Sex on the beach

2 10 2018
Female green turtles (Chelonia mydas) spawning (top) and diving (bottom) on Raine Island (Great Barrier Reef, Queensland, Australia) — photos courtesy of Ian Bell. This species is ‘Endangered’ globally since 1982, mainly from egg harvesting (poaching conflict in Mexico for olive ridley Lepidochelys olivacea featured by National Geographic’s video here), despite the success of conservation projects (39). Green turtles inhabit tropical and subtropical seas in all oceans. Adults can grow > 150 kg and live for up to ~ 75 years. Right after birth, juveniles venture into the open sea to recruit ultimately in coastal areas until sexual maturity. They then make their first reproductive migration, often over 1000s of km (see footage of a real dive of a camera-equipped green turtle), to reach their native sandy beaches where pregnant females will lay their eggs. Each female can deposit more than one hundred eggs in her nest, and in several clutches in the same season because they can store the sperm from multiple mating events.

When sex is determined by the thermal environment, males or females might predominate under sustained climatic conditions. A study about marine turtles from the Great Barrier Reef illustrates how feminisation of a population can be partitioned geographically when different reproductive colonies are exposed to contrasting temperatures.

Fortunately, most people in Western societies already perceive that we live in a complex blend of sexual identities, far beyond the kind of genitals we are born with. Those identities start to establish themselves in the embryo before the sixth week of pregnancy. In the commonest scenario, for a human foetus XY with one maternal chromosome (X) and one paternal (Y) chromosome, the activation of the Sry gen (unique to Y) will trigger the differentiation of testicles and, via hormonal pathways, the full set of male characteristics (1).

Absence of that gene in an XX embryo will normally lead to a woman. However, in just one of many exceptions to the rule, Sry-expression failure in XY individuals can result in sterile men or ambiguous genitals — along a full gradient of intermediate sexes and, potentially, gender identities. A 2015 Nature ‘News’ feature echoes two extraordinary cases: (i) a father of four children found to bear a womb during an hernia operation, and (ii) a pregnant mother found to host both XX and XY cells during a genetic test – with her clinical geneticist stating “… that’s the kind of science-fiction material for someone who just came in for an amniocentesis” (2). These real-life stories simply reflect that sex determination is a complex phenomenon.

Three ways of doing it

In nature, there are three main strategies of sex determination (3) — see scheme here: Read the rest of this entry »

Food for sex

18 03 2013
Quercus_KakFeed Photo
Kakapo are unique among the ~ 400 parrot species (Psittaciformes) for being flightless, nocturnal and extremely long-lived (up to 100 years!). Additionally, they are herbivorous (seeds, fruits, polen, plants), males can weigh up to 2-4 kg (40% heavier than females), and females lay their eggs on the ground or cavities – i.e., 3 eggs in a single clutch annually, although 2 clutches might occur if the nest fails at the beginning of the reproductive season or if the eggs are taken for artificial incubation.Native to New Zealand, kakapo once inhabited the subalpine fringes of forest and scrub. Polynesians (1000 years ago) and Europeans (mostly in the XIX Century) arrived in the archipelago accompanied by dogs, cats, rats and mustelids that cornered kakapo populations in the Fiordland region (south-west of the South Island) where it was declared extinct in 1989. In 1977, a population of some 200 individuals was found on Stewart Island – this population was already in decline to the claws and jaws of feral cats. By the 1980s, the failure of captive breeding programs prompted the transfer of 60 individuals from Steward to carnivore-free islands. The global (known) population ‘rocketed’ from 50 individuals in 1999 to 126 in the 2012 censuses and, consequently, the kakapo’s IUCN status changed in 2000 from ‘Extinct in the Wild’ to ‘Critically Endangered’. Under the management of the Kakapo Recovery Programme, kakapo are now present on the islands of CodfishAnchor and Little Barrier.

Inbreeding, system shocks caused by fire or cyclones (for example), or demographic stochasticity (by which two or more outcomes are possible) such as how many males and females will be born in a single year, are all factors that threaten the persistence of small and fragmented populations. They can, however, be reverted by conservation actions.

If you have ever taken dancing classes, you will be familiar with the scarcity of male partners and how this can jeopardize group learning. When reproduction, rather than salsa pirouettes, is at stake, a biased sex ratio can compromise the persistence of species. For instance, when females are unable to find males (or vice versa), fertility rates can collapse as a result – a well-known cause of an Allee effect (1). Curiously, natural selection can promote such bias by favouring a species’ investment in litters dominated by one of the two genders. The evolutionary formulation of such scenario is that females can adjust the sex ratio of their offspring depending on the amount of available resources (2) – see contrasting cross-taxa studies on this subject (3-5). Thus, when resources abound (e.g., food), mothers can afford the offspring’s gender requiring more resources to reach adulthood or once adulthood is reached, is less likely to reproduce successfully (6). This predisposition to one gender or another can be key to the conservation of endangered species (7).

The kakapo case

At the end of the 1990s, the New Zealand Department of Conservation placed dispensers of supplementary food in the territories of some kakapo (a rather enormous, flightless parrot Strigops habroptilus) to encourage their reproduction. Back then, only 60 individuals were left of the entire species . Unfortunately, those females with access to the supplemental food conceived 67% of male chicks (so exacerbating the fact that kakapo populations are naturally male-biased), while those females without extra feeding had 71% of female chicks (8). Something wasn’t working. Read the rest of this entry »