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Open Access Publications from the University of California

Space Use and Reproductive Success of Male Sea Otters

  • Author(s): Tarjan, Lily Maxine
  • Advisor(s): Tinker, Martin T
  • Estes, James A
  • et al.

Animal space use determines access to resources, such as food and mates, and has implications for animal mating systems. Polygynous mating systems emerge when males, who typically exhibit low parental investment, monopolize females, who typically exhibit high parental investment, by defending patches of resources or aggregations of females. Defense of resource-containing territories incurs energetic costs and detracts time from essential activities such as foraging. Understanding mating systems is therefore key to interpreting processes that limit population growth in threatened species such as sea otters. Reproductive success of male sea otters may be related to defense of small territories (2–6 km2) in shallow coastal waters. The absence of strong seasonal synchrony in female estrus requires that males defend territories year-round, suggesting a high cost to territory defense. My objectives were to: (1) characterize male space use, (2) quantify male reproductive success, and (3) describe the sea otter mating system. This research provided a new method of estimating animal space use in restricted habitats. The method estimates the probability of space use based on features of the physical environment (e.g. water depth) and effectively excludes unsuitable areas from home range estimates. I applied this method to VHF radio-telemetry data of 72 male sea otters in California and distinguished between three distinct space-use tactics, constituting territorial, satellite, and transient tactics. Tissue samples were collected from 67 males and 215 females in Monterey, California in 1999–2012. I assigned 37 microsatellite nuclear DNA markers to these samples, conducted paternity analyses, and provided the first estimate of male reproductive success in sea otters. Paternity analyses matched 40 father-pup pairs. Paternity assignments were low overall; individual males were assigned a maximum of 3 pups. Reproductive success peaked at seven years of age, and continued to 14. Territorial males who defended abundant kelp canopy experienced elevated siring success, but satellite males also sired pups. The sea otter mating system is thus characterized by resource defense polygyny. This work provides a foundation for synthesizing the energetic and reproductive tradeoffs of space-use tactics and better understanding the population biology of sea otters.

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