Animal Behavior Notes

Maladaptive signal responses

• Novel environment hypothesis: The current environment differs significantly from the environment in which the behavior evolved, so species haven't had enough time to adapt.

• Net benefit hypothesis: Sensory mechanisms may cause fitness losses for some individuals sometimes, but the average fitness gain from reacting to a sender outweighs these losses.

Illegitimate Signals

• Thynnine wasps pollinate elbow orchids because the orchids send illegitimate signals.

Elbow Orchids

• Elbow orchids exploit male wasps' ability to respond to olfactory and visual cues of female wasps.

• The orchid's lip petal emits odors similar to those released by calling females, attracting sexually motivated males.

Preexisting biases and evolution of deceitful signals

• Illegitimate senders can evolve if there's a legitimate communication system to exploit.

• Orchids exploit a preexisting sensory bias in male wasps that evolved sensitivity to the appearance and odor of conspecific females.

• The plant mimics the appearance and odor of female wasps.

• Plants that feed rather than deceive pollinators evolved "come-and-get-it" signals after pollinators' sensory capacities evolved.

Eavesdropping on others

• Male Photinus fireflies can be eaten when predators deceptively lure them to their deaths.

• Calling túngara frogs sometimes attract fringe-lipped bats instead of females.

• Predators that take advantage of prey signals are illegitimate receivers, listening in on individuals that lose fitness.

• Key difference between firefly and túngara frog examples: [unclear from the context]

Risk of Attack and Male frogs

• The risk of attack to male frogs is greater if the male call includes one or more chucks as well as the intro whine.

• Males in small groups are more likely to drop the chucks from their calls compared to males in larger groups due to greater risk of predation.

Ground-nesting birds

• Ground-nesting birds that encounter more mammalian predators tend to produce higher frequency calls that do not travel as far, better concealing the individual senders.

Predation risk and begging

• Clay eggs advertised by begging calls of the tree-nesting black-throated blue warbler were found and bitten significantly more often by eastern chipmunks than were the eggs associated with the calls of the ground-nesting ovenbird.

Reproductive behavior

• The two sexes of satin bowerbird look different and take on very dissimilar approaches to reproduction

• In most animals, males do the courting and females do the choosing

Sexual selection

• Why do extravagant courtship behaviors and morphological traits (e.g., ornaments and armaments) persist over evolutionary time?

• Sexual selection – “the advantage certain individuals have over others of the same sex and species in exclusive relation to reproduction” (Darwin 1871)

Reproductive skew

• Reproductive skew – unequal partitioning of reproduction within a population or social group; the sex with higher skew also tends to have greater variance in reproductive success

• In satin bowerbirds, there is high skew and reproductive variance among males, but much lower skew and reproductive variance among females, why?

• Very few (if any) females use the sperm of more than one male to fertilize their eggs

The fundamental asymmetry of sex

• Why is it more common for males to do the courting, and females to do the choosing?

• Anisogamy – the occurrence of gametes that differ greatly in size, which generates many observable sex differences

Bateman’s principle

• In fruit flies, a male’s reproductive success increases with the number of mates; also referred to as the Bateman gradient

• A female’s reproductive success is more dependent on partner quality (and ability to produce eggs and care for offspring) than on the number of male mates

• Bateman’s principle – males tend to have higher reproductive variance than females as a result of sex differences in mating behavior

• Slope of the gradient is indicative of the strength of sexual selection that a given sex experiences

Bateman’s principle - Patricia Gowaty

• Patricia Gowaty was unable to replicate Bateman’s experiments; she concluded that his method produced biased offspring number estimates of each sex by overestimating subjects with no mates and underestimated subjects with one or more mates

• Gowaty’s findings do not eliminate the key insights previously outlined by Bateman:

  • Higher variance in the number of mates and in offspring production are both indicative of the sex experiencing stronger sexual selection

  • Bateman gradients are typically steeper in the sex experiencing stronger sexual selection

• These insights apply equally well to both sexes; e.g., the same principles that act on male traits also act on female traits in many species

Are sperm always cheap?

• Spermatogenesis is not limitless and ejaculation can be costly

• Males of many species need to recover after ejaculating and often have fewer sperm in subsequent ejaculations

• In Soay sheep, predicted copulation rates (based on a male’s horn and hindleg length) were negatively related to the number of sperm per ejaculate

Cost-benefit approach to parental care

• Sex differences do not end with the investment in gametes and reproductive behavior; they also carry over to investment in offspring care

• Putting resources into large gametes and helping offspring become adults increases the probability that offspring will live long enough to reproduce and pass on parental genes to the next generation

• What a parent supplies to one offspring cannot be used to make additional offspring down the road; the parent must make a potential trade-off in current versus future reproduction

• Parental investment – the weighing of time/energy expenditures and risks taken by a parent to help existing offspring at the expense of reducing future reproductive opportunities

Parental investment takes many forms

• Parental investment takes many forms

Parental investment

• In most species, females are more likely to derive a net benefit from taking care of offspring because it is extremely likely to carry her genes

• A male’s paternity is often less certain, given that females of many species are inseminated by more than one male; males have less incentive to be parental if they lose fertilization opportunities in the process

• Although males and females of diploid species may have the same average reproductive success, they will differ in reproductive variance due to divergence in reproductive skew

Parental investment - sex ratio

• There are generally fewer sexually active females at any given time, creating a male-biased operational sex ratio

• Females that have already mated usually have less to gain by subsequent copulations (or because it is more energetically expensive to replenish eggs than sperm)

• Sex differences in behavior have apparently evolved in response to differences in the size and number of gametes produced, which is often amplified by variation in the degree to which a female and male provide parental care for their putative offspring