Effects of food, kinship, and density on the longevity of spiderlings

Cannibalism is a behavioral characteristic found in a wide variety of animal groups. Although the rates of cannibalism can vary from one group to another, studies indicate that the main factors contributing to an increase in the frequency of such behavior are the availability of food, population density, the behavior and availability of victims, and environmental stress. We carried out different laboratory experiments to assess whether different factors such as the presence or absence of food among siblings and non-siblings, and at different densities among conspecific and heterospecific individuals, affect longevity of recently emerged Brazilian brown recluse (Loxosceles intermedia Mello-Leitão, 1934) and Chilean recluse (Loxosceles laeta (Nicolet, 1849)) spiderlings during periods of starvation. The results revealed that the survivorship of L. laeta during starvation was significantly higher than that of L. intermedia and that the addition of conspecific individuals increased survival rates by 1.5- and 1.6-fold, respectively. The tolerance of conspecifics differed between the two species, and generally, cannibalism was not observed, probably due to the risk of predation and limited consumption by weakened spiders, which coincided with the continued availability of endogenous vitelline reserves, thus indicating that the use of these spiderlings as a food resource may act to regulate starvation in more resistant spiders. The greater longevity and conspecific tolerance of L. laeta may be important factors contributing to the establishment of large populations of this spider in restricted areas, whereas for L. intermedia, hunger probably functions as a trigger for dispersal.

Microhabitat and spatial complexity predict group size of the whip spider Heterophrynus batesii in Amazonian Ecuador

The process by which solitary animals evolve to live in groups is a central question in biology. Conspecifics that compete for patchy resources may evolve incipient social behaviours such as group formation and conspecific tolerance when persistent aggressive interactions reduce resource access. In this investigation, a facultative group living species of whip spider was studied to understand the microhabitat resources that support group formation. Although most species of whip spider are solitary and oftentimes cannibalistic, Heterophrynus batesii sometimes aggregate in small groups at the bases of tree trunks. Twenty-five groups of whip spiders and associated tree buttress microhabitats were surveyed at Tiputini Biodiversity Station bordering Yasuní National Park in Eastern Amazonian Ecuador. Tree buttress complexity, surface area and other microhabitat variables were recorded. Heterophrynus batesii aggregated in groups of 2–8 animals and used large, buttressing, complex trees with more leaf litter relative to those available in the environment. This study showed that large groups of whip spiders require more complex microhabitats than smaller groups that were associated with more variable microhabitat parameters. These microhabitats act as patches of limited resources important for the species, and may have led to the evolution of tolerance and facultative group living.