L2- Counter Intuitive

Looking at paradoxical behaviours in the context of Darwinian evolution:

Q→ why do humans eat so many sweets and soft drinks if they are so harmful?

looking at the proximate and ultimate reasons:

Evolutionary hypothesis for behaviours at two different levels:

Group selection
Individual selection

e.g. Langur monkeys infanctide:

live in social groups, 1 male, multiple females
male is challenged, displaced and then all the infants are found dead
new male is the main suspect, if caught the females will attack him
Q→ why do males kill the offspring?
3 hypotheses:
- Non-adaptive→ social overcrowding
- Favoured by natural selection at the individual level→ females become pregnant sooner, more offspring before being overthrown
- Favoured by natural selection at the group level→ reduces overcrowding and overexploiting of resources
evidence:

is unlikely as this only happens after a takeover and not more in large groups
most likely true
group selection can theoretically occur but the circumstances that favour group selection are usually very rare

When can group selection occur?

individuals have certain behaviours that are adaptations that assist the survival of the group
e.g. buffalos reduce their reproductive rate when the group is too populated, reduces the number of individuals, avoids overexploitation
groups with individuals that have the self-restraining gene survive better than group with individuals that have only the selfish gene
however, individual selection is usually stronger than group selection

Individual selection:

red individuals→ non-restraining, selfish, reproduce as fast as possible
blue individuals→ restraining, altruistic, defend, reproduce slower
→ individual selection favours red individuals, red outnumbers the blue, red pass on the selfish gene, spreads through the population

Group selection:

two different groups compete for space/resources → the group with more blue individuals has the advantage as they have more defenders
within group level favours red (individual selection)
between group level favours blue (group selection)
group selection is only favoured when:
- groups are long lived compared to indivduals
- inter-group competition is important
→ langars are at an individual level (hypothesis 2 is correct)

Other examples of infanticide:

lions, langars→ males compete for females, males commit infanticide
giant waterbugs, jacana birds→ females compete for males, females commit infanticide
meerkats:
- live in social groups, 1 dominant female, subordinate females
- the dominant and some subordinate females breed
- pregnant females carry out the infanticide
- dominant offspring:
  - when no others are pregnant, there is nearly 100% survival of dominant offspring
  - when others are pregnant, ~55% survive
    → survival of dominant females litter is significantly lower if others are pregnant in the group
- subordinate offspring:
  - when no others are pregnant, there is ~70% survival rate
  - when dominant is pregnant, there is ~15% survival rate
  - when other subordinates are pregnant, ~30% survival rate
  - when both dominant and subordinate are pregnant, ~10% survival rate
    → survival of subordinate females litter is significantly lower if others are pregnant in the group

Why do meerkats carry out infanticide?

infanticide is adaptive
pregnant females kill other offspring so they cannot compete with their unborn offspring for resources
increases the fitness by increasing the eventual quality of offspring

Other ways meerkats solve this issue:

dominance can suppress subordinate females from breeding in the first place
has costs and benefits

Self-sacrifice in Worker Honey Bees:

when a bee stings another animal, she leaves the sting and part of her abdomen behind too (autotomy), then dies
the sting has complex adaptive behaviours when not attached to bee:
- is easy to go in
- is hard to get out
- has 3 barbed stings that pump venom into the intruder
- has an alarm pheromone that guides other worker bees to the intruder and makes them more prone to sting the intruder→ this drives the intruder away from the nest
Q→ why do honey bees sacrifice themselves?
- worker bees carry out reproductive altruism→ do not reproduce, rear close relatives, genes are passed on indirectly through kin selection
  → by sacrificing themselves, they save their reproductive investments instead and pass on genes by protecting kin

Sex change in Anthias Fish:

when a large dominant female dies, this leaves an opening in the population
a female grows a whole new set of gonads and changes into a male
Q→ why do female anthias fish change into males?
- increases individual fitness
- is more beneficial for them to be a dominant male to mate with lots of females

Spiders:

can detect smallest movements of prey
prey are a similar size to male spiders
male spiders have many strategies to not be eaten by female spiders:

Garden spider→ finds a female, waits for her to mature, plucks strands of the web as he approaches her, does not always work
Crab spider→ male grabs female, waits until she stops struggling, ties her up, moves underneath her, copulates, leaves her to get out
Nuptial gift→ food offerings of insects wrapped in silk, female is distracted with eating whilst the male copulates

Latrodectus mactans→ North American black widow, female eats male after copulation
Latrodectus hasselti→ Australian redback, sister species, male is deliberately eaten whilst copulating
→ Latro- latin for servant/robber, Dektres- greek for biter

Redback spiders (studied by Maydiane Andrade):

the female is much larger than the male (f- 450mg, m- 5mg)
the female lives longer than the male (f- 2 years, m- 2-4 months)
males mature at a younger age than females
females recopulate with many males

Redback spider copulation:

male aligns himself facing forward on her abdomen
male inserts sperm transferring organ into her reproductive tract
male elevates himself on his hindlegs, summersaults backwards into her jaw
female eat male whilst the sperm transfer is taking place
Q→ what is the fitness benefit of being eaten by the female?
- female eating male does not increase egg production
- benefit depends on if male is the first or second to copulate:

First male:

if first male is eaten, 67% females reject a second male
if first male is not eaten, 4% females reject a second male
→ female is highly significantly less likely to remate if she eats the first male, increases males chance of paternity

Second male:

males that are cannibalised copulate for longer
the longer the second male copulates, the more offspring he will gain

→ both the first and second males are eaten to increase their paternity, males are unlikely to find a second female in their short lifetime

Bean Weevil Copulation (Callosobruchus maculatus):

male penis tip is covered in spines that penetrate the lining of the female reproductive tract
male physically abuses the female so they do not copulate again
findings:
- females that have mated with one male have significantly more damage to their reproductive tract than virgins
- females use back legs to kick males towards end of copulation
- when the back/front legs of females are removed, females are significantly less likely to dislodge males
- females that kick have significantly less tract damage
- there is no difference in number of eggs laid in females that had one vs two damaging matings
- females that mates once live significantly longer than females who mates twice
conclusions:
- damage to the reproductive tract discourages remating
- kicking reduces the amount of time spent copulating
- kicking reduces the amount of damage caused
- the damage does not affect fecundity
- damage does reduce lifespan though
→ males are currently winning this battle of the sexes (no fitness cost to them but significant fitness cost to females that die earlier) but it could switch in favour of the females in the future though
- dynamic flux system→ females always trying to overcome the males, males always trying to overcome the females