Document: IAN-3-65-48

Extreme variation in body size along a relatively narrow altitude gradient in the alpine tree weta Hemideina maori (Orthoptera: Stenopelmatidae).

JAMIESON, I.G.* and J.ROCK

University of Otago, Dunedin, New Zealand ¹

Abstract:
Evolutionary ecologists have long been aware of latitudinal and altitudinal variation in body size in ectotherms. The most common pattern found is as temperatures decrease with increasing latitude/altitude, maturation tends to be delayed and thus adults emerge at a larger body size. We have recently discovered a body size gradient over a relatively small increase in altitude (450 m) in the alpine tree weta Hemideina maori, a large flightless cricket-like insect endemic to New Zealand. Although there are advantages of having large body size for mate competition in male weta and increased fecundity for female weta, adults found at lower altitudes (~1000 m) not only have significantly smaller body sizes but mature two instars earlier than weta found near the summit (1450 m). We tested two hypotheses derived from life-history theory to explain this pattern. 1) Increased temperature can cause rates of metabolism to increase, bringing age-related adult mortality forward and under such conditions, selection would favour early maturation; and 2) The distribution of weta at their lower/warmer altitude range overlaps with the common gecko, Haplodactylus maculatus, a potential predator of juvenile weta (but not adults), and increased risks in juvenile mortality would also select for early maturation. Data on adult survival rates, predator distribution and weta body size did not support either of these hypotheses. Further common-environment and translocated experiments are planned but in the meantime we conclude that size variation in weta is possibly a non-adaptive developmental consequence of temperature induced heterochrony.

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This abstract is being presented at: 8:45 AM in session:
Oral Session #4: Herbivore Responses to Plants.