Factors Affecting the Spatial Distribution of Oviposition Sites for Tandem Black Saddlebags Dragonflies (Odonata: Libellulidae).

Behavioral decisions made during the selection of oviposition sites may have a strong effect on the fitness of both parents and offspring (Resetarits 1996). Behaviors associated with oviposition site selection are particularly interesting in animals such as odonates (i.e., dragonflies and damselflies), because the terrestrial adults must find optimal sites for the success of their aquatic offspring. Buskirk and Sherman (1985) suggested that a female’s choice of oviposition site may be strongly affected by the ecology of their aquatic larvae. Furthermore, individual female odonates may concentrate most of their eggs in one location (e.g., Perithemis tenera; Jacobs 1955) or spread them out over a larger area within or among breeding sites (e.g., Pantela flavescens; Schenk et al. 2004). This spatial distribution of oviposition sites may relate to egg predation and intraspecific competition among larvae that a species experiences (Buskirk and Sherman 1985), and widely distributed oviposition sites may be a form of bet hedging, whereby the female spreads risk to cope with environmental uncertainty (Buskirk and Sherman 1985, Hopper 1999). The particular choice of oviposition sites, and the spatial distribution of sites, may also be affected by the interactions a female has with competing individuals (Koch 2006). For some species these interactions are positive, such as the aggregating behavior in ovipositing females of Sympetrum vicinum (McMillan 2000) and Argia moesta (Byers and Eason 2009). In these species, the presence of conspecific females may indicate a good oviposition site (McMillan 2000). Interactions may also negatively affect a female’s oviposition success. For example, during times of high male density, Koch (2006) observed some females adjusting by hovering closer to the water, a change that decreased male harassment but increased their risk of predation. Such male harassment is a consequence of sperm competition (Simmons 2001). Sperm competition exists in many odonates because the last male to copulate with the female prior to oviposition has a higher probability of fertilizing her eggs (Waage 1984, Alcock 1994). Odonate species with last male advantage often guard their females either by ovipositing in tandem (in physical contact with female), or by accompanying the female without contact (Waage 1984, Corbet 1999, Schenk et al. 2004). Some species participate in both types of mate guarding depending on environmental and social factors (Latty 2006). The type of mate guarding may also relate to the oviposition pattern of the female (Buskirk and Sherman 1985; Schenk et al. 2004). For instance, species that perform tandem oviposition typically have many widely distributed oviposition sites within a limited area (Conrad and Pritchard 1992), whereas noncontact guarding species localize oviposition sites (Schenk et al. 2004). Thus, links may exist between a species’ oviposition site choice and its mating system (Buskirk and Sherman 1985, Ware et al. 2012). Most studies of the spatial distribution among oviposition sites in odonates have focused on species that oviposit in multiple breeding sites (e.g., different ponds; Schenk et al. 2004). Relatively few studies have quantified the distribution of oviposition sites within a breeding site, especially for those species that have tandem oviposition, nor have studies typically investigated how this distribution is affected by interactions with the guarding male or other individuals. In this study, we investigated the oviposition site selection of the black saddlebags dragonfly (Tramea lacerata Hagen). T. lacerata is a widely distributed migratory species (Needham et al. 2000). The territorial males often patrol large areas along the edges of ponds or lakes, and the females arrive at the breeding site to copulate and oviposit in tandem with a male or, more rarely, to oviposit alone (Dunkle 1989). The oviposition behavior used by most Tramea sp. is particularly interesting because tandem formation is interrupted continually during oviposition (Young 1967, Sherman 1983). During tandem oviposition the male guides the female to a site, releases the tandem grip, and the female dips to the surface of the water to release eggs. Following oviposition the male grabs her again and the tandem proceeds to a different location (Sherman 1983, Dunkle 2000). Dunkle (1989) observed that T. lacerata tandems occasionally chase other dragonflies while searching for oviposition sites. Oviposition for T. lacerata typically occurs in open water or on algae mats (Dunkle 1989), and the larvae are often found among stems of aquatic vegetation or masses of algae (Needham et al. 2000). Specifically, in this study, we examine the following questions: (1) Do tandems choose particular locations in which to oviposit? (2) Are these locations concentrated or dispersed at the breeding site? (3) Is oviposition site selection affected by interactions with other dragonflies? (4) Does oviposition behavior differ between females ovipositing in tandem and those ovipositing alone? Based on the answers to these questions, we speculate on how oviposition site selection represents an outcome of selection pressures on the offspring, the male, and the female.