The evolutionary loss of paternal care is associated with shifts in female life history traits

Parental care can increase the fitness of parents through increased
offspring survival but can also reduce reproductive output by limiting
time and energy allocated to additional mating opportunities. The
evolutionary origin of parental care is often associated with shifts in
life history traits (e.g., high investment in few, large offspring, slow
offspring growth), but little is known about whether the evolutionary loss
of care is associated with reciprocal shifts in the same life history
traits. Here, we capitalize on the divergence of parental care between
ecotypes of three-spined stickleback (Gasterosteus aculeatus) to test for
associations between parental care and life history traits. While males
from most stickleback populations provide care, an unusual “white” ecotype
has recently lost paternal care. We found support for the hypothesis that
the evolutionary loss of paternal care is associated with shifts in female
life history traits; relative to females of the ecotype with paternal
care, females of the white ecotype that lack paternal care produced
clutches with a similar overall mass and a greater number of smaller eggs,
despite their smaller body size, suggesting lower per-offspring
investment. We did not detect an ecotypic difference in embryonic
development rate, metabolic rate or offspring age at hatching, contrary to
the ‘safe harbor hypothesis’. These results support the theory that
behavioral traits such as parental care co-evolve with other life history
traits and highlight opportunities for future study of the underlying
causal mechanisms.

亲本照料（parental care）可通过提升后代存活率来增加亲本的适合度，但同时也会因限制了额外交配机会可分配的时间与能量，而降低繁殖产出。亲本照料的进化起源通常与生活史性状（life history traits）的转变相关（例如，对少量大型后代的高投入、后代生长缓慢），但目前学界对照料行为的进化丢失是否会伴随同类生活史性状的反向转变仍知之甚少。本研究利用三刺鱼（Gasterosteus aculeatus）不同生态型间亲本照料行为的分化，来检验亲本照料与生活史性状之间的关联。尽管多数刺鱼种群的雄性会提供父本照料（paternal care），但一种罕见的“白色”生态型近期已丢失了父本照料行为。我们的研究结果支持“父本照料的进化丢失与雌性生活史性状转变相关”这一假说：相较于保留父本照料的生态型雌性，缺失父本照料的白色生态型雌性尽管体型更小，但其产卵团的总质量相近，且产下的卵数量更多、个体更小，这表明其对单尾后代的投入更低。与“安全港假说（safe harbor hypothesis）”相悖的是，我们未在胚胎发育速率、代谢速率或后代孵化日龄上检测到生态型间的差异。上述结果支持“亲本照料等行为性状会与其他生活史性状协同进化”这一理论，并为后续探究其潜在因果机制提供了研究方向。