to the prey may be predictable enough that natural selection can shape responses not dependent on experience. On the other hand for generalist’s predators they will have to learn the signals and gain experience to be able to track down their prey. Just like colouration is a warning signal for predators, some vibration signals may play a similar role allowing predators to learn which signals to track down and which to avoid.

Overall, little is known about the how predators use vibrations to track down their prey and the responses in which predators make remain speculative. As a greater amount of research goes in to deciphering the extent to which predators use the prey vibrations, the speculation will surely change.

Links

http://news.bioscholar.com/2011/03/killer-spiders-prey-on-insects-that-use-vibration-to-woo-mates.html

Cocroft, R. B. (2011) The public world of insect vibrational communication. Molecular Ecology, 20, 2041-2043.

Sexual Cannibalism in Wolf Spiders

Cannibalism occurs in a number of vertebrate and invertebrate species.  It is believed to have evolved as a way to gain a high-quality meal. Wilder and Rypstra did an experiment to test this hypothesis by looking at Wolf Spiders (Hogna helluo) and testing the amount of protein and lipid (fat) that a female would gain from a male H.helluo and a cricket. It was discovered in a previous study done in 2008 that only one third of females would engage in sexual cannibalism, which is a relatively low number if it was the best source of energy.

In the experiment Wilder and Rypstra placed female H.helluo into an arena and then added a male.  They then removed the male and placed in a cricket. Most food-limited females did not consume the male H.helluo but would eat the cricket as soon as it was placed in the arena. Wilder and Rypstra then went on to diagnose the protein and lipid value of both the male H.helluo and the cricket. It was found that the female consumed 51% of the male’s body but 72% of the cricket (which was the same size as the male H.helluo).

The female H.helluo took in similar amounts of protein from both the male and the cricket. Male H.helluo have a greater amount of protein than the cricket, yet with 47% of the protein present in males ingested and 67% of the cricket’s present protein ingested ,the totals were very similar. The difference in nutrition was when it came to the amount of lipids that the females gained from the prey. In both prey items, all the lipids that were present were ingested, but crickets have almost four times the amount of lipids than the male H.helluo.

This lipid to protein ratio becomes important when it is compared to the nutritional needs of egg production.  Egg production is limited not by protein, but by lipids, which could be an explanation of why females would prefer to prey on crickets rather than the males of their own species. Lipids also cause a higher proportion of females to be larger and heavier, allowing healthier and stronger individuals.  Producing egg sacs takes a lot of time and energy and a single cricket or male H.helluo doesn’t allow the females to produce any egg sacs. When the female H.helluo were fed a cricket every three to four days in the lab, they produced egg sacs after 85 days, give or take 11 days either side.

There are a number of other reasons why females may not feed on their mates.  These include risk of disease transmission and the risk of male retaliation. If the males were to fight back it could turn out negatively, with the females getting injured. This could account for why females are more likely to attack smaller males. There is very much a ‘cost versus benefit’ debate that goes on with cannibalism and could explain why hungry females avoid attacking males of the same species. There needs to be more studies in the future to get a better and fuller picture of why sexual cannibalism occurs, including looking at other types of nutrients.

Links:

http://www.springerlink.com.ezproxy.lincoln.ac.nz/content/e16756544g5l4336/fulltext.pdf