title: Within species support for the expensive tissue hypothesis: a negative association between brain size and visceral fat storage in females of the Pacific seaweed pipefish.
abstract: The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the high cost of brain development and maintenance is predicted to constrain adaptive brain size evolution (the expensive tissue hypothesis, ETH). Here, we test the ETH in a teleost fish with predominant female mating competition (reversed sex roles) and male pregnancy, the pacific seaweed pipefish Syngnathus schlegeli. The relative size of the brain and other energetically expensive organs (kidney, liver, heart, gut, visceral fat, and ovary/testis) was compared among three groups: pregnant males, nonpregnant males and egg producing females. Brood size in pregnant males was unrelated to brain size or the size of any other organ, whereas positive relationships were found between ovary size, kidney size, and liver size in females. Moreover, we found that the size of energetically expensive organs (brain, heart, gut, kidney, and liver) as well as the amount of visceral fat did not differ between pregnant and nonpregnant males. However, we found marked differences in relative size of the expensive organs between sexes. Females had larger liver and kidney than males, whereas males stored more visceral fat than females. Furthermore, in females we found a negative correlation between brain size and the amount of visceral fat, whereas in males, a positive trend between brain size and both liver and heart size was found. These results suggest that, while the majority of variation in the size of various expensive organs in this species likely reflects that individuals in good condition can afford to allocate resources to several organs, the cost of the expensive brain was visible in the visceral fat content of females, possibly due to the high costs associated with female egg production.
authors: Department of Ecology and Genetics/Animal Ecology Evolutionary Biology Centre Uppsala University Norbyvägen 18D SE-75236 Uppsala Sweden.
authors: Masahito
authors: 0000-0002-0144-2893
authors: M
authors: Tsuboi
authors: Center for Field Science of the Seto Inland Sea Hiroshima University 5-8-1, Minatomachi 725-0024 Takehara City Hiroshima Japan.
authors: Jun
authors: J
authors: Shoji
authors: Department of Biology Faculty of Agriculture and Life Science Hirosaki University 1-1, Bunkyo-cho 036-8560 Hirosaki Aomori Japan.
authors: Atsushi
authors: A
authors: Sogabe
authors: Department of Ecology and Genetics/Animal Ecology Evolutionary Biology Centre Uppsala University Norbyvägen 18D SE-75236 Uppsala Sweden.
authors: Ingrid
authors: I
authors: Ahnesjö
authors: Department of Zoology/Ethology Stockholm University Svante Arrhenius väg 18B SE-10691 Stockholm Sweden.
authors: Niclas
authors: N
authors: Kolm
keywords: Brain size evolution; Syngnathidae; sex‐role reversal; the expensive tissue hypothesis; trade‐off